Department
Of
Electronics & Communication Engineering
Laboratory Manual
Communication Systems (EE-226-E)
Applied College of Management and Engineering
Affiliated to
Communication System Laboratory
(EE-226-E)
S.N.
Name of Experiment
1.
Study of amplitude modulation (AM).
2.
Determine the modulation index of amplitude modulated (AM) wave.
3.
Study of Double Sideband AM Reception.
4.
Study of frequency modulated (FM) wave.
5.
Study the demodulation of frequency modulated (FM) wave.
6.
Determine the modulation index of frequency modulated (FM) wave.
7.
Study of pulse amplitude modulation (PAM).
8.
(a) Study of pulse code modulation (PCM) transmitter.
(b) Study of Pulse code modulation (PCM) receiver.
9.
Study of amplitude shift keying (ASK) modulator and demodulator.
10.
Study of Frequency shift keying (FSK).
11.
Study of various data formatting methods.
EXPERIMENT NO. 1
Study of amplitude modulation (AM).
Aim: To trace the wave shape of the electrical signal at the input and output terminals of
amplitude modulator, using CRO.
Apparatus: AM trainer, CRO (20 MHz), connecting leads.
Theory:
In case of AM, the amplitude of high frequency carrier wave is varied in accordance with
the modulating signal i.e.
if carrier wave is C (t) = Ac Cos ωc t and modulating signal is m (t) = A Cos ωm t, then
AM signal
S (t) = (Ac + m (t)) Cos ωc t
or
S (t) = Ac (1 + m Cos ωm t) Cos ωc t
Where m is called the modulation index and its values lies between 0 & 1. on
2
Figure-1
Figure-2
Procedure:
This experiment investigates the generation of double sideband amplitude modulated
(AM) waveforms, using the ST2201 module. By removing the carrier from such an AM
waveforms, the generation of double sideband suppressed carrier (DSBSC) AM is also
investigated. To avoid unnecessary loading of monitored signals, X10 oscilloscope
probes should be used throughout this experiment.
3
1. Ensure that the following initial conditions exist on the board.
a. Audio input select switch in INT position:
b. Mode switch in DSB position.
c. Output amplifier's gain pot in full clockwise position.
d. Speakers switch in OFF position.
2. Turn on power to the ST2201 board.
3. Turn the audio oscillator block's amplitude pot to its full clockwise (MAX) position,
and examine the block's output (t.p.14) on an oscilloscope.
This is the audio frequency sine wave which will be as our modulating signal. Note that
the sine wave’s frequency can be adjusted from about 300 Hz to approximately 3.4 KHz,
by adjusting the audio oscillator's frequency pot.
Note also that the amplitude of this audio modulating signal can be reduced to zero, by
turning the Audio oscillator's amplitude present to its fully counterclockwise (MIN)
position. Return the amplitude present to its max position.
4. Turn the balance pot, in the balanced modulator & band pass filter circuit 1 block, to
its fully clockwise position. It is this block that we will use to perform double-side band
amplitude modulation.
5. Monitor, in turn, the two inputs to the balanced modulator & band pass filter circuits 1
block, at t.p.1 and t.p.9.
Note that:
a. The signal at t.p.1 is the audio-frequency sine wave from the audio oscillator block.
This is the modulating input to our double-sideband modulator.
b. Test point 9 carries a sine wave of 1MHz frequency and amplitude 120mVpp approx.
This is the carrier input to our double-sideband modulator.
6. Next, examine the output of the balanced modulator & band pass filter circuit 1 block
(at t.p.3); together with the modulating signal at t.p.1. Trigger the oscilloscope on the t.p.
1 signal.
Check that the waveforms as shown in fig 3.
Fig 3
The output from the balanced modulator & band pass filter circuit 1 block (at t.p. 3) is a
double-sideband. AM waveform, which has been formed by amplitude-modulating the
1MHz carrier sinewave with the audio-frequency sinewave from the audio oscillator.
The frequency spectrum of this AM waveform is as shown below in fig. 2, where fm is
the frequency of the audio modulating signal.
4
Fig. 4
7. To determine the depth of modulation, measure the maximum amplitude (Vmax) and
the minimum amplitude (V min) of the AM waveform at t.p.3, and use the following
formula:
Percentage Modulation 
VMAX  VMIN
VMAX  VMIN
Where Vmax and Vmin are the maximum and minimum amplitudes shown in Fig.3
8. Now vary the amplitude and frequency of the audio-frequency sinewave, by adjusting
the amplitude and frequency present in the audio oscillator block. Note the effect that
varying each pot has on the amplitude modulated waveform.
The amplitude and frequency amplitudes of the two sidebands can be reduced to zero by
reducing the amplitude of the modulating audio signal to zero. Do this by turning the
amplitude pot to its MIN position, and note that the signal at t.p.
