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Preliminary Design ReviewNovember 12, 2004
Team 05512Brian Gonzales
Naanzem HoomkwapWilliam Lambert
Surat Teerakapibal
Department of Electrical EngineeringKate Gleason College of Engineering
Rochester Institute of Technology76 Lomb Memorial Drive
Rochester, NY 14623-5604
Executive Summary
Modern cars frequently come equipped with remote keyless entry systems. These
systems allow the automobile owner to perform a variety of tasks, including unlocking
doors, opening doors, opening trunks, arming alarms, and setting off panic alarms from a
maximum distance of 15 to 100 feet. Secondary uses for these systems have evolved as
their presence has become ever more ubiquitous. One such application is in locating a
vehicle lost in a parking lot – the automobile owner presses the “lock” or “panic” button,
causing the car horn to emit a beep. The user is then directed by sound to his or her car.
Some of these RKE applications benefit from a range which exceeds many of the
standard systems. The purpose of this project is to implement a non-intrusive method of
extending existing RKE systems ranges. In order to accomplish this, several versions of
repeaters are proposed. The advantages and disadvantages of each approach are
discussed, and a design for the circuitry is developed.
After feasibility assessments, an approach which could repeat RKE systems for a
wide variety of cars was developed. A receiver is implemented with high performance
components. A microcontroller then detects the incoming digital data at a much higher
rate than is being sent. The data is filtered then stored. When the end of the transmission
is detected, the microcontroller activates a small transmitter which repeats the signal to
the car.
The system developed in this paper will function for all RKE systems operating in the
315 MHz band using ASK modulation (covering most cars in the US). The system is
compact, battery powered, and requires no connections to the automobile that will
contain it.
05512 RKE Repeater 2
1. INTRODUCTION...............................................................................................................................7
1.1 BACKGROUND................................................................................................................................81.1.1 RKE Systems..................................................................................................................................81.1.2 Repeater Systems...........................................................................................................................9
1.2 PROJECT DESCRIPTION.........................................................................................................................101.3 PROJECT OBJECTIVE.............................................................................................................................101.4 PROJECT SCOPE....................................................................................................................................111.5 FUNDING..............................................................................................................................................11
2. SPECIFICATIONS..................................................................................................................................12
2.1 ANTENNA SPECIFICATIONS..................................................................................................................122.2 RECEIVER SPECIFICATIONS..................................................................................................................122.3 TRANSMITTER SPECIFICATIONS............................................................................................................132.4 CONTROL SPECIFICATIONS...................................................................................................................132.5 HOUSING SPECIFICATIONS...................................................................................................................142.6 LEGAL SPECIFICATIONS.......................................................................................................................15
3. CONCEPT DEVELOPMENT................................................................................................................16
3.1 ANTENNA.............................................................................................................................................163.1.1 Antenna Theory............................................................................................................................163.1.2 Antenna Simulation......................................................................................................................19
3.2 RECEIVER.............................................................................................................................................233.2.1 Receiver Theory...........................................................................................................................233.2.2 Receiver Possibilities...................................................................................................................27
3.3 REPEATER.............................................................................................................................................283.4 TRANSMITTER......................................................................................................................................323.5 FILTERS................................................................................................................................................34
3.5.1 Preselector...................................................................................................................................343.5.2 Intermediate Frequency Filter.....................................................................................................353.5.3 Transmitter Output Filter............................................................................................................353.5.4 Filter Designs...............................................................................................................................353.5.5 Active Filters................................................................................................................................363.5.6 Passive Filters..............................................................................................................................363.5.7 SAW Filters..................................................................................................................................40
3.6 SYSTEM CONTROL................................................................................................................................413.7 HOUSING..............................................................................................................................................42
4. FEASIBILITY ASSESSMENT...............................................................................................................43
4.1 ANTENNA.............................................................................................................................................434.2 RECEIVER.............................................................................................................................................434.3 REPEATER.............................................................................................................................................474.4 TRANSMITTER......................................................................................................................................494.5 FILTERS................................................................................................................................................51
4.5.1 Preselector...................................................................................................................................514.5.2 Intermediate Frequency Filter.....................................................................................................524.5.3 Transmitter Output Filter............................................................................................................53
4.5 CONTROLLER........................................................................................................................................544.6 REPEATER HOUSING.............................................................................................................................54
5. ANALYSIS AND DESIGN......................................................................................................................56
5.1 ANTENNA DESIGN................................................................................................................................575.2 RECEIVER DESIGN................................................................................................................................59
5.2.2 Low Noise Amplifier....................................................................................................................605.2.3 Mixer and IF Preamp...................................................................................................................60
05512 RKE Repeater 3
5.2.4 IF Limiting Amplifier with RSSI..................................................................................................615.3 TRANSMITTER......................................................................................................................................635.4 FILTERS................................................................................................................................................68
5.4.1 Preselector...................................................................................................................................685.4.2 Intermediate Frequency Filter.....................................................................................................695.4.3 Transmitter Output Filter............................................................................................................70
5.5 T/R SWITCH..........................................................................................................................................715.6 HOUSING DESIGN.................................................................................................................................725.7 CONTROL SYSTEM DESIGN..................................................................................................................755.8 IMPEDANCE MATCHING NETWORK......................................................................................................92
6. WORK COMPLETED............................................................................................................................93
BIBLIOGRAPHY.........................................................................................................................................97
APPENDIX A – FCC REGULATIONS.....................................................................................................99
APPENDIX B – BILL OF MATERIALS................................................................................................101
APPENDIX B – BILL OF MATERIALS................................................................................................101
Appendix C – Complete Circuit Schematic.................................................................................................102
05512 RKE Repeater 4
Table of FiguresTABLE 2.1: CONTROLLER RESPONSIBILITIES..................................................................................................14FIGURE 3.1: MININEC PROGRAM USED FOR ANTENNA MODELING...............................................................20FIGURE 3.2: RADIATION PATTERN OF ¼ WAVE DIPOLE..................................................................................21FIGURE 3.3: DIRECTIVE GAIN PATTERN OF THE QUARTER WAVE ANTENNA.................................................22FIGURE 3.4: CURRENT DISTRIBUTION OF THE QUARTER WAVE ANTENNA.....................................................23TABLE 3.1: ELECTRICAL CHARACTERISTICS FOR A QUARTER WAVE ANTENNA.............................................23TABLE 3.2: IMPORTANT MIXER PARAMETERS [RF DESIGN GUIDES]............................................................26TABLE 3.3: RECEIVER IC POSSIBILITIES........................................................................................................27FIGURE 3.6: DIGITAL WAVEFORM (ABOVE) AND CARRIER (BELOW) WHICH COMPRISE THE SIGNAL FOR ASK
...............................................................................................................................................................30FIGURE 3.7: EXAMPLE OF ASK MODULATION OF THE SEQUENCE “1 0 1 0 1 1 0 0”.....................................30FIGURE 3.8: SIGNAL CORRUPTED BY GAUSSIAN NOISE.................................................................................31FIGURE 3.9: SIGNAL AFTER GOING THROUGH THE ENVELOPE DETECTOR......................................................32FIGURE 3.10: TRANSMITTER BLOCK DIAGRAM...............................................................................................33TABLE 3.4: TRANSMITTER IC POSSIBILITIES..................................................................................................34FIGURE 3.11: A PASSIVE BANDPASS FILTER....................................................................................................37FIGURE 3.12: FREQUENCY RESPONSE OF A PASSIVE BANDPASS FILTER.........................................................37FIGURE 3.13: SOME LOWPASS IF FILTERS......................................................................................................38FIGURE 3.14: FREQUENCY RESPONSE OF VARIOUS IF LOWPASS FILTERS.......................................................38FIGURE 3.15: SOME TRANSMITTER PA LOW PASS FILTERS FOR DIFFERENT CUTOFF FREQUENCIES.............39FIGURE 3.16: FREQUENCY RESPONSE OF THE TRANSMITTER LOW PASS FILTERS...........................................39FIGURE 3.17: SAW FILTER.............................................................................................................................40FIGURE 3.18: SAW FILTER IMPEDANCE MATCHING......................................................................................41TABLE 4.1: WEIGHTED ANTENNA FEASIBILITY ANALYSIS..............................................................................43TABLE 4.2: FEASIBILITY ANALYSIS OF THE RECEIVER....................................................................................47TABLE 4.3: FEASIBILITY ASSESSMENT FOR THE REPEATER............................................................................49TABLE 4.4: FEASIBILITY ASSESSMENT FOR THE TRANSMITTER......................................................................51TABLE 5.1: ESTIMATED POWER DRAW FOR CIRCUITRY..................................................................................56TABLE 5.2: LINK BUDGET ANALYSIS.............................................................................................................57FIGURE 5.1: DIRECTIVE GAIN PATTERN OF THE QUARTER WAVE ANTENNA.................................................58FIGURE 5.2: CURRENT DISTRIBUTION OF THE QUARTER WAVE ANTENNA.....................................................58TABLE 5.3: ELECTRICAL CHARACTERISTICS FOR A QUARTER WAVE ANTENNA.............................................59FIGURE 5.3: THE FINAL ANTENNA DESIGN......................................................................................................59FIGURE 5.4: RFRD0420 PIN DIAGRAM...........................................................................................................60FIGURE 5.5: FULL RECEIVER SCHEMATIC......................................................................................................62FIGURE 5.6: TRANSMITTER SCHEMATIC.........................................................................................................63FIGURE 5.7: TRANSMITTER OUTPUT CIRCUITRY............................................................................................64FIGURE 5.8: SIMULATION OF TRANSMITTER OUTPUT CIRCUITRY.................................................................64TABLE 5.2: MAX1472 PIN DESCRIPTION.......................................................................................................66FIGURE 5.9: COMPLETE TRANSMITTER SCHEMATIC.......................................................................................67FIGURE 5.11: SAW FILTER FREQUENCY RESPONSE.......................................................................................69FIGURE 5.12: INTERMEDIATE FREQUENCY LOW PASS FILTER.......................................................................69FIGURE 5.13: IF FILTER FREQUENCY RESPONSE............................................................................................70FIGURE 5.14: TRANSMITTER OUTPUT FILTER.................................................................................................70FIGURE 5.15: TRANSMITTER FILTER FREQUENCY RESPONSE.........................................................................71FIGURE 5.16: TRANSMITTER/RECEIVER SWITCH............................................................................................72TABLE 5.4: ADG918 PIN DESCRIPTION..........................................................................................................72FIGURE 5.17: SPECIFICATIONS FOR THE HOUSING..........................................................................................74TABLE 5.5: CONTROL SYSTEM SPECS [MICROCHIP]......................................................................................76TABLE 5.6: CONNECTIONS NEEDED FOR MICROCONTROLLER.......................................................................76TABLE 5.7: PIN CONNECTIONS TO THE MICROCONTROLLER [MICROCHIP]....................................................77FIGURE 5.18: PIC16F87 MICROCONTROLLER SCHEMATIC DIAGRAM...........................................................78FIGURE 5.19: HIGH LEVEL OVERVIEW OF CONTROL SYSTEMS......................................................................79TABLE 5.8: PIN STATES AT POWER ON...........................................................................................................80
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FIGURE 5.20: POWER ON SEQUENCE..............................................................................................................81FIGURE 5.21: INTERRUPT HANDLING FOR CHANGE ON RB4...........................................................................82FIGURE 5.22: ROUTINE TO CHECK FOR VALID RECEIVED DATA AND ENTER RECEIVE MODE.........................84FIGURE 5.23: RATE INDEPENDENT DETECTION OF THE ASK SIGNAL.............................................................86FIGURE 5.24: ROUTINE FOR RECEIVING VALID DATA.....................................................................................87FIGURE 5.25: EXAMPLE OF CORRUPTED DATA...............................................................................................88FIGURE 5.26: THE BINARY FILTERING SUBROUTINE.......................................................................................89FIGURE 5.27: THE TRANSMIT ROUTINE...........................................................................................................91FIGURE 5.28: IMPEDANCE MATCHING NETWORK............................................................................................92FIGURE 6.2: TANK FILTER..............................................................................................................................94FIGURE 6.3: TANK FILTER FREQUENCY RESPONSE........................................................................................94Figure 6.4: First Generation Receiver Schematic...........................................................................................95
05512 RKE Repeater 6
1. Introduction
A large percentage of automobiles manufactured in the last 15 years have come
equipped with Remote Keyless Entry (RKE) systems. These systems allow the owner of
the automobile to perform basic functions such as locking the automobile, unlocking the
automobile, opening the trunk, starting the car, opening doors, setting alarms, or even
setting off a panic alarm by pressing a button of a remote control. The remote control is
often a small device attached to the keychain called a “key fob”. Having the ability to
perform these functions remotely allows the user to open a car quickly if needed, open a
trunk if his hands are full, unlock doors for everyone so they do not have to wait around
in inclement weather, and so on. Because of the usefulness of these features, RKE
systems have become nearly ubiquitous.
In addition to their intended functions, RKE systems have become commonly used as
locator devices for automobiles in parking lots. If an owner leaves his or her car at the
mall and can remember the general location of the car, he or she can press the “lock” or
“panic” functions, causing the automobile to sound the horn. This can greatly aid the user
in finding his or her car.
