lab2 am fm radio autumn2009

8/13/2019 LAB2 AM FM Radio Autumn2009

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Autumn 2009: Radio Electronics (TSEK-02) 1/18

Electrical Engineering Department (ISY) Linköping University, Linköping Sweden

Date: ________ Student Name: _________________________ Lab Supervisor: ___________

Personal Number:- Signature: _______________

LAB-2

Measurements on AM / FM Radio

Receiver Building Blocks

Prepared by

Rashad.M.Ramzan

[email protected]

Note: The gain values in above block diagram are typical values.





1. Objectives

• To understand how the radio broadcast AM/FM receivers work.

• To understand the RF receiver building blocks; their functionality and tradeoffs.

• To familiarize with basic RF measurement techniques and instrumentation likeoscilloscope, spectrum analyzer and RF signal generator.

2. Instructions

2.1. You should read the LAB manual carefully and answer all questions labeled as PE

(Preparatory Exercise) before starting the lab. If you fail to do this, it might not be possible for you to complete the lab assignments within the scheduled hours.

2.2. Spectrum Analyzer in the LAB should be used with extreme care; it must not be

used without DC blocker (already connected to RF out) and must not be exposed to

power levels higher than 30dBm. Failing to do so might permanently damage theequipment!

3. Equipment Used

The ElencoTM

Superhet 108 AM/FM radio receiver board

Power Supply: SN16A

Oscilloscope: HP 54600B

Spectrum analyzer: R&S FSL or R&S FS315 or HP 8562E

Signal generator: R&S SM300 or R&S SMB100A or HP E4432B

DVM (Digital Voltmeter), cables and connectors.

4. Introduction:

In this lab you will investigate characteristics and the behaviour of functional blocks used

in traditional AM and FM receivers. You will also learn how to use the RF measurementequipment and how to carry out the measurements on radio circuits.

The function of the broadcast radio receiver is to recover the audio signal that was

modulated onto the RF carrier at the radio station, and apply it to the speaker, reproducing

the sounds of the announcer. There are various ways to combine the carrier frequency andthe audio signal together. This process is called modulation. The most commonly used

modulation methods are amplitude modulation (AM), frequency modulation (FM), and

phase modulation (PM). Also digital signals can be modulated onto radio frequency carrier.When the signal is transmitted, there is a number of impairments that degrade the signal on

the way until it gets to the receiver. We can broadly categorize those impairments as





channel impairments and impairments in transmitter and receiver. The typical impairments

are: The absorption of radio signal power, air and surrounding environment absorb signal.

Signal reflections caused by the ground and obstacles, signal detected in the receiver is

a sum of direct and reflected waves which can cause an effect known as fading. Co-channel interference, distant radio transmitters on the same or close to frequency of

interest will disturb the reception of a neighbouring station. Image-channel interference which deteriorates the receiver sensitivity. Non-linearities and saturation effects in different transmitter and receiver blocks give

rise to intermodulation distortion.

Noise, background noise in receiver, thermal noise generated by the receiver electronics

itself, atmospheric noise (burst noises from thunder storms and similar) and industrialnoise (RF noise from electronics, sparks)

The radio receiver ElencoTM Superhet 108 AM/FM to be investigated in this lab is shownin Fig-1. To facilitate measurements it is mostly built of discrete components loosely

spaced on the board.

Fig-1: AM/FM radio receiver

The receiver is a “superheterodyne” of the standard AM and FM broadcast frequencies. Its

block diagram with depicted 9 sections is shown in Fig-2. The receiver sections can be

characterized as follows. Section-1, the Audio Amplifier Stage, is used to increase the power of the audio signal

received from either AM or FM detector to a power level capable of driving the 8Ω

speaker.

Section-2 includes the AM detector circuit and the AGC (automatic gain control) stage.

The AM detector converts the amplitude modulated IF (intermediate frequency) signal

to audio signal. The AGC stage feeds back a DC voltage to the first AM IF amplifier in

order to maintain a near constant level of audio at the detector.





Fig-2: Block diagram of AM/FM radio receiver

Section-3 is the second AM IF amplifier. The second AM IF amplifier is tuned to 455

kHz and has a fixed gain of 50 at this centre frequency.

Section-4 is the first AM IF amplifier which has a variable gain that depends on theAGC voltage received from the AGC stage. The first AM IF amplifier is also tuned to

455 kHz.

