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Experiment-1
AMPLITUDE MODULATION AND DEMODULATION

Aim: To study the process of amplitude modulation and demodulation and to calculate the depth
of modulation.
Apparatus:
1. Hi-Q Trainer Kit for AM
2. Oscilloscope 20MHz Dual channel / DSO
3. Patch Cords
Theory: Amplitude Modulation is defined as a process in which the amplitude of the carrier
wave c (t) is varied linearly with the instantaneous amplitude of the message signal m(t). The
standard form of amplitude modulated (AM) wave is defined by
s(t)=A (1+ m
a
m(t))cos (2f
c
t)
Under modulation:
In this case m
a
<1.Here the maximum amplitude of the baseband signal is less than the
maximum carrier amplitude A i.e. | x(t)|
max
< A.
The envelope does not reach the zero amplitude axis of the AM waveform and so the
baseband signal may be fully recovered from the envelope of AM wave. Modulating signal is
preserved in the envelope only if the percentage modulation is less than or equal to 100
percentage.
100% modulation:
In this case m
a
=1.Here the amplitude of the message signal is equal to the amplitude of
the carrier signal, i.e. |x(t)|
max
=A.
Over modulation:
In this case m
a
>1.Here the amplitude of the baseband signal exceeds the maximum
carrier amplitude, i.e. |x(t)|
max
>A. Here the percentage modulation is greater than 100,the
baseband signal is preserved in the envelope. The message signal recovered from the envelope
will be distorted. This distortion is called envelope distortion.
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Circuit diagram:


Fig : Circuit dig of AM modulator



Fig : Circuit dig of AM demodulator using envelope detector

Procedure:

Modulation:
1) Connect the circuit as shown in the diagram.
a) Output of modulating signal generator to modulating signal input TP2.
b) The frequency of the modulating signal is adjusted to 1 kHz and amplitude to 1V.
2) Switch ON the power supply.
3) Observe the amplitude modulated signal at TP3.
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4) Try varying the amplitude of modulating signal by varying the amplitude pot and observe the
AM output for all types of modulation.
5) Remove the modulating signal input and observe the output at TP3 which is the carrier signal.
6) Switch OFF the power supply.

Demodulation:

1) Connect the circuit as shown in the diagram.
a) Output of modulating signal generator to modulating signal input TP2.
b) AM output at TP3 is connected to TP4(input of diode).
c) Diode output TP5 is connected to input of low pass filter TP6.
d) Output of low pass filter TP7 to input of amplifier TP8.
2) Observe the demodulated output at TP9 after switching ON the power supply.
3) Switch OFF the power supply.

Observations:

V
c


f
m


f
c




S.No.
Amplitude of modulating
signal V
m
(volts)
V
max

(volts)
V
min
(volts)

% modulation




% Of modulation =
Vmax Vmin
Vmax + Vmin
100


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Waveforms:



Fig: AM wave in time domain


Fig: Under modulated, 100% modulated, Over modulated AM wave in time domain
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Precautions:

1) The connections must be tight and accurate.
2) Make sure that there are no short circuits.
3) Check the circuit before switching ON the power supply.
4) Switch OFF the power supply before making or breaking connections.
5) Note down the readings without the parallax error.


Result:

Amplitude modulation and demodulation are performed and the outputs are verified by
varying the amplitude of the modulating signal and the depth of modulation is calculated.


















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Experiment-2
FREQUENCY MODULATION AND DEMODULATION