RKE systems are often implemented in lowest cost and least obtrusive manner
possible. Because of this, the systems on some automobiles are restricted to a very short
range, down to 15 feet, while others will function up to and over 100 feet. Sometimes, the
short range of the systems can be an unfortunate limiter to their utility. For instance,
someone looking for his or her car might want to be able to sound the horn from a
distance away. People with car alarms which go off frequently might want to be able to
turn them off without walking all the way out to their car.
05512 RKE Repeater 7
Such applications make extending the range of the repeater desirable. There are
obvious ways of doing this, for instance modifying the car to have a better antenna for the
RKE device. However, the average user does not necessarily want to modify his or her
car to improve RKE performance. Instead, a non-intrusive way of extending an RKE
systems range is desired. The best solution for to this problem was determined to be an
RKE repeater. The repeater would have a high performance antenna and receiver capable
of substantial gain over the system included in the automobile. The repeater would listen
for a transmission, store it, and then retransmit it to the car, allowing the car to perform
the requested operation. In this way, the range of the system could be transparently
extended with no modifications to the automobile required.
By using careful design, a cost effective, marketable product could be produced that
could ultimately be sold at an electronics store. If the range extension is significant, the
product would be useful to a wide range of consumers looking for the added advantages
of a more robust RKE system.
1.1 Background
1.1.1 RKE Systems
RKE systems generally communicate in an unlicensed portion of the spectrum
reserved for intermittent transmission of control data. This frequency is typically 315.0
MHz for US systems and 433.92 MHz for European systems. They key fob is generally
an extremely low power device, on the order of 1 mW.
When a key is pressed on the key fob, it translates it into a digital code of a few bits
in length. This code is combined with a pseudo-random hopping code (for security
05512 RKE Repeater 8
purposes). The whole code is then modulated into a radio signal, most often using
amplitude shift keying (ASK) modulation. Upon receipt of a transmission, the vehicle
verifies the security code and then executes the command.
The code that is sent has a random part and a fixed part. The fixed part of the code
tells the vehicle what function to implement. The random part is used by the vehicle to
verify that the signal it is receiving is from the owner and not from someone else’s key
fob. The key fob and the vehicle both have random number generators that are seeded the
same. The vehicle stores the next 256 possible numbers and checks to see of the signal
sent has one or those numbers. If it does, then the vehicle will respond to the signal. If it
does not, then receiver ignores the signal. The random code ensures that vehicle does not
respond to anything other then the owner’s key fob.
1.1.2 Repeater Systems
A repeater is a device used in communication systems to extend the range of an
existing communication system. Repeaters are used often in today’s wide cell phone
networks and radio networks. Repeater stations and towers are common throughout the
United States. Because of this there are a wide range of standard repeater designs. These
designs all have the same basic process - a signal is received through an antenna, the
frequency of the signal is shifted, and the signal is retransmitted through the same
antenna.
If the signal in a standard repeater system is not shifted it would be impossible to
simultaneously receive and then retransmit the signal through the same antenna because
the power out of the transmitter would overload the receiver. The receiver for the repeater
05512 RKE Repeater 9
and the transmitter for the receiver must be isolated. Both the receiver and the transmitter
will have a narrow bandpass filter in front of them (called a duplexer). As long as the
output signal is shifted enough to be attenuated by the receivers filter, then there will be
sufficient isolation between the receiver and the transmitter. Without sufficient isolation
there will be positive feedback in the repeater.
If retransmission is on the same frequency, two approaches may be used. First, two
highly isolated antennas may be used. In such a system, the gain of the repeater must not
exceed the isolation between the antennas. In the second system, the signal is recorded
until the end is reached. Once the end is reached, the signal is transmitted again. This is
called a “Parrot” repeater. According to FCC regulations, such a device is not a repeater.
1.2 Project Description
Existing RKE systems work at ranges between 15 and 100 feet. This purpose of this
project is extend the range of the existing RKE system by designing a repeater that will
receive the signal from the key fob and retransmit it to the vehicle’s receiver. The range
of RKE systems should be extended to greater than 200 feet. To do this, the repeater must
be able to receive the signal from the key fob at a greater range then vehicle and then be
able to rapidly retransmit that signal to the vehicle.
1.3 Project Objective
An affordable RKE repeater capable of extending the range of existing RKE systems
with out modification to the automobile will be constructed. The repeater will be
independent of the vehicle and the existing RKE system, including the power supply. The
05512 RKE Repeater 10
device will be small, inexpensive, easily transferable between automobiles, and work
with most existing systems.
1.4 Project Scope
The scope of the project will be limited to systems functioning on 315 MHz using
ASK modulation. Other types of RKE systems will not be covered by the repeater.
1.5 Funding
According to the financial constraint specified by the sponsor, the RF repeater is
expected to have a competitive price in its market, determined by the developer to be
$30-50. A total of $200.00 has been committed by the sponsor for the development of the
product. More funds are available if needed.
For the prototype, the antenna will require approximately $20 since the only
connectors would need to be purchased. Mixers, filters and housing for the receivers were
purchased for roughly $80 in order to conduct preliminary test circuits. These parts will
also be used when building the prototypes. The controller part of the prototype should
cost about $10 if a programmer does not need to be purchased. The rest of the funds
would be allocated towards the receiver and transmitter circuitry.
05512 RKE Repeater 11
2. Specifications
2.1 Antenna Specifications
The antenna required for this project needs to be omni-directional (radiating
equally well in all directions in one plane), based on the assumption that the user is not
going to be using the device from an elevation significantly above or below the vehicle.
The design frequency for this project is assumed to be 315MHz, which means that the
wavelength is 0.952m. For this wavelength, many different antennas can be considered,
such as the quarter or half wave dipole, the helix antenna, and the loop antenna among
others.
2.2 Receiver Specifications
The chief requirement for the RKE repeater is that it be able to receive and retransmit
all RKE devices. RKE devices on cars in the United States operate primarily in two
bands: at 315MHz (American/Japanese cars) and 433MHz (European cars). The repeater
will only be able to function at one of these frequencies at a time, so the receiver will be
designed accordingly.
In order for the receiver to function for all the different available RKE systems, it
must be capable of receiving and retransmitting different modulation types at different
data rates. The two modulation types that are likely to be encountered are ASK and FSK.
These two modulations types are very different – because of this the receiver must be
limited to recovering one or the other. Research indicates that the vast majority of RKE
systems use ASK, therefore the receiver will be designed to detect ASK.
05512 RKE Repeater 12
2.3 Transmitter Specifications
Because of the proximity of the repeater to the car’s receiver, very few requirements
are placed on the transmitter. It is merely required to transmit an ASK signal over a range
for a couple of feet to the automobile’s receiver. The transmitter must be a low power
device and capable of transmission at sufficient speed.
2.4 Control Specifications
For all repeater designs in which the repeater does not continuously retransmit
constantly, some sort of control unit is necessary. The controller will be responsible for
activating and deactivating the different portions of the circuit. When no signal is
incoming, the controller must keep the receiver turned on and listening while keeping
itself in a minimum power draw state.
When a valid transmission is being received, the controller must ensure that the data
from the transmission is being captured and stored in memory. After this, the controller
must activate the transmitter and send the data.
The power management done by the controller is crucial in making the final product
marketable – if the power consumption is too high for AA batteries, the repeater will
either have an unnecessarily short lifetime or will require more sizeable batteries.
05512 RKE Repeater 13
Controller Responsibilities
1. Power Management
2. T/R Switch operation
3. Received data detection or A/D conversion
4. Data storage
5. Transmitter/Receiver operation
Table 2.1: Controller Responsibilities
2.5 Housing Specifications
In order for the RKE to be an effective consumer product, some amount of thought
must go into its physical form. It should be very small (consumers are unlikely to
purchase something that would prove unsightly in their expensive cars), easy to insert and
remove, and look attractive. Because a high quality antenna will be one of the best ways
of improving the range of the system, it is necessary that an attractive enclosure that still
facilitates the antenna to be designed.
The design specifies that the unit must be completely independent of the automobile
it will be used in. It must require no installation other than placing it appropriately in the
car and it must require no connections to the automobile. These requirements lead to the
following list of specifications for the housing:
1. Must contain room for batteries (preferably a standard size, such as AA or AAA).
2. Must not exceed 6” in length or width, excepting the antenna.
05512 RKE Repeater 14
The housing should also facilitate its final placement. The only known specification
for placement of the unit at this time is that it should be above the window level.
2.6 Legal Specifications
The operation of the repeater within the United States is subject to FCC regulations,
Title 47, Chapter 1, Part 15, Section 231 – Periodic Operation including the band of the
repeaters operation. It specifies, “The provisions of this section are restricted to periodic
operation within the band 40.66-40.70 MHz and above 70 MHz. Except as shown in
paragraph (e) of this section, the intentional radiator is restricted to the transmission of a
control signal such as those used with alarm systems, door openers, remote switches,
etc.” [47CFR15.231].
The use of a repeater is not specifically granted by the FCC regulations. However, so
long as adequate circuitry is included in the design to ensure that the repeater does not
continuously transmit noise, no rules are being broken.
The rules govern maximum transmitted field strength. The transmission, however,
will only be going from the repeater to the receiver in a car, a distance that will, in the
worst case scenario of a minivan, not exceed a couple of meters. In order to best
conserve power the transmitter will operate at an extremely low voltage so the field
strength limitations will not be a concern.
It is likely that the device would require FCC approval in order to go to market. This
step should be taken in tandem with the rest of the design process in order to ensure that
no legal issues are encountered. Should legal issues be encountered, no part of the current
design would be usable for the system.
05512 RKE Repeater 15
3. Concept Development
3.1 Antenna
Available antenna designs include the loop, the quarter wave antenna, and the half
wave antenna. In the case of the loop, the input impedance is in the order of a few
thousand ohms; unfortunately a loop doesn't offer a good impedance match to a coaxial
transmission line. Using two identical loops side by side with a few inches spacing
between them reduces the impedance. Space does not permit this though. The directional
pattern becomes asymmetrical and the nulls off the side may be only a few dB down from
the peak of the radiation pattern. An unbalanced, unshielded loop can also pick up
conducted interference from the feed line. The half wave antenna is similar to the quarter-
wave and even though the half-wave antenna's impact on installation is minimal, it is
taller than a quarter-wave antenna cut for the same frequency.
3.1.1 Antenna Theory
For theoretical purposes a finite length dipole will be analyzed to find the radiation
characteristics. It will be assumed that the dipole has a negligible diameter smaller than
the operating wavelength. Hence the current distribution for this dipole can be described
by the following equations:
(3.1.1)
Using the far field approximations given by the equation below,
05512 RKE Repeater 16
(3.1.2)
Where,
dz’ = length of an infinitesimal dipole
The electric field can be obtained by integration.
(3.1.3)
The resulting expression for the electric field takes the form of
(3.1.4)
Using the relationship between E and H, H can be found and can be written as
(3.1.5)
Since a quarter wave dipole is being examined l can be replaced by /4 and k= 2/ in
equation (3.1.4).
The quarter wave antenna was simulated using EXPERT MININEC, an engineering
tool for the design and analysis of wire antennas. MININEC’s solution is based on the
numerical solution of an integral equation representation of electric fields given in
equation (3.1.4) above. MININEC assumes that the wire radius is very small with respect
05512 RKE Repeater 17
to the wavelength and the wire length and the wire must be subdivided into short
segments so the radius is assumed small with respect to segment lengths. MININEC
makes use of the boundary condition on tangential electric field at the surface of a perfect
conductor, namely that the electric field must be zero. Based on the initial assumptions
that the wires must be thin, the total axial electric field on the wire is forced to zero. The
three sources of the tangential electric field on the wire are:
Currents and charges on the wires and on nearby wires.
Incoming waves from distance or nearby radiators.
Local sources of electric field on the wire.
Voltage sources or current sources are local sources that connect to the wires.
MININEC uses the moment method (MM) solution, which is a numerical procedure
for solving electric field integral equation. An important step in the MM is the choice of
basis functions; basis functions are chosen to represent the unknown currents. The
triangular basis function also known as the piecewise linear function is chosen in this
case. The piecewise linear function is defined by
(3.1.6)
Testing functions are also chosen to enforce the integral equation on the surface of the
wire [Antenna Theory]. A typical but not unique inner product is given by
(3.1.7)
05512 RKE Repeater 18
Where the weighting testing function is represented by w in the equation above and S
is the surface of the structure to be analyzed. With the choice of basis and testing
functions, a matrix approximating the integral is defined. To achieve the matrix a set of N
testing functions {wm} = w1, w2…wN are defined in the domain of the operator. If this
matrix is inverted and multiplied by the local sources of electric field, the complex
magnitudes of the current basis functions are derived.[Antenna Theory]
3.1.2 Antenna Simulation
To run MININEC for a complete analysis of the current, impedance and radiation
patterns of the quarter wave antenna some parameters have to be defined.