Section-5 includes the AM mixer, AM oscillator and AM antenna stages. When the

radio wave passes through the antenna, it induces a small voltage across the antennacoil. This voltage is coupled to the mixer and is down-converted to the IF frequency of

455 kHz. This change is accomplished by mixing the radio frequency signal with the

oscillator signal.

Section-6 is the FM ratio detector. In the AM detector section we observed that the

audio was detected from changes in the amplitude of the incoming signal. In FM

detection, the audio is detected from changes in frequency of the incoming signal. The

so called ratio detector is used here, which has a built-in limiting circuit to limit thesignal amplitude so that noise at the carrier will be minimized. The FM ratio detector

has a fixed gain of about 20.

Section-7 is the 2nd FM IF amplifier. This amplifier is tuned to 10.7 MHz and has afixed voltage gain of approximately 20. The 3dB bandwidth of this stage is

approximately 350 kHz.

Section-8 is the 1st FM IF amplifier. This amplifier is also tuned to 10.7 MHz with 3dB

bandwidth of 350 kHz and has a voltage gain of approximately 10.

Section-9 includes the FM mixer, FM oscillator, FM RF amplifier, AFC (Automatic

Frequency Control) stage and FM antenna. The incoming radio signals are amplified by

the FM RF amplifier, which is tuned to a desired radio station in the FM frequency





bandwidth of 88 MHz to 108 MHz. These amplified signals are then coupled to the FM

mixer stage to be down-converted to an IF of 10.7 MHz. This change, as in AM, isaccomplished by mixing radio frequency signal with the oscillator signal. The AFC

stage feeds back a DC voltage to the FM oscillator to minimize the oscillator frequency

drift.

Preparatory Exercise

• PE: What portions of the frequency spectrum are allocated for typical AM and FM broadcasts?

___________________________________________________________________

___________________________________________________________________

• PE: Why is the IF fixed the superhet receiver, when we tune to the certain radio

station by tuning its RF stage?

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

• PE: Draw a simplified diagram of the ratio detector to illustrate its working

principle by a phasor diagram.

Hint : Refer to R. Blake pp. 247-248 or http://tpub.com/neets/book12/51d.htm





• PE: Draw a simplified diagram of the AM envelope detector and illustrate its

working principle in the time domain.

PE: Harmonic distortion is defined as distortion components which are integer

multiples of the fundamental signal frequency (i.e. they are harmonically related to

the fundamental frequency). Symmetric distortion is usually represented by odd

harmonics while asymmetric distortion instead corresponds to both even- and oddharmonics. Draw a tone signal with typical symmetrical distortion (caused by

clipping) in the time domain and the corresponding spectrum. Similarly consider

asymmetrical distortion (e.g. one side clipping).

Hint : You can refer to http://hyperphysics.phy-astr.gsu.edu/hbase/audio/amp.html)and click Harmonic Distortion inside the page. For more confidence on the

asymmetric case you can also support your answer by Matlab simulations.

Asymmetric Distortion Symmetric Distortion





• PE: Define selectivity of a radio receiver. What does it depend on ?

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

• PE: Define sensitivity of a radio receiver. What does it depend on ?

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

• Describe the tradeoff between selectivity and sensitivity in a typical superhetreceiver.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

• PE: A strong and a weak radio transmitter are transmitting very close on frequencyscale. If you want to listen to the to weak radio station with good fidelity, whatenhancement will you recommend for present radio receiver design?

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________





Part-I: AM Radio Receiver

Power and current measurement of AM receiver (use DVM)

Fix the supply voltage to 9V and measure the current and calculate the power.

Supply current (receiver off) ___________ mA

Supply current (with moderate volume, FM/AM switch on AM position)

__________ mA Calculate Power = _____________________ mW

4.1. Audio Ampli fier Gain and BW (Section-1)

The purpose of the audio amplifier is to increase the signal power to the level sufficient to

drive the 8Ω speakers. LM386 is a general-purpose audio amplifier with two pins provided for external gain control.

Fig-3: Test setup for audio amplifier measurements

Gain Measurement

Power up the AM/FM radio receiver board with 9V, 0.1A current limited supply.

Set AM/FM switch to FM position and use the LF output of signal generator forgain measurement.

Setup the circuit as shown in the Fig-3. Keep the radio volume knob at

approximately middle position.

Set the signal generator frequency at 1 kHz and voltage at 20mV (RMS).

(VRMS = V pp/2√2)

Connect the signal generator output to TP2 and oscilloscope CH1 to jumper J3

(Input of amplifier).