Aim:
To study the process of frequency modulation and demodulation and to calculate the modulation
index
Apparatus:
1) Hi-Q test equipment Pvt. Ltd Frequency modulation and demodulation trainer kit
2) Oscilloscope 20MHz dual channel / DSO
3) Patch cords
Theory:
Frequency modulation (FM) is the encoding of information in a carrier wave by varying
the instantaneous frequency of the wave. (Compare with amplitude modulation, in which the
amplitude of the carrier wave varies, while the frequency remains constant).
In analog signal applications, the difference between the instantaneous and the base
frequency of the carrier is directly proportional to the instantaneous value of the input-signal
amplitude.
Frequency modulation is used in radio, telemetry, radar, seismic prospecting, and monitoring
newborns for seizures via EEG. FM is widely used for broadcasting music and speech, two-way
radio systems, magnetic tape-recording systems and some video-transmission systems. In radio
systems, frequency modulation with sufficient bandwidth provides an advantage in cancelling
naturally-occurring noise.
A sine wave which is the modulating signal is generated by using the IC 8038(U1).An 8 pin
IC LF356(U4) is used as an amplifier which amplifies the sine wave which is generated by using
IC 8038(U7) which is inbuilt.
In this circuit IC 8038(U7) is used to generated FM.The frequency of the waveform
generator is a direct function of the DC voltage at pin 8.By altering this voltage,FM is
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performed.For small deviations the modulating signal will be applied directly at pin 8 and for
larger FM deviations,the modulation signal is applied between the positive supply voltage at pin
8.An IC 741(U6) is the unity gain amplifier which is used as a buffer.The FM output is taken at
pin 6 of U6.
In the demodulation section it comprises of PLL and AC amplifier.The output of modulator
is given as input to PLL.
The PLL output is obviously less and it is fed to the AC amplifier which comprises of single
operational amplifier and whose output is amplified.
Circuit diagram:


Fig: FM Modulator using Ic 8038

Fig: FM Demodulator circuit


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FM Waveforms

Fig : FM waveform

Fig : FM waveforms
Procedure:
Modulation:
1) Connect the circuit as shown in the diagram.
a) The sine wave from the modulating signal generator TP4, to the modulating signal input
TP 5.
b) Adjust the amplitude of the modulating signal generator 1 to 2 V and frequency of
modulating signal to 100Hz to 2 kHz by varying the respective pots.
2) Switch ON the power supply.
3) Observe the frequency modulator output at TP6.
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4) Switch OFF the power supply.
Demodulation:
1) Connect the circuit as shown in the diagram.
a) The sine wave from the modulating signal generator TP4, to the modulating signal input
TP5.
b) Adjust the amplitude of the modulating signal generator 1 to 2 V and frequency of
modulating signal to 100Hz to 2 kHz by varying the respective pots.
c) Frequency modulator output from TP6 to PLL input TP8.
d) Output of PLL TP9 to AC amplifier input TP10.
2) Switch ON the power supply.
3) Observe the demodulated output at TP11 and the output is the exact replica of the input.
4) Switch OFF the power supply.

Observations:

S.No. Description Amplitude Frequency




S.No. A
m
T
max
T
min
f
min
f
max
= f
max
- f
min
/ 2






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Precautions:
1) Make the connections tightly and accurately.
2) Switch OFF the power supply before making.
3) Make sure that there is no short circuit.
4) Check the circuit before switching ON the power supply.
Result:
The frequency modulation is obtained for different values of the modulating signal and the
carrier signal and the demodulated signal is also obtained.




















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Experiment-3
FREQUENCY MODULATION AND DEMODULATION



























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Experiment-4
PRE-EMPHASIS AND DE-EMPHASIS
Aim:
To find the frequency response characteristics of pre-emphasis and de-emphasis circuits

Equipments Required:

S.No. Component Range Quantity
1. Transistor AC128 1
2. Capacitor 0.01F,47F 2
3. Resistor 1k,10k,100k 4
4. Inductor 0.1mH 1
5. RPS (0-30)V/2A 1
6. CRO 30MHz 1
7. Audio frequency oscillator 1
8.
Breadboard &Connecting
wires
1