Figure 3.1: MININEC Program used for antenna modeling
05512 RKE Repeater 19
The antenna is placed on the z-axis as shown in the figure above and fed at the end of
the antenna at z =0. The antenna is simulated on a ground plane, because it is going to be
mounted on the metal top of the housing unit. The ground plane is also used to limit the
downward radiation of the antenna.
Given that the repeater is transmitting and receiving at 315MHz the wavelength can
be obtained as thus
(3.1.8)
The length therefore is quarter of the wavelength resulting in l = 0.238m. After a few
iterations the length had to be changed to get the optimal impedance and gain, the
optimum length was found to be 0.226m. Two geometry points are then defined as (x1, y1,
z1)=(0, 0, 0) and (x2, y2, z2)=(0, 0, 0.226). The method of moments requires that the wire
be broken into segments, the greater the number of segments the more accurate the result.
The number of segments for this antenna was set to 40; the points at which the different
segments of the wire are connected are identified as current nodes. The program was then
run to obtain the following results.
05512 RKE Repeater 20
Figure 3.2: Radiation pattern of ¼ wave dipole
The Radiation pattern, which is a “mathematical function or a graphical
representation in this case of the radiation properties of the antenna as a function of space
coordinates”. The radiation pattern seen in figure 2 above sweeps from 0 and from
0 because the antenna is on a ground plane half of the radiation pattern is not
shown.
There are two measurements of gain, namely directive gain and power gain.
Directive gain is the ratio of the power density radiated in a particular direction to the
power density radiated to the same point by a reference antenna, assuming both antennas
are radiating the same amount of power. The power gain is the same as directive gain
except that antenna efficiency is taken in to account and the total power fed to the
antenna is used in the calculations. It is assumed that the antenna and the reference have
the same input power and the reference is lossless [Antenna Theory]. The power gain is
equal to the directive gain if an antenna is lossless (it radiates 100% of the input power).
The gain of an antenna is often used as a figure of merit. For the quarter-wave antenna
05512 RKE Repeater 21
the as obtained from the simulation is seen the figure below, with the maximum at
5.15dB.
Figure 3.3: Directive Gain pattern of the Quarter wave antenna
05512 RKE Repeater 22
Figure 3.4: Current distribution of the quarter wave antenna
Freq(MHz)
Resistance()
Reactance()
Impedance()
Phase(Deg)
VSWRdB
S11dB
S12dB
315 35.789 -.79302 35.798 -1.27 1.3978 -15.603 -1.5756
Table 3.1: Electrical characteristics for a quarter wave antenna
Technically, antenna impedance is the ratio at any given point in the antenna of
voltage to current at that point. Depending upon height above ground, the influence of
surrounding objects and other factors, a quarter-wave antenna with perfect ground
exhibits a nominal input impedance of around 36 ohms [A.R.R.L Antenna Book], which
is pretty close to the value seen in the table above.
3.2 Receiver
3.2.1 Receiver Theory
After the signal is received from the antenna, it is passed to the first filter. This
particular filter is known as the preselector. The preselector is designed to limit the
bandwidth of spectrum reaching the RF amplifier and mixer to minimize distortion. The
receiver spurious responses can also be attenuated using the preselector. The preselector
must also be able to suppress local oscillator energy originating in the receiver. A
possibility of the RF preselector filter is a highly selective, cavity tuned filter, cascaded
with a low-pass filter. As this filter will encounter the highest RF levels, it should possess
a high intercept point.
The RF amplifier is required to mainly isolate filter 1 and filter 2 from each other in
order to maintain the overall selectivity. Due to this fact, a high reverse isolation
05512 RKE Repeater 23
amplifier is necessary. Other characteristics of the RF amplifier such as the noise figure,
gain and intercept point are determined by the receiver performance requirements.
The received signal is then passed to another filter, Filter 2. It is usually called the
image filter because of its nature to rejects image noise. This filter attenuates the receiver
spurious response frequencies, direct IF frequency pickup and noise at the image
frequency caused by the RF amplifier. The second harmonic occurred in the RF amplifier
and local oscillator energy leaking back to the antenna can also be suppressed by this
filter. Moreover, due to the fact that the mixer usually has very little rejection for odd
harmonics of the receive frequency that may leak to the system, it is extremely important
for this filter to not have any return responses at high frequencies.
To further maximize the intercept performance, a diplexer network can be added in
order to reject any signals that would reflect back into the mixer. This network has the
ability to suppress the local oscillator harmonics that might disturb the functionality of
the mixer.
Due to the fact that the LO signal has to have a relatively high amplitude, the mixer
will generate its own harmonics as it operates. As a result, double balanced mixer should
be used since they are internally balanced and so would not cause this particular problem.
To improve the mixer performance through optimizing the second-order intercept point,
externally generated second harmonics of the LO signal should be suppressed using the
injection filter.
The receiver’s channel selectivity is determined by the single-sideband (SSB) phase
noise of the first local oscillator. It can also be affected by the wideband noise which is
measured at the frequency offsets that are greater than the SSB phase noise. In addition, it
05512 RKE Repeater 24
is very crucial to have a slow spurious signals in the LO signal to prevent the
corresponding receiver spurious responses. A LO synthesizer can be used to limit the
circuit block for frequency change lock time. As the LO signal is very significant to the
system performance as a whole, it must be able to oscillate regardless of temperature and
power supply variations.
After the signal is mixed, it is then passed to the first IF stage. The function of this
filter is to protect the following stages from close-in IM signals. It also has to provide
adjacent channel selectivity and attenuates the second image. Although two different
characteristics of the filter must be met, the number of poles required is determined by
the required second-image selectivity. This is due to the fact that the requirement on the
second-image selectivity is much more stringent than that of the adjacent channel
selectivity. The equivalent noise bandwidth of the IF chain is also a very important
receiver property as it determines the level of noise that reach the detector and the
modulation bandwidth that can be received. Group delay must also be compensated by
either software or hardware in order to minimize the group delay distortion. To improve
mixer’s IM performance, the maximum impedance presented to the high impedance
mixer on the filter skirts must be limited. This can be done by isolating the filter from the
mixer by an impedance inverter network. It is very crucial to select the first IF crystal
with a good IM.
Another high gain stage is then followed. This IF amplifier should possess a high
intercept point. However, if the earlier mentioned IF filter stage is present, the required
intercept point does not necessary have to be as high. [RF Design Guides]
05512 RKE Repeater 25
Mixer Parameter Affected Receiver SpecificationConversion loss Receiver sensitivityThird-order intercept point Intermodulation distortionSecond-order intercept point Half IF spurious response rejectionHigher-order intercept point High-order spurious rejectionNoise balance Receiver sensitivity, AM noise rejectionLO to RF isolation Conducted LO energy propagating toward
antennaRF to IF isolation Susceptibility to direct IF frequency pickup
Table 3.2: Important Mixer Parameters [RF Design Guides]
Receiver Design Procedures
1. Allocate approximate gains and losses as needed to meet the required receiver
sensitivity specification and IM distortion requirements.
2. Select the first IF frequency.
3. Select the first LO injection side.
4. Investigate the mixer.
5. Based on mixer performance, design the injection filter and select LO technology.
6. Investigate filter topologies
7. Design the RF amplifier
Figure 3.5: Basic Receiver Design
05512 RKE Repeater
Filter # 1 Filter # 2
RF Amplifier
Injection Filter
1st IF Stages
1st IF Amplifier
Detector
1st Local Oscillator
26
3.2.2 Receiver Possibilities
The receiver can be built using discrete parts that meet the specifications as noted in
an earlier section. Although discrete parts were used in order to support the preliminary
assumption, existing receiver integrated circuit chips would reduce the cost in the final
prototype assembly. Further detailed discussions will be made in the Feasibility section
of this report.
rfRXD0420 MAX7033 RXM-315-LC-PManufacturer Microchip Maxim LinxFrequency Range 300-450 MHz 300-450 MHz FSK/ASKPower Consumption 8.2mA 5.2mA 5mAIF Frequency 455kHz to 21.4MHz 10.7MHz Not AvailableModulation Mode ASK/FSK/FM ASK ASKPrice $2.79 $4.53 $17
Table 3.3: Receiver IC Possibilities
3.3 Repeater
All of the RKE systems that will work with the repeater are designed for a single
frequency at 315MHz, which means that in order for the car to open, a 315MHz signal
must be received and retransmitted on the same frequency.
This leads to three basic repeater designs:
1. Simultaneous Retransmission - The received signal is simultaneously
retransmitted on a separate, highly isolated antenna. This is the simplest
system, requiring essentially no control circuitry. It requires, however,
directional antennas which are highly isolated from one another if feedback is
to be avoided.
05512 RKE Repeater 27
2. Waveform Storage – The incoming waveform is mixed down to a low IF and
then sampled with an A/D converter. The resulting data is stored till the end of
the transmission is detected. The waveform is then mixed back to the RF
frequency and retransmitted. This technique would retransmit ASK and FSK,
but would offer little gain advantage and frequently transmit extra noise. High
performance signal processing capability along with a large memory would be
required.
3. Demodulating the signal – The incoming signal is demodulated and stored.
After the end of the transmission is detected, the data is output to an ASK
transmitter and sent again as before. This approach is limited to ASK and
would require novel circuitry if it is to work with more than a very small
variety of automobiles. This could be implemented using either a
microcontroller or a DSP.
3.3.1 ASK Theory
Amplitude Shift Keying (ASK) is the simplest form of bandpass digital
communication. The premise is simple – a carrier at the frequency of transmission is
adjusted to different levels in order to represent different sequences of digital data.
The binary form of ASK (OOK, or on/off keying) is even simpler – the carrier is
turned on and off. When the carrier is on, a “1” is being sent. When the carrier is off, a
zero is being sent. In the example below, a simple sample sequence “1 0 1 0 1 1 0 0” is
modulated. A 2 Hz sine wave is sent. It is then multiplied by bit value being sent for each
bit in the sequence. Because the data in the example is being sent at 1 bit/s, the sinusoid is
multiplied by each bit for 1 second. In figure 3.6, an example baseband digital signal is
05512 RKE Repeater 28
seen along with a carrier generated at the given frequency. By multiplying the carrier by
the baseband waveform, a bandpass digital signal is generated. [Digital Communications]
Mathematically, this can be expressed as:
(3.3.1)
0 1 2 3 4 5 6 7 8
0
0.5
1
Message m(t)
Time [s]
0 1 2 3 4 5 6 7 8-1
-0.5
0
0.5
1Carrier s(t)
Time [s]
Figure 3.6: Digital waveform (above) and carrier (below) which comprise the signal for ASK
0 1 2 3 4 5 6 7 8
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Time
Mag
nitu
de
ASK Modulated Data
"1" "1""1""1""0" "0""0" "0"
Figure 3.7: Example of ASK modulation of the sequence “1 0 1 0 1 1 0 0”
05512 RKE Repeater 29
Obviously the modulated signal is not band limited. Therefore, filtering is performed
before the signal is finally transmitted. This filtering leads to distortion, but at slow data
rates with relatively wide bandwidth, like those encountered in RKE systems, the effect
on communication is negligible.
Once the signal has been transmitted, it is corrupted by noise. The simplest noise
model is white, Gaussian noise.
(3.3.2)
For example:
0 1 2 3 4 5 6 7
x 10-3
-4
-3
-2
-1
0
1
2
3
4
5Noise Corrupted Signal
Time [s]
Figure 3.8: Signal Corrupted by Gaussian Noise
The signal is then filtered heavily. Finally, an envelope detector filters and rectifies
the signal:
05512 RKE Repeater 30
0 1 2 3 4 5 6 7
x 10-3
-0.2
0
0.2
0.4
0.6
0.8
1
1.2Envelope Detected Signal
Time [s]
Figure 3.9: Signal after going through the envelope detector
In spite of what figure (3.9) may look like, the data can be successfully demodulated
and stored.
3.4 Transmitter
The final stage of this repeater system will have to be a transmitter. The primary
function of the transmitter is to amplify the power in the signal so that it will be able to
reach the vehicles existing antenna. There does not have to be a lot of power in the signal
being retransmitted from the repeater system to the vehicle’s receiver. This is because the
signal does not have to travel very far. The signal only has to be able to go from the
repeater system in vehicle, most likely in the back seat of the vehicle, to the receiver in
05512 RKE Repeater 31
the vehicle, normally placed under the dashboard. In fact, a less powerful transmitter is
desired since it would be a smaller current draw.
There are two basic design concepts for the transmitter. It can be built using discrete
components or the transmitter can be bought from a company that manufactures RF
transmitters.
The transmitter for this project does not have to be very complicated since the total
power amplification requirements are small. Therefore, it is possible to build a transmitter
from discrete components. The basic design for a transmitter is shown below in figure 8.
Figure 3.10: Transmitter block diagram
This design includes a buffer, a power amplifier, and an oscillator. To design this
system discretely means that each one of these components would be designed separately
and then put together to make the transmitter. The design of the individual components
would most likely mean buying the components from RF manufactures. To design these
components from transistors up is beyond the scope of this project.
