Nor Farhani Zakaria

Microelectronic Eng.

WEEK TOPICS

1 Introduction

2 Sensors / Transducers

3 Sensors / Transducers ctd.

4 Signal conditioning circuits

5 Data Handling & Display

6 Measurement Setup

7 Data conversion

8 Microprosessor & Microcontroller

Objectives:

To realize the important of measurement setup in

electronic applications

To develop understanding of measurement tools to

obtain microwave frequencies

To get to know briefly on measurement of sensitivity

and selectivity

What will we learn in this

chapter?

• Measurement of:

- Microwaves and the frequencies

- The strength of the radio waves

- Sensitivity & selectivity

- Frequency modulation

- Amplitude modulation

Microwave Measurement

Wavemeter Introduction:

• Usage: to measure frequency

• Use any type of resonant circuit that compatible with

frequency range (i.e: lumped constant, co-axial two lines or

cavities circuit)

equipped with means

for indicating the

current induced in it.

frequency is

determined from

reaction produced

by the wavemeter

upon the system

wavemeter is used

as a coupling device

in a system to

transmits power from

a generator to a load.

Principle of Operation

Cavities of uniform circular / rectangular

cross-section resonates when axial length

equals an integral number of half guide

wavelength:

gnL 2

1

L = axial length of cavity

n = 1,2,3,.. Order of resonance

λg = guide wavelength

Example: cavity wavemeter as an absorption wavemeter

f

c

cf

Free space wavelength, λ is distance travelled by wavefront of the

electromagnetic waves. It is related to frequency by:

c = 3 x 108 m/s

Waveguide travel in form of distinctive wave pattern known as

modes, and guided transmission known as guide wavelength, λg.

For circular and rectangular waveguide :

22

222

111

c

cg

cg

λc = cut-off wavelength

λg = guide wavelength

λ = free space wavelength

Example of

Wavemeter

CO-AXIAL CAVITY

Co-Axial Wavemeter frequency range : 600 to 10,000 MHz lower frequencies : cavity becomes excessively long higher frequencies : dimension becomes small. have two coupling loops 1) feeding power into the line through a co-axial cable 2) coupling crystal rectifier indicator to the oscillators in the cavity. tuning process is accomplished by varying the position of the short circuit plunger by using a lead screw. Accuracy could be quite high : at value of 0.05%.

Cavity Wavemeter

Advantages: high accuracy, mechanical simplicity, large

physical size in proportion to the wavelength

The microwave energy inputs is fed through one of two

inputs, A or B.

The crystal rectifier rectifies signal which indicated on current

meter M.

Frequency resonance variation set by plunger, that

mechanically connected to a micrometer mechanism.

Into Cavity Out from cavity

reduces cavity

size

increase cavity

size

Increases

resonant

frequency.

reduces resonant

frequency

Plunger movement:

EXAMPLE 1

Figure shows a practical way of using cavity as a cavity

wavemeter. The circular waveguide is 28.33[mm] in

diameter and having first resonance at L= 4 [mm]. The cut

off wavelength is at 1.71 D. Calculate:

i) cut-off wavelength, λc

ii) Cut-off frequency, fc

iii) Source frequency, f

of the wavemeter.

Microwave Strength

Measurement: Strength meter • Technically a power density meter

• RF Field strength meter detects electric

field 0.5 MHz up to 3GHz and express the

field strength as power density

[0.0001μW/cm2]

• Meter is directional & only detects

components of electric field that has same

polarization along the axis meter.

Standard field generator

/ substitution method

General

method to

determine

field

strength

standard antenna method

Voltage induced when antenna received microwave

field, and generate induce wave.

Equivalent induced voltage in antenna is measured

•Strength of the radio waves is compared with a field of known strength

produced by field generator. (intended for use at very high frequencies)

•input of coaxial line can be switched from the receiving antenna(in term

of unknown strength) to a signal generator (in terms of power).

•Power delivered by signal generator & from antenna is compared.

•Signal generator adjusted to deliver similar power like from antenna

G

Pr

2

2480

ε=field strength, λ=wavelength (radio wave)

Pr=load power(generator), G=antenna power gain

Sensitivity Measurement

• The sensitivity of a radio receiver = ability to amplify weak

signals.

• Often defined in the form of voltage that must be applied to

the receiver input terminals to give a standard output power

(μV or decibel miliwatt dBm below 1V).