Theory:
In processing electronic audio signals, pre-emphasis refers to a system process designed
to increase (within a frequency band) the magnitude of some (usually higher) frequencies with
respect to the magnitude of other (usually lower) frequencies in order to improve the overall
signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation
distortion or saturation of recording media in subsequent parts of the system. The mirror
operation is called de-emphasis, and the system as a whole is called emphasis.
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Pre-emphasis is achieved with a pre-emphasis network which is essentially a calibrated filter.
The frequency response is decided by special time constants. The cutoff frequency can be
calculated from that value.
Pre-emphasis is commonly used in telecommunications, digital audio recording, record cutting,
in FM broadcasting transmissions, and in displaying the spectrograms of speech signals.
One example of this is the RIAA equalization curve on 33 rpm and 45 rpm vinyl records.
Another is the Dolby noise-reduction system as used with magnetic tape.
In high speed digital transmission, pre-emphasis is used to improve signal quality at the output of
a data transmission. In transmitting signals at high data rates, the transmission medium may
introduce distortions, so pre-emphasis is used to distort the transmitted signal to correct for this
distortion. When done properly this produces a received signal which more closely resembles the
original or desired signal, allowing the use of higher frequencies or producing fewer bit errors.
Pre-emphasis is employed in frequency modulation or phase modulation transmitters to equalize
the modulating signal drive power in terms of deviation ratio. The receiver demodulation process
includes a reciprocal network, called a de-emphasis network, to restore the original signal power
distribution.
In telecommunication, de-emphasis is the complement of pre-emphasis, in the antinoise system
called emphasis. Emphasis is a system process designed to decrease, (within a band of
frequencies), the magnitude of some (usually higher) frequencies with respect to the magnitude
of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by
minimizing the adverse effects of such phenomena as attenuation differences or saturation of
recording media in subsequent parts of the system.
Special time constants dictate the frequency response curve, from which one can calculate the
cutoff frequency.
Pre-emphasis is commonly used in audio digital recording, record cutting and FM radio
transmission.
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In serial data transmission, de-emphasis has a different meaning, which is to reduce the level of
all bits except the first one after a transition. That causes the high frequency content due to the
transition to be emphasized compared to the low frequency content which is de-emphasized. This
is a form of transmitter equalization; it compensates for losses over the channel which are larger
at higher frequencies. Well known serial data standards such as PCI Express, SATA and SAS
require transmitted signals to use de-emphasis.
Circuit Diagram:

Fig: Pre-emphasis circuit

Fig: De-emphasis circuit

Procedure:
1. Connect the circuit as per circuit diagram as shown in Fig..
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2. Apply the sinusoidal signal of amplitude 20mV as input signal to pre emphasis circuit.
3. Then by increasing the input signal frequency from 500Hz to 20KHz, observe the output
voltage (V
o
) and calculate gain 20 log
10
(V
o
/ V
i
)
4. Plot the graph between gain Vs frequency.
5. Repeat above steps 2 to 4 for de-emphasis circuit (shown in Fig.2). by applying the sinusoidal
signal of 30mV as input signal


Observations:
Pre-emphasis:
S.No. Frequency (Hz) V
o
Gain = V
0
/V
i
Gain in dB = 20log
10
(V
0
/V
i
)






De-emphasis:
S.No. Frequency (Hz) V
o
Gain = V
o
/ V
i
Gain in dB = 20log
10
(V
o
/ V
i
)




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Waveforms:


Precautions:
1. Check the connections before giving the power supply
2. Observations should be done carefully
Result:
Thus the frequency response characteristics of pre-emphasis and de-emphasis circuits are
determined and are plotted on semi log graph.












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Experiment-5
SINGLE SIDEBAND MODULATION AND
DEMODULATION