There are large numbers of existing RF transmitters that have been designed for
RKE systems. It would be possible for this system to take an existing transmitter,
05512 RKE Repeater
Directional coupler
Lowpass filter
Power Controller
Modulation Input
DiagnosticsForward
Reverse
32
designed for RKE systems and use as our transmitter. The only design involved would be
picking the transmitter that best fit the specifications for this project. Some the
possibilities are as follows:
Part # ADF7012 MAX1472 TH7107 rfHCS362F
Manufacture Analog Decives Maxim Melexis Microchip
Modulation Mode FSK/ASK ASK FSK/ASK ASK, FSK
Frequency Range
50-1000 MHz
300-450 MHz
315/433 MHz
310-480 MHz
Maximum Data Rate 150kbs 100kbs 40kbs 3334bps
Output Power -16dBm to +13dBm +10dBm -12dBm
to +2dBm-12dBm to
+2dBmPower
Consumption 21mA 5.3mA 4.8 to 11.5 mA
4.8 to 11.5 mA
Price: $1.89 $3.74(free Samples) $6.04 Samples
Available
Table 3.4: Transmitter IC Possibilities
3.5 Filters
One of the most important design considerations for this project is the amount of
noise interfering with the system. If large enough, this noise can completely distort the
signal and prevent the repeater from communicating the signal to the vehicle’s receiver.
To keep the noise from dominated the system analog filters must be incorporated into the
design. There are three important filters needed in the system. These are the preselector,
the intermediate frequency (IF) filter, and the transmitter output filter. These can be
implemented using active filters, passive filters, or SAW filters.
3.5.1 Preselector
05512 RKE Repeater 33
The first filter needed on the receiver is called the preselector or front end filter. This
should be a bandpass filter centered at the operation frequency of the system, in this case
315MHz. The purpose of this filter is to reduce the amount of noise coming into the
system by only allowing a narrow band around the given center frequency into the
system.
The preselector must be a bandpass filter with a center frequency at 315MHz. It must
have a bandwidth of less then 600 kHz. It has to have a fairly good attenuation outside of
the bandwidth. Finally, it must match the 50Ω load on the input and a 50Ω load on the
output since the T/R switch will have 50Ω impendence and the input the receiver circuit
will be designed to have 50Ω impedance.
3.5.2 Intermediate Frequency Filter
The second filter need on the receiver is on the output of the receiver. This is should
be low pass intermediate frequency filter (IF). The purpose of this filter is to reduce
prevent the image frequencies from continuing to detection. The image frequencies are a
result of the signal being mixed down from an RF signal to an IF signal.
3.5.3 Transmitter Output Filter
The final filter required for this system is a low pass filer. This time it is on the
output of the transmitter. It is there to prevent the image frequencies from being
transmitted. The image frequencies are a result of the signal being mixed up from an IF
signal to an RF signal.
3.5.4 Filter Designs
05512 RKE Repeater 34
To implement the different filters needed for this project there are several different
filter designs that can be used, the active filter, the passive filter or the SAW filter. For
each of these types of designs, there is a board range of designs for each filter
specifications.
3.5.5 Active Filters
Active filters are filters that are designed using a mathematical approximation to meet
the desired specifications. These filters then implement the approximation using
operational amplifiers, resistors, and capacitors. To implement an active filter requires
some input power to the operational amplifiers. These filters can be designed to include a
gain. Examples of some mathematical approximations for the filters are:
Butterworth Approximation: (3.5.1)
Chebyshev Approximation: (3.5.2)
For these approximations α is the attenuation in decibels at some frequency ω. The
designer uses these to find n which is the order of the filter.
3.5.6 Passive Filters
Passive filters are implemented using only resistors, capacitors, and inductors. These
are components that do not require any power to operate. These filters are designed using
the same mathematical approximations as the active filter. After these filters have been
designed as an active filter a transformation can be preformed on them to make them into
05512 RKE Repeater 35
a passive filter. This is called a lossless-ladder transformation, since the combination of
passive components is called a ladder and to make it lossless is important. Some
examples for passive filter are:
C 1 2.0 0 1 5 p
C 1 3.0 0 1 5 p
1 2L 6
8 u H
1
2
L 71 6 uH
1
2
L 81 6 u H
C 1 4
. 0 3 2 p
R 1 3
5 0
R 1 45 0
V 71 V a c0 V d c
0
V
Figure 3.11: A passive bandpass filter
Passive Bandpass Filter
0
0.1
0.2
0.3
0.4
0.5
0.6
300
301
302
302
303
304
305
306
306
307
308
309
310
311
311
312
313
314
315
315
316
317
318
319
319
320
321
322
323
324
324
325
326
327
328
328
329
330
Frequency (MHz)
Mag
nitu
de (V
)
Figure 3.12: Frequency response of a passive bandpass filter
05512 RKE Repeater 36
C 31 . 6 n
1 2L 2
8 uH
C 41 . 6 n
R 35 0
V 21V a c0 V d c
R 4
5 0
0
C 73 . 2 n
1 2L 4
1 6 u
C 83 . 2 n
R 75 0
V 41V a c0 V d c
R 8
5 0
0
Design for f=1MHzDesign for f=2MHz
C 1 03 . 2 n
R 115 0
V 61 V a c0 V d c
R 1 2
5 0
Single C Design for f=1MHz0
C 91 . 6 n
R 95 0
V 51 V a c0V d c
R 10
5 0
0
V
VV
V
Single C Design for f=2MHz
Figure 3.13: Some lowpass IF Filters
IF Passive LowPass Fitlers
0
0.1
0.2
0.3
0.4
0.5
0.6
10 280
550
820
1090
1360
1630
1900
2170
2440
2710
2980
3250
3520
3790
4060
4330
4600
4870
5140
5410
5680
5950
6220
6490
6760
7030
7300
7570
7840
8110
8380
8650
8920
9190
9460
9730
1000
Frequency (KHz)
Mag
nitu
de (V
)
Single C f=2Mhz Single C f=1MHz Design f=2Mhz Degisn f=1Mhz
Figure 3.14: Frequency response of various IF lowpass filters
05512 RKE Repeater 37
C 1 01 0 p
1 2L 5
5 0 n H
C 1 11 0 p
R 1 15 0
V 61 V a c0 V d c
R 1 2
5 0
0
Design for f=315MHz
C 12 0 p
1 2L 1
2 0 n H
C 22 0 p
R 15 0
V 11 V a c0 V d c
R 2
5 0
0
C 38 p
1 2L 2
4 0 n H
C 48 p
R 35 0
V 21 V a c0 V d c
R 4
5 0
0
V
V
V
VV
V
C 56 p
1 2L 3
3 2 n H
C 66 p
R 55 0
V 31 V a c0 V d c
R 6
5 0
0
C 75 . 3 p
1 2L 4
2 6 . 5 n H
C 85 . 3 p
R 75 0
V 41 V a c0 V d c
R 8
5 0
0
Adapted Design for f=315MHz
Design for f=600MHz
Design for f=500MHz
Design for f=400MHz
C 91 0 . 1 p
R 95 0
V 51 V a c0 V d c
R 1 0
5 0
0
Single C Design for f=315MHz
Figure 3.15: Some Transmitter PA Low Pass Filters for different cutoff frequencies
Low Pass Filters
0
0.2
0.4
0.6
0.8
1
1.2
1 28 55 82 109
136
163
190
217
244
271
298
325
352
379
406
433
460
487
514
541
568
595
622
649
676
703
730
757
784
811
838
865
892
919
946
973
100
Frequency (MHz)
Am
plitu
de (V
) for
1V
inpu
t
Adapted Design f=315Mhz Design f=400MHz Design f=500MHzDeisgn f=600MHz Deign f=315MHz Design Single C f=315MHz
Figure 3.16: Frequency response of the transmitter low pass filters
05512 RKE Repeater 38
3.5.7 SAW Filters
A surface acoustic wave (SAW) filter is a passive filter which does not use normal
discreet elements such as resistors, capacitors, or inductors. It is a thin metal film
structure deposited on top of a piezoelectric crystal substrate, definition found at
http://www3.sympatico.ca/colin.kydd.campbell/. An example of a simple SAW filter is
shown below:
Figure 3.17: SAW Filter
The filter only resonates at a specific frequency. Because it only resonates at a given
frequency, it acts as a very narrow band filter centered at that frequency. This type of
filter is advantageous because it has a very narrow pass band and does not require any
power to implement. The disadvantage is that it is expensive. To implement a SAW filter,
an impedance match must be designed. Below is an example of a SAW filter
implemented with impendence matching to 50 Ohm:
05512 RKE Repeater 39
Figure 3.18: SAW Filter Impedance Matching
3.6 System Control
There are several important functions that require control circuitry. These include
enabling the transmitter and receiver and controlling the T/R switch. A microcontroller is
required to perform these operations. The microcontroller will also need to store the
signal received. The controller must operate fast enough to decode the signal, have
sufficient memory to store the decoded data, and have enough outputs to control all of the
other circuitry.
The microcontroller will control the enable for the transmitter; the transmitter
only needs to be on when the signal is transmitting so that power is not wasted. It also
controls the enable for the receiver. The receiver will only turn off when the transmitter is
transmitting; this will increase the isolation and prevent feedback. The microcontroller
will control the T/R switch. The switch needs to be set to receiver when the receiver is
enabled and set to the transmitter when the transmitter is enabled. Finally, it will send the
stored data to the transmitter during retransmission.
The microcontroller needs to operate fast enough to decode the input data. If the
data rate is too fast for the microcontroller then it will store an incorrect signal and
repeater will not function. It also needs to have a stable time reference. This time
05512 RKE Repeater 40
reference will most likely be set by an external crystal - if the time base varies then the
microcontroller’s internal clock will be disrupted, preventing a successful storing and
retransmission of the signal.
3.7 Housing
The housing will be as small and attractive as possible. It should not occupy any
excess space and should be able to be attached to the parts of the car as appropriate.
Further development of the housing solution cannot be completed until the antenna and
circuit board have been completed.
05512 RKE Repeater 41
4. Feasibility Assessment
4.1 Antenna
The size and housing unit of this project limits the length of the antenna design. Since
space does not permit for a non-wire antenna, the quarter-wave antenna was the most
feasible for this project because of its length. Since the user should be able to use the key
fob from all directions it is required that the antenna be omni directional as opposed to
directional antenna. Using the weighted method of feasibility the quarter-wave antenna
was the most feasible. The quarter wave antenna is also an optimum length because the
gain diminishes with a decrease in antenna length.
Evaluate each additional concept against the baseline, score each attribute as: 1 = much worse than baseline
concept 2 = worse than baseline 3 = same as baseline 4 = better than baseline 5= much better than baseline
Hal
f-w
ave
Ant
enna
Hal
f-w
ave
Ant
enna
Qua
rter-
Wav
e A
nten
naQ
uarte
r-W
ave
Ant
enna
Loop
Ant
enna
Loop
Ant
enna
Rel
ativ
e W
eigh
tR
elat
ive
Wei
ght
Sufficient Student Skills?Sufficient Student Skills? 5 5 5 11%
Sufficient Lab Analysis Equipment?Sufficient Lab Analysis Equipment? 4 4 4 11%
Cost of Materials?Cost of Materials? 5 5 5 3%
Cost of Purchased Components?Cost of Purchased Components? 5 5 5 6%
Complete within 2 quarters?Complete within 2 quarters? 4 4 4 11%
Complete by 1 student?Complete by 1 student? 5 5 5 17%
Has a similar technology been used before?Has a similar technology been used before? 5 5 5 0%
Is it theoretically possible?Is it theoretically possible? 4 4 4 19%
Size?Size? 3 5 4 22%
0%
Weighted ScoreWeighted Score 4.1 4.6 4.4
Normalized ScoreNormalized Score 90.3% 100.0% 95.2%
Table 4.1: Weighted antenna feasibility analysis
4.2 Receiver
05512 RKE Repeater 42
An estimation of the required gains, losses, and intercept points must be calculated in
order to show the feasibility of the system. In general, the RF amplifier gain should not
exceed 20 dB as it would create many problems. These issues include unavailability of a
single device, instability of the system and the unachievable required amplifier intercept
point. A filter insertion loss of 3 dB or less could also be implemented without any
problems.
The selection of IF frequency is also a very crucial process because it would
determine the location of the image and the half IF spurious response frequencies. The
choice of IF frequency, however, is not totally flexible as crystals and IF filters are only
manufactured in certain standard center frequencies. The IF frequency must also be
different from harmonics of the other discrete frequencies such as the digital clock
operating frequency and reference frequency.
When selecting the first LO injection side, a few considerations must be thought of.
High-order spurious responses and self-quieting frequencies may favor on or the other
injection side, once the IF frequency has been chosen. Also, higher-frequency oscillators
typically have worse SSB phase noise but the required voltage controlled oscillator
(VCO) tuning range (in percent) for synthesized sources is less for high-side injection
than for low-side injection. The chosen mixer may have a limited frequency of operation,
forcing low-side injection. A lower-frequency LO that is multiplied up in frequency may
sometimes offer advantages over a high-frequency LO without frequency multiplication
as well.