3 becomes an un-modulated sine wave of frequency 1 MHz, indicating that only the
carrier component now remains. Return the amplitude pot to its maximum position.
Now turn the balance pot in the balanced modulator & band pass filter circuit 1 block,
until the signal at t.p. 3 is as shown in Fig.5
Fig. 5
The balance pot varies the amount of the 1 MHz carrier component, which is passed from
the modulator's output.
By adjusting the pot until the peaks of the waveform (A, B, C and so on) have the same
amplitude, we are removing the carrier component altogether. We say that the carrier has
been 'balanced out' (or 'suppressed') to leave only the two sidebands.
Note that once the carrier has been balanced out, the amplitude of t.p.3's waveform
should be zero at minimum points X, Y; Z etc. If this is not the case, it is because one of
the two sidebands is being amplified more than the other. To remove this problem, the
5
band pass filter in the balanced modulator & band pass filter circuit 1 block must be
adjusted so that it passes both sidebands equally.
This is achieved by carefully trimming transformer T1, until the waveform's amplitude is
as close to zero as possible at the minimum points.
The waveform at t.p.3 is known as a double-side suppressed carrier (DSBSC) waveform,
and its frequency spectrum is as shown in Fig.6.
FIG-6 Frequency Spectrum of DSBSC Wave Form
Note that now only the two sidebands remain, the carrier component has been removed.
Precautions:
1. Make sure that all the instruments are properly connected.
2. The various grounds should be properly connected.
EXPERIMENT- 2
Determine the modulation index of amplitude modulated (AM) wave.
6
Aim: To calculate the modulation index of amplitude modulated (AM) wave for three
different cases (m=0, m<1, m>1) which are obtained by changing the amplitude of input
sinusoidal signal.
Apparatus: AM trainer, CRO (20 Mhz), function generator (20 V p-p), connecting leads.
Theory:
AM wave is represented by S(t) = (Vc + Vm Cos Wm t) Cos Wc t
= Vc (1 + m Cos Wm t) Cos Wc t
where,
m = Vm / Vc
Fig-1 Amplitude Modulated (AM) wave
From the above figure,
Vm = (Vmax – Vmin) / 2 and Vc = ( Vmax + Vmin ) / 2
m = (Vmax - Vmin )/( Vmax + Vmin)
Procedure:
1. Perform the experiment no. 1 up to step no. 6
2. Now apply the modulated waveform to the Y input of the oscilloscope and the
modulating signal to the X input.
3. Press the XY switch; you will observe the waveform similar to the one given below:
Fig. 2
7
Calculate the modulation index by substituting in the formula value of modulation index
using
V
 VMIN
Percentage Modulation  MAX
VMAX  VMIN
4. Some common trapezoidal patterns for different modulation indices are as
shown:
Fig-2
Observation:
Table 1
S.NO.
Vmax
Vmin
m =
(Vmax Vmin)/(
Vmax +
Vmin)
:
Precautions:
1. Make sure that all the instruments are properly connected.
2. The various grounds should be properly connected.
EXPERIMENT NO-3
Study of Double Sideband AM Reception.
Aim: This experiment investigates the reception and demodulation of AM waveforms
Apparatus: AM trainer, CRO (20 MHz), function generator (20 V p-p), connecting
leads.
Theory:
This experiment investigates the reception and demodulation of AM waveforms by
the ST2201/ ST2202 module. Both AM broadcast signals, and AM transmissions
from ST2201, will be examined, and the operation of automatic gain control at the
receiver will be investigated.
A diode operating in a linear region of its V-I characteristics can extract the envelope of
an AM wave. This type of detector is known as envelope detector. A capacitor is
connected as shown in fig1. For the positive half cycle the diode conducts and capacitor
is charged to peak value of the carrier voltage .For the negative half cycle the diode is
reverse biased, it does not conduct. So, the capacitor starts discharging through the
resistance R with a time constant T=RC. The time constant has to be chosen suitably, its
8
value must be selected in accordance with the relation 1/RCWmMa, (Wm is the
frequency of the modulating signal and Ma is the modulation index) otherwise it
produces the distortion in the demodulated signal.
Procedure:
1. Position the ST2201 & ST2202 modules, with the ST2201 board on the left, and a gap
of about three inches between them.
2. Ensure that the following initial conditions exist on the ST2201 board.
a. Audio oscillator's amplitude pot in fully clockwise position.
b. Audio input select switch in INT position.
c. Balance pot in balanced modulator & band pass filter circuit 1 block, in full clockwise
position;
d. Mode switch in DSB position.
e. Output amplifier's gain pot in full counter-clockwise position.
f. TX output select switch in ANT position:
g. Audio amplifier’s volume pot in fully counter-clockwise position.
h. Speaker switch in ON position.
i. On-board antenna in vertical position, and fully extended.
