Acmanual ww8

ENGINEERING 2-2 AC LAB MANUAL ONLY FOR REFERENCE

Expt: 1
AMPLITUDE MODULATION
AIM
To study the operation of Amplitude modulator
To calculate the modulation index of the Amplitude modulated wave.

APPARATUS
Experimental boards of frequency modulation, dual trace CRO, AF generator.

THEORY

AMPLITUDE MODULATION

It is a type of modulation in which the amplitude of high frequency sine wave
(carrier) is varied in accordance with the instantaneous value of the modulating signal.
Consider a sine wave signal Vm(t) with frequency f
Vm(t) = Em ? Sin(2? f .t )
And another sine wave signal Vc (t) with frequency F
Vc(t) = Ec ? Sin(2? F .t )
In this signal Vm(t) is called modulating signal and the signal Vc(t) is called carrier
signal. Vary the amplitude of the carrier Vc(t) after adding the modulating signal Vm(t) to
Ec. You obtain a signal Vm(t) amplitude modulated, which can be expressed by:
Vm(t) = ?Ec ? k ? Em.Sin(2? f .t ? ? Sin(2? F .t ) ? Ec?1 ? m.Sin(2? f .t )? ? Sin(2? F .t )
Percentage modulation signal is defined as the value:
K .Em
m? .100
Ec
With reference to fig 1.1C the modulation index m can be calculated in this way.
E max ? E min
m? ? 100
E max ? E min
It is evident that larger the spectrum of the modulating signal, larger is the band width Bw
occupied by the modulating signal. The resulted BW is equal to the double of the modulating
signal:
BW = 2.fm where fm=modulating signal frequency

AMPLITUDE DEMODULATION

The extraction of the modulating signal from an AM signal can be carried out using
the envelop detector. Consider, in fact the AM signal shown in figure 2.1 and note that the
modulating signal constitutes the envelop if the waveform reported.
The most common envelop detector consists of a diode followed by RC filter. Its
operation is analogous to the one of a half-wave rectifier, as the output voltage follows the

1



maximum values of the carrier. As the amplitude of the carrier is variable, by properly
choosing R and C, the output of the detector can be faithfully reproduced.

DISTORTIONS OF THE DETECTED SIGNAL

The demodulated signal can have two kinds of distortions:
1) If the time constant R.C is too small with respect to the time period of the carrier, the
envelop is approximated to a waveform affected by ripple, the more evident the lower
value of R.C
2) If the time constant R.C too high with respect to the period of the modulating signal.
The detected signal always follows the behavior of the envelop, but sometimes it
takes an exponential decreasing law (distortion by diagonal cutting)

MINIMUM VALUE OF R.C (RIPPLE MINIMIZATION)

A first criterion for the choice of RC has the purpose to reduce the amplitude of the
ripple, which approximates the envelop to the minimum. To obtain this result the time
constant RC must be very big with respect to the period T of the carrier, so to avoid an
excessive discharge of C between two consecutive peaks of the AM signal.
R.C>>T
Fig. 2.4 shows two examples of detected signals, obtained with different values of R.C cannot
be increased as you like, because too high values can cause distortion, too.

MAXIMUM VALUE OF R.C (DISTORTION BY DIAGONAL CUTTING)

A further consideration for the proper choice of time constant R.C is detected
referring to fig 2.4.
If the time constant is too high it can happen that, from a certain moment t1, the
output of the detector does not follow the envelope of the modulated signal any more, but
continues to diminish with the discharge law of C, which is different to the law according to
which the envelope varies. Then, the diode detector keeps, cut off until the instant t2, in
which the outputs reach the envelope. In the interval (t2-t1), in fact, the output is constantly
higher than the maximum values of the modulated signal, and the diode is inversely biased.
The distortion presented by the detected signal is known as Distortion by diagonal
cutting.
The maximum value of RC is calculated supposing that the capacitor C discharges on
the resistor R to a higher equal rhythm to the one in which the envelope of the modulated
signal drops. The detected conditions as follows.
1 V (1 ? m 2 )
R.C ? .
f max m
With m = modulation index: fmax = maximum frequency of the modulating signal
Note that for m = 1 the condition cannot be satisfied, and so the detected signal will
be certainly distorted.

2



LAST CONDITIONS ON THE CHOICE OF R.C IN THE ENVELOP DETECTOR

? To obtain a high detection efficiency the resistance R must be much higher than the
differential resistance of rd of the diode (R>> rd).
? To minimize the ripple in the detected signal the product R.C must be much higher
than the period T of the carrier (R.C >> T)
? To avoid distortion by diagonal cutting the product RC must be smaller or equal to a
value, which depends on the modulating frequency, and on the modulation index. In
the case of the modulating signal with variable amplitude and frequency, the
maximum values of the frequency as well as of modulation index must be considered.

VCC = 12V
CIRCUIT DIAGRAM

N1 N2 AM Output 47k?
100k?

4.71? F
2N2369

+
4.71? F
2N2369 Modulating Signal
+
4.7k? 2.7k?
Carrier
100?

Figure : a. AM Modulator
OA 79

DEMODULATED
AM SIGNAL 1KPF RL SIGNAL

Figure : b. AM Demodulator

3



CIRCUIT DESCRIPTION

MODULATOR
The carrier is fed to transistor Q2 at its base of fig 1. This produces a collector
current in Q2 at carrier frequency. The impedance in the collector circuit Q2 is decided by the
transistor Q1. modulating signal is fed to the base of Q1, which changes the impedance
offered by Q1 at the modulating frequency. The output is taken through a transformer
coupling. When the modulating signal is zero Q1 offers a fixed impedance in the collector
circuit of Q2, so that the output is a constant unmodulated carrier. As the modulating signal is
applied to Q1 the impedance changes and the amplitude of the carrier at output also changes
in accordance with above equation.

DEMODULATOR
The circuit consists of a half wave rectifier followed by a capacitor input filter. Input
to the circuit is a AM wave. The diode cuts off the negative going portion of the AM wave.
Capacitor charges up to the peak of the carrier cycle through the low resistance of the rd
(forward resistance of the diode) and then during the negative half cycle tried to discharge
through relatively high resistance RL. Capacitor value is so chosen that the discharge is so
small in the time between carrier half cycles. Hence the capacitor voltage tends to follow the
envelop of the carrier and the voltage available across RL is simply the modulation envelop
superimposed on the carrier. A dc level in the output comes because the current through
diode flows in the form of pulses occurring at the peak of each carrier cycle.

PROCEDURE
1) Make a study of the board.
2) Measure the internal carrier frequency by connecting the CRO at the carrier terminals.
3) Connect the carrier to the carrier input terminals, an AF generator to modulating
signal terminals and a CRO at the AM output terminals
4) Keeping the AF signal level to zero, switch on power supply of the board. Observe
that un-modulated carrier will appear on CRO screen. Measure its peak amplitude, Ec.
5) Keeping the CRO range to modulation frequency, slowly increase the AF generator
signal level and note down the amplitude Em. Amplitude modulated waveform will
appear on CRO.
Em
6) Compare the theoretical value of modulation given by % m ? ? 100, with the
Ec
E max ? E min
practical value given by %m ? ? 100
E max ? E min
TABULATION FOR MODULATION
Practical Theoretical
Base band E max ? E min Em
S.No Emax Emin Em Ec %m ? ? 100 %m ? ? 100
frequency fm
E max ? E min Ec

4



MODEL WAVEFORMS
Em

Fig:A t
EC

t
Fig:B

Emax

Emin
t

Fig:C

Emax

Fig:D
Em t

Fig:E t

Figure A: Modulating Signal
Figure B: Carrier
Figure C: Modulated Signal
Figure D: Diode Detector output
Figure E: Demodulated Signal

5



DEMODULATION

1) Connect the AM wave available at the output terminal to the demodulator input
terminals.
2) Connect the load to the circuit.
3) Connect the CRO to the detector output terminals. See that with the variation of the
modulating signal amplitude the amplitude of the detected output changes and
measure its frequency.