Aim:
To study the process of single side band modulation and demodulation.
Apparatus:
1) Hi-Q test equipment Pvt .Ltd Single sideband modulator and demodulator
2) CRO / DSO
3) Patch cords
Theory:
In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-
carrier (SSB-SC) is a refinement of amplitude modulation that more efficiently uses transmitter
power and bandwidth. Amplitude modulation produces an output signal that has twice the
bandwidth of the original baseband signal. Single-sideband modulation avoids this bandwidth
doubling, and the power wasted on a carrier, at the cost of increased device complexity and more
difficult tuning at the receiver.
SSB was also used over long distance telephone lines, as part of a technique known as
frequency-division multiplexing (FDM). FDM was pioneered by telephone companies in the
1930s. This enabled many voice channels to be sent down a single physical circuit, for example
in L-carrier. SSB allowed channels to be spaced (usually) just 4,000 Hz apart, while offering a
speech bandwidth of nominally 3003,400 Hz.
Amateur radio operators began serious experimentation with SSB after World War II. The
Strategic Air Command established SSB as the radio standard for its aircraft in 1957.It has
become a de facto standard for long-distance voice radio transmissions since then.
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One method of producing an SSB signal is to remove one of the sidebands via filtering, leaving
only either the upper sideband (USB), the sideband with the higher frequency, or less commonly
the lower sideband (LSB), the sideband with the lower frequency. Most often, the carrier is
reduced or removed entirely (suppressed), being referred to in full as single sideband suppressed
carrier (SSBSC). Assuming both sidebands are symmetric, which is the case for a normal AM
signal, no information is lost in the process.
The front end of an SSB receiver is similar to that of an AM or FM receiver, consisting of a
super heterodyne RF front end that produces a frequency-shifted version of the radio frequency
(RF) signal within a standard intermediate frequency (IF) band.
To recover the original signal from the IF SSB signal, the single sideband must be frequency-
shifted down to its original range of baseband frequencies, by using a product detector which
mixes it with the output of a beat frequency oscillator (BFO). In other words, it is just another
stage of heterodyning (mixing down to base band).
Block Diagram

Fig: Block dig of SSBSC generation
Procedure:
Modulation:
1) Switch ON the power supply.
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2) Observe the outputs of modulating signal generator i.e.FM and FM+90
0
using respective
pots and set the amplitude 0.3V (P-P).
3) Observe F
c
and F
c
+90
0
using pots and adjust

amplitude 0.3V(P-P).
4) Connect the circuit as shown in wiring diagrams.
5) Connect F
m
signal to F
m
input of DSB GEN1 & F
c
signal to

F
c
input of DSB GEN2.
6) Observe the output of DSB GEN1 by varying the carrier adjust pots make even DSB
loops. Here amplitude is 0.6V(P-P).
7) And also observe the output of DSB GEN2 by varying the pots make when DSB loops.
Here amplitude is 0.6V(P-P).
8) Connect the outputs of DSB GEN1 &DSB GEN2 to input of adder circuit and observe
the SSB output. You will get single carrier frequency at SSB output. Here amplitude is
0.4V.
9) In this method of SSB GEN both the LSB signals get added as they are in phase and USB
get cancelled. As they are out of phase by 180
0
. This is the output of LSB SSB.
10) For obtaining USB as the SSB signal connect the circuit as shown in wiring diagrams 3.
11) Connect FM signal to FM signal to input of DSB GEN1 and F
c
+90
0
to F
c
input of DSB
GEN1.
12) Connect the F
m
+90
0
to F
m
input of DSB GEN2 and F
c
signal to F
c
input of DSB GEN2.
13) Connect the outputs of DSB GEN1 to the inputs of adder circuit and observe the SSB
output.
14) In this method of SSB GEN the lower side band get added as they are in phase. This is
the output of USB SSB at SSB output.
Demodulation:
1) For demodulation connect SSB output to SSB demodulation input and also connect F
c

signal to F
c
input of SSB demodulation and observe its output.
2) Connect SSB demodulation output to input of filter and observe filter output as smooth
modulating signal.
For observing USB and LSB effects:
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1) Connect point carrier signal A to F
c
input of DSB GEN1 and point carrier signal B to
F
c
input of DSB GEN2.
2) Keep frequency of modulating signal 2MHz and amplitude of F
m
and F
m
+90
0
is 0.3V(P-
P).
3) Observe and measure the frequency of the SSB output.
4) Interchange point A and point B and the frequency of SSB.
Waveforms:


Precautions:
1) Check the connections before giving the power supply.
2) Observations should be done careful.
Result:
Thus the process of single sideband modulation and demodulation is studied and waveforms are
plotted on a graph.