There are a few differences between passive and active mixers. It is one of the most
important selection that would significantly affect the receiver overall performance.
05512 RKE Repeater 43
While passive mixers possess better IM performance, it requires much higher local
oscillator power and do not provide conversion gain. Active mixers are directly opposite.
Active mixers require less local oscillator power and still do not have a much better noise
figure. The second-order intercept point of the mixer will determine the necessity of the
RF filtering for the half IF spurious response. Also, with higher amplification of the VCO
signal, the wide band noise will be higher. As a result, an injection filter may be needed
in order to suppress image noise to achieve better sensitivity. This particular filter does
not necessary have to be highly complicated. A simple low-pass filter would be adequate
to suppress the second harmonic from the LO signal and help balancing the mixer by
improving the mixer second intercept point.
The LO technology selection is probably one of the most flexible part of the receiver
which relies heavily on the receiver application. For a single-frequency receiver, such as
this project, a simply crystal oscillator can be used. Although in many other systems, a
frequency synthesizer or a LC discrete inductor-capacitor oscillator circuit could be
possible candidates.
The RF filter must be chosen to correspond with the determined IF frequency and the
first LO injection side. Then, a filter topology that rejects the appropriate signal must be
selected. For this specific application, a high-side injection must be used in order to reject
the high-frequency noise that is coupled with the signal. Since the selectivity is a trade off
for insertion loss, the selected filter’s selectivity must be sacrificed as the input to the RF
amplifier must have low insertion loss.
05512 RKE Repeater 44
The RF amplifier is the last block of the receiver circuitry. It is there to fine-tuned the
signal properties after all of the other earlier mentioned parts are determined. It is much
more feasible to shape the signal in this stage than in other stages.
As this particular receiver design is aimed to have low power consumption, there are
more constraints on its operation. With high tendency of the receiver to overload and a
possibility of IM distortion, it is critical to design the receiver to have as narrowband as
possible. This type of receiver usually alternately switches itself on and off to conserve
battery power. [RF Design Guides]
When the feasibility assessment was carried out to compare the benefits between
receiver construction from discrete components and existing receiver IC, it would be cost
and time effective to purchase the IC. Moreover, all of the necessary components of a
receiver could be found in existing receiver IC. In the final design, rfRXD0420 will be
used. This IC will results in a receiver system that will match the need of this project at a
reasonable price. However, this existing receiver IC does not include the filters that are
necessary. As a result, additional discrete SAW and low pass filters must be purchased.
Information on filters could be found in the filter section of this report.
05512 RKE Repeater 45
Evaluate each additional concept against the baseline, score each attribute as: 1 = much worse than baseline concept 2 = worse than
baseline 3 = same as baseline 4 = better than baseline 5= much better than baseline
Dis
cret
e Pa
rtsD
iscr
ete
Parts
Exis
ting
Parts
Exis
ting
Parts
Rel
ativ
e W
eigh
tR
elat
ive
Wei
ght
Sufficient Student Skills?Sufficient Student Skills? 4 4 11%
Sufficient Lab Analysis Equipment?Sufficient Lab Analysis Equipment? 4 4 3%
Cost of Materials?Cost of Materials? 2 4 6%
Cost of Purchased Components?Cost of Purchased Components? 2 4 8%
Complete within 2 quarters?Complete within 2 quarters? 3 4 14%
Complete by a student?Complete by a student? 3 4 17%
Has a similar technology been used before?Has a similar technology been used before? 4 5 0%
Is it theoretically possible?Is it theoretically possible? 4 5 19%
Does it use the spectrum well?Does it use the spectrum well? 4 5 22%
0%
Weighted ScoreWeighted Score 3.4 4.4
Normalized ScoreNormalized Score 77.4% 100.0%
Table 4.2: Feasibility analysis of the receiver
4.3 Repeater
Three approaches for repeater design were identified. Simultaneous retransmission
was almost immediately ruled out due to the omnidirectional antenna requirement.
Because of it, the isolation between the transmit and receive antennas would be minimal
– the gain would be far greater than the isolation. This would create feedback, making the
system useless.
The IF waveform storage approach was initially the most promising. It appeared
relatively easy to implement while being extremely robust (independent of modulation
type). However, many problems existed. First, the key fobs do not have a particularly
stable frequency source. Thus, the bandwidth which would need to be stored would be
very large, requiring an extremely high performance A/D converter. The circuitry
05512 RKE Repeater 46
required to implement such a device would consume a large amount of power.
Furthermore, since no form of data detection is implemented, the repeater would be
repeating noise frequently, which would be an irresponsible use of the spectrum.
Demodulating the data was the most restrictive method investigated but also the most
feasible. It limits the scope to only ASK type transmitters. It has the advantage of
requiring much lower performance parts than the IF approach as well as the ability to
detect if real data is being received. A microcontroller was decided upon over a DSP for
this approach because of power considerations. The weighted analysis agreed well with
the reasoning, as seen below:
Evaluate each additional concept against the baseline, score each attribute as: 1 = much worse than baseline concept 2 = worse than baseline 3 = same as baseline 4 = better than
baseline 5= much better than baseline Sam
ple
Sign
alSa
mpl
e Si
gnal
Dem
odul
atio
n M
icro
cont
rolle
rD
emod
ulat
ion
Mic
roco
ntro
ller
Dem
odul
atio
n D
SPD
emod
ulat
ion
DSP
Sim
alta
nous
Ret
rans
mis
sion
Si
mal
tano
us R
etra
nsm
issi
on
Rel
ativ
e W
eigh
tR
elat
ive
Wei
ght
Sufficient Student Skills?Sufficient Student Skills? 3 3 3 3 2%
Sufficient Lab Analysis Equipment?Sufficient Lab Analysis Equipment? 3 2 2 1 2%
Cost of Materials?Cost of Materials? 4 3 1 4 22%
Cost of Purchased Components?Cost of Purchased Components? 4 3 1 4 22%
Complete within 2 quarters?Complete within 2 quarters? 4 3 3 4 2%
Complete by a student?Complete by a student? 3 3 3 3 2%
Has a similar technology been used before?Has a similar technology been used before? 3 3 3 3 2%
Is it theoretically possible?Is it theoretically possible? 2 4 4 1 22%
Power ConsumptionPower Consumption 3 4 1 3 22%
Does it use the spectrum well?Does it use the spectrum well? 3 4 4 3 2%
Weighted ScoreWeighted Score 3.2 3.4 1.9 3.0
05512 RKE Repeater 47
Normalized ScoreNormalized Score 94.2% 100.0% 55.2% 86.6%
Table 4.3: Feasibility assessment for the repeater
4.4 Transmitter
There were two basic ideas for transmitter design presented in section 3.4. These
were to either design a transmitter from discrete components or to buy a transmitter chip
that has been designed for RKE systems. Both of these ideas are feasible from a technical
point of view. To design the transmitter discretely would simply require finding the
correct parts and making sure that they operated together properly to get the desired
output. To buy a transmitter chip would only require finding the chip that best suited the
desired output.
There are two key factors for feasibility other then the technical factor. These are the
price of the design and the power consumption of the design. To design the transmitter
from discrete parts would cost more. To get a good power amplifier cost almost as much
as the entire transmitter chip. The mixer and the demodulator will also add to the cost.
The cost of the discrete parts is going to be significantly greater, therefore, than the cost
of the transmitter chip. The power consumption for the discrete parts may or may not be
less then the transmitter chip. That depends on which chip is used and what parts are used
for the discrete design. To get less power consumption in the discrete parts will drive up
the cost for the discrete parts.
Building the transmitter out of discrete parts will result in a better transmitter since
each of the parts can be higher quality. However, it will cost more and may have higher
power consumption. Since the transmitter for this project does not have to be very good,
05512 RKE Repeater 48
but it does have to be cheap and low in power, the transmitter chip idea is more feasibly
to the design.
05512 RKE Repeater 49
Evaluate each additional concept against the baseline, score each attribute as: 1 = much worse than baseline concept 2 = worse than baseline 3 = same as baseline 4 = better than
baseline 5= much better than baseline
Dis
cret
e C
ompo
nent
sD
iscr
ete
Com
pone
nts
Tran
smitt
erTr
ansm
itter
Rel
ativ
e W
eigh
tR
elat
ive
Wei
ght
Sufficient Student Skills?Sufficient Student Skills? 4 4 11%
Sufficient Lab Analysis Equipment?Sufficient Lab Analysis Equipment? 4 4 3%
Cost of Materials?Cost of Materials? 2 4 6%
Cost of Purchased Components?Cost of Purchased Components? 2 4 8%
Complete within 2 quarters?Complete within 2 quarters? 3 3 14%
Complete by a student?Complete by a student? 3 4 17%
Has a similar technology been used before?Has a similar technology been used before? 4 5 0%
Is it theoretically possible?Is it theoretically possible? 4 5 19%
Power ConsumptionPower Consumption 1 5 22%
Does it use the spectrum well?Does it use the spectrum well? 4 5 22%
Weighted ScoreWeighted Score 3.6 5.4 0.0
Normalized ScoreNormalized Score 67.6% 100.0% #
Table 4.4: Feasibility assessment for the Transmitter
4.5 Filters
4.5.1 Preselector
There are three different design concepts for the preselector. These are an active
filter, a passive filter, or a SAW filter, as discussed in section 3.5. The key factors in the
05512 RKE Repeater 50
feasibility for the preselector are the pass band, the cost of the filter, the physical
capability to implement the filter, and the power consumption of the filter. Due to the
fact that power consumption is such a big concern for this project the active filter is ruled
out.
For the preselector a passive filter will be very hard to actually implement. This is
because at it will be very hard to get the desired bandwidth it passive components. The
problem with bandwidth of the filter is that it relies completely on the load. The load for
this circuit is very small, only 50 Ω. This makes the values of the inductor and capacitors
very small and therefore more expensive and less reliable. SAW filters give amazingly
good response with very little loss and no input power. The bandwidth for SAW filter is
very narrow and therefore meets a key specification for the preselector.
Overall the SAW filter is best option for the preselector. This is because it is easy
to implement, does not require any input power and has a very narrow bandwidth.
4.5.2 Intermediate Frequency Filter
There are only two different design concepts for the IF filter. The SAW filter can
not be used for the IF filter since this filter should be a lowpass filter rather then a
bandpass filter. Therefore, the two design concepts are either using a passive filter or an
active filter. The active filter is much easier to design and would give a gain rather then a
loss. However, the active filter is going consume power and since power consumption is
an important design consideration, passive filters will better meet the design
specifications for this project.
Figure (3.13) displays all of the different passive filter designs that were
considered for this project. Figure (3.14) shows the result from the simulation of these
05512 RKE Repeater 51
different filters. The simulation in this case is an important deciding factor. For the low
pass filter there is a significantly better response due to the addition of an inductor and
capacitor. The single capacitor filter has a much slower fall time. Therefore the two
capacitor and inductor filter will be used for this project
Due to power considerations a passive filter will be used to the IF filter. To get a
sharper cut off in the filter a second order, two capacitor and inductor, filter will be used.
4.5.3 Transmitter Output Filter
There are only two different design concepts for the transmitter output filter. The
SAW filter cannot be used for this filter since this filter should be a lowpass filter rather
then a bandpass filter. Therefore, the two design concepts are either using a passive filter
or an active filter. The active filter is much easier to design and would give a gain rather
then a loss. However, the active filter is going consume power and since power
consumption is an important design consideration, passive filters will better meet the
design specifications for this project.
Figure (3.15) in displays all of the different passive filter designs that were considered
for this project. Figure (3.16) in the same section shows the result from the simulation of
these different filters. The simulation in this case is an important deciding factor. For the
low pass filter there is a significantly better response due to the addition of an inductor
and capacitor. The single capacitor filter has a much slower fall time. Therefore the
second order, two capacitor and inductor, filter will be used for this project. There are
different designs for the second order filter. Each of these designs is based upon a
different cutoff frequency. For this project the magnitude at 315 MHz should be large and
then cutoff sharp after that. The modified 315 MHz design therefore works best. This
05512 RKE Repeater 52
design is based upon a Butterworth approximation that was modified in the simulation to
move the frequency slightly.
Due to power considerations a passive filter will be used to the transmitter output
filter. To get a sharper cut off in the filter a second order, two capacitor and inductor,
filter will be used. To get a good response at 315 MHz and a good attenuation after that
the modified 315 MHz filter will be used.
4.5 Controller
There are a numerous controllers from various companies capable of handing the
processing necessary for the repeater. The specifications will be determined by the rest of
the system. After that, it is merely a matter of choosing an appropriate microcontroller
that has low current consumption and meets the other design needs.
4.6 Repeater Housing
Commercial viability considerations dominate the feasibility analysis of the RKE
repeater. Optimal performance might well be achieved by placing a large antenna array
on the top of the car. This, however, would lead to increased price, system complexity,
and most importantly to the end-user, an unsightly mess on the top of the car.