TABULATION FOR DEMODULATION

S.No Modulation signal Demodulated signal Modulating signal Demodulated
voltage (Em) voltage (Em) frequency (f m) signal frequency

QUESTIONS:

1) Why modulation is an essential process of communication system?
2) Mention some applications of AM?
3) What are the distortions that are likely to be present in the demodulated output when
diode detector is used?
4) Explain how negative peak clipping occurs in the demodulated signal when diode
detector is used?

6



Expt:2
FREQUENCY MODULATION AND DEMODULATION

AIM
1. To study the process of frequency modulation and measure the modulation
index.
2. To study the demodulation of FM signal.

APPARATUS

Experimental board of frequency modulation, dual traces CRO, AF generator.

THEORY

Frequency modulation is a system in which the frequency of the carrier is varied in
accordance with the signal amplitude.

Let’ assume for the moment that the carrier of the transmitter is at its resting
s
frequency (no modulation) of 100MHz and we apply a modulating signal. The amplitude of
the modulating signal will cause the carrier to deviate from this resting frequency by a certain
amount. If we increase the amplitude of this signal, we will increase the deviation to a
maximum of 75 kHz as specified by the FCC. If we remove the modulating voltage, the
carrier shifts back to resting frequency (100MHz). From this we can say that the deviation of
the carrier is proportional to the amplitude of the modulating voltage. The shift in the carrier
frequency from its resting point compared to the amplitude of the modulating voltage is
called the deviation ratio (a deviation ratio of 5 is the maximum) allowed in commercially
broadcast FM)

The rate at which the carrier shifts from its resting point to a nonresting point is
determined by the frequency of the modulating signal. The interaction between the amplitude
and frequency of the modulating signal on the carrier is complex and requires the use of
Bessel’ function to analyze the results).
s

If the modulating signal is 15kHz at a certain amplitude and the carrier shift is 75
kHz, the transmitter will produce eight significant sidebands. This is known as the maximum
deviation ratio.

Deviation ratio = (fdev (max) / fAF(max) )
If the frequency deviation of the carrier is known and the frequency of the modulating signal
is known then

Modulation index = fdev / fAF

7



CIRCUIT DESCRIPTION

FM generator is basically a sine wave oscillator whose frequency could be varied
by modulation signal. If the inductance or capacitance of a LC oscillator is varied then
frequency modulation will result. In this board an integrated circuit called the function
generator (8038) has been used for generation of FM. A ramp voltage is generated by
charging a capacitor from a constant current source. A triangle waveform can be generated by
alternate charging and discharging of a capacitor with constant current source and sink. The
frequency of triangle waveform depends on the value of capacitance and the current of the
source/sink. Now, if the capacitance is kept fixed and the current of the source and sink is
varied according to the amplitude of a modulating signal then we shall get a waveform whose
frequency is directly proportional to the amplitude of the modulating signal. This triangular
waveform then can be converted into a sinusoidal one with the help of wave shaping circuits
to get the true FM.

Fig below shows the block diagram of the IC function generator. Two current sources,
alternatively charge and discharge an external capacitor. When the current source 1 drives the
triangle voltage above positive reference voltage, comparator 1 triggers the flip-flop. This
connects the current source 2 which acts as a sink for the capacitor. This discharge rate is
same as the charge rate. When triangle waveform falls below the negative reference level
then the comparator 2 triggers the flip-flop and source 2 gets disconnected. The voltage
across capacitor becomes triangle output. The sine wave is then obtained from the triangle
wave through a converter circuit. The frequency of this sine wave is modulated by letting the
modulating signal control the value of current source.

Slope detection is one way of demodulating FM signals. The frequency selective
nature of tuned circuit curve converts the frequency modulated carrier into an amplitude
modulated one. After this the signal can be extracted simply by diode detector circuit which
is used to detect an AM wave. The slope detector will work efficiently only if the frequency
deviations are limited to the linear range of tuned curve.

8



CIRCUIT DIAGRAM

VCC +12V

6

U1

IC 8038
7 -
2 ++
8 10k
FM output
Modulation signal 11

Figure a: FM Modulator

OA 79
C1

4.7K Demod Output
FM s ignal 0.01uf N1 N2 2.2kpf

Figure b: FM Demodulator

PROCEDURE

STUDY OF MODULATION

1. Connect one input of the dual trace CRO between T1 and ground.
2. Switch on the power to the board.
3. Measure the frequency of sine wave carrier which appears on CRO screen. It shall be
around 20 kHz. Record this as fc.
4. Apply MOD signal at Em keeping control switch of CRO in DC position.
5. Note the variation in the frequency of the carrier as the amplitude of the DC is varied.
6. Next put the control switch in the sine wave position.

9



MODEL WAVEFORMS
Em

t
Fig :A

Ec

t

Fig :B
fmax fmin
Ec

v
t

Fig :C

v
Em

t
Fig :D

Figure A: Modulating Signal
Figure B: Carrier
Figure D: Demodulated Signal

10



7. Obtain a stationary wave form of FM signal on CRO screen by adjusting the
amplitude and frequency of the modulating signal. At this stage use the other trace of
the CRO to observe the modulating signal FM simultaneously.
8. Observing the FM wave form and modulating signal simultaneously on CRO screen,
measure the frequency of the FM signal around the time of positive peak of the
modulating signal. Record this frequency as fmax.
9. (fmax-fc) gives the frequency deviation.
10. Calculate the modulation index of the signal.

DEMODULATION

1. Generate FM signal using sine wave modulating signal. Keep the
the modulating signal small.
2. Connect T1 to T2. FM signal gets connected to the demodulator circuit.
3. Observe the AM signal prior to the detector diode on CRO.
4. Connect the CRO at Demodulated output terminals. The original modulating signal
appears here after the envelop detection. Measure its frequency, is should be same as
that of the modulating signal connected at Em.
5. Increase the amplitude of the modulating signal and observe its effect on the Demod
output. At larger amplitudes distortion is produced. Give reasons for this in your
notebook.

TABULATION
MODULATION

Amplitude A B
S.No. of Mod. signal(v) fmax(kHz) fmin(kHz) ? f=A-B ? = ? f/fm

DEMODULATION
Mod-voltage Modulating Demodulated Demodulated
S.No. (Em) in mv Frequency in (kHz) Signal voltage Signal frequency

11



Expt: 3
BALANCED MODULATOR
AIM
To study the process of Balanced Modulation and the corresponding waveforms. To
study the demodulation of Balanced modulated signal.

APPARATUS
Experimental board of Balanced Modulator and Demodulator. AF signal generator for
modulating signal, CRO.

CIRCUIT DIAGRAM
100 100
+ Vcc
C1

100
Carrier 680
10u 8 2 3.9 K 3.9 K
10 3
470uf IC 1496
1
6
Mod.Signal Modulated Signal
4
R1

12
4.7K 5 14
560
10K

560
P1

- Vcc
Figure :a. DSB-SC Modulator
820 1.8K
Vcc
C3 200

10K 3.9K 3.9K
C6

R2

2 3
Carrier 8
C1 5
10
BM Signal C2 IC 4196
IC 1496 6
1
Demod. output
4 12
14
100 100

C4 C5

Figure :b. DSB-SC Demodulator
12



THEORY

Balanced Modulator is essentially an Amplitude modulation with no carrier power at
output and is the heart of all the methods of single side band suppressed carrier modulation
and demodulation. Any circuit which can produce in its output the product of two separate
input signals can be used as a balance modulator. In balance modulation only sidebands are
transmitted while carrier is fully suppressed.