3. Ensure that the following initial conditions exist on the ST2102 board:
a. RX input select switch in ANT position.
b. R.F. amplifier's tuned circuit select switch in INT position.
c. R.F amplifier's gain pot in fully clock-wise position;
d. AGC switch in INT position.
e. Signal Detector switch in diode position.
f. Audio amplifier's volume pot in fully counter-clockwise position.
g. Speaker switch in ON position.
h. Beat frequency oscillator switch in OFF position.
i. On-board antenna in vertical position, and fully extended.
4. Turn on power to the modules.
5. On the ST2202 module, slowly turn the audio amplifier's volume pot clockwise,
until sounds can be heard from the on-board loudspeaker. Next, turn the vernier tuning
dial until a broad cast station can be heard clearly, and adjust the volume control to a
comfortable level.
6. The first stage, or 'front end' of the ST2202 AM receiver is the R.F amplifier
stage. This is a wide -bandwidth tuned amplifier stage, which is tuned into the wanted
station by means of the tuning dial. Once it has been tuned into the wanted station, the
R.F. amplifier, having little selectivity, will not only amplify, but also those frequencies
that are close to the wanted frequency. As we will see later, these nearby frequencies will
be removed by subsequent stages of the receiver, to leave only the wanted signal.
Examine the envelope of the signal at the R.F. Amplifier’s output (at t.p. 12), with an a.c.
- coupled oscilloscope channel. Note that:
a. The amplifier's output signal is very small in amplitude (a few tens of mill
volts at the most). This is because one stage of amplification is not sufficient to bring
the signal's amplitude up to a reasonable level.
b. Only a very small amount of amplitude modulation can be detected, if any. This
is because there are many unwanted frequencies getting through to the amplifier output,
which tend to 'drown out' the wanted AM Signal.
You may notice that the waveform itself drifts up and down on the scope
display, indicating that the waveform's average level is changing. This is due to the
operation of the AGC circuit, which will be explained later.
7. The next stage of the receiver is the mixer stage, which mixes the R.F.
Amplifier’s output with the output of a local oscillator. The Frequency of the local
oscillator is also tuned by means of the tuning dial, and is arranged so that its frequency
9
is always 455 KHz above the signal frequency that the R.F. amplifier is tuned to.
This fixed frequency difference is always present, irrespective of the position of
the tuning dial, and is arranged so that its frequency is always 455 KHz above the
signal frequency that the R.F. amplifier is tuned to. This fixed frequency difference is
always present, irrespective of the position of the tuning dial, and is known as the
intermediate frequency (IF for short). This frequency relationship is shown below, for
some arbitrary position of the tuning dial.
Fig-1
Examine the output of the local oscillator block, and check that its frequency
varies as the tuning dial is turned. Re-tune the receiver to a radio station.
8. The operation of the mixer stage is basically to shift the wanted signal down to the IF
frequency, irrespective of the position of the tuning dial. This is achieved in two stages.
a. By mixing the local oscillator's output sine wave with the output from the R.F.
amplifier block. This produces three frequency components:
The local oscillator frequency = (f sig + IF)
The sum of the original two frequencies, f sum = (2 f sig + IF)
The difference between the original two frequencies,
f diff = (f sig + IF - f sig) = IF
These there frequency components are shown in Fig.2.
Fig-2
b. By strongly attenuating all components. except the difference frequency, IF this is
done by putting a narrow-bandwidth band pass filter on the mixer's output.
The end result of this process is that the carrier frequency of the selected AM station is
shifted down to 455 KHz (the IF Frequency), and the sidebands of the AM signal are now
either side of 455 KHz.
10
9. Note that, since the mixer's band pass filter is not highly selective, it will not
completely remove the local oscillators and sum frequency components from the
mixer's output. this is the case particularly with the local oscillator component,
which is much larger in amplitude than the sum and difference components.
Examine the output of the mixer block (t.p. 20) with an a.c. coupled oscilloscope
channel, and note that the main frequency component present changes as the
tuning dial is turned. This is the local oscillator component, which still dominates the
mixer's output, in spite of being attenuated by the mixer's band pass filter.
