DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
EXPERIMENT NO. 1
PULSE AMPLITUDE MODULATION AND DEMODULATION
AIM : To study pulse amplitude modulation and demodulation.
EQUIPMENT REQUIRED:
1. Pulse amplitude modulation and demodulation trainer kit.
2. Cathode ray oscilloscope (0-20 MHz)
3. BNC probes and patch cords
4. Function generator (0-1MHz)
THEORY: Pulse amplitude modulation is a modulation where the amplitude of carrier
containing is varied linearly with amplitude of message signal, even through there are
three types of PAM i.e. ideal type or flat top type. PAM is most popular and widely used
because during the transmission the noise is interfered at the top of the transmission pulse
which can be early removed if the PAM pulse is flat-top.
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
PROCEDURE :
1. Switch on pulse amplitude modulation and demodulation trainer.
2. In clock generator section connect pin 6 of 555 IC to the 100pf capacitor terminal.
3. Check the clock generator (RF) output signal.
4. Connect RF output of clock generator to the RF input of modulator circuit.
5. Connect an AF signal with an arbitrary amplitude and frequency from function
generator to the AF input of modulator section.
6. Short the 10f terminal and 10K terminal of modulator.
7. connect 10 K terminal to pin 1 of IC 4016.
8. Connect the CRO to modulated output of modulator section.
9. Adjust the 1kpotnetiometer to vary the amplitude of the modulated signal.
10. Adjust the AF and clock signal frequency and amplitude to get stable output wave
form (such that clock frequency is very much grater than AF frequency). The
output observed is single polarity PAM.
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
11. During demodulation, connect the modulated output to the PAM.
12. During demodulation, connect the modulated output to the PAM input of
demodulator section.
13. Connect channel 1 of CRO to modulating signal and channel – 2 to demodulated
output. Adjust the AF signal frequency to observe the clear output.
14. observe the two wave forms that they are 1800 out of phase, since the transistor
detector operates in CE configuration.
OBSERVATIONS:
SIGNAL
PARAMETER TIME PERIOD
AMPLITUDE(in
(in seconds)
volts)
FREQUENCY
(in Hz)
X(t)
S(t)
Xd(t)
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
EXPECTED WAVE FORMS:
RESULTS:
PRECAUTIONS:
1. Check all the components and devices used in the circuit for its proper
functioning.
2. Rig-up the circuit with utmost care and avoid loose connections
3. Check the circuit connection thoroughly before switching ON.
4. Handle the equipments carefully.
5. Operate the measuring instruments gently, as they are sensitive.
6. Switch off the supply when not in use.
VIVA VOCE QUESTIONS:
1. Define pulse Amplitude Modulation
2. Give few appliation of PAM
3. Disccuss the Bandwidth requirements of PAM.
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
EXPERIMENT NO:2
PULSE WIDTH MODULATION AND DEMODULATION
AIM : To study pulse width Modulation and Demodulation.
EQUIPMENT REQUIRED:
1. Pulse width modulation and demodulation trainer.
2. Cathode ray oscilloscope (0-20 MHz)
3. BNC probes and patch cords
4. Function generator (0-1MHz)
THEORY:
In pulse width modulation, the signal to be transmitted as sampled as in pulse
amplitude modulation. In pulse time modulation amp. Of pulse is of pulse is proportional
to amp of signal at the sampling instant. Then we have two types of pulse time
modulation.
1. PWM
2. PPM
This is also know as pulse duration modulation (POM) the variations of pulse
width modulation are pulse is held constant and change in pulse width with signal is
measured with respect to the leading edge.
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
CIRCUIT DIAGRAM OF PULSE WIDTH MODULATION
PROCEDURE:
1. Switch on pulse width modulation and demodulation trainer.
2. Apply clock and AF signal with an arbitrary frequency and amplitude by using
function generator and connect them to the respective terminals of PWM
modulation.
3. Observe the PWM output at pin 3 of 555 IC on CRO.
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
4. By varying frequency and amplitude of the modulating signal, observe the
corresponding change in the width of the output pulses.
5. during demodulation, connect PWM output of PWM modulation to the PWM
input of PWM demodulation.
Observe the demodulation output at AF signal of PWM demodulation on CRO.