MODULATION

1. Make a study of the layout of the board.
2. Measure the internal carrier frequency and amplitude by CRO.
3. Connect the built in carrier to the carrier input terminal and AF generator (MOD
SIGNAL) between input signal terminal and ground terminal.
4. Keep AF signal level to zero and frequency approximately 4k Hz, switch ON
power supply on the board.
5. Keeping the CRO sweep in kHz/ m sec. Range, slowly increase the AF generator
signal level and obtain a stationery balance modulated waveform on CRO by
adjusting the amplitude of the carrier and the amplitude and frequency of the
modulating signal.
6. Trace the envelope of the modulated waveform from CRO screen.

DEMODULATION

1. Connect one output of the balance modulator to Mod I/P terminal and carrier of
the modulator to the carrier terminal of the demodulator.
2. Connect the CRO at any one Demod output point. The demodulated signal
appears here. Measure its frequency. It should be same as that of the modulating
signal.

13



Model Waveforms
Em

t

Fig A:

Ec

t

Fig B:

Phase reversal
Emax

t

Fig C:

Em

t

Fig D:
Figure A: Modulating Signal Figure B: Carrier Signal
Figure C: Modulated Signal (DSBSC representation)
Figure D: Demodulated Signal (Message Signal)

14



PROCEDURE

QUESTIONS
What are the factors on which the bandwidth of a side band in balance
modulator output depends?

TABULAR FORMS
MODULATION

S.No Mod. Freq (fm) Emax Emin Em Ec Practical % m Theoretical % m

DEMODULATION

S.NO Modulating signal Demodulating signal Modulating frequency Demodulating frequency
Voltage Emin v voltage Em in v Fm in Hz Fm in Hz

15



Expt: 4
PRE -EMPHASIS& DE - EMPHASIS
AIM
To draw the pre-emphasis and de-emphasis circuits and plot the frequency response of
the circuit.

APPARATUS

Signal Generator, power supply, transistor, resistors, capacitors, DIB.

THEORY

The noise triangle shows that noise has greater effect on the higher frequencies than
on lower ones. Thus if higher frequencies were artificially boosted at the transmitter and
correspondingly cut at the receiver, an improvement in noise immunity could be expected.
This boosting of the higher modulating frequencies in accordance with a prearranged curve,
is termed as pre-emphasis and the compensation at the receiver is called de-emphasis.

CIRCUIT DIAGRAM
Vcc

L R 75k C2 10n

R1 Rc
10 u Signal generator
+

+
10 u 5
CRO
BC 547 C 1n
+

+

Signal Generator 100 u
CRO
R2
RE

PRE-EMPHASIS DE-EMPHASIS

Model graph

Pre-emphasis
Gain in dB

Frequency

De- emphasis

16



PROCEDURE

PRE-EMPHASIS

1. Design a self bias circuit.
2. Let T=75? sec. Let Rc= 1k? , select L T=L/RC
3. Connect the circuit as per the diagram.
4. Apply the input signal from a signal generator.
5. Vary the frequency of the input signal from 20Hz to 20kHz.
6. For each value of the input signal note down the output and calculate the gain.
7. Draw the graph between frequency Vs gain (Gain = 20 log (V0 / Vi ).

DE-EMPHASIS
1. Let T=RC, T=75 ? sec., Let C=1000pf, R= 75k?

DESIGN

Determine the value of Rc, Re, R1, R2 for the network for the operating point
indicated.

VCC = 12V ICQ = 5mA VCEQ = 6V
VEN = VCC / 10 = 1.2 V
RE = VEN / IE = VEN / IC =
RC = VRC / IC = (VCC – VCEQ – VEN ) / IC
VEN = VBE + VE =
R2 ? (1 / 10) ? ? ? RE.
Find R2, VBN = (R2 / (R1 + R2)) ? VCC find R1

TABULAR FORM

PRE-EMPHASIS DE-EMPHASIS

S.No Freq O/P (V) Gain S.No Freq O/P (V) Gain
(20 log(Vo/Vi) (20log(Vo/Vi)

17



Expt:5
CHARACTERISTICS OF MIXER
AIM
To study the performance of mixer circuit (frequency converter)

APPARATUS
ACT 01, 03. ACT power supplies, CRO

THEORY:
The radio frequency signal (RF) detected by antenna is amplified by an RF amplifier
and transferred, via a frequency mixer and a local oscillator (LO), to a lower and fixed
frequency (Intermediate Frequency IF). The translation to a fixed and lower frequency than
the received on enables to:
? Use a fixed band amplifier for the amplification of all the received signals,
independently from their frequency.
? Operate on lower frequency signals
The frequency translation is some times used in communication systems where for example,
the modulation is carried out at an intermediate frequency having value lower than that of the
effectively transmitted frequency.
As already said, to carry out the frequency translation a locally generated translation
is necessary, called local oscillation, which interacts with the radio frequency signal so as to
produce the new frequency.
If the oscillation is produced by the same operating circuit translation device, it is
normally called as converter; in the reverse case we speak respectively of local oscillator and
mixer. The frequency translation is differently indicated as frequency conversion, mixing,
beating, and heterodyne process. The techniques used are many, and depend on the device
used and on the operating frequencies.

MIXER CIRCUiT

100 K

18



RF/FM RF IN
OUT
P6 P7 MIXER
Freq. LEVEL (ACT 03)
0.2 VP-P
400 KHz – 1500KHz
VCO2 (ACT 01)

500KHz 1500KHz

P4 P5
RF/FM LO
OUT
Freq. LEVEL
0.2 VP-P
VCO 1(ACT 01)
MIXER CONNECTIONS USING ACT 01

PROCEDURE

CIRCUIT CONNECTIONS:

1. Connect the power supply with proper polarity to the kits ACT -01 &ACT -03 and
switch it on.
2. Refer to the above block diagram and carry out the following connections and switch
settings.
3. Carry out the following presetting in ACT -01
? Keep the switch S3 in 1500 KHz position
? Using the pot P4 keep VCO1 frequency at 1000 KHz.
? Using the pot P5 keep VCO1 amplitude at 1 V p-p
? Using the pot P6 keep VCO2 frequency at 550 KHz
? Using the pot P7 keep VCO2 amplitude at 10omV p-p
4. Connect the output of VCO2 (act -01) RF/FM OUT post to the input of mixer (ACT -
03) RF IN post
5. Connect the output of VCO1 (act -01) RF/FM OUT post to the other input of mixer
(ACT -03) LO IN post.

19



6. Connect the oscilloscope to the output post of the mixer IF OUT post and accurately
vary the frequency of the local oscillator using the pot P4 until you detect a sine wave
signal.
7. Measure the frequencies of the two input signals and the output signals, and check
that the last is the difference of the first two fIF = fLO – fRF.

20



Expt:6
RECEIVER MEASUREMENTS
AIM
1. Study of AM superhetrodyne radio receiver.
2. To study the characteristics of AM superhetrodyne radio receiver.
3. To plot the I/P and O/P waveforms of detector E1 power amplifier.

APPARATUS
AM superhetrodyne receiver multimeter, CRO.