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Experiment-6
PULSE AMPLITUDE MODULATION &
DEMODULATION

Aim:
To study pulse amplitude modulation & demodulation and observe the waveform.
Apparatus:
1. Transistor BC107 1
2. Resistor 10K - 2, 22K - 1
3. Capacitor 0.01F - 1
4. Function Generator 2
5. Digital Storage Oscilloscope 25MHz
Theory:
Pulse modulation may be used to transmit analog information, such as continuous speech or
data. It is a system in which continuous waveforms are sampled at regular intervals.
Information regarding the signal is transmitted only at the sampling times, together with any
synchronizing pulses that may be required. At the receiving end, the original waveforms may be
reconstituted from the information regarding the samples, if these are taken frequently enough.
Despite the fact that information about the signal is not supplied continuously as in AM and FM,
the resulting receiver output can have negligible distortion.
Circuit diagram:


Fig: PAM Modulation


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Fig: PAM Demodulation

Procedure:
1. Connections are made as per the circuit diagram.
2. Modulating signal of 3V, 100KHz is given to collector.
3. Carrier signal in the form of pulses of high frequency of 4V, 20KHz is given to the base of
the transistor.
4. Output is measured at the emitter.
5. Connect the circuit to the CRO, to the emitter of the transistor and observe the waveforms
at the CRO.
Output waveforms

Table:

m(t) volts s(t) volts



Precautions:
1. All the connections must be made correctly & tightly.
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2. Make sure to switch OFF power supply, before making or breaking connections.
3. Note down the readings with any parallax error.
4. Take the output at the emitter junction.
Result:
Thus, the pulse amplitude modulation and demodulation are studied and the waveforms are
plotted on graph.














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Experiment-7
SAMPLING & RECONSTRUCTION
Aim:
1. To study the sampling theorem & it s reconstruction.
2. To study the effect of amplitude & frequency variation of modulating signal on the
output.
3. To study the effect of variation of sampling frequency on the demodulated output.

Apparatus:

1. Hi-Q test Equipment Pvt. Ltd Sampling & Reconstruction kit.
2. Oscilloscope & Reconstruction trainer kit.
3. Patch cords.

Theory:

The statement of sampling theorem can be given in two parts as:
i. A band-limited signal of finite energy, which has no frequency component higher
than f
m
Hz, is completely described by its sample values at uniform intervals less or
equal to f
m
.
ii. A band-limited signal of finite energy, which has no frequency components higher
than f
m
Hz, may be completely recovered from the knowledge of its samples taken
at the rate of 2f
m
samples per second.
A continuous time signal may be completely represented in its samples & recovered back if
the sampling frequency is f
s
>>2f
m
. Here f
s
is the sampling frequency and f
m
is the maximum
frequency present in the signal.

Circuit Diagram:





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Output waveform















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Procedure:

Sampling:
1. Connect the circuit as shown in the diagram 1.
a) Output of modulating signal generator to modulating signal input in sampling
circuit keeping the switch in 1KHz position & amplitude pot to max position.
b) Output of pulse generator to sampling pulse input in sampling circuit keeping the
switch 16KHz position(Adjust the duty cycle pot to mid position i.e. 50%)
2. Switch ON the power supply.
3. Observe the outputs of sampling , sampling hold & flat top output. By varying the
amplitude pot also observe the effect on outputs.
4. By varying duty cycle pot observe the effect on sampling outputs (Duty cycle is varying
from 10-50%)
5. Vary the switch position in the pulse generator circuit to 32KHz and now observe the
outputs. By varying the amplitude pot also observe the effect on outputs.
6. Now, vary the switch position in modulating signal generation to 2KHz & repeat all the
above steps 3&4.
7. Switch OFF the power supply.