Placing the antenna on top of the car would lead to optimal system performance. With
the antenna mounted in such a location, it would be free from the reflections and
diffraction that it will likely experience if placed inside of the car. This, however, would
lead to a significantly more complicated housing design, as it would have to withstand
the harsh environment outside of a car (including high winds, water, temperature
extremes, collisions with bugs and other debris, and a variety of other events). The car
05512 RKE Repeater 53
top location of the antenna would likely be considered unsightly by most consumers as it
would need to be placed on the roof of the car and would likely not match the paint color
of the car or the styling of the car. Additionally, some method of affixing the device to
the top of the car would be required (for example, permanent magnets). These would
likely add to the cost of the device and make installation more cumbersome, particularly
for short consumers or people with vans or sport utility vehicles.
Making the antenna a permanent part of the unit and then placing the unit in a
specified portion of the car would seem to be the most commercially viable option. As
long as the repeater is sufficiently small, the final assembly could be quite unobtrusive. It
could be little more than a small box strategically placed in the car that user does not see
or think about except to change batteries every so often.
05512 RKE Repeater 54
5. Analysis and Design
From a system overview perspective, one of the most important considerations is
power consumption. Since the device should have a long lifetime on only a couple of
batteries, careful attention is paid to power consumption. Based on the design presented,
the following power budget is calculated:
Component Description Power On Current Standby CurrentSources: AA Bateries Energizer AA Bateries1 2850 mAH
Sinks: rfRXD042 Microchip 8.2mA <100nA MAX1472 MAXIMIC 5.3mA <350nA PIC16F87 Microchip 150uA <100nA T/R Switch Analog Devices <1uA
Max Current 8.351mAMax Standby 8.201mATransmitting 5.45mA
Table 5.1: Estimated power draw for circuitry1Based on two AA Energizer batteries, http://data.energizer.com/PDFs/e91.pdf
Also important in any communications system is the link budget. After analyzing the
system, the following budget was tabulated:
05512 RKE Repeater 55
Component Value Power
1. Transmitter Power (dBW) 1mW -60.0
2. Circuit Loss (dB) 0
3. Transmit Antenna Gain 0
4. Terminal EIRP [-60.0]
5. Path Loss <65.93>
6. Other TX losses <10>
7. Received Isotropic Power [-134.978]
8. Receive Antenna Gain 5.15
9. Receive Signal Power [-131.978]
10. Noise Spectral Density <192.5>
11. Received Pr/No (dB/Hz) [62.672]
12. Data rate (dB-bit/s) (40kb/s) 46
13. Received Eb/No (dB) [16.651]
14. Implementation loss (dB) <1.5>
15. Required Eb/No (dB) <10.0>
16. Margin (dB) [[5.15]]
Table 5.2: Link Budget Analysis
5.1 Antenna Design
Based on the analysis seen in previous sections and the following information on the antenna:
An quarter wave antenna with the length adjusted to 0.226m for optimal gain and
impedance. A solid AG wire of size 18 and a wire diameter .0403in would be used for the
quarter-wave antenna attached to a BNC connector. The following are the characteristics
for the antenna:
05512 RKE Repeater 56
Figure 5.1: Directive Gain pattern of the Quarter wave antenna
Figure 5.2: Current distribution of the quarter wave antenna
05512 RKE Repeater 57
Freq(MHz)
Resistance()
Reactance()
Impedance()
Phase(Deg)
VSWRdB
S11dB
S12dB
315 35.789 -.79302 35.798 -1.27 1.3978 -15.603 -1.5756
Table 5.3: Electrical characteristics for a quarter wave antenna
The final antenna design is seen in the following figure:
Figure 5.3: The final antenna design
5.2 Receiver Design
The final receiver prototype would be implemented by a combination of filters
and the existing rfRXD0420 receiver IC.
05512 RKE Repeater 58
Figure 5.4: rfRD0420 Pin Diagram
5.2.1 Bias Circuitry and Frequency Synthesizer
In this particular chip, the receiver enable input (ENRX) is located at pin 28. It has
the ability to pull down to Vss. This is crucial to the bias circuitry which provides the
bandgap biasing and shutdown capabilities. Pin 26 is used in order to provide the
reference frequency to the PLL using a crystal locator. An external loop filter is
connected to pin 29 to control the dynamic behavior of the PLL.
5.2.2 Low Noise Amplifier
The input to the Low Noise Amplifier (LNA) is connected to pin 31, which then
produces an output at pin 3. The mode of the LNA is controlled by connecting different
voltages to pin 2. The built-in LNA has the capability to be in either high gain or low
gain mode.
5.2.3 Mixer and IF Preamp
05512 RKE Repeater 59
After the signal is amplified, the signal is then passed through 1IFIN pin (pin 4) in
order to step down the signal to the Intermediate Frequency (IF). The mixer is biased
through pins 6 and 7. These inputs would keep the mixer balanced. The 1IFOUT (pin 9) has
an impedance of 330 ohms in order to perfectly match with the cost effective ceramic IF
Filters.
5.2.4 IF Limiting Amplifier with RSSI
IF Limiting Amplifier stabilizes the signal after it passes through the IF Preamp to
prepare it for demodulation. The signal input is fed into 2IFIN (Pin 11). At the same time
Pin 21 also generates the Received Signal Strength Indicator (RSSI). In this stage, a 390
ohm resistor is placed paralleled to the 2IFIN pin in order to match the output impedance
of 330 ohm ceramic IF filters.
Due to the fact that this project deals with amplitude shift keying (ASK), RSSI is
compared to a reference voltage. The output of this pin has an internal resistance of 36k
ohms which converts the RSSI current to voltage. [Microchip Specification Sheet]
The following figure is the complete receiver schematic, including the filters:
05512 RKE Repeater 60
Figure 5.5: Full Receiver Schematic
05512 RKE Repeater 61
5.3 Transmitter
The final transmitter design will be based around the MAXIM/IC MAX1472 ASK
transmitter. The transmitter has a sufficiently high data rate, desired low output power,
and very low power consumption. It requires minimal external circuit that lends easily to
integrate with the remainder of the circuit.
Figure 5.6: Transmitter Schematic
The only additional circuitry required will be a crystal which controls the frequency
for the output of the transmitter and a filter. The filter shown here is a low pass filter and
the values are discussed in the filter section of the report, section 3.5.3. The other added
circuitry is for power adjustment. This output circuit is simulated and is shown below in
figure 5.7.
05512 RKE Repeater 62
C 12 0 p
C 22 0 p
C 36 8 0 p
C 42 2 0 p
1 2L 1
2 0 n H
1
2
L 21 0 u H
R 25 0
R 3
5 0
V 23 V d c
0
0
0
V 3
F R E Q = 3 1 5 0 0 0 0 0 0V A M P L = 1V O F F = 0
V
R 4
Figure 5.7: Transmitter Output Circuitry
Simulation From Output Circuitry
-0.5
0
0.5
1
1.5
2
2.5
3
0
0.063
4928
1.301
6163
3.285
7991
5.269
9818
7.254
1646
9.238
3473
11.22
253
13.20
6713
15.19
0896
17.17
5078
19.15
9261
21.14
3444
23.12
7627
25.11
1809
27.09
5992
29.08
0175
31.06
4358
33.04
854
35.03
2723
37.01
6906
39.00
1088
40.98
5271
42.96
9454
44.95
3637
46.93
7819
48.92
2002
Time (ns)
Mag
nitu
de (V
)
10 Ohms 100 Ohms 1k Ohms
Figure 5.8: Simulation of Transmitter Output Circuitry
This chart shows the results from the simulation for the transmitter circuitry. The two
50Ω resistors simulate the output impedance from the transmitter chip and input
impendence to the antenna. The other resistor is a variable resistor. This resistor controls
the power amplification to the signal. The result of changing this resistor is shown above
05512 RKE Repeater 63
in figure 20. The smaller the resistor the higher the gain in the system is going to be. This
was simulated with a 315 MHz signal so the filter will not affect this simulation. The
simulation for the filter part of this circuit is shown below in section 5.4.3.
The transmitter chip used in this design is MAX1472. This is a low power, 300 to 450
MHz transmitter that has been designed for RKE systems. This transmitter includes a
phase lock loop that is controlled by an external crystal. The input signal, data-in signal,
and an enable signal are sent through an ADD gate. This means that the data-in is only
amplified if the enable is high. The transmitter also includes a power amplifier. The level
that the power is amplified by is set by an external circuit. That external circuit includes a
low pass filter and control circuitry for the power amplifier. The circuitry draws the
power amplifier to a certain level. The resistor R1 seen in figure (5.7) controls the level
of the amplifier. In figure (5.7) R1 is set to 270Ω which makes the transmitter amplify the
signal by approximately 10dBm.
In the transmitter, MAX1472, pin 1 is called XTA1 and connects the chips
internal oscillator to the external crystal. Pin 8 is called XTA2 and it connects the internal
oscillator to the other side of the crystal. Pin 2 is called GND and it connects the chips
internal ground to the external ground. Pin 7 is called VDD and it provides the power to
the chip. In figure 19 is connected to a capacitor which projects the chip and to power.
The power into this chip can vary from 2.1 to 3.6 VDC and the current in is normally
5.3mA and a maximum input current of 16.4mA. Pin 3 is called PAGND is the power
amplifier ground; it is connected to the same ground as pin 2. Pin 4 is called PAOUT it is
the output from the power amplifier and it is the output from the transmitter. Pin 5 is
called ENABLE and it is turns on the chip. Enable is connected to the microcontroller; if
05512 RKE Repeater 64
low the transmitter sleeps and draws very little current, when high the transmitter accepts
the input signal and amplifies it. Pin 6 is called DATA and it is the input to the
transmitter. This is all shown on table (5.2).
Pin LABLE Description 1 XTA1 Connected to external Crystal2 GND Connected to external ground3 PAGND Connected to external ground4 PAOUT Transmitter Output5 ENABLE Microcontroller control6 DATA Transmitter input7 VDD Connected to external power8 XTA2 Connected to external Crystal
Table 5.2: MAX1472 Pin Description
To actual simulate the transmitter is not possible, the only way to know for certain
that this transmitter will work it to buy it and test it. However, we are confident that this
transmitter will work well in this system because it has the gain that is required and the
rest of the transmitter is fairly simple.
05512 RKE Repeater 65
Figure 5.9: Complete Transmitter Schematic
05512 RKE Repeater 66
5.4 Filters
5.4.1 Preselector
It was decided that the best preselector filter to use was a SAW filter. This means that
design for this filter involved finding a SAW filter that meets the needs of this project.
The SAW filter needs to have a wide enough bandwidth to ensure that any small variance
in the frequency is included. To do this bandwidth needs to be at least 600 kHz with a
center frequency of 315 MHz. The SAW filter needs to be a passive SAW filter. It needs
to have a small insertion loss.
SAW filters can not be easily designed and manufactured given the tools that students
have, therefore it has to be bought. The SAW filter that was finally settled on is the
162988 produced by COM DEV SAW Products. This filter has a pass band of 830 kHz, a
center frequency of 315 MHz, and an insertion loss of 2.5dB. This is the filter:
Figure 5.10: SAW Filter
The frequency response for this filter could not be simulated using Pspice since there
is no working SAW component in Pspice. Therefore, the frequency response for this
filter is taken from the data sheet and are shown below in figure (5.11).
05512 RKE Repeater 67
Figure 5.11: SAW Filter Frequency Response
5.4.2 Intermediate Frequency Filter
It was decided that the best low pass filter to use for the intermediate frequency was a
second order passive filter. This filter’s cutoff frequency was designed for 1MHz since
anything more than that is not necessary. The receiver should be able to bring down the
final input signal to below 1MHz and therefore, this filter will attenuate the noise in
higher frequencies and pass the mixed down signal to the microcontroller stage of the
repeater. The design for the second order 1MHz low pass filter is shown below in figure
(5.12) and the results from the simulation are also shown below in figure (5.13).
C 73 . 2 n
1 2L 4
1 6 u
C 83 . 2 n
R 75 0
V 41 V a c0 V d c
R 8
5 0
0
Design for Intermediate Frequency Low Pass Filter
V
Figure 5.12: Intermediate Frequency Low Pass Filter
05512 RKE Repeater 68
Intermidate Frequency Low Pass Filter
0
0.1
0.2
0.3
0.4
0.5
0.6
0
0.16
0.32
0.49
0.65
0.81
0.97
1.14 1.
3
1.46
1.62
1.78
1.95
2.11
2.27
2.43
2.59
2.76
2.92
3.08
3.24
3.41
3.57
3.73
3.89
4.05
4.22
4.38
4.54 4.
7
4.86
5.03
5.19
5.35
5.51
5.68
5.84 6
Frequency (MHz)
Mag
nitu
de (V
)
Figure 5.13: IF Filter Frequency Response
5.4.3 Transmitter Output Filter
The transmitter output circuitry is shown above in figure (5.7). Part of this
circuitry is a second order low pass filter. The design for this filter was done using
315MHz as the cutoff frequency and then modifying the design to shift the frequency in
such a way that 315MHz was passed at full strength. The final design for the filter is
shown below in figure (5.14). The results for the simulation of this filter are shown below
in figure (5.15).