Connect oscilloscope CH2 to audio amplifier output point TP1 (output of

amplifier).





Record the input and output voltage levels (Please note that gain varies with

volume control).

Voltage Gain = _______= ________

Bandwidth Measurement

Use LF output of signal generator for BW measurement.

Set the signal generator frequency at 10 kHz and voltage at 20mV (RMS).

Connect the signal generator output to TP2.

Connect oscilloscope to monitor input and output signals at TP2 and TP1

respectively.

Change the volume control so that output voltage is 2 V pp.

Now, slowly decrease the frequency to lower 3dB corner frequency so that output

is approx. 0.7 x 2 V pp = 1.4 V pp , note down:

f low,3dB = ________

Now slowly increase the frequency to higher 3dB corner frequency so that output

is 0.7 x 2 V pp = 1.4 V pp , note down:

f high,3dB = ________

Bandwidth = f high,3dB - f low,3dB = _________

4.2. AM Detector and AGC (Section-2)

The purpose of the detector is to change the amplitude modulated IF signal to an audio

signal. First, the amplitude modulated IF signal is applied to a diode in such a way as toleave only the negative portion of that signal (see Fig-6). When the diode is in

conduction, it will force the capacitors C33 and C38 to charge to approximately the same

voltage as the negative peak of the IF signal.

Fig-6: AM detector and AGC stage





After conduction stops in the diode, the capacitors will discharge through resistors R36

and R42. The discharge time constant must be small enough to follow the audio signalotherwise high frequency audio distortion will occur. The discharge time constant must be

large enough, however, to remove the intermediate frequency (455 kHz) and leave only

the audio as shown in Fig-6.

The purpose of the automatic gain control (AGC) circuit is to maintain a constant level atthe detector, regardless of the strength of the incoming signal. Without AGC, the volume

control would have to be adjusted for each station and even moderately strong stations

would saturate the 2nd

IF amplifier causing audio distortion. AGC is accomplished by

adjusting the DC bias of the first IF amplifier to lower its gain as the signal strengthincreases. There is negative DC bias present with audio signal. This negative bias

corresponds to the strength of the incoming signal. The large signal results in the more

negative the DC component. At test point five (TP5), the audio is removed by a low passfilter, R36 and C32, leaving only the DC component. Resistor R35 is used to shift the

voltage at TP5 high enough to bias the base of transistor Q8 to the full gain position when

no signal is present. Resistors R35 and R36 also forward bias diode D4 just enough to

minimize “On Condition” threshold voltage.

AM Detector Bandwidth Measurement

Connect the circuit as shown in Fig-7 and set the switch to AM position.

Set the signal generator for AM at 455 kHz, 80% modulation, 1 kHz and 300 mV

(RMS) EMF.

Connect the RF output of the signal generator to TP3 via 0.001uF.

Connect the CH1 of oscilloscope to TP3

Connect the CH2 of oscilloscope to TP2. Set the volume of radio receiver at comfortably audible level.

Increase/decrease the signal generator amplitude until output on oscilloscope CH2

(demodulated signal) is approximately 200 mV peak, make sure that it’s free ofdistortion.

Leave the frequency of signal generator at 455 kHz.

Increase the modulation frequency of 1 kHz until output drops to approx.

200mV x 0.7 (3dB) = 140 mV

f high,3dB = ________

3 dB BW of AM detector = f high,3dB = _________

Connect the spectrum analyzer to TP3 and observe AM spectrum for 455 kHz,80% modulation, 1 kHz and 300 mV (RMS) EMF.

Identify the carrier and sidebands.

Change the depth of modulation between 10% and 100% and observe the effect

both on the oscilloscope and spectrum analyzer.





Fig-7: AM detector bandwidth measurement setup

4.3. 1st IF Amplifier Gain Measurement (Section-3 and 4)

The operation of the 1st IF amplifier is the same as the second IF amplifier with one

important difference. The gain of the first IF amplifier decreases after the AGC threshold iscrossed to keep the audio output constant at the detector and prevent overload of the second

IF amplifier. This is accomplished by making the voltage on the base of transistor Q8 lower

as the signal strength increases. Since the voltage from base to emitter is fairly constant, the

drop in voltage at the base produces a similar drop in voltage at the emitter of Q8. Thisdrop lowers the voltage across R37 and thus, reduces the DC current through R37. Since all

of the DC current from the emitter of Q8 must go through R37, the DC current in Q8 is

therefore lowered. When the DC current in a transistor is lowered, its effective emitterresistance increases. The AC gain of transistor Q8 is equal to the AC collector load of Q8

divided by its effective emitter resistance. Raising the value of the effective emitter

resistance, thus, lowers the AC gain of Q8.