THEORY
The superheterodyne circuit is used in almost all the transistor radios (excepting a
few used for receiving local stations). In this system the frequency of incoming signals are
changed to a fixed frequency known as intermediate frequency IF. The main work of
amplifying the signal and of selecting the desired station, is done in IF section. This permits
higher amplification per stage and better selectivity.
The block diagram of Radio receiver is shown in below. It consists of the following
parts / stages.

1. frequency converter (Mixer and Oscillator)
2. I.F. amplifier
3. Detector
4. Audio Preamplifier
5. Driver
6. Output
7. Power supply

FREQUENCY CONVERTER

It has two sections Mixer and Oscillator. Usually only one transistor is used for both
sections.
The oscillator produces oscillations at a frequency which is higher than the frequency
of the station being received. The difference equals the intermediate frequency (I.F).
Out of the signals interrupted by the aerial, the signals from the desired stations are
selected by a tuned circuit at the input of the mixer stage. These signals and the oscillations
from the oscillator are mixed by the mixer. The mixing of these signals results in the
production of their sum (f1+f2) and the difference (f1-f2) frequencies. The difference is IF
signal. It contains the same modulation as contained by the received signals. The IF signals
are given to the IF amplifier.

21



IF AMPLIFIER

It amplifies the IF signals. It provides a large part of the gain and selectivity. Two
stages are used in this section to obtain the required amplification. Tuned transformers known
as IFT are used as a collector load and for interstage coupling in the IF amplifier. The
amplified IF signals are given to detector.

DETECTOR

The detector separates the audio signal from the modulated carrier. A diode is
generally used for detection.

AUDIO POWER AMPLIFIER

The audio signals received from the detector are given to the audio preamplifier stage.
This stage amplifies these signals and gives them to the driver stage.

DRIVER

The signals obtained from the preamplifier are not strong enough to drive the output
stage. The driver stage further amplifies these signals and gives them to the output stage.

OUTPUT STAGE

The output stage amplifies the audio signals obtained from the driver and develops
sufficient power to drive the loud speaker. To obtain maximum economy in battery
consumption, pushpull output stages using two transistors in class B circuit is normally used.

POWER SUPPLY

Most of the transistor radios operate on a few cells/battery. The supply voltages from
3 to 9V are common. Some transistor radios can be operated from batteries as well as mains.
In these transistor the mains voltage is converted into D.C. by diode rectifier.

22



GENERAL BLOCK DIAGRAM OF SUPERHETRODYNE RADIO RECEIVER
Antenna

Frequency 1st IF 2nd IF Audio Driver
converter amplifier amplifier Detector preamplifier

AGC
(mixer + LO)
Q1 Q2 Q3 D1 Q4 Q5
BF 194B BF 195C BF 195D 0A79 BC148A BC148B

Output

Q6,Q7
AC128 Loud
Speaker

BLOCK DIAGRAM OF AM/FM/SW RADIO RECEIVER

AM tuner AM IF AM
amplifier Detector
Audio Audio Audio
preamplifi Driver O/P
er
FM tuner FM IF FM
amplifier Detector

PROCEDURE
1. Study the radio receiver kit.
2. Draw the block diagram of A.M. superhetrodyne receiver.
3. Identify all stages according to the block diagram and note down the RF, IF
transistors with their specifications.

23



4. Study the following stages
? Frequency converter stage
? IF amplifier stage
? Detector and AGC stage
? Audio driver and output stage
? Power supply stage
5. Measure the sensitivity (I/P and O/P voltages of frequency converter) and selectivity
(I/P and O/P voltages of IF stage)
6. Measure the test point voltages
7. Observe the test point waveforms

Table of test point voltages

Transistor Emitter Base Collector Normal current
No signal condition
Q1
Q2
Q3
Q4
Q5
Q6
Q7

24



IF SIGNAL AT COLLECTOR OF Q1

1KHz
?V
450KHz

0V

1KHz

450KHz

1KHz

450KHz 10mV

0V

25



Expt: 7
PHASE LOCKED LOOP
AIM
To study design and find the lock range and capture range of phase locked loop

APPARATUS
Phase locked loop trainer board, function generator, CRO.

CIRCUIT DIAGRAM
Vcc

RT

R3 C
C1

10 8

7 Demodulated output
2
Input Signal NE 565 6
Reference output
3
4
9 1 5

CT

- VEE

THEORY

The phase locked loop principle has been used in the applications such as FM tracking
filters, frequency synthesized transmitters, FM demodulators, FSK decoders and generation
of local oscillator frequencies in TV and FM tuners.

This block diagram in its basic form contains 1. Phase detector, 2. Low pass filter 3.
VCO (Voltage Controlled Oscillator). The function of phase detector is to compare the input
signal fin and fout. The output of the phase detector is DC voltage and is often referred as the
error voltage. The output signal of phase detector is fed to the input of low pass filter. The

26



function of the low pass filter is to remove the high frequencies and it allows only low
frequency signals. It produces a DC level. The output of low pass filter is fed to the VCO.
The VCO frequency is compared with the input signal and adjusted until it is equal to the
input frequencies. If no signal is applied, the VCO starts to change until it equals the input
frequency. And the PLL is said to be in phase locked state.

PROCEDURE
1. Connect the circuit of PLL as shown in figure.
2. Measure the free running frequency of VCO at pin No. 4, with the input signal Vin set
equal to zero. Compare it with the calculated value = 0.25 RT.CT.
3. Now apply the input signal of 1V(P-P) square wave at 1 KHz at pin No.2. connect one
channel of CRO and display this signal on the scope.
4. Gradually increase the input signal frequency till the PLL is locked to the input
frequency. This frequency f1 gives the lower end of capture range. Go on increasing
the input frequency, till the PLL tracks the input signal say to a frequency f2, gives the
upper end of the lock range. If the frequency is increased further, the loop will get
unlocked.
5. Now gradually decrease the input frequency till the PLL is again locked, this is the
frequency f3, the upper end of the capture range. Keep on decreasing the input
frequency until the loop is unlocked. This frequency f4 gives the lower end of lock
range.
f
6. The lock range ? f L ? ( f 2 ? f 4 ). Compare it with the calculated value of 7.8 o
12
7. The capture range ? f C ? ( f 3 ? f 1 ) . Compare it with the calculated value of capture
? ? fL ?
range. ? f C ? ? .? ?
? 2? ? 3.6 ? 10 ? C ?
3

TABULATION

fo.= --------------------.

S.No. f1 f2 f3 f4 ? fc ? fc ? fL ? fL
in Hz in Hz in Hz in Hz theoretical practical theoretical practical

27



Expt: 8
SYNCHRONOUS AM DETECTOR

Aim
To study the performance of the synchronous am detector

Apparatus
Modules ACT-01, ACT-02, power supplies for module, CRO

THEORY
In synchronous Am detector, also known as coherent detector, the signal modulated in
amplitude is mixed to a coherent carrier signal and the result then, passes across a lowpass
filter, which supplies the desired demodulated signal. The signal vM can be expressed by the
following relation

vM(t) = Vm(t).sin (2 ? F.t)

Where Vm(t) = A.[1+m.sin(2 ? F.t)] is the amplitude of the envelop of the Am signal, and F
is the frequency of the carrier.

If the AM signal is multiplied with a un-modulated signal which has the same frequency
and phase, the output of the multiplier is as composite signal Vo(t)which can be
expressed in this way

Vo (t) = [ AC.sin( 2? F .t )].[Vm(t ).sin( 2? F .t )] ? Ko.Vm.[1 ? cos(2? .2F .t )]
Where Ko is considered as the gain of the multiplier circuit.