OBSERVATIONS:
SIGNAL
PARAMETER
TIMEPERIOD
FREQUENCY
AMPLITUDE ( in
(in seconds)
(in Hz)
volts)
X(t)
C(t)
XPWM (t)
(Diff.
Pulsewidths)
Xd(t)
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
EXPECTED WAVEFORM:
AF SIGNAL
^
RESULTS
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
PRECAUATIONS:
7. Check all the components and devices used in the circuit for its proper
functioning.
8. Rig-up the circuit with utmost care and avoid loose connections
9. Check the circuit connection thoroughly before switching ON.
10. Handle the equipments carefully.
11. Operate the measuring instruments gently, as they are sensitive.
12. Switch off the supply when not in use.
VIVA VOCE QUESTIONS:
1. Define Pulse width Modulation
2. Give few applications of PWM.
3. Discuss the Bandwidth requirements of PWM.
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
EXPERIMENT NO.3
PULSE POSITION MODULATION AND DEMODULATION
AIM : To study Pulse Position Modulation and Demodulation.
EQUIPMENT REQUIRED:
1. Pulse position modulation and demodulation trainer.
2. Cathode ray oscilloscope (CRO) – (0.20MHz)
3. BNC probes and patch cords.
4. Function generator (0-1 MHz)
THEORY :
Pulse position modulation is one of the pulse time modulation scheme. In PPM
the analog sample value determines the position of a naeron pulse relate to the clocking
time. Techniques for generating PPM are also related and it is seen that PPM is easily
obtained from PWM by a monostable multivibrator circuit in the literature on PPM
systems. The comparator level V, is often called the slicing lead.
PWM or PPM signals may be converted back to the corresponding analog signal
by a receiving signal for PWM detection PWM signal is used to start and stop the
integration of an integrator that is, the integrator is reset to zero because relatively wide
band channel is needed especially for PPM.
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
CIRCUIT DIAGRAM:
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DEPARTMENT OF ECE
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PROCEDURE:
1. Switch on PPM modulator and demodulator trainer.
2. Apply clock and AF signal with an arbitrary frequency and amplitude by using
function generator and connect them to pin 2 and pin 5 of 555 IC respectively.
3. Observe the PWM output across R2 resistor.
4. Observe the PPM output at pin 3 of second 555 IC on CRO and note down the
PPM waveform w.r.t. PWM and clock signal.
5. Connect the PPM output to the PPM input of demodulation.
6. Observe the demodulated output on CRO.
OBSERVATIONS:
SIGNAL
X(t)
C(t)
XPWM(t)
XPPM(t)
Xd(t)
Amplitude
(in volts)
Timperiod
(in seconds)
Frequency
(in Hz)
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
RESULTS:
PRECAUTIONS:
1) Check all the components and devices used in the circuit for its proper
functioning.
2) Rig-up the circuit with utmost care and avoid loose connections
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
3) Check the circuit connection thoroughly before switching ON.
4) Handle the equipments carefully.
5) Operate the measuring instruments gently, as they are sensitive.
Switch off the supply when not in use.
VIVA-VOCE QUESTIONS:
1. Define Pulse position modulation
2. Give few applications PPM.
3. What are the Bandwidth requirements of PPM.
4. Compare PAM, PWM and PPM.
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
EXPERIENCE NO.4
SAMPLING THEOREM - VERIFICATION
AIM : To study and verify the sampling theorem and reconstruction of sample
waveform.
EQUIPMENT REQUIRED:
1. Verification of sampling theorem trainer kit.
2. Function generator (0 – 1 MHz)
3. Cathode ray oscilloscope (CRO) (0-20MHz)
4. BNC probes and patch cords.
THEORY:
Sampling process is used to convert analog signal to digital form initially it is converted
to discrete signal. It is done by a process called sampling. After defined at discrete time
interval and hence discontinuous signal formed basically sampling involves the
multiplication of base band signal with an impulse train which is periodic with period. TS
in continuous ware modulation some parameters of modulation ware varied continuous
with the message signal and we have two types of sampling.
1. Natural sampling
2. Flat top sampling
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
CIRCUIT DIAGRAM:
10kΩ_5%
10KΩ
10kΩ_5%
10kΩ_5%
PIN DIAGRAM ICLM324
PROCEDURE:
1. Connection are made as per the circuit diagram.
2. Apply the modulating signal with an arbitrary frequency (fm) and amplitude using
function generator.