C 72 0 p
1 2L 4
2 0 n
C 82 0 p
R 75 0
V 41 V a c0 V d c
R 8
5 0
0
Design for Transmitter Output Filter
V
Figure 5.14: Transmitter Output Filter
05512 RKE Repeater 69
Transmitter Output Filter
0
0.1
0.2
0.3
0.4
0.5
0.6
0.1
0.13
0.17
0.21
0.28
0.35
0.46
0.59
0.76
0.98
1.26
1.62
2.09
2.69
3.47
4.47
5.75
7.41
9.55
12.3
15.8
20.4
26.3
33.9
43.7
56.2
72.4
93.3
120
155
200
257
331
427
550
708
912
Frequency (10MHz)
Mag
nitu
de (V
)
Figure 5.15: Transmitter Filter Frequency Response
5.5 T/R switch
Due the fact that only one antenna is used for this design a transmitter/receiver
switch is needed in front of the repeater to indicate the path the signal needs to take. This
switch needs to have good isolation to keep the signal from crossing over into the other
side of the system. To keep the design cheap and at the same time keep the isolation good
a switch from Analog Devices is going to be used. This switch, ADG901, has an isolation
of 43 dB and has a very low power consumption of less then 1μA. The switch is shown
below in figure (5.16).
05512 RKE Repeater 70
Figure 5.16: Transmitter/Receiver Switch
This chip is controlled by the microcontroller. If the control signal into pin 2 is
low then a path between the antenna and the receiver is established. If the control signal
into pin 2 is high then a path between the antenna and the transmitter is established. The
pins configuration is described in table (5.3).
PIN LABLE Description1 VDD Connected to external power2 CTL Microcontroller control3 GND Connected to external ground4 RFC Connected to Antenna5 RF2 Connected to Receiver6 GND Connected to external ground7 GND Connected to external ground8 RF1 Connected to Transmission
Table 5.4: ADG918 Pin Description
5.6 Housing Design
The housing will be the minimum size necessary to contain the entire unit, including
the antenna and the batteries. The housing will be constructed from a durable material
capable of withstanding the temperature extremes that could be encountered in an
05512 RKE Repeater 71
automobile as well as long term exposure to sunlight. It will be a neutral color. The final
housing will fit well in a wide variety of vehicles.
The antenna size is the primary consideration in trying to make sure the unit is not
cumbersome. Because a ¼ antenna is desired, a helical or folded dipole configuration will
likely be implemented in order to conserve space.
A prototype housing design is presented below. The final design may have to be
modified to take into account the repeaters final placement in the car. The housing will be
different if, for instance, it is found that the performance is better when attached to the
rear window versus resting behind the rear seat.
05512 RKE Repeater 72
Figure 5.17: Specifications for the housing
05512 RKE Repeater 73
Final dimensions for the housing will be determined after a working prototype of the
repeater has been produced. Then it will be possible to optimize the board layout in order
to minimize housing size and maximize antenna effectiveness.
Placement of the housing in the automobile will be determined upon completion of a
working prototype. The unit will be placed in various locations in a number of different
common automobiles. The automobiles will be placed in a controlled environment. The
repeater will then be tested in each automobile from a variety of different angles and
distances. At each location the different key fob functions will be transmitted and the
ability of the repeater to cause the automobile to operate as desired will be noted. The
repeater will be moved to different locations in the vehicle and the affect on performance
will be noted. In this way, a list of proffered locations for the repeater will be empirically
formed. The complex environment of a parking lot and the interior of the car are too
complicated to allow for an adequate model to be formed.
5.7 Control System Design
The controller selected is the Microchip PIC16F737. It is a 28 pin device capable of
operating at speeds between DC and 20 MHz, containing 368 bytes of RAM. The
pertinent features along with reasoning behind their selection are below:
Feature ImportanceInternal Oscillator
Accurate internal oscillator eliminates need for external clock
Analog Comparators
Needed for dynamic detection of received signal if receiver comparator is not used.
Number of digital outputs
Many of the pins are assigned multiple functions. Other similar devices with lower pin-counts lacked a sufficient
05512 RKE Repeater 74
number of outputs to control all of the other devices.Memory With 368 bytes of RAM available, external memory is
not necessary, simplifying design and ensuring faster access time.
Speed With operation up to 20 MHz, having enough time to perform operations between detections is not a problem.
Cost At such a low cost the device offers an affordable solution.
Table 5.5: Control System Specs [Microchip]
Since each byte of memory will contain the number of ones or zeros encountered in a
row from the receiver, the will account for a total of up to 368 byes of received data from
the key fob. This is far larger than any known RKE system encountered up to this point
and should allow for some margin of error on the part of the receiver. Because the
PIC16F87 is based on true Harvard architecture, reading and writing to its internal
memory takes only one cycle, eliminating the timing concerns that other approaches may
have required.
The controller must control the state of the T/R switch (connected to the transmitter
or receiver), the receiver enable, and the transmitter enable. It must also store the detected
data from the receiver and send it to the transmitter. The following connections are
necessary for the microcontroller:
Connection TypeRX ENABLE OUTPUTTX ENBALE OUTPUTT/R SWITCH OUTPUTDATA OUT OUTPUT
RX DATA IN INPUT
Table 5.6: Connections Needed for Microcontroller
05512 RKE Repeater 75
Name Pin Use Connection DescriptionRA2/AN2/Cvref/Vref 1 RA2 TX ENABLE Held high for transmission
RA3/AN3/Vref+/C1OUT 2 RA3 TX DATAASK Data sent by transmitter. High keys transmitter
RA4/AN4/T0CK1/C2OUT 3 *RA5/MCLR/Vpp 4 *Vss 5 Vss VssRB0/INT/CCP1 6 *RB1/SDI/SDA 7 *RB2/SDO/RX/DT 8 *RB3/PGM/CCP1 9 *RB4/SCK/SCL 10 RB4 RX DATA IN Data sent to MCU from receiverRB5/SS/TX/CK 11 *RB6/AN5/PGC/T1OSO/T1CKI 12 *RB7/AN6/PGD/T1OSI 13 *Vdd 14 Vdd VddRA6/OSC2/CLKO 15 *RA7/OSC1/CLKI 16 *
RA0/AN0 17 RA0 TR SWITCHHeld low for antenna connected to RX, high for TX
RA1/AN1 18 RA1 RX ENABLEHold high to enable receiver, always except when TX is on
*Pins not connected. If alternate architecture is necessary, pins may be used.
Table 5.7: Pin connections to the Microcontroller [Microchip]
05512 RKE Repeater 76
Figure 5.18: PIC16F87 Microcontroller Schematic Diagram
Below is an overview of the control process:
05512 RKE Repeater 77
Figure 5.19: High Level Overview of Control Systems
The controller rests in a low power state while the transmitter is powered off and the
antenna is connected to the receiver. Once a “1” is detected from the receiver, the
controller starts recording data bits and storing them to memory. This continues until a
sequence of zeros is detected. The receiver is then turned off, the antenna is switched to
the transmitter, and the transmitter is turned on. The transmitter transmits the data, and is
then turned off. The antenna is switched back to the receiver, which is then powered on
again and the system reverts to the listening state.
05512 RKE Repeater 78
When the repeater is powered on, it should default to a low power state where the
receiver is monitoring incoming signals. The PIC has a default power on state, and so
each of the output pins must be set corresponding to its function. The following settings
are necessary at power on:
Pin Type OutputRA0 OUTPUT 0RA1 OUTPUT 1RA2 OUTPUT 0RA3 OUTPUT 0RA4 - -RA5 - -RA6 - -RA7 - -RB0 - -RB1 - -RB2 - -RB3 - -RB4 INPUT -RB5 - -RB6 - -RB7 - -
Table 5.8: Pin States at power on
Seen below is the power on coding sequence. It sets the appropriate pins as input or
outputs, then sets the received signal change as an interrupt. After everything is set, the
receiver is turned on and the signal is listened for.
05512 RKE Repeater 79
Figure 5.20: Power On Sequence
Once the PIC16F87 is in idle mode, it will draw very little current until it is
interrupted by a change in the value on the RB4 (Receiver data) pin. During this time, the
antenna is held by the receiver and the receiver continuously receives data.
When the receiver detects a one, the value of pin RB4 changes. This calls the
microcontroller interrupt, which executes the code at address 0x04. At this location a
subroutine will be called in order to input data that is being received. The routine will
sample the detector output at a rate of 40,000 samples/s. The subroutine, as seen below,
05512 RKE Repeater 80
will indicate the reception of a 1 by putting 1 in the first memory location through setting
the indirect register.
Figure 5.21: Interrupt handling for change on RB4
Once the interrupt has been executed, the reception begins. A RAM location name
stream stores the last detected bit, be it a “1” or a “zero”. As reception progresses, the
stream of incoming data is rotated through “STREAM”. The data that is rotated out has
already been accounted for in the counter RAM locations. It is only held in STREAM so
that the binary filtering operation can be performed on the data.
05512 RKE Repeater 81
The first order of business is to make sure that actual data is being received and it was
just not some random noise. This will ensure that random transmissions are infrequent.
Four bits are read, spaced by 1/40000 s. These bits are each rotated into STREAM. If
there is only one high bit in stream, actual data is not being transmitted, so the receive is
canceled and the system returns to a listening state. If valid data is detected, the receiver
enters the receiver loop.
05512 RKE Repeater 82
Figure 5.22: Routine to check for valid received data and enter receive mode.
05512 RKE Repeater 83
The receive loop checks pin RB4 (output of the receiver) and records its state. This
state is rotated into the STREAM memory. Using indirect addressing, the RAM positions
are cycled through. Because the data is being detected at a very high rate, a number of 1’s
and 0’s will be detected in a row every time will be quite high. This makes memory
allocation most efficient by recording the number of 1’s and zero’s detected from the
receiver in a row. An example is seen below:
05512 RKE Repeater 84
Figure 5.23: Rate independent detection of the ASK signal
As long as the data is retransmitted at the same rate it is collected at, a nearly identical
signal to the one received will be transmitted.
05512 RKE Repeater 85
The bit that has just been detected is not the one, however, that is transferred to the
counter in memory. Instead, a binary filtering subroutine is called. Upon completion of
this filtering, the bit detected four cycles ago is collected. If 0’s or 1’s are being counted
currently and a 0 or 1, respectively, is received, the current counter is simply
implemented. If zeros are being counted, it is checked to see if 128 0’s in a row have
been detected. If this is the case, then receive mode is exited and the data is transmitted.
If the opposite signal is detected, a transition has occurred. The RSF register is
incremented. If RSF is now at the last memory location, the sequence is too long to be
correct so receipt is aborted. Otherwise, a one is put in the new counter and the loop
continues.
Figure 5.24: Routine for receiving valid data
05512 RKE Repeater 86
The binary filtering routine is intended to recover some of the sensitivity that is lost
by receiving at a much higher data rate. For instance:
0 10 20 30 40 50 60
0
0.2
0.4
0.6
0.8
1
Received Data
Figure 5.25: Example of corrupted data
Because the data rate will never be greater than 5000bps, if a detection of a 0 or 1
only lasts for one or two cycles, it is too short to be a real transition. These bits can safely
be assumed to be errors in detection.
A simple subroutine is listed below for doing this. In the subroutine, the STREAM is
compared to several known bit error patterns. If the STREAM matches one of these
known error patterns, it is corrected and the correct value is placed in stream. In this way,
knowledge that the actual data rate is less than the receive rate can be used in order to
help have more error free detection.
05512 RKE Repeater
Data in error
87
Figure 5.26: The binary filtering subroutine.
Once this is completed, the data is then set for transmission, and the transmit routine
is called.
The transmit routine is extremely simple. Because each location in RAM stores the
number of ones or zeros corresponding to that particular section of the waveform, the
task of the processor is merely to hold at high or low the output to the transmitter for the
specified amount of time. This is accomplished by recalling each of the RAM locations
where a number of 1’s and 0’s has been stored. The number is then decremented while
holding the TX output pin high or low. A delay corresponding to the data rate the data
was detected at is also contained in the path. The transmitter sends the recorded data until
0x7F is encountered, which indicates an end to the received data. When this is detected,
05512 RKE Repeater 88
the receiver is turned back on, the antenna is switched back to the receiver, and the
transmitter is turned off.
05512 RKE Repeater 89
Figure 5.27: The transmit routine
05512 RKE Repeater 90
5.8 Impedance Matching Network
After the quarter wavelength antenna was chosen, it was necessary to match it to the
50 Ohm input impedance of the T/R switch. The miniNEC simulation showed that the
impedance of the antenna is 35.78-j0.798 ohms. In order to connect the antenna to the
T/R switch which has the impedance of 50 ohms, an LC matching circuit was designed.
The circuit consists of a 72nH inductor and a 98nF capacitor.
C 19 8 n F
1 2L1
72 n H
Switch Antenna
Figure 5.28: Impedance matching network
05512 RKE Repeater 91
6. Work Completed
6.1 First Generation Receiver
To verify that the ideas for the repeater design will actually work, a basic receiver
circuit was designed and an implemented using ideal components. This receiver followed
the diagram shown below in figure 40. The design included a band pass filter a low noise
amplifier, a mixer and an intermediate frequency low pass filter.