Gain and Bandwidth Measurement

Set the volume of radio receiver at minimum level.

Connect the circuit as shown in Fig-8 and short TP3 to R38 as shown.

Set the signal generator at 455 kHz, no modulation, 60 mV (RMS) EMF approx.and connect RF output to TP6 via 0.001uF capacitor.

Connect oscilloscope to TP4 for output monitoring. The oscilloscope must have

probe capacitance of 12 pF or less to avoid loading and detuning of IF AMP.

Change the signal generator amplitude so that the output is 4 VPP.





Fig-8: Test setup for 1st IF amplifier stage

Record the input voltage (base of Q8) and output voltage (TP4).

Voltage Gain = _______ = ________

Change the Signal Generator frequency and record the 3 dB bandwidth

(4VPP x 0.7 = 2.82VPP)

Bandwidth = f high,3dB - f low,3dB = ________-________ = __________

Remove the short circuit and observe what happens to the output voltage at TP4(AGC effect)?

4.4. AM Mixer and Oscil lator (Section-5)

In a superheterodyne receiver, the radio waves at the antenna are amplified and then mixed

with the local oscillator to produce the intermediate frequency (IF). Transistor Q7 amplifiesthe RF signal, and simultaneously oscillates at a frequency 455 kHz above the desired radio

station frequency. So the RF amplifier, local oscillator, and mixer are in one circuit here.

To make the oscillator a positive feedback from the collector to the emitter of Q7 is

provided by coil L5 and capacitor C31. During the mixing process the following fourfrequencies are present at the collector of Q7.

1. The local oscillator frequency, LO.

2. The RF carrier or radio station frequency, RF.

3. The sum of these two frequencies, LO + RF.

4. The difference of these two frequencies, LO – RF.





The “difference frequency” is used as the intermediate frequency in AM radios. The

collector of Q7 also contains an IF transformer (T6) tuned to the difference frequency. This

transformer rejects all frequencies except those near 455 kHz. T6 also couples the 455 kHzsignal to the base of Q8 to be processed by the IF amplifiers. The antenna and the oscillator

coils are the only two resonant circuits that change when the radio is tuned for different

stations. Since a radio station may exist 455 kHz above the oscillator frequency, it isimportant that the antenna filter rejects this station and selects only the station 455 kHz

below the oscillator frequency that we refer to as the image problem. If the selectivity ofthe antenna (Q factor) is high, the image will be reduced sufficiently. The oscillator circuitmust also change when the radio is tuned in order to remain 455 kHz above the tuning of

the desired radio station. The degree of accuracy in keeping the oscillator frequency exactly

455 kHz above the tuning of the antenna is called tracking accuracy.

Measurement of Tracking Range of Local Oscillator

Connect the circuit as shown in Fig-9.

Connect the Oscilloscope to the collector of Q7.

Fig-9: Test setup for measurement of Phase noise and Tracking Range

Turn the Tuning Knob from one extreme to another extreme and note the oscillator

frequency.

f min ___________ , f max ____________.

Tracking Range: _________________________

Calculate the AM frequency band reception possible for an IF of 455 kHz.

f min - 455 kHz = ___________ , f max - 455 kHz = ____________

Connect the spectrum analyzer at R33 and observe the spectrum of the local

oscillator (LO) signal and its harmonics.





Down-Conversion

Set the signal generator for AM at 1 MHz, 80% modulation, 5 kHz and 60 mV(RMS) EMF and connect RF output to TP7.

Connect the oscilloscope to the collector of Q7 to observe the oscillator output.

Turn the Tuning Knob to adjust the frequency of oscillator to 1.455 MHz whichafter mixing with the AM signal (from signal generator) produces an IF of 455kHz.

Connect the spectrum analyzer to R33 and observe and identify the signal

components between:

900 kHz to 1.1 MHz: _________________________

1.4 MHz to 1.5 MHz: _________________________

400 kHz to 500 kHz: _________________________

5. Part-II: FM Radio Receiver

The architecture of FM radio receiver is same as AM in Part-I. The fundamental difference

is the type of the demodulator (detector) used to detect the base-band signal. In AM

receiver the AGC was used to keep the signal power constant for a certain station at theinput of the detector.