10. Tune in to a strong broadcast station again and note that the monitored signal
shows little, if any, sign of modulation. This is because the wanted component, which is
now at the IF frequency of 455 KHz, is still very small in component, which is now at
the IF frequency of 455 KHz, is still very small in comparison to the local oscillator
component. What we need to do now is to preferentially amplify frequencies
around 455KHz, without amplifying the higher-frequency local oscillator and SUM
components. This selective amplification is achieved by using two IF amplifier
stages, IF amplifier 1 and IF amplifier 2, which are designed to amplify strongly a
narrowband of frequencies around 455 KHz, without amplifying frequencies on either
side of this narrow band. These IF amplifiers are basically tuned amplifiers which have
been pre-tuned to the IF frequency-they have a bandwidth just wide enough to
amplify the 455
KHz carrier and the AM sidebands either side of it. Any frequencies outside this narrow
frequency band will not be amplified. Examine the output of IF amplifier 1 (at. t.p. 24)
with an a.c.-coupled oscilloscope channel, and note that:
a. The overall amplitude of the signal is much larger than the signal amplitude at
the mixer's output, indicating that voltage amplification has occurred.
b. The dominant component of the signal is now at 455 KHz, irrespective of any
particular station you have tuned into. This implies that the wanted signal, at the IF
frequency, has been amplified to a level where it dominates over the unwanted
components.
c. The envelope of the signal is modulated in amplitude, according to the sound
information being transmitted by the station you have tuned into.
11. Examine the output of IF amplifier 2 (t.p.28) with an a.c.-coupled oscilloscope
channel, noting that the amplitude of the signal has been further amplified by this
second IF amplitude of the signal has been further amplified by this second IF amplifier
stage. IF amplifier 2 has once again preferentially amplified signals around the IF
frequency (455 KHz), so that:
a. The unwanted local oscillator and sum components from the mixer are now so
small in comparison, that they can be ignored totally,
b. Frequencies close to the I F frequency, which are due to stations close to the wanted
station, are also strongly attenuated. The resulting signal at the output of IF amplifier
2 (t.p.28) is therefore composed almost entirely of a 455 KHz carrier, and the A.M.
sidebands either side of it carrying the wanted audio information.
12. The next step is extract this audio information from the amplitude variations of the
signal at the output of IF amplifier 2. This operation is performed by the diode detector
block, whose output follows the changes in the amplitude of the signal at its input.
To see how this works, examine the output of the diode detector block (t.p.31), together
with the output from. IF amplifier 2 (at t.p.28). Note that the signal at the diode detector's
output:
· Follows the amplitude variations of the incoming signal as required:
· Contains some ripple at the IF frequency of 455 KHz, and
11
· The signal has a positive DC offset, equal to half the average peak to peak amplitude of
the incoming signal. We will see how we make use of this offset later on, when we look
at automatic gain control (AGC).
13. The final stage of the receiver is the audio amplifier block contains a simple
low-pass filter which passes only audio frequencies, and removes the highfrequency ripple from the diode detector's output signal. This filtered audio signal is
applied to the input of an audio power amplifier, which drives on board loudspeaker (and
the headphones, if these are used). The final result is the sound you are listening to! The
audio signal which drives the loudspeaker can be monitored at t.p. 39 (providing
that the audio amplifier block's volume pot is not in its minimum volume
position). Compare this signal with that at the diode detector's output (t.p. 31), and
note how the audio amplifier block's low pass filter has 'cleaned up' the audio signal.
You may notice that the output from the audio amplifier block (t.p. 39) is
inverted with respect to the signal at the output of the diode detector (t.p. 31) this
inversion is performed by the audio power amplifier IC, and in no way affects the
sound produced by the receiver.
14. Now that we have examined the basic principles of operation of the ST2202
receiver for the reception and demodulation of AM broadcast signals, we will try
receiving the AM signal from the ST2201 transmitter.
Presently, the gain of ST2201's output amplifier block is zero, so that there is no output
from the Transmitter. Now turn the gain pot in ST2201's output amplifier block to
its fully clockwise (maximum gain) position, so that the transmitter generates an
AM signal. On the ST2201 module, examine the transmitter's output signal (t.p.13),
together with the audio modulating signal (t.p.1), triggering the 'scope with the signal'.
Since ST2201 TX output select switch is in the ANT position, the AM signal at t.p.13 is
fed to the transmitter's antenna. Prove this by touching ST2201's antenna, and
nothing that the loading caused by your hand reduces the amplitude of the AM
waveform. at t.p.13.
The antenna will propagate this AM signal over a maximum distance of about 1.4 feet.
We will now attempt to receive the propagated AM waveform with the ST2201/ ST2202
board, by using the receiver's on board antenna.
15. On the ST2201 module, turn the volume pot (in the audio amplifier block)
clockwise, until you can hear the tone of the audio oscillator’s output signal, from
the loudspeaker on the board.
Note: If desired, headphones may be used instead of the loudspeaker on the board. To
use the headphones, simply plug the headphone jack into the audio amplifier
block's headphones socket, and put the speaker switch in the OFF position. The
volume from the headphones is still controlled by the block's volume pot. Turn the
volume pot to the full counter-clockwise (minimum volume) position.