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
3. Sampling clock signal with a variable frequency should be connected across the
terminals which is indicated on the kit (or) by using function generator.
4. Now observe the sampling output of the circuit at the output terminal by adjusting
the amplitude and frequency of modulating and clock signal (amplitude of
modulating signal is limited to 6V p-p).
5. By using capacitors and resistors provided on the trainer, reconstruct the signal
and verify it with the given input.
6. Reconstructed signal voltage will be depends on capacitor value.
7. Vary the sampling frequency and study the change in reconstructed signal.
8. If the sampling clock frequency (fs) is below 2fm where fm is the modulating
frequency, the distorted outputs is observed.
(Note: for clear output take modulating signal of high frequency during sampling
(<<fs) and low frequency during reconstruction).
OBSERVATION:
SIGNAL
PARAMETER
TIMEPERIOD
FREQUENCY
AMPLITUDE ( in
(in seconds)
(in Hz)
volts)
X(t)
S(t)
Xd(t)
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
RESULTS:
PRECAUTIONS:
1. Check all the components and devices used in the circuit for its proper
functioning.
2. Rig-up the circuit with utmost care and avoid loose connections
3. Check the circuit connection thoroughly before switching ON.
4. Handle the equipments carefully.
5. Operate the measuring instruments gently, as they are sensitive.
6. Switch off the supply when not in use.
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
VIVA-VOCE QUESTIONS:
1. State sampling theorem
2. Explain under sampling, critical sampling and oversampling
3. Define Nyquist rate
4. What are the effects of undersampling
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
EXPERIMENT NO.:5
TIME DIVISION MULTIPLEXING
AIM : To verify the operation of Time – Division Multiplexing.
EQUIPMENT REQUIRED:
1. Time-division multiplexing and de-multiplexing trainer.
2. Cathode Ray Oscilloscope (CRO) – (0.20MHz)
3. BNC probes and patch cords.
THEORY :
Analog time division multiplexing (digital) as the number of message to be
transmitted increases, the frequency division techniques. The number of sub carries
needed increases and stabilses of which problem can rise. Additional circuit is record
(or) required at both side is transmitting and receiving ends to handle each added channel.
The band width requirement increases directly with number of channels. These problems
are eliminated to guest extent by using time division multiplexing together with pulse
modulation.
In TDM each intelligence is said to be transmitted is sampled sequentially and the
resulting pulse code is used to modulate the carrier the same carrier frequency is used to
transmit different pulse sequentially one after another each intelligence to transmitted has
been allotted a given time slot only or small modulates the carrier at any time no added
equipment and no increase in bandwidth is need when multiplexing in bandwidth is need
when multiplexed. The number of sequential channel that can be handled is limited.
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
CIRCUIT DIAGRAM:
PROCEDURE :
1. Switch on time division multiplexing and de multiplexing trainer.
2. Observe the individual waveform on channel 1 of CRO and notedown the
corresponding amplitude and frequency.
3. Connect the sine wave to CH1, square wave to CH2 and triangular wave to CH3
terminal of 8 to 1 MUX.
4. Observe the multiplexer output on channel 1 of CRO.
5. Connect MUX output to DEMUX input.
6. Observe the corresponding signal outpu’s at channel 2 of CRO.
7. Repeat the above steps by applying different inputs using function generator.
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OBSERVATIONS:
SIGNAL
Amplitude
(in volts)
Timperiod
(in seconds)
Frequency
(in Hz)
X(t)
C(t)
XPWM(t)
XPPM(t)
Xd(t)
X
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
OBSERVATIONS:
SIGNAL
Amplitude
(in volts)
Timperiod
(in seconds)
Frequency
(in Hz)
X(t)
C(t)
XPWM(t)
XPPM(t)
Xd(t)
RESULTS:
PRECAUTIONS:
1. Check all the components and devices used in the circuit for its proper
functioning.
2. Rig-up the circuit with utmost care and avoid loose connections
3. Check the circuit connection thoroughly before switching ON.
4. Handle the equipments carefully.
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
5. Operate the measuring instruments gently, as they are sensitive.
6. Switch off the supply when not in use.
VIVA-VOCE QUESTIONS:
1. Explain Time Division Multiplexing
2. Name few applications of TDM
3. Compare TDM with FDM.
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Expt No. 12
PULSE DURATION MODULATION USING
MONOSTABLE MULTIVIBRATOR
AIM:
To produce pulse duration modulation using Monostable multivibrator.