Figure 6.1: First Generation Receiver
The band pass filter used in this design two simple tank circuits. The reason a tank
circuit was used instead of a band pass filter is that the size inductors available were not
small enough to build a proper band pass filter. A tank circuit is simply a capacitor and an
inductor in shunt. These act as a very wide-band bandpass filter depending on the values
used for the inductor and capacitor. To make the bandwidth a little more narrow a second
tank filter was added in series and separated by a .5Ω resistor. The reason the bandwidth
is so wide is because the load on the filter is so small. The bandwidth of the filter depends
entirely on the output resistor. The two tank filter circuit is shown below in figure 6.2 and
the simulation for this filter is in figure 6.3.
05512 RKE Repeater
Bandpass Tank Filter
Lowpass Filter
Function Generator
LNA MixerScope
92
C 12 . 7 p
C 22 . 7 p
1
2
L 1.1 uH
1
2
L2.1 u H
R 1
. 5
R 25 0
R 3
5 0V 11 V a c0 V d c
0
V
Figure 6.2: Tank Filter
Input Bandpass Filter
0.00E+00
1.00E-01
2.00E-01
3.00E-01
4.00E-01
5.00E-01
6.00E-01
1
2.34
4
5.49
5
12.8
8
30.2
70.7
9
166
389
912
2138
5012
1174
9
2754
2
6456
5
2E+0
5
4E+0
5
8E+0
5
2E+0
6
5E+0
6
1E+0
7
3E+0
7
6E+0
7
1E+0
8
3E+0
8
8E+0
8
2E+0
9
4E+0
9
1E+1
0
2E+1
0
5E+1
0
1E+1
1
3E+1
1
7E+1
1
2E+1
2
4E+1
2
9E+1
2
Fequency (Hz)
Mag
nitu
de (V
)
Figure 6.3: Tank Filter Frequency Response
The amplifier and mixer used for this receiver were both ordered from
www.minicircuits.com. The amplifier used for this receiver was MAN-1LN which has a
gain of 10dBm. The mixer used for this receiver was TUF-1LH. The amplifier was
powered by a 12.5 DC voltage and a maximum current of 40mA. The mixer was a
passive component. It is a ring diode mixer. It needs 10dBm in to operate effectively. It
has a LO input for the oscillator, a RF input for the input signal, and a IF output for the
mixed down signal. The output filter was just a simple capacitor in shunt with the output.
This capacitor acted as a very simple low pass filter which helped to prevent the image
05512 RKE Repeater 93
frequencies from interfering with the output signal. The final receiver circuit used for this
receive followed the schematic seen in figure 6.4.
Figure 6.4: First Generation Receiver Schematic
This schematic was built using the discrete parts described above. The circuit was
then tested using a function generator to simulate the input RF signal and to provide the
signal from the oscillator. The circuit was also simulated using a quarter wave antenna
for 315 MHz signal. This antenna was plugged in and the signal from the key fobs was
looked at.
All these simulations had a lot of noise. This is not what one would expect with an
ideal input, an ideal oscillator, and a very high quality mixer. However, there is still a
tremendous amount of noise in the output signal. The reason there is some much noise in
this system is because the system was constructed on a perforated board and the
components were connected with wires. This is a radio frequency system and therefore
the wires are acting as small antennas. The signal is interfering with itself. This is
because the original input signal is radiated out of the input line and is received into the
output line. To get a better and cleaner signal a printed circuit board with 50Ω lines needs
to be designed and used.
05512 RKE Repeater 94
Even though the signal coming out of this receive is very messy it is still possible to
tell by the simulations that the signal being set was at 315MHz and that the signals are
being modulated using ASK modulation. These simulations verified the assumptions
made in the design for this repeater.
6.2 Work Planned
The next step then is to design a printed circuit board, with 50Ω lines, for the receiver
and repeat the experiment. This time more key fobs will be used and the key fobs will be
baked, frozen, and pressed many times to look at the frequency shifts due to the change
done to the key fob. The resulting data will allow us to modify our design to ensure that it
will work in all environments and remain stable even if the communicating key fob is not
stable.
This must be done before the design can be implemented. If the signal of multiple key
fobs is not used then there is no guarantee that the system will work as desired. Once the
range of frequencies and power levels has been recorded then the microcontroller code
can be finished. This is the only remaining design left.
Once all this is done then the implementation of the design can be done. The parts
need to be bought and a circuit board needs to be designed. The board needs to have 50 Ω
lines between the components to prevent a loss in signal power. After the board is
designed the circuit can be tested and the design can be modified to ensure that results
meet the customer’s specifications.
05512 RKE Repeater 95
Bibliography
Books:
Vizmuller, Peter, RF Design Guide Systems, Circuits, Equations, Artech House, Boston, Ma., 1995
Carr, Joe, RF Components and Circuits, Newnes, Oxford, 2002
Carr, Joe, Secrets of RF Circuit Design Second Edition, McGraw-Hill Inc., New York, 1997
Balanis, Constantine, Antenna – Theory and Design, John Wiley & Sons Inc., New York,
1997
The A.R.R.L Antenna Book
MiniNEC Introduction to Modelling
Data Sheets:
rfRXD0420: http://ww1.microchip.com/downloads/en/DeviceDoc/70090a.pdf
PIC16F87: http://ww1.microchip.com/downloads/en/DeviceDoc/70090a.pdf
MAX1742: http://pdfserv.maxim-ic.com/en/ds/MAX1472.pdf
SAW Filter 162988: http://www.saw-device.com/pdfs/datasheets/162988%20315%20MHz%20RF%20Filter%20Data%20Sheet-P0.pdf
ADG919: http://www.analog.com/UploadedFiles/Data_Sheets/456430626ADG918_9_a.pdf
RKE Theory:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1774
http://www.mwrf.com/Articles/ArticleID/7760/7760.html
http://www.mwrf.com/Articles/Index.cfm?ArticleID=7760&pg=2
05512 RKE Repeater 96
Repeater Theory:
http://www.mwrf.com/Articles/ArticleID/8319/8319.html
http://www.mobilecomms-technology.com/contractors/inbuilding/ems/
http://www.bvkhawaii.com/billyg/amareptr.htm
http://www.wtsn.binghamton.edu/bara/classes/Element2Summarypart1.pdf
PeopleWe would like to acknowledge the following people for their support, insight, and
advice in helping us to establish a preliminary design.
Dr. Phillips
Dr. Venkataraman
Professor Slack
Paul Jacobs
Jan Van Nekeerk
05512 RKE Repeater 97
Appendix A – FCC Regulations
The following FCC regulations govern the usage of the spectrum the repeater operates on:
[Code of Federal Regulations][Title 47, Volume 1][Revised as of October 1, 2003]From the U.S. Government Printing Office via GPO Access[CITE: 47CFR15.231]
[Page 745-746] TITLE 47--TELECOMMUNICATION CHAPTER I--FEDERAL COMMUNICATIONS COMMISSION PART 15--RADIO FREQUENCY DEVICES--Table of Contents Subpart C--Intentional Radiators Sec. 15.231 Periodic operation in the band 40.66-40.70 MHz and above 70 MHz.
(a) The provisions of this section are restricted to periodic operation within the band 40.66-40.70 MHz and above 70 MHz. Except as shown in paragraph (e) of this section, the intentional radiator is restricted to the transmission of a control signal such as those used with alarm systems, door openers, remote switches, etc. Radio control of toys is not permitted. Continuous transmissions, such as voice or video, and data transmissions are not permitted. The prohibition against data transmissions does not preclude the use of recognition codes. Those codes are used to identify the sensor that is activated or to identify the particular component as being part of the system. The following conditions shall be met to comply with the provisions for this periodic operation: (1) A manually operated transmitter shall employ a switch that will automatically deactivate the transmitter within not more than 5 seconds of being released. (2) A transmitter activated automatically shall cease transmission within 5 seconds after activation. (3) Periodic transmissions at regular predetermined intervals are not permitted. However, polling or supervision transmissions to determine system integrity of transmitters used in security or safety applications are allowed if the periodic rate of transmission does not exceed one transmission of not more than one second duration per hour for each transmitter. (4) Intentional radiators which are employed for radio control purposes during emergencies involving fire, security, and safety of life, when activated to signal an alarm, may operate during the pendency of the alarm condition (b) In addition to the provisions of Sec. 15.205, the field strength of emissions from intentional radiators operated under this section shall not exceed the following:
------------------------------------------------------------------------ Field strength of Field strength of Fundamental frequency (MHz) fundamental spurious emissions (microvolts/meter) (microvolts/meter)------------------------------------------------------------------------40.66-40.70..................... 2,250............. 22570-130.......................... 1,250............. 125130-174......................... \1\ 1,250 to 3,750 \1\ 125 to 375174-260......................... 3,750............. 375260-470......................... \1\ 3,750 to \1\ 375 to 1,250 12,500.Above 470....................... 12,500............ 1,250------------------------------------------------------------------------
05512 RKE Repeater 98
\1\ Linear interpolations.
[[Page 746]]
(1) The above field strength limits are specified at a distance of 3 meters. The tighter limits apply at the band edges. (2) Intentional radiators operating under the provisions of this section shall demonstrate compliance with the limits on the field strength of emissions, as shown in the above table, based on the average value of the measured emissions. As an alternative, compliance with the limits in the above table may be based on the use of measurement instrumentation with a CISPR quasi-peak detector. The specific method of measurement employed shall be specified in the application for equipment authorization. If average emission measurements are employed, the provisions in Sec. 15.35 for averaging pulsed emissions and for limiting peak emissions apply. Further, compliance with the provisions of Sec. 15.205 shall be demonstrated using the measurement instrumentation specified in that section. (3) The limits on the field strength of the spurious emissions in the above table are based on the fundamental frequency of the intentional radiator. Spurious emissions shall be attenuated to the average (or, alternatively, CISPR quasi-peak) limits shown in this table or to the general limits shown in Sec. 15.209, whichever limit permits a higher field strength. (c) The bandwidth of the emission shall be no wider than 0.25% of the center frequency for devices operating above 70 MHz and below 900 MHz. For devices operating above 900 MHz, the emission shall be no wider than 0.5% of the center frequency. Bandwidth is determined at the points 20 dB down from the modulated carrier. (d) For devices operating within the frequency band 40.66-40.70 MHz, the bandwidth of the emission shall be confined within the band edges and the frequency tolerance of the carrier shall be [plusmn]0.01%. This frequency tolerance shall be maintained for a temperature variation of -20 degrees to +50 degrees C at normal supply voltage, and for a variation in the primary supply voltage from 85% to 115% of the rated supply voltage at a temperature of 20 degrees C. For battery operated equipment, the equipment tests shall be performed using a new battery. (e) Intentional radiators may operate at a periodic rate exceeding that specified in paragraph (a) of this section and may be employed for any type of operation, including operation prohibited in paragraph (a) of this section, provided the intentional radiator complies with the provisions of paragraphs (b) through (d) of this section, except the field strength table in paragraph (b) of this section is replaced by the following:
------------------------------------------------------------------------ Field strength of Field strength of Fundamental frequency (MHz) fundamental spurious emission (microvolts/meter) (microvolts/meter)------------------------------------------------------------------------40.66-40.70..................... 1,000............. 10070-130.......................... 500............... 50130-174......................... 500 to 1,500 \1\.. 50 to 150 \1\174-260......................... 1,500............. 150260-470......................... 1,500 to 5,000 \1\ 150 to 500 \1\Above 470....................... 5,000............. 500------------------------------------------------------------------------\1\ Linear interpolations.
In addition, devices operated under the provisions of this paragraph shall be provided with a means for automatically limiting operation so that the duration of each transmission shall not be greater than one second and the silent period between transmissions shall be at least 30 times the duration of the transmission but in no case less than 10 seconds.
[54 FR 17714, Apr. 25, 1989; 54 FR 32340, Aug. 7, 1989]
05512 RKE Repeater 99
Appendix B – Bill of Materials
# Part Description Source Cost
1 rfRXD0420 Receiver Micochip $2.79 1 MAX1472 Transmitter Maxim $3.74 1 PIC16F87 Microcontroller Micochip $2.26 1 ADG918 T/R switch Analog Devices $1.07 1 162988a SAW Filter COM DEV $1.67 3 445-1268-1-ND .1uF Capacitor Degikey $0.09 1 311-1026-1-ND 220pF Capacitor Degikey $0.10 1 PCC2129CT-ND 680pF Capacitor Degikey $0.06 2 311-1153-1-ND 20pF Capacitor Degikey $0.10 1 490-2104-ND 270Ω resistor Degikey $0.89 1 M7825-ND 10uH inductor Degikey $0.54 1 TK4231-ND 20nH inductor Degikey $1.95 1 TK4229-ND 16uH inductor Degikey $1.30 2 DN10680CT-ND 60nH inductor Degikey $0.75
Total: $23.31
05512 RKE Repeater 100
Appendix C – Complete Circuit Schematic
05512 RKE Repeater 101