Make the signal vo(t) pass across the low pass filter, to remove the component with 2F.t
frequency, the resulting signal Vout is

Vout = Ko.Vm (t)
Which corresponds to the Am detected signal.

A PLL (Phase Locked Loop) system is generally used to regenerate a signal in the receiver,
which is exactly synchronous to the carrier of Am signal. In the applications where a high
quality is required, the use of AM synchronous detector presents some important
advantages with respect to the envelop detector
? low distortion
? capacity to properly demodulate the Am signal with high modulation depth, or with
quick variations of the modulation signal (as for example in pulse modulations)
? capacity to provide gain instead of attenuation introduced by diode.

28



P1 P2
Freq Level
1-10 KHz 0-2Vpp
0-1KHz
S1 S2
Function Generator (ACT-01) P1 P2
CAR. NULL level
P1
BALANCED MODULATOR 1 CAR. NULL level
(Act-02) BALANCED MODULATOR 2

29
(Act-02)
LOW PASS FILTER
P6 P7
FREQ
LEVEL
0-
2Vpp
400 KHz- 1500KHz
SYNCHRONOUS AM DETECTOR


Em

t

Fig :A

EC

t

Fig :B
Emax
Emin

t

Fig :C

Emax
Emin

Fig :D t
Em

t

Fig :E

Figure A: Modulating Signal Figure B: Carrier
Figure D: Balaced Moulator output at the detector Modulating Signal
Figure E: Demodulated Signal

30



PROCEDURE

1. Connect the power supply with proper polarities to kits and switch it on.
2. Refer to the figure and carry out the following connections and settings
3. In ACT-01
? Keep switch S1 in sine wave position
? Keep switch S2 in upper position (1-10KHz).
? Using pot P1 keep sine wave signal at 1KHz.
? Using pot P2 keep sine wave signal amplitude at 0.5 V pp
? Using pot P6 keep VCO2 frequency at 450KHz
? Using pot P1 keep VCO2 amplitude at 1Vpp.
4. In ACT-02
? Keep BALANCED MODULATOR 1 CARRIER NULL rotated completely
clockwise or slightly lower than clockwise pot P1
? Keep BALANCED MODULATOR 1 LEVEL at 20mVpp using pot P2.
? Keep the BALANCED MODULATOR 2 CARRIER NULL in center position, so
that the circuit operates as frequency converter (balanced modulator with suppressed
carrier) using pot P3.
? Keep the BALANCED MODULATOR 2 LEVEL at maximum using pot P4.
5. Connect the output of function generator (ACT -01) OUT1 post to the input of balanced
modulator 1 (ACT-02) SIGNAL IN post.
6. Connect the output of VCO2 (ACT -01) RF/FM OUT post to the input of balanced
modulator 1 (ACT-02) CARRIER IN post.
7. Connect the output of balanced modulator1 (ACT -02) MOD OUT post to the input of
balanced modulator 2 (ACT-02) SIGNAL IN post.
8. Connect the carrier of balanced modulator1 (ACT -02) CARRIER IN post to the input of
balanced modulator 2 (ACT-02) CARRIER IN post.
9. Connect the output of balanced modulator 2 (ACT -02) MOD OUT post to the input of
low pass filter (ACT-02) IN post.
10. Connect the CRO to the input of the demodulator post SIGNAL IN of the balanced
modulator 2 and to the output post MOD OUT. The waveform detected across the output
is the product between AM signal and the carrier that in the actual application is locally
regenerated in the receiver. Note that this waveform consists of a frequency double the
carrier, to which the waveform of the modulating signal is added.
11. The lowpass filter eliminates the high frequency component, supplying a clean detected
signal. As the filter has a cutting frequency of 3.4 KHz, all the superior frequencies are
attenuated.
12. Increase the modulation depth of AM signal and check that the detected signal does
always faithfully follow the envelope. There is no distortion by diagonal cutting.

31



Expt: 9
SSB SYSTEM
Aim
To study the performance of the SSB system.
Apparatus
SSB transmitter and receiver kits, CRO.
Theory

Single-sideband modulation is a form if amplitude modulation in which the carrier has
been cancelled out with a balanced modulator, and one of the sidebands has been removed by
using any one of the filtering methods. The most common of these is the filter method. SSB
is made possible because in “ordinary” AM, the carrier contains no information while two
sidebands are mirror images of each other. From an information-carrying point of view, one
of the sideband is superfluous. The main advantages of the SSB over DSBFC are that it
requires much less power to transmit and occupies only half the bandwidth. Its main
disadvantage is that it requires very much greater system frequency stability for satisfactory
operation, and this feature is particularly demanding on receivers.

Compared with DSBFC, SSB is decidedly a latecomer. Because of the receiver
frequency stability requirement, SSB is not used in broadcasting, although sporadic work on
compatible SSB goes on. In compatible SSB a proportion of the carrier would be transmitted
and could be received by ordinary DSBFC receivers without undue distortion. Because of its
power and bandwidth savings, SSB is used in a multitude of communication systems, notably
long distance point-to point communication and mobile communications(including ship-to-
shore and shore-to ship) at frequencies below about 30MHz. SSB is also a basic building
block in frequency division multiplexing, a broad band communication technique used for
multi channel communication in microwave links, terrestrial and submarine coaxial cables
and satellite up- and down-links.

The filter system

The filter system is simplest system of three-after the balanced modulator the unwanted
sideband is removed (actually heavily attenuated) by a filter. The filter may by LC, crystal,
ceramic or mechanical, depending on the carrier frequency and other requirements. A block
diagram of an SSB transmitter employing this system is shown below.

The key circuits in this transmitter are balanced modulator and the sideband
suppression filter. The special considerations involving sideband suppression will now be
examined.

32



Such a filter must have a flat bandpass and extremely high attenuation outside the
bandpass. There is no limit on this; the higher the attenuation, the better. In radio
communication systems, the frequency range used for voice is 300 to about 2800 Hz in most
cases. If it is required to suppress the lower sideband and if the transmitting frequency is f,
then the lowest frequency that must be passed through this filter without attenuation is f +
300Hz, whereas the highest frequency that must be fully attenuated is f – 300Hz. In other
words, the filter’ response must change from zero attenuation to full attenuation over a range
s
of only 600Hz. If the transmitting frequency is much above 10MHz, this is virtually
impossible. This situation becomes even worse if lower modulating frequencies are
employed, such as the 50Hz minimum in AM broadcasting. In order to obtain a filter
response curve with skirts as steep as those suggested above, the Q of the tuned circuits used
must be very high. As the transmitting frequency is raised, so must the Q be raised, until a
situation is reached where the necessary Q is so high that there is no practicable method of
achieving it.

Looking at the situation from the other end, we find that there must be an upper
frequency limit for any type of filter circuit used. It has been found, for instance, that
multistage LC filters cannot be used for RF values such greater than about 100kHz. Above
this frequency the attenuation outside the bandpass is sufficient. LC filters may still be
encountered in currently used HF equipment, but they have otherwise tended to be
superseded by crystal, ceramic or mechanical filters, mainly because of the bulky size of
components and great improvements in mechanical filters. Mechanical filers have been used
at frequencies up to 500 kHz, and crystal or ceramic filters up to about 20MHz.

Of the three major types of SSB filters, the mechanical filters seems to be the one
with the best all-around properties, small size, good bandpass, very good attenuation
characteristics and an adequate upper frequency limit are its chief advantages. Crystal or
ceramic filters may be cheaper, but are preferably at frequencies above 1 MHz.