16. On the ST2201/ST2202 receiver, adjust the volume pot so that the receiver's
output can be clearly heard. Then adjust the receiver's tuning dial until the tone generated
at the transmitter is also clearly audible at the receiver (this should be when the tuning
dial is set to about 55-65 and adjust the receiver's volume pot until the tone is at a
comfortable level. Check that you are tuned into the transmitter's output signal, by
varying ST2201's frequency pot in the audio oscillator block, and nothing that the tone
generated by the receiver changes.
The ST2201/2202 receiver is now tuned into AM signal generated by the ST2201
transmitter. Briefly check that the waveforms, at the outputs of the following
receiver blocks, are as expected :
R. F. Amplifier (t.p.12)
Mixer (t.p.20)
12
I.F. Amplifier 1 (t.p.24)
I.F. Amplifier 2 (t.p.28)
Diode Detector (t.p.31)
Audio Amplifier (t.p.39)
By using the microphone, the human voice can be used as transmitter's audio
modulating signal, instead of using ST2201's audio oscillator block. Use DSB and not
DSBSC. Connect the microphone’s output to the external audio input on the
ST2201 board, and put the audio input select switch in the EXT position.
Precautions:
1. Make sure that all the instruments are properly connected.
2. The various grounds should be properly connected.
EXPERIMENT NO. 4
Study of frequency modulated (FM) wave.
Aim: To trace the wave shape of the electrical signal at the input and output terminals of
frequency modulator with DC and AC modulating voltage, using CRO.
Apparatus: FM training kit, CRO (20 MHz), function generator (20 V p-p), connecting
leads.
Theory:
The FM wave is represented by S (t) = Ac Cos [Wc t +k m(t)] where ,Ac Cos Wc t is the
carrier wave and m (t)is the modulating signal. Hence, in FM the frequency of the
carrier wave varies in accordance with the instantaneous value of modulating signal.
Carrier
Signal
13
FM signal
Fig- F M Modulated Waveforms
Function
Frequency
Modulator
Generator
CRO
GND
Fig-2 Test set-up
Procedure:
1. Make the setup as shown in fig 2.
2. Give a dc signal of fixed amplitude (say 5V) at the input of FM training kit.
3. Observe the modulated wave on CRO .Now trace the input and output waveforms as
observed on CRO using trace paper.
4. Vary the amplitude of dc signal in small steps and observe the effect on the modulated
wave for different values of dc amplitude. Note down the readings in the table1.
5. Set function generator frequency at 10 kHz. The carrier oscillator is in-built. Measure
its frequency. It should be 100 kHz.
Repeat the steps (1-5) by giving (i) square signal and then (ii) sinusoidal signal at the
input of FM kit.
Table 1
S.No.
Input dc voltage(v)
output frequency (Hz)
Precautions:
1. Make sure that all the instruments are properly connected.
2. The various grounds should be properly connected
14
EXPERIMENT NO.5
Study the demodulation of frequency modulated (FM) wave.
Aim: To trace the wave shape of the electrical signal at the input and output terminals of
Foster Seelay demodulator, using CRO.
Apparatus: FM training kit, CRO (20 MHz), function generator (20 V p-p), connecting
leads.
Theory:
The circuit consists of an inductively coupled double-tuned circuit in which both
primary and secondary coils are tuned to same frequency(intermediate frequency).The
primary voltage V3(i.e. signal voltage)thus appears across the inductor L.The center
tapping of the secondary coil has an equal and opposite voltage across each half winding.
Hence V1 and V2 are equal in magnitude but opposite in phase. The RF voltages Va1
and Va2 applied to the diodes D1 and D2 are expressed as
Va1=V3+V1 and Va2=V3-V1
Voltages Va1 and Va2 depend upon the phasor relations between V1,V2 and V3.
The RF voltage Va1 and Va2 are separately rectified by the diodes D1 and D2
respectively to produce voltage Vo1 and Vo2.The voltage Vo1 and Vo2 then represent
the amplitude variations of Va1 and Va2 respectively. The diodes are so arranged that the
output voltage Vo is equal to the arithmetic difference of Vo1 and Vo2.
Vo= Vo2 - Vo1
15
Fig 1: FM Demodulator
CRO
Function
generator
Frequency
modulator
FM
Demodulator
Fig 2: Test set-up
Procedure:
1. Make test setup as shown in fig 2.
2. Set the function generator frequency at 10 kHz. Give a sinusoidal signal of fixed
amplitude (say 5V) at the input of FM training kit from function generator.
3. Feed the modulator output to the input of FM demodulator. Observe the demodulated
wave on CRO.