EQUIPMENT REQUIRED: DC Power supply, IC74121, Thermistor, Capacitor
2200pF, Pulsar.
THEORY:
Pulse modulation is widely used in digital communications. In this type of modulation,
the information to be transmitted in converted into pulses of various sizes and shapes and
then transmitted to appropriate stations. The process of converting data into pulses can be
carried out in a variety of ways and is called encoding.
Three of the most common types of encoding or modulation are discussed here. In pulse
amplitude modulation or PAM, the data controls the height of the pulse while in PDM or
pulse duration modulation, the pulse width is proportional to the data value. Pulse
position modulation or PPM is the process in which the position of a pulse with respect to
a reference pulse is proportional to the data value. These processes are illustrated in
Fig.(1).
The essential difference between pulse modulation and the conventional AM or FM is
that in AM or FM some parameter of the carrier wave varies continuously by the
information, whereas in pulse modulation some parameter of a pulse is varied by the
sampled message. The pulses are usually of very short duration and as such a pulse
modulator is ‘OFF’ most of the time, i.e., low duty cycle. The low duty cycle operations
coupled with the facility of utilization of the time interval between two consecutive
pulses of a particular message for transmitted other messages makes pulse modulation
superior to AM or FM.
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The PDM and PPM systems are not usually as susceptible to noise as a PAM system.
Figure 3 shows a simple circuit to produce PDM. It is a monostable multivibrator with
thermistor as an element that controls the width of the output pulse. As a result, the
output pulse width varies in accordance with the temperature of the thermistor, i.e.,
output is pulse width modulated. One can obtain PPM by differentiating and rectifying
the PDM signals.
Fig.3: PDM using a monostable multivibrator
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DEPARTMENT OF ECE
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PROCEDURE:
1. Rig up the circuit as per the circuit diagram.
2. Pulses are given at Pin 4.
3. Observe the waveform at Q.
4. Plot the Pulse duration waveforms.
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DEPARTMENT OF ECE
DIGITAL COMMUNICATIONS LAB
Expt No. 13
GENERATION OF PDM AND PPM USING PLL
AIM: To generate Pulse duration modulation and Pulse Position modulation waveform
using Phase Lock Loop.
EQUIPMENT REQUIRED:
(1) CRO
(2) Audio oscillator
(3) Square-wave generator (15 kHz)
(4) ICs: 565, 7486 with bases, Transistor 2N2222
(5) Dual power supply (6-0-6V)
(6) Capacitors and resistors.
THEORY:
The PLL can be used to produce PDM and PPM with the help of the circuit
shown in Fig.1. The phase-locked loop remains locked if the VCO input remains constant
such that fi = fo = fo’. the modulating signal applied to pin 7 (input of VCO) upsets this
equilibrium. This causes the phase detector output to change in order to maintain the
VCO input at this level. A change in the output of the phase detector means a change in
the phase difference between its two input signals. Thus, the VCO output has a phase
shift proportional to the modulating signal amplitude, i.e., it is pulse position modulated.
This signal is amplified by T before using the same for some purpose.
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DEPARTMENT OF ECE
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Fig.1: Generation of PDM and PPM using PLL
The input signal and the PPM constitute the two inputs of the exclusive OR gate.
It compares the PPM signal and the original pulse leading to production of PDM at twice
the frequency of the original signal,
The centre frequency of the VCO is adjusted by R3 while R4 may be adjusted to
control the duty cycle of PDM signal.
EXPERIMENTAL PROCEDURE
1. Wire the circuit as shown in Fig.1.
2. Use the square-wave generator to feed the input signal vc (pin2)
at about 15 kHz.
3. Use the audio oscillator as the source of modulating voltage (200 Hz to 2 kHz).
4. Adjust R3 such that the free-running frequency of the VCO is close to the input
signal frequency.
5. Monitor PDM and PPM outputs using CRO.
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6. Vary the modulating signal frequency and observe (a) pulse width of PDM and
(b) pulse position in PPM.
Questions
1. What is pulse modulation? How is it different form AM or FM?
2. What are the advantages of pulse modulation?
3. Distinguish between PAM, PDM and PPM.
4. Explain how a PLL can be used to generate PPM and PDM.
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