All these filters ( even the crystal ) have the same disadvantage –their maximum
operating frequency is below the usual transmitting frequencies. This is the reason for the
balanced mixer shown in below (it is very much like a balanced modulator, except that the
sum frequency is much farther from the crystal oscillator frequency than the USB was from
the carrier, so that it can be selected with a tuned circuit.) In this mixer, the frequency of the
crystal oscillator or synthesizer is added to the SSB signal from the filter, the frequency thus
being raised to the value desired for transmission. Such an arrangement also allows the
transmitter to be tunable. If the transmitting frequency is much higher than the operating
frequency of the sideband filter, then two stages of mixing will be required. It becomes too
difficult to filter out the unwanted frequencies in the output of the mixer.

It might be noted that the mixer is followed by linear amplifiers. The reason is that the
amplitude of the SSB signal is variable and must be fed to a class C amplifier, which would
distort it. A class B RF amplifier (Push-Pull) is used instead because it is more efficient than

33



class A amplifier. The name linear amplifier is not confined to SSB systems. Linear
amplifiers are used in any AM system in which low or no signal distortion is a requirement.

SSB out to linear
Crystal amplifiers
oscillator

Buffer Balanced Sideband Balanced mixer
modulator suppression filter

AF in Audio amplifier Filter for other Crystal oscillator of
sideband synthesizer

Filter method of sideband suppression

SSB Generation

Ensure the following initial conditions on ST2201 module
1. a) AUDIO INPUT SELECT switch in INT position.
b) MODE switch in SSB position.
c) OUTPUT Amplifier’ GAIN preset in fully clockwise position.
s
2. Turn the AUDIO OSCILLATOR block’ AMPLITUDE preset to its fully clock wise
s
(max) position, and examine the block’ output (t.p.14) on CRO. This is the
s
modulating signal whose frequency can be adjusted from about 300Hz to about 3.4
KHz. By adjusting the AUDIO Oscillator’ FREQUENCY preset.
s
3. Adjust the FREQUENCY preset for an audio frequency of 2KHz. (approx. mid way)
4. Monitor the two inputs to the BALANCE MODULATOR block, at t.p. 15 and t.p.6.
5. The signal at t.p. 15 is the audio frequency sine wave from audio oscillator block.
This is the modulating signal to the balance modulator.
6. The signal at t.p. 6 is a sine wave whose frequency is slightly less than 455 KHz. It is
generated by the 455 kHz OSCILLATOR, and is the carrier input to the BALANCED
MODULATOR block.
7. Examine the output of the balanced modulator block(t.p.17) together with modulating
signal (t.p.15)
8. The waveforms should be as shown in fig 1.
9. Note that it may be necessary to adjust the BALANCED MODULATOR blocks
BALANCE preset, in order to ensure that the peak of t.p.17’ waveform envelop
s
(labeled A,B,C ) all have equal amplitudes this results in removing the carrier
component from resulting AM signal by adjusting the balance preset. Here we get a
DSBSC waveform

34



10. The DSBSC waveform is now passed on to the CERAMIC FILTER block whose
purpose is to pass the upper sideband but to lock the lower sideband.
11. Monitor the output of the CERAMIC BANDPASS FILTER block at t.p.20. Note that
the envelope of the signal a t.p. 20 now has fairly constant amplitude. If the amplitude
is not reasonably constant adjust the BALANCE preset in the BALANCE
MODULATOR block to minimize the variations in the signal’ amplitude.
s
12. Now observe the CERAMIC BANDPASS Filter’ output signal(t.p.20) and note that
s
the signal is good clean sine wave , indicating that the filter is passed upper sideband
only.
13. Turn the AUDIO OSCILLATOR block’ FREQUENCY preset throughout its range.
s
Note that for most audio frequencies, the waveform is a good, clean sine wave,
indicating that the lower sideband has been completely rejected by the filter. For low
audio frequencies the signal is not such a pure sinusoid. This is because the upper and
lower sidebands are now very close to each other.
14. Note that, by passing only the upper sideband, the frequency of the modulating signal
is shifted by an amount equal to carrier frequency.
15. Adjust the audio oscillators frequency to 2KHz, turn the block’ AMPLITUDE preset
s
to min position, and note that the amplitude of the signal at CERAMIC BANDPASS
Filter’ output drops to zero. This highlights one on the main advantages of SSB
s
amplitude modulation if there is no modulating signal , then the amplitude of the
SSB waveform drops to zero, so that there is no power wastage.
16. The CERAMIC BANDPASS FILTER is used here to pass only the upper sideband,
but to attenuate the lower sideband which is produced during the AM modulation. But
the disadvantage of this type of filter is that the range of pass band is fixed during the
filter’ manufacture and cannot be altered subsequently. The filter we are using here
s
has a pass band which is centered at 455 kHz.
17. The ST2201/ST2202 receiver will accept radio-frequency signals in the AM broadcast
band i.e. signals which fall in the frequency range of 525 kHz. To overcome this
problem the narrow band of frequencies must be shifted up, so that it falls within AM
broad cast range. This frequency shifting operation is performed by the BALANCED
MODULATOR & BANDPASS FILTER CIRCUIT 2, block, which contains a
balanced modulator followed by a tuned circuit.
18. Now monitor the output of the BALANCED MODULATOR & BANDPASS
FILTER CIRCUIT 2 block at t.p.22, and check that the waveform is pure sine wave
of frequency approximately 1.45 MHz. check that the waveform is reasonably good
sinusoid for audio modulating frequencies. If this is not the case adjust the
BALANCE preset in BALANCED MODULATOR & BANDPASS FILTER
CIRCUIT 2 block to remove the residual carrier frequency at 1MHz.
19. Now the signal has been shifted to AM broad cast range of 525kHz to 1.6MHz.
20. Now monitor the frequency at t.p 22. Reduce the amplitude of the modulating signal
to zero and observe that the output at t.p.22 follows the drops to zero.
21. This is the SSB signal which is applied to antenna for transmission.

35



SSB reception

1. Position the ST 2201 & ST2202 modules, with the ST2201 board on the left, and a
gap of about three inches.
2. ensure the following initial conditions exists on the ST2201 module
a) AUDIO Oscillator’ AMPLITUDE preset in fully clockwise position.
s
b) AUDIO INPUT SELECT switch in INT position.
c) MODE switch in SSB position.
d) OUTPUT Amplifier’ GAIN preset in fully clockwise position.
s
e) TX OUTPUT SELECT switch in ANT position.
f) AUDIO Amplifier’ VOLUME preset in fully counter-clockwise position.
s
g) SPEAKERS switch in ON position.
h) On board antenna in vertical position, fully extended.
3. Ensure the following initial conditions exist on the ST2202 module.
a) RX INPUT SELECT switch in ANT position.
b) RF Amplifier’ TUNED CIRCUIT SELECT switch in INT position.
s
c) RF Amplifier’ GAIN preset in fully clockwise position.
s
d) AGC switch in OUT position.
e) DETECTOR switch in PRODUCT position.
f) AUDIO Amplifier’ VOLUME preset in fully counter clockwise position.
s
g) SPEAKERS switch in ON position.
h) BEAT FREQUENCY OSCILLATOR switch in ON position.
i) On-board antenna in vertical position fully extended.
4. Turn on the power to the modules.
5. On the ST 2201 module, examine the transmitter’ output signal (t.p.13), and make
s
sure that the signal is reasonably good sine wave.
6. Turn the ST2201’ amplitude preset (in AUDIO OSCILLATOR block) to its fully
s
counter clockwise (minimum) position, and note that the amplitude if the monitored
signal from ST2201 (at t.p.13) drops to zero. Which illustrates that SSB waveform
contains no carrier-power. Now return the amplitude to its maximum position.
7. Since ST2201’ TX OUTPUT SELECT is in the ANT position the SSB signal is fed
s
to the antenna.
8. On the ST2202 module, monitor the output of the IF AMPLIFIER 2 block (t.p.28)
and turn the tuning dial until the amplitude of the monitored signal is at its greatest.
9. Since the incoming signal has no carrier component, the receiver AGC circuit cannot
make use of incoming carrier amplitude in order to control the gain of the receiver.
This means that the receiver’ AGC CIRCUIT cannot be used for SSB reception, and
s
must be switched out.
10. Turn the GAIN preset in ST2201’ OUTPUT AMPLIFIER block, so that the preset’
s s
arrowhead is horizontal and pointing to the left. This ensures that the amplitude of the
transmitted SSB signal is small.