4. Now trace the input and output waveforms as observed on CRO using trace paper.
5. Vary the sinusoidal signal amplitude in steps of 1V. Repeat steps 3 and 4.
Precautions:
1. Make sure that all the instruments are properly connected.
2. The various grounds should be properly connected.
EXPERIMENT NO. 6
Determine the modulation index of frequency modulated (FM) wave.
16
Aim: To determine the modulation index of two different FM waves which are obtained
by changing the amplitude of input sinusoidal signal.
Apparatus: FM training kit, CRO (20 MHz), function generator (20 V p-p), connecting
leads, frequency counter.
Theory:
Frequency modulation is a system in which the amplitude of carrier wave is kept constant
and frequency is varied in accordance to the modulating signal. The shift in carrier
frequency from its resting point compared to that of modulating frequency is called
deviation ratio.
For FM wave,
V (t) = Ac Cos [ Wc t + K ∫ m (t) dt ]
Let, m (t) = Am Cos Wm t
W = d/dt [ Wc t + K ∫ m (t) dt]
= Wc + Kd/dt (∫ m (t)dt )
= Wc + K(Am Cos Wmt )
f = fc + K’Am Cos Wm t
∆ f = f – f c = K’ Am Cos Wm t
Frequency
Modulation Index,  = ∆ f / f m
counter
CRO
Frequency
Modulator
Function
Generator
GND
Fig 1:Test set-up
Procedure:
1.
2.
3.
4.
5.
6.
Make the connections on the FM kit as shown in fig 1.Give the input from
function generator (say 5V, 10 kHz).Note down its frequency (fm). Set the
frequency counter to read in 100 kHz range.
The frequency of the carrier will be maximum when the amplitude of input is
maximum and the frequency of the carrier will be minimum when the
amplitude of input is minimum.
Note down the maximum and minimum frequency of FM wave.
Measure maximum frequency derivation
∆ f = f - f c=( fmax - f min)/2
Calculate modulation index  = ∆ f / fm, and record the value in table 1
Repeat the steps 1-5 for another input (say 6V, 10 kHz).
Table 1
S.NO.
fmax
fmin
 ==∆f
/ fm
17
Precautions:
1. Make sure that all the instruments are properly connected.
2. The various grounds should be properly connected
EXPERIMENT NO.7
Study of Pulse Amplitude Modulation (PAM).
Aim: To trace the wave shape of the electrical signal at the input and output terminals
pulse amplitude modulator, using CRO.
Apparatus: Pulse amplitude modulator, function generator (20V p-p), CRO (20 MHz),
connecting leads.
Theory:
Pulse amplitude modulation, the simplest form of pulse modulation, is illustrated in fig 2. It
forms an excellent introduction to pulse modulation in general. Pulse amplitude modulation
is a pulse modulation system in which the signal is sampled at regular intervals, and each
sample is made proportional to the amplitude of the signal at the instant of sampling. As
shown in fig 2. The two types are double polarity pulse amplitude modulation, which is selfexplanatory and single polarity pulse amplitude modulation, in which a fixed DC level is
added to the signal, to ensure that the pulses are always positive.
18
It is very easy to generate and demodulate pulse amplitude modulation. In a generator, the
signal to be converted to Pulse Amplitude Modulation is fed to one input of an AND gate.
Pulses at the sampling frequency are applied to the other input of the AND gate to open it
during the wanted time intervals. The output of the gate then consists of pulses at the
sampling rate, equal in amplitude to the signal voltage at each instant. The pulses are then
passed through a pulse shaping network, which gives them flat tops.
Figure-1 PAM signal
Procedure:
1. Connect the circuit as shown in fig. 2
a. Output of sine wave to modulation signal IN in PAM block keeping the switch in 1 kHz
position.
b. 8 kHz pulse output to pulse IN.
c. Connect the sample output low pass filter input
d. Output of low pass filter to input of AC amplifier. Keep the gain pot in AC amplifier
block in max position.
2 Monitor the output of AC amplifier. It should be a pure sine wave similar to input.
3. Try varying the amplitude of input, the amplitude of output will vary.
4. Similarly connect the sample & hold & flat top outputs to low pass filter and see the
demodulated waveform at the output of AC amplifier.
5. Switch ON the switched faults No. 1, 2, 3, 4, 5 & 8 one by one and see their effects on
output.
6. Try to locate the fault and explain the reason behind them.
7. Trace the various waveforms and compare them.
8. Switch OFF the power supply.
Precautions:
1. Make sure that all the instruments are properly connected.
2. The various grounds should be properly connected.
19
Figure-2
20
EXPERIMENT NO.8 (a)
Study of Pulse Code Modulation (PCM) transmitter.