Reconstruction:

1. Connect the circuit as shown in the diagram 2
a) .Output of modulating signal generator to modulating signal input in sampling
circuit keeping the switch in 1KHz position & amplitude pot to max position.
b) Output of pulse generator to sampling pulse input in sampling circuit keeping the
switch 16KHz position(Adjust the duty cycle pot to mid position i.e. 50%)
c) Connect the sample output to the input of low pass filter.
d) Output of low pass filter to input of AC amplifier, keep the gain pot in AC
amplifier to max position.
2. Switch the power supply.
3. Observe the output of AC amplifier. The output will be the replica of the input. By
varying the gain pot observe the demodulating signal amplification.
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4. Similarly connect the sample and hold output and flat top output & observe the
reconstructed signal.
5. Vary the switch position in sampling frequency circuit to 32KHz & now repeat the steps
3&4.
6. Vary the switch position in the modulating signal generator to 2KHz & repeat all the
above steps.
7. Switch OFF the power supply.

Precautions:
1. All the connections must be made correctly & tightly.
2. While noting the readings from the CRO, note the readings without parallax error.
3. Make sure to switch OFF the power supply before making or breaking connections.

Result:
The sample output, sample & hold output and the reconstruction outputs have been verified
& sampling theorem & its reconstruction have been verified.




















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Experiment-8
PULSE WIDTH MODULATION & DEMODULATION

Aim:
1. To study the pulse width modulation and demodulation techniques.
2. To study the effect of amplitude and frequency of modulating PWM output.
Apparatus:
1. Hi-Q test equipment pulse width modulation & demodulation.
2. Oscilloscope 20MHz Dual channel.
3. Patch cords.
4. 555 Timer 1
5. Resistors 47K - 1, 10K - 1
6. Capacitor 0.01F 1
7. Regulated power supply (0-30)V/2A.
8. Function Generator 10MHz
9. Bread board.
10. Connecting Wires.

Theory:

Pulse Width Modulation is also known as Pulse Duration Modulation (PDM). Three
variations of the pulse width are possible. In one variation, the leading edge of the pulse is held
constant and change in pulse width with signal is measured with respect to leading edge. In
other variation, the tail edge is held constant and with respect to it, pulse width is measured. In
the third variation, centre of the pulse is held constant and pulse width changes on either side of
the centre of the pulse.

The Pulse Width Modulation is basically a monostable multivibrator with a modulating input
signal applied at the control voltage input. Internally, the control voltage is adjusted to the 2/3
Vcc. Externally applied modulating signal changes the control voltage, and hence the threshold
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voltage level. As a result, the time period required to charge the capacitor up to threshold
voltage level changes, giving pulse modulated signal at the output.

Unlike Pulse Amplitude Modulation, noise is less since in PWM, amplitude is held constant.
Signal and noise separation is very easy in case of PWM. PWM communication does not require
synchronization between transmitter and receiver.

In PWM, pulses are varying in width and therefore their power contents are available. This
requires that the transmitter must be able to handle the power contents of the pulse having
maximum pulse width. Large bandwidth is required for the PWM communication as compared
to PAM. The pulse width is controlled by the input signal voltage, and we get the pulse width
modulated waveform at the output.

Circuit Diagram:


Waveforms

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Procedure:
Modulation:
1. Connect the circuit as shown in the diagram.
2. Switch ON the power supply.
3. Observe the output of pulse width modulation block.
4. Vary the modulating signal generator frequency by switching the frequency selector switch
to 2 KHz.
5. Now again observe PWM output.
6. Repeat the same steps for frequency at 32 KHz pulse.
7. Switch OFF the power supply.

Demodulation:
1. Connect the circuit as shown in the diagram.
2. Switch ON the power supply.
3. Observe the output at LPF & AC amplifier. The output will be the replica of the input.
4. Now vary the position of the switch of the switch in modulating signal generator to 2 KHz
& observe the output.
5. Repeat the above steps for pulse frequency of 32 KHz.
6. Switch OFF the power supply.

Table:

S.NO Control Voltage (V
P-P
) Output Pulse Width(msec)




Precautions:
1. Make all the connections correctly and tightly.
2. Make sure to switch OFF the power supply before making or breaking connections.
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3. While connecting the RPS, put the current knob in between maximum and minimum
position.

Result:
Thus, the pulse width modulation and demodulation techniques are studied and the effect
of amplitude and frequency of modulating signal of PWM output are studied.


