• More gain receiver it had, the smaller input signal necessary

to produce desired output power.

• In practice, sensitivity test is conducted before selectivity test.

(same setup with selectivity test)

Sensitivity, selectivity, fidelity experimental setup

• To measure sensitivity, radio source and load is standardized to

prevent variation in measurement condition

• Standard AM signal of 30% modulation & 400 MHz modulating

frequency is applied through dummy antenna.

• Input voltage to produce 50 mW of standard output power is

measured.

• During measurement, input is increased full volume till power

output dissipated the load resistance

• Input in μV of carrier is the measure of the sensitivity

Sensitivity varies over tuning band

At 1000Hz, sensitivity =12.7 μW or -98 dbV

Note:

Measurement of Selectivity • selectivity = ability to only received the radio stations that the

radio is tuned into while rejecting (adjacent) unwanted signals.

• Same experimental setup as sensitivity measurement, except

the frequency is varied to either side(increase decrease) of

the tuning.

• Output of receiver falls (attenuation increases) because

frequency is incorrect input voltage need to be increased to

maintain standard output

• The ratio of the voltage required at resonance to voltage

required when the generator is tuned is calculated to give

selectivity curve.

• Selectivity is determine by the ratio of inductive reactance to

resistance, Q.

• Bandwidth of tuned circuit therefore

• Narrower the bandwidth = the better selectivity need to have

high Q

RXQ L /

Q

LC

Q

fB r

w

)2/(1

fr = resonance frequency

Q = ratio inductive resistanc

to resistance

Fig: selectivity curve of typical tuned circuit

Bandwidth = difference between upper

Frequency, f2 and lower cut-off frequency, f1

which located 3dB on sensitivity curve.

EXAMPLE 2

A tuned circuit is having a 15 μH coil with a

resistance of 25 Ω is connected in parallel

with a 67.6 pF variable capacitor.

i)Calculate bandwidth of the tuned circuit.

ii) Calculate the value of resistance of the

coil that must be reduced to get 50kHz

bandwidth using the same circuit.

MEASURING FREQUENCY

RESPONSE IN AUDIO AMPLIFIER • Frequency response = range of audio frequencies an audio

component can reproduce. Humans can hear between 20 Hz

and 20,000 Hz.

•Frequency response measurement is

essential for filters, couplings circuits,

amplifier stages and overall audio circuit

Fig: frequency response in filters

• Example: transistor audio stage experimental setup (to determine

the band of frequencies that this stage can efficiently handle)

Input from signal generator is coupled by C1 to amplifier stage. R1 used to

develop signal to amplifier gate. RL is the drain load of gate to develop output

signal which is coupled by C2. Voltmeter is connected across output load

resistor RL

• Voltmeter set on ac and as sinusoidal wave frequency approach

the audio band, small voltage indication appears on the

voltmeter.

• Voltage levels off to a steady reading as frequency reaches

maximum response of the amplifier (around 20 Hz)

• Voltage drops as frequency increases above this value

• Voltage vs frequency is plot as frequency response of the

amplifier stage

Frequency response curve

Actual response = band pass

Frequencies being between low f & high f

Cutoff = Half power points =half of the squared voltage =

0.707 times peak voltage

Frequency response = 12Hz to 75kHz

Nor Farhani Zakaria

Microelectronic Eng.

MODULATION

Modulator

Demodulator

Transmission

Channel

Input

transducer

Transmitter

Receiver

Output

transducer

Carrier

EM waves

(modulated signal)

EM waves

(modulated signal)

Information signal

(electrical signal)

Information signal

(electrical signal)

modulation & demodulation : range shifting procedure of

baseband frequencies to suitable transmission frequency, &

corresponding shift back to original frequency range

FREQUENCY MODULATION

Unmodulated Carrier

Information signal

Modulated wave Frequency varying-

amplitude constant

Large amplitude>0V:

high frequency Small amplitude <0 V:

low frequency

• The frequency fi of the information signal controls the rate at which the carrier frequency increases and decreases. fi must be less than fc.

• Information voltage reaches maximum value = change in carrier frequency reached maximum deviation above nominal value. Information reaches a minimum = the carrier frequency at its lowest below the nominal value. When the information signal is zero, then no deviation 0 of the carrier will occur.

• frequency deviation, fd =The maximum change in frequency to the carrier, fc .