Aim: To trace the wave shape of the electrical signal at the input and output terminals of
PCM transmitter, using CRO.
Apparatus: PCM transmitter kit, connecting leads, CRO.
Theory:
The block diagram of PCM transmitter is as follows:
SAMPLER
QUANTIZER
ENCODER
PCM O/P
Analog
Input
Figure-1 PCM Transmitter
The sampling, quantizing and encoding operations are usually performed in the same
circuit which is called an analog – to – digital converter.
Figure-2 Sampling and Quantization
TX
TIMING LOGIC
TY
S/L
CH1
CH 2
SHIFT REGISTER
D6-D0
O/P
logic
21
PCM O/P
INPUT 1
SC
Error check code
generator
EC
INPUT 2
A/D CONVERTER
Fig 3: Test set up
Procedure:
1. Make the connections as shown in fig-3.
2. Give one modulating signal to input-1 and another modulating signal to input-2 from
the in-built function generator. These two signals are time division multiplexed.
3. Observe the PCM output on the CRO.
4. Store the waveform, trace a print and analyze.
Precautions:
1 Make sure that all the instruments are properly connected.
2. The various grounds should be properly connected
EXPERIMENT NO. 8 (b)
Study of Pulse Code Modulation (PCM) receiver.
Aim: To trace the wave shape of the electrical signal at the input and output terminals of
PCM receiver, using CRO.
Apparatus: PCM transmitter kit, PCM receiver kit, connecting leads, CRO (20 Mhz).
Theory:
The block diagram of PCM receiver is shown belowQuantiser
Decoder
Filter
22
PCM
INPUT
m(t)
TIMING
LOGIC
CH-1
LATCH
D/A
converter
PCM
DATA
Shift
register
CH -2
Error
detection or
correction
logic
Fig 1: Test set up
Procedure:
1. Make the connections as shown in fig 1.
2. Give the PCM signal from PCM transmitter kit to the input of receiver kit.
3. Observe the modulating signal on the CRO. The two TDM signals can be observed at
CH1 and CH2 respectively. Trace them.
4. Compare the receiver CH1 and CH2 output with the inputs at PCM transmitter kit.
Precautions:
1. Make sure that all the instruments are properly connected.
2. The various grounds should be properly connected.
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\
EXPERIMENT NO. 9
Study of amplitude shift keying (ASK) modulator and demodulator.
Aim: To trace the wave shape of the electrical signal at the input and output terminals of
ASK modulator, and demodulator using CRO.
Apparatus: Data formatting and carrier modulation transmitter trainer kit, Connecting
leads, CRO (20 MHz), PCM transmitter kit.
Theory:
The simplest method of modulating a carrier with a data stream is to change the
amplitude of the carrier wave every time the data changes. This modulation technique is
known as amplitude shift keying. The simplest way of achieving ASK is by switching
‘ON’ the carrier whenever the data bit is a ‘1’ and switching ‘off’ when the data bit is a
‘0’. Fgure-1 & 2, shows the ASK modulator and Demodulator block diagram
respectively. Figure-3 &4, shows the ASK modulated and demodulated waveforms
respectively.
Carrier Sine wave
Carrier I/P
ASK waveform
PCM
Tx
Unipolar Data
Stream
Modulator I/P
Fig-1: ASK modulator
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Fig-2: ASK demodulator
Procedure:
For ASK Modulator
1. Give the PCM output to the input of ASK modulator as shown in fig 1.
2. Connect a 960 KHz carrier (in -built) to the input of ASK modulator.
3. See the waveform on the CRO.
4. Trace the ASK wave as observed on the CRO.
For ASK Demodulator
1. Give the ASK modulated output to the input of ASK Demodulator and the leads as
shown in fig 2.
2. See the waveform on the CRO.
3. Trace the ASK Demodulated wave as observed on the CRO.
Figure-3 ASK Modulation waveforms
Figure-4 ASK Demodulation waveforms
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Precautions:
1. Make sure that all the instruments are properly connected.
2. The various grounds should be properly connected
EXPERIMENT NO. 10
Study of Frequency Shift Keying (FSK).
Aim: To trace the wave shape of the electrical signal at the input and output terminals of
FSK transmitter and receiver, using CRO.
Apparatus: Data formatting and carrier modulation transmitter trainer, connecting leads,
CRO (20 MHz), PCM transmitter trainer, Data formatting and carrier modulation
receiver trainer.
Theory:
In FSK the carrier frequency is shifted in accordance to the digital modulation signal. If
the higher frequency is used to represent a data 1 and lower frequency to represent data 0
the resulting frequency shift keying (FSK) waveform is obtained as shown in figure-1.