36



11. On ST 2202 module, fine tune the tuning dial until the amplitude of the monitored
signal is at its greatest.
12. The receiver’ BFO produces a sine wave at the IF frequency of 455 KHz. This
s
455KHZ is the input to the receiver’ PRODUCT DETECTOR block, where it is
s
mixed with the SSB from IF amplifier.
13. The PRO0DUCT DETECTOR block mixes the output from the BFO with the output
from the IF amplifier block. Mixing process results in the generation of two new
frequency components: 1) sum of two frequencies 2) difference of those two.
14. The low-pass filter at the output of the PRODUCT DETECTOR accepts the
difference frequency. This audio frequency is then converted into sound by the
receivers AUDIO AMPLIFIERS BLOCK.
15. To demodulate out the incoming signal, we tune the Receiver’ LOCAL s
OSCILLATOR so that the output frequency range from IF AMPLIFIER 2 is slightly
below the 455KHz BFO frequency, such that the difference frequency generated by
the PRODUCT DETECTOR is same that of the modulating frequency. Then as the
frequency of the modulating signal changes the PRODUCT DETECTOR should
follow it.
16. On ST 2202/1 modules adjust the volume preset so that the receiver’ output is clearly
s
audible.
17. Slowly turn the tuning dial and notice that the tone at the receiver’ output changes.
s
On the ST2201 module turn the volume preset clockwise, until you can hear the tone
of the audio oscillator’ output signal. Find the position of the TUNING dial till two
s
tones are approximately same (i.e. of transmitter and receiver).
18. Now turn the frequency preset in ST2001’ AUDIO OSCILLATOR lock, throughout
s
its range noting that the frequency of the tone generated by the ST2202 remains close
to that generated by ST2201 for all preset positions.
19. connect the CRO at the output of Audio Oscillator block and at the output of the
receiver and note if there is any frequency deviation in these signals.

37



Model waveforms
Em

t
Fig : A

EC

t

Fig : B
a b c d
Em a a a

t

Fig : C
Em

t

Fig : D
Em

t

Fig : E
Figure :A Modulating Signal at t.p.15
Figure :B Carrier at t.p. 6
Figure :C Balanced modulator o/p at t.p. 17
Figure :D Balanced modulator and bandpass filter circuit 2 o/p at t.p.22
Figure :E Demodulated signal at t.p. 15

38



Expt:10
SPECTRUM ANALYZER
AIM
To analyze the AM and FM waveform using Spectrum Analyzer.

The oscilloscope is the most common device used to display the signals, with time as
x-axis. The signal which require time as x-axis, to display them are time domain signals. The
signals which require frequency as x-axis, to display them are called frequency domain
signals. Frequency domain display of signal consists of information of energy distributed of
the signal. The analysis of such a frequency domain display of the signal is called spectral
analysis of the signal. Thus the study of the energy distribution across the frequency spectrum
if a given signal is defined as the spectral analysis.

The instrument which graphically provides the energy distribution of a signal as a function of
frequency on its CRT is called spectrum analyzer. It provides a calibrated graphical display
with the frequency on horizontal axis and the signal component on the vertical axis, the
sinusoidal components of which, the signal is made up of, are displayed as the vertical lines
against frequency coordinates. The frequency of each vertical line gives the absolute
amplitude if the component while the horizontal location gives that particular frequency.

Introduction
The analysis of electrical signals is a fundamental problem for many engineers and
scientists. Even if the immediate problem is not electrical, the basic parameters of interest are
often changed into electrical signals by means of transducers. The rewards for transforming
physical parameters to electrical signals are great, as many instruments are available for the
analysis of electrical signals in the time and frequency domain.

The traditional way of observing electrical signals is to view them in time domain
using oscilloscope. The time domain is used to recover relative timing and phase information
which is needed to characterize electrical circuit behavior. How ever, not all circuit can be
uniquely characterized from just time domain information. Circuit elements such as
amplifiers, oscillators, mixers, modulators, detectors and filters are best characterized by their
frequency response information. This frequency information is best obtained by viewing
electrical signals in frequency domain. To display the signal in the frequency domain requires
a device that can discriminate between frequency domains is the spectrum analyzer. It
graphically displays the voltage or power as a function of frequency on a CRT.

In the time domain, all frequency components of a signal are seen summed together.
In the frequency domain, complex signals are separated in to their frequency components,
and power level at each frequency is displayed. The frequency domain is a graphical
representation of signal amplitudes as a function of frequency. the frequency domain contains
information not found in time domain.

39



Types of spectrum analyzers

There are two basic types of spectrum analyzers, swept-tuned and real time analyzers.
The swept-tuned analyzers are tuned by electrically sweeping them over their frequency
range. Therefore the frequency components of a spectrum are sampled sequentially in time.
This enables periodic and random signals to be displayed, but makes it impossible to display
transient response. Real time analyzers, on the other hand, simultaneously display the
amplitude of all signals in the frequency range of the analyzer: hence the name real-time.
This preserves the time dependency between signals which permits information to be
displayed. Real time analyzers are capable of displaying transient response as well as periodic
and random signals. The swept tuned analyzers are usually of the trf (tuned radio frequency)
or superheterodyne type. A trf analyzer consists of a frequency range, a detector to produce
vertical deflection on a CRT, and a horizontal scan generator used to synchronize the tuned
frequency to the CRT horizontal deflection. It is a simple, inexpensive analyzer with wide
frequency coverage, but lacks resolution and sensitivity. Because trf analyzers have swept
filter they are limited in sweep width

Applications of spectrum analyzers

1) modulation measurements: When the frequency scan of spectrum analyzer is set to zero
and x-axis is representing time instead of frequency, it operates as a fixed tuned receiver to
measure amplitude against time. This is called its synchroscope mode. When analyzer is
tuned to carrier frequency with bandwidth at least twice that of modulation frequency and
with a linear display, the envelop of an AM signal is observed. Measuring the peak Vp and
through Vt, modulation index can be determined.
When operated in normal mode, two sidebands separated from the carrier by
modulation frequency fm are observed. The modulation index can be calculated from the
sidebands and carrier amplitude. Similarly it can be used to calculate the distortion occurring
in modulation process. The sideband configuration in frequency modulation enables observer
to calculate the frequency modulation index.
2) Continuous wave signal frequency stability
The frequency drift of a signal can be measured by observing the excursions of the signal
across the display. Over period of minutes, it gives long term stability while over period of
seconds it gives short term stability.
3) Harmonic distortion measurement
The distortion affects the frequency components of a signal to be transmitted. The harmonics
appear as the additional signals in the spectrum analyzer at multiples of the carrier frequency.
To keep it low, its measurement plays an important role. The spectrum analyzer can be used
to make such distortion measurements.
4) noise measurement

40



The noise can be measured with very straightforward method using the spectrum analyzer.
Similarly the measurement of impulse noise also can be measured using spectrum analyzer.
The examples of impulse noise in the generation of voltage spikes due to engine ignition and
electric motor commutation.
Examining pulse modulation
This is the first application of spectrum analyzer. The spectrum analyzer can be used to
measure or evaluate the quality of the pulse modulation. The difficult task of measuring pulse
modulation of radar transmitters is possible due to spectrum analyzer.
Apart from these common applications it is used in the following applications as well.
1) in the fields of biomedical electronics, geological surveying, oceanography. It is used
to analyze the water and air pollution.
2) It is used to measure the antenna pattern.
3) It is used to tune the parametric amplifier.
4) It is used to examine the vibration signals from the automobiles, airplanes, space
vehicles bridges and other mechanical systems. It provides useful information about
mechanical integrity, unbalance and bearing, gear wear.
5) It finds number of applications in the field of electronic testing related to trouble
shooting and quality control.

