1. LAB - 3
PULSE AMPLITUDE MODULATION & DEMODULATION
3.1 OBJECTIVE
In this lab, you will learn how to perform the pulse amplitude modulation and demodulation
and to calculate the modulation index for various modulating voltages.
3.2 HARDWARE REQUIRED
.
Transistor, AFO, IC NE566, Resistors, Capacitor, CRO, RPS.
3.3 PREPARATION (PRE-LAB)
Do the complete revision of Pulse Amplitude Modulation and Demodulation theory.
3.4 THEORY
Pulse amplitude modulation is a scheme, which alters the amplitude of regularly spaced
rectangular pulses in accordance with the instantaneous values of a continuous message signal.
Then amplitude of the modulated pulses represents the amplitude of the intelligence.
A train of very short pulses of constant amplitude and fast repetition rate is chosen the
amplitude of these pulse is made to vary in accordance with that of a slower modulating signal the
result is that of multiplying the train by the modulating signal the enve lope of the pulse height
corresponds to the modulating wave .the Pam wave contain upper and lower side band frequencies
.besides the modulating and pulse signals.
The demodulated PAM waves, the signal is passed through a low pass filter ha ving a cut –
off frequencies equal to the highest frequency in the modulating signal. At the output of the filter is
available the modulating signal along with the DC component
PAM has the same signal to noise ratio as AM and so it is not employed in practical
circuits
3. Fig 3-2 PAM Model Graph
Fig 3-3 PAM Demodulation Model Graph
3.6 TEST PROCEDURE
3.6.1 Modulation
1. Make the circuit as sho wn in circuit diagram
2. Set the pulse generator output to be 41Vpp at 100HZ
3. Set AFO output at 2 Vpp 100HZ
4. Observe the output wave form on a CRO
5. Tabulate the reading.
4. 3.6.2 Demodulation
1. Connect the circuit as shown in figure
2. Given the modulated output with AFO used to the input of the circuit.
3. Vary the potentiometer so that modulating signal is obtained.
4. Measure the amplitude of the signal and verify with that of the input.
3.7 LAB RESULT
Thus the pulse amplitude modulation was performed and its corresponding demodulation was
also performed.
3.8 POST LAB QUESTIONS
1. What is the process of sampling an analog signal at a high rate ?
2. Which multiplexing technique is used to transmit both digital and analog signals?
3. Why PAM is not preferable in digital transmission?
4. Which device is used to track PAM frequency variations in the clock recovery circuit ?
4. What kind of switches are commonly used in PAM multiplexers?
ASSESSMENT
Prelab
: 30 marks
(design (10), circuit (10), tabulatio n (10))
Lab performance
: 30 marks
Viva-voce
: 15 marks