The demodulation of FSK waveform can be carried out by a phase locked loop as shown
in fig 2. The phase locked output tries to lock to output frequency. It achieves this by a
generating o/p voltage to be fed to voltage-controlled oscillator. The output from PLL
contained the carrier component. Therefore the signals are passed to low pass filter to
remove them.
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Figure-1 FSK Modulation waveforms
Input 1
Carrier I/P f1
Hz
ASK
PCM
modulator1
f1> f2
Tx
Modulation
I/P
Carrier I/P f2
Input 2 Hz
Figure 2
ASK
Invert
modulator2
er
Inverted
datdataModu
lation I/P
Summing amp
o/p-1
FSK
o/p-2
Fig-2 FSK modulator
Fig-3 FSK Generation
To generate FSK wave, we use two ASK modulators. At the input of first ASK
modulator we give data (PCM output) and a carrier of frequency f1, hz. . At the input of
second ASK modulator we give inverted data and a carrier of frequency f2, Hz
(f1>f2).The outputs of two ASK modulators are added in summing amplifier to get FSK
waveform as shown in figure-3.
FSK
Wave
PLL detector
LPF
Voltage
comparator
Data output
m(t)
27
Fig 3:FSK demodulator
Procedure:
For FSK modulator
1.Give the PCM output from PCM transmitter trainer to the input of ASK
modulator 1 as shown in fig- 2.
2. Connect a 1.4 MHz carrier to the input1 of ASK modulator 1.
3.Give the inverted PCM output from PCM transmitter trainer to the input of ASK
modulator 2.
4.Connect a 960 kHz carrier to the input2of ASK modulator 2.
5.The outputs of two ASK modulators are applied in summing amplifier to get
FSK waveform.
6.Observe the output of summing amplifier in CRO.
7.Trace the FSK wave as observed
For FSK demodulator:
1. Make connections as shown in figure-3 for FSK demodulator.
2. The modulating signal is obtained at the output.
3. Observe the wave on CRO and trace it.
Precautions:
1. Make sure that all the instruments are properly connected.
2. The various grounds should be properly connected
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EXPERIMENT NO. 11
Study of various data formatting methods.
Aim: To trace the waveforms of Non return to Zero Signaling (Unipolar) ,Bipolar NRZ
Signaling, Unipolar Return to Zero (RZ) signaling, Bipolar RZ signaling, Alternate Mark
inversion (AMI) RZ signaling, Split phase (Manchester) signaling.
Apparatus: Data formatting and carrier modulation transmitter trainer, connecting leads,
CRO (20 MHz), PCM transmitter trainer.
Theory:
Digital data can be transmitted by various pulse waveforms. The important parameters to
be considered in selecting a signaling format is the spectral characteristics, immunity of
the format to noise, bit synchronization capacity, cost and complexity of implementation.
Different formatting techniques are explained below:
(a) Non return to Zero Signaling (Unipolar):
Symbol is represented by transmitting a pulse of constant amplitude for the entire
duration of the bit interval, and symbol O is represented by no pulse. NR Z indicates that
the assigned amplitude level is maintained throughout the entire bit interval.
(b)Bipolar NRZ Signaling:
Symbol and 0 are represented by pulses of equal positive and negative
amplitudes. In either case, the assigned pulse amplitude level is maintained throughout
the bit interval.
(c)Unipolar Return to Zero (RZ) signaling:
A positive pulse that returns to zero before the end of the bit interval represents
symbol 1 and symbol 0 is represented by the absence of pulse.
(d)Bipolar RZ signaling:
Positive and negative pulses of equal amplitude are used for symbols 1 and 0,
respectively. In either case, the pulse returns to 0 before the end of the bit interval.
(e)Alternate Mark inversion (AMI) RZ signaling:
Positive and negative (pulses of equal amplitude) are used alternately for
symbol 1 and no pulse is used for symbol 0. In either case the pulse returns to 0
before the end of the interval.
(f)Split phase (Manchester) signaling:
Symbol is represented by a positive pulse followed by a negative pulse, with both
pulses being of equal amplitude and half bit duration; for symbol 0, the polarities
of these pulses are reversed.
PCM Transmitter
Carrier
modulation
and data Tx kit
CRO
(1-6 terminals)
Fig1:Test set up
29
Procedure:
1. Connect the output of PCM transmitter trainer to the input of data formatting and
carrier modulation transmitter trainer as shown in fig1.
2. Connect the CRO to select one of the different type of data formatted signals (NRZ,
RZ, AMI, Manchester) by connecting CRO to one of the six output terminals of data
formatting and carrier modulation transmitter trainer.
3. Draw the waveforms as observed on the CRO.
Precautions:
1. Make sure that all the instruments are properly connected.
2. The various grounds should be properly connected
30