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Experiment-9
PULSE POSITION MODULATION & DEMODULATION
Aim:
1. To study generator of PPM signal and its demodulation.
2. To study the effect of amplitude & frequency of modulating signal on its output and
observe waveforms.
Apparatus:
1. Modulation and Demodulation trainer.
2. Oscilloscope 20MHz, Dual trace.
3. Patch cords.
4. 555 Timer 2
5. Resistors 10K - 2, 1K - 1 , 22K - 1
6. Capacitor - 0.1F 2 , 0.01F 1
7. Regulated Power Supply (0-30V)/2A
8. Breadboard
9. Connecting Wires
10. Function Generator 30 MHz
Theory:
In Pulse Position Modulation, the amplitude & width of the pulse are kept
constant, while the position of each pulse, with reference to the position of a reference pulse,
is changed according to the instantaneous sampled value of the modulating signal. Thus, the
transmitter has to send synchronizing pulses to keep the transmitter and receiver in
synchronism. As the amplitude and width of the pulses are constant, the transmitter handles
constant power output, a definite advantage over the PWM. But the disadvantage of the
PPM system is the need for transmitter-receiver synchronization. Pulse position modulation
is obtained from pulse width modulation. Each trailing edge of PWM pulse is a starting
point of the pulse in the PPM. Therefore, position of the pulse is 1:1 proportional to the
width of pulse in PWM and hence it is proportional to the instantaneous amplitude of the
sampled modulating signal.

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The PPM generation consists of a differentiator and a monostable multivibrator. The input
of the differentiator is a PWM waveform. The differentiator generates positive and negative
spikes corresponding to leading and trailing edges of the PWM waveform. Diode D
1
is used
to bypass the positive spikes. The negative spikes are used to the trigger monostable
multivibrator. The monostable multivibrator then generates the pulses of same width and
amplitude with reference to trigger to given pulse position modulated waveform. In case of
pulse-position modulation, it is customary to convert the received pulses that vary in
position to pulses that vary in length. Like PWM, in PPM amplitude is held constant thus
less noise interference. Like PPM, signal & noise separation is very easy. Because of
constant pulse widths and amplitudes, transmission of power of each pulse is same.

Circuit Diagram:


Fig: PPM Modulator

Fig: PPM Demodulaton
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Procedure:
Modulation:
1. Connect the circuit as shown in the diagram.
a) Output of sine wave to modulation signal input in PPM block.
b) Keep switch in 1KHz position and amplitude pot in the maximum position.
2. Switch ON the power supply.
3. Observe PWM output, differentiated output signal at TP3.
4. Now, monitor the PPM output at PPM out.
5. Try varying amplitude and frequency of sine wave by varying the amplitude pot and
frequency selection switch to 2 KHz & observe the PPM output.
6. Switch OFF the power supply.

Demodulation:
1. Connect the circuit as shown in the circuit diagram.
a) Sine wave output of 1 KHz from modulating signal generator to modulating
signal input.
b) Connect the PPM output to input of LPF.
2. Switch ON the power supply.
3. Observe the demodulated signal at the output of LPF out.
4. Thus recovered signal is true replica of the input signal.
5. As output of LPF has less amplitude, connect the output of LPF to input of AC
amplifier.
6. Observe demodulated output on oscilloscope and also observe amplitude of
demodulated signal by varying the gain pot. This is the amplified demodulated
output.
7. Repeat steps for the modulating signal for frequency 2KHz (with switch in position
2KHz)
8. Switch OFF the power supply.



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Table:

Modulating Signal
Amplitude (V
P-P
)
Time Period (msec) Total Time Period
(msec) Pulse Width ON
(msec)
Pulse Width OFF
(msec)





Precautions:
1. Make all the connections correctly & tightly.
2. Make sure to switch OFF the power supply before making or breaking connections.
3. While connecting the RPS, put the current knob in between maximum & minimum
position.
Result:
The pulse position modulation and demodulation are performed and the waveforms are
observed by changing the amplitude and frequency of the modulating signal.