• fd sets the dynamic range (i.e bandwith, number of sidebands) of transmission.

FREQUENCY DEVIATION

Measuring Frequency Deviation

• Using FM receiver with a beat frequency oscillator (BFO)

• Modulating signal = variable dc voltage. (degree of deviation depends on amplitude, so frequency variables is not needed)

• FM receiver tuned to minimum/maximum frequency allow excursion (swing).

• Dc voltage is slowly increased until a zero beat is obtained in receiver (corresponding dc voltage = maximum allowable amplitude of modulated wave)

• Once the degree of deviation has been determined, the

modulation index can be calculated:

β<1 spectrum β=1 spectrum

i

d

i

c

f

f

f

f

βi: modulation index

fd= frequency deviation

fi: frequency of modulating signal

only two significant sidebands,

and thus the spectrum looks very

similar to that for an AM carrier.

Number of significant sidebands

has increased to four.

Number of significant sidebands = )1(2

• FM radio uses β > 1= wideband FM.

• practical bandwidth is going to be given by the number of

significant sidebands multiplied by the width of each

sideband

ic

i

i

c

iFM

ff

ff

f

fBandwidth

2

12

12

icFM ffBandwidth 2

*NOTE: Alternatively, bandwidth can be measured by

frequency response of an audio receiver circuit:

RECALL measuring frequency response

EXAMPLE 1

In national radio broadcasts using FM, the

frequency deviation of the carrier,Δfc is

chosen to be 75 kHz, and the information

baseband is the high fidelity range 20 Hz

to 15 kHz. Calculate the bandwidth of the

radio.

AMPLITUDE MODULATION

Unmodulated Carrier

Information signal

Modulated wave Amplitude varying-frequency

constant

MEASURING AMPLITUDE

MODULATION USING CRO • CRO : cathode ray oscilloscope present visual monitoring

& accurate measurement of modulation percentage

• Modulation index can be practically calculated using 3

methods:

- wave envelope

- trapezoidal

-double ellipse

Carrier

signal

Modulating

signal

Modulator

CRO

wave envelope pattern

t

Emax

Emin

Ec

Em

Em

Amplitude modulated signal

If carrier 100% modulation,

Em =Emin = Emax (symmetrical

wave)

M=Em/Ec

Em = Emax - Emin/2 ….1

Ec=Emax – Em

=Emax -( Emax-Emin/2 )

=Emax+Emin/2 …2

Dividing (1) by (2)

M = Emax-Emin …3

Emax+Emin

Modulation index:

standard method of evaluating

modulation index

c b

a

100 bc

b-c % M

Trapezoidal pattern

In this method the Am signal is

connected vertical deflection

and the modulation signal is

applied to the horizontal

deflection plate of the

oscilloscope. The display on the

screen of oscilloscope is

trapezoidal as shown in fig.

c=2(Ec + Em) ……1

b=2( Ec-Em) ……2

Adding (1) and (2), c+b=4Ec

Ec=(c + b) / 4 ……3

subtracting (1) and (2), c-b=4Em

Em= (c-b) / 4 …….4

Modulation index:

M = Em/ Ec = (c-b) / ( c+b ) 100

bc

b-c % M

t

Em

Ec

c b

Double ellipse method

• Shape is due to phase difference between the signal applied

to Vertical and Horizontal deflection plates.

• Phase shift is done by 50kΩ resistor & capacitor in horizontal

amplifier

• Modulation < 100% a blank ring in the middle, 100% above a

disc in the centre ellipse

100 ab

a-b % M

a: minimum amplitude of modulation

b: maximum amplitude of modulation

Exercise 1

A 100 MHz carrier is frequency modulated

by a 10 V peak to peak signal of 10 kHz.

The instantaneous carrier frequency varies

between 99.95 and 100.05 MHz.

i)Calculate: the modulator sensitivity, the

modulation index.

ii) What will be the new value for the

frequency and modulation index if the

modulating signal is changed to one of 6 V

peak to peak and frequency 8 kHz.

Exercise 2

Speech is incorporated into a TV channel as

an FM signal. If the maximum allowed

frequency deviation is 25 kHz and the

maximum modulating frequency is 15 kHz,

what is the modulation index and the

bandwidth of the output FM signal

Exercise 3

i) State two method to measure a bandwidth

of FM audio receiver.

ii) Explain one of the method above.