In FM receiver AFC (Automatic Frequency Control) is used to keep the FM oscillator

frequency stable and to avoid drift with time. The signal of interest in engraved infrequency of modulated signal instead of amplitude. So amplitude variations which are in

certain bound will have no bad effect on quality of FM reception.We will describe the FM ratio detector which is used here. The remaining FM receive

stages are very similar to those of the AM receiver.

Fig-10: FM Radio Receiver

Power and current measurement of FM receiver (use DVM)

Fix the supply voltage to 9V and measure the current and calculate the power.

Supply current (receiver off) ___________ mA





Supply current (with moderate volume, FM/AM switch on FM position)

__________ mA Calculate Power = _____________________ mW

FM Ratio Detector

In FM detection, the audio is detected from changes in frequency of the incoming

signal. The ratio detector has built-in limiting action which limits the signal so that

any noise riding on the FM carrier will be minimized. When an incoming signal is

present at T4 and T5, a current flows through D2, D3, R26, R27, and a largecapacitor C25 which is a bypass for R28. For the IF signal R26 and R27 are

practically connected to ground (C24 is like a short circuit). At no modulation the

circuit is in resonance, and the currents through the diodes D2 and D3 are equal because T5 is center tapped and the voltages on the secondary of T5 are

perpendicular to the IF voltage across the secondary of T4 (also referred to ground

by C23 acting as a short). Thus, no current is drawn through C23 resulting in zeroaudio output voltage.

Fig-11: FM Ratio Detector

When the incoming signal is modulated, its frequency is different from theresonant frequency and the current through one diode will be larger than the other

because the voltages across the secondary of T5 are not any more perpendicular to

the voltage across the secondary of T4. Then, like in the Foster-Seeley

discriminator the driving voltage for one diode branch is different from the voltage

applied to the other. This causes a current to flow in C23 which will produce anaudio voltage across C23 (the same current flows through C24 as well). If themodulation is of opposite direction than before, more current will flow in the other

diode, which will again cause current to flow in C23 but in opposite direction

resulting in an audio voltage being produced across C23. The ratio detector isdecoupled by the resistor R23 and capacitor C21.





Bandwidth Measurement of Ratio Detector

Connect the circuit as shown in Fig-12.

Set the AM/FM switch to FM position.

Set the volume of radio receiver at comfortable audio level.

Set the generator for FM at 10.7 MHz modulated at 1 kHz, 22.5 kHz deviationwith minimum voltage out (EMF ∼ 20mV (RMS)).

Connect the signal generator RF output to base of Q6 (Input of the ratio detector).

Connect the oscilloscope to TP2 to observe the output of detector.

Slowly increase the amplitude of the generator until a 1 kHz sine wave is

observable on the scope.

Connect the spectrum analyzer to base of Q6 to observe the FM spectrum as well.

Fig-12: Test Setup for FM Ratio Detector (Scope to TP2)

Increase/Decrease the deviation frequency, what happens at the output and why?

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

For 22.5 kHz deviation change the amplitude of FM signal so that the detector

output is 400 mVPP.

Increase the modulation frequency and record the 3 dB bandwidth

(400 mVPP x 0.7 = 282.8 mVPP)





Bandwidth = f high,3dB = __________

Measurement of Tracking Range of Local Oscillator

Connect the spectrum analyzer to the emitter of Q3.

Set the start and stop frequency for spectrum analyzer to 70 MHz and 130 MHz,respectively.

Set the marker setting for spectrum analyzer to auto maximum peak .

Rotate the Tuning Knob from one extreme to another extreme and note theminimum and maximum oscillator frequency.

f min ___________ , f max ____________.

Tracking Range: _________________________

Down-Convertion

Set the signal generator for FM at 100 MHz modulated at 1 kHz, 22.5 kHzdeviation and 60 mV (RMS) EMF and connect RF output to TP13.

Connect the spectrum analyzer to the emitter of Q3 to observe the oscillator

output.

Turn the Tuning Knob to adjust the frequency of oscillator to 110.7 MHz which

after mixing with the FM signal (from signal generator) produces an IF of 10.7MHz.

Observe the spectrum and identify different signals between 1.0 MHz and 130

MHz at:

100 MHz: _________________________

110.7 MHz: _________________________

10.7 MHz: _________________________

Congratulations! You have completed the LAB.

lab2 am fm radio autumn2009

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