EXPERIMENT NUMBER 3
Pulse Amplitude Modulation & Demodulation (PAM)
AIM: Conduct an experiment to generate PAM signal by varying the amplitude of the modulating signal and
frequency of the sampling signal. Also design a circuit to demodulate the obtained PAM signal and verify
sampling theorem. Plot the relevant waveforms.
LEARNING OBJECTIVE: To understand the waveform of PAM and:
To understand the use of transistor as pulse amplitude modulator.
To understand the use of operational amplifier and switching device (FET) as pulse amplitude
modulator.
To understand the classification of pulse modulation.
PRIOR CONCEPTS: Modulation and its types, Pulse modulation, Sampling T heorem, Nyquist Rate.
EQUIPMENT REQUIRED COMPONENTS REQUIRED
THEORY: Pulse-amplitude modulation is the simplest form of signal modulation and analog to digital
conversion method where the message infor mation is encoded in the amplitude of a series of signal pulses. It is
a modulation technique in which the amplitude of each pulse is controlled by the instantaneous amplitude of the
modulation signal at the time of each pulse. This technique transmits data by varying the voltage or po wer
amplitudes of individual pulses in a timed sequence of electromagnetic pulses. In other words, the data to be
transmitted is encoded in the amplitude of a series of signal pulses. Modulating a sine-wave carrier makes it
possible to keep the frequency content of the transferred signal as close as possible to the centre frequency
(typically the carrier frequency) of the pass band. There are two operations involved in the generation of PAM
signal.
i. Instantaneous sampling of modulating signal m(t) every Tsseconds where the sampling rate fs= 1/Tsis
chosen in accordance with the sampling theorem.
ii. Lengthening the duration of each sample so obtained to some constant value T.
These two operations are jointly referred to as SAMPLE and HOLD.
Demodulation is performed by detecting the amplitude level of the carrier at every symbol period.
PROCEDURE
1. Connections are made as shown in the circuit diagram.
2. Apply the square wave carrier signal of around 5V (p-p) amplitude with frequency fc= 5 KHz at the base.
3. Apply sine wave modulating signal with frequency fm= 100Hz with 2V (p-p) amplitude (use function
generator) at the collector of the transistor.
Note: frequency ranges mentioned above may vary from kit to kit. These values are just for your
guidance.
4. Output is taken at the emitter.
5. Observe the PAM output.
6. Modulated signal is fed to the input of demodulation circuit.
7. Observe the demodulated signal at the output.
8. Repeat the steps 2 to 5 for fc= 2fmand fc< 2fm.