HAMEG 5010 SPECTRUM ANALYZER

41



Expt:11
FIBER OPTIC ANALOG LINK
AIM
1. To setup a Fiber Link for the transmission of analog signals and voice
signals
2. To find the frequency response and band width of the fiber.
3. To determine the bending loss and attenuation of the fiber.

THEORY
Fiber Optic Links can be used for transmission of digital as well as analog signals.
Basically a fiber optic link contains three main elements, a transmitter and optical fiber and a
receiver. The transmitter module takes the input signal in electrical form and then transforms
it into optical (Light) energy containing the same information. The optical fiber is a medium
which carries this energy to the receiver. At the receiver, the light is converted back into
electrical form with the same pattern as originally fed to the transmitter.

TRANSMITTER
Fiber optic transmitters are typically composed of a buffer, driver & optical source.
The buffer electronics provides both an electrical connection and isolation between the
transmitter and the electrical system supplying data. The driver electronics provides electrical
power to the optical source in a fashion that duplicates the pattern of data being fed to the
transmitter. Finally the optical source (LED) converts the electrical current to light energy
with the same pattern. The LED SFH 450V supplied with the kit operates outside the visible
spectrum. It’ optical output is centered at the near infrared wave length of 950nm. The
s
emission spectrum is broad, so a faint red glow can usually seen when the LED is on in a
dark room. The LED SFH450V used in the kit is coupled to a transistor driver in the common
emitter mode. The driver is preceded by the amplifier buffer. The amplifier in this case is a
LM 741 operational amplifier configured as voltage amplifier. In the absence of input signal
half of the supply voltage appears at the base of the transistor. This biases the transistor near
midpoint within the active region for linear applications. Thus LED emits constant intensity
of light at this time. When the signal is applied to the amplifier it over rides the DC level at
the base of the transistor which causes the Q point of the transistor to oscillate about the mid
point. So the intensity of the LED varies about it’ previous constant value. This variation in
s
the intensity has linear relation with the input electrical signal. Optical signal is then carried
over by the optical fiber.

OPTICAL FIBER

Optical fibers are available in different variety of materials. These materials are
usually selected by taking into account their absorption characteristics for different
wavelengths of light. In case of optical fiber, since the signal is transmitted in the form of

42



light which is completely different in nature as that of electrons, one has to consider the
interaction of matter with the radiation to study of losses in fiber. Losses are introduced in
fiber due to various reasons. As light propagates from one end of fiber to another end, part of
it is absorbed in the material exhibiting absorption loss. Also part of the light is reflected back
or in the same other direction from the impurity particles present in the material contributing
to the loss if the signal at the other end of the fiber. In general terms it is known as
propagation loss. Plastic fibers have higher loss of the order of 180dB/ KM. when ever the
condition of angle of incidence of the incident light is violated the losses are introduced due
to refraction of light. This occurs when fiber is subjected to bending. Lower the radius of the
curvature more is the loss. Other losses are due to coupling of fiber at LED and photo
detector ends.

RECEIVER
The function of the receiver is to covert the optical energy into electrical energy
which is then conditioned to reproduce the transmitted electrical signal in its original form.
The detector SFH250V used in the kit has a diode type output. The parameters usually
considered in the case of detector are it’ responsivity at peak wavelength and response time.
s
SFH250V has responsivity of about 4? A per 10? W of incident optical energy at 950nm and
it has rise time and fall time of 0.01? sec.
PIN photo diode is normally reverse biased. When optical signal falls on the diode,
reverse current starts to flow, diode acts as closed switch and in the absence of light intensity
it acts as an open switch. Since PIN diode usually has low responsivity, a Trans impedance
amplifier is used to convert this reverse current into voltage. This voltage is then amplified
with the help of another amplifier circuit. This voltage is the duplication of the transmitted
electrical signal.

Signal source Amplifier Optical
Section Transmitter

CRO Optical
receiver

Optical Cable
PROCEDURE
Initial settings :
1. Connect the supply to the kit.

43



2. observe the sine wave at the SIN O/P point and adjust the amplitude of the wave to
0.1V(p-p) by using pot.
3. Apply this signal to the amplifier input post and adjust the pot in amplifier section to
get necessary gain. The amplified signal is available at the AMP O/P point.
4. This signal is fed to the transmitter input and the light source emits the intensity
modulated light signal.
5. Now slightly unscrew the light source o/p knob and properly insert the fiber.
6. In the similar fashion, connect the other end of the fiber to the receiver.
7. observe the detected signal at the AC O/P point. This should be exact replica of the
input signal. If it is not, adjust the intensity control pot and gain control pot to get the
required signal.

Frequency response of the fiber
1. After the initial settings, apply the sine wave from external function generator with
amplitude of 0.1V (p-p).
2. Keep this amplitude as constant and vary the frequency of the input signal.
3. Note the output signal amplitude for various values of input frequencies. (50Hz –
1MHz)
4. Calculate the gain and plot frequency Vs gain graph.
5. Obtain the 3-dB frequencies and calculate the bandwidth of the fiber.

Measurement of bending losses in fiber
1. After initial settings apply the sine wave from external function generator with
amplitude of 0.1 V (p-p) and frequency of 1KHz.
2. Keep this input as constant, fold the fiber cable into circular shape and measure the
output voltage.
3. Then decrease the diameter of the circular cable (only upto 2 CM) and note the
corresponding variation in output voltage.
4. calculate the gain and plot the graph of diameter Vs gain

Measurement of attenuation of a fiber cable
1. After the initial settings, apply the sine wave from external function generator with
amplitude of 0.1V (p-p) and frequency of 1 KHz.
2. Measure the output voltage (V1) for a 1 meter fiber cable.
3. Now replace the 1 meter fiber by 3-meter fiber and note the output voltage (V2).
4. Calculate the attenuation of the fiber by using the formula 1 ? exp .?? ? ?L1 ? L2 ??
V
V2
where ? = attenuation of fiber and its unit is nepers / meter
L1 and L2 are the lengths of the fiber i.e., 1 and 3 respectively.
Setting up a voice link
1. Connect the microphone to the socket marked MIC JACK in the audio preamplifier
section of the kit.

44



2. Connect the speakers to the socket marked SPEAKER JACK in the audio amplifier
section of the kit.
3. Now Apply the output of the amplified microphone output which is available at MIC
O/P post in the kit to the AMP I/p post.
4. Connect the detected output which is available at DETECTOR O/P to AUDIO O/P
post.
5. Adjust the optical power control pot P6 and voice control pot P8 to set up voice link.

45


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