13 Jun 2026

Pulse Amplitude Modulation and Demodulation

practical pg-iv electronics pam communication

Aim

To generate a pulse-amplitude-modulated signal and recover the message signal by demodulation.

Apparatus

PAM trainer, message oscillator, pulse generator, sampling circuit, low-pass filter, CRO, and connecting leads.

Experimental arrangement

The message and sampling pulses enter the modulator; the receiver low-pass filter recovers the message waveform.

Theory

A communication system must represent a continuously varying message in a form that can be transmitted and recovered. In pulse-amplitude modulation, a periodic switch samples the message at equally spaced instants, so the amplitude of each pulse follows the instantaneous message voltage. If $m(t)$ is the message and $p(t)$ is a periodic sampling pulse train, the sampled signal is $s(t)=m(t)p(t)$.

The spectrum contains shifted replicas of the message spectrum around harmonics of the sampling frequency. If the highest message frequency is $f_m$, faithful recovery requires

\[f_s\geq2f_m.\]

A low-pass filter selects the original baseband replica and reproduces the message. When $f_s<2f_m$, spectral replicas overlap and aliasing makes the recovered waveform distorted.

Observations

Message frequency (kHz)	Sampling frequency (kHz)	Demodulated waveform
1	10	faithful sine wave
2	10	faithful sine wave
6	10	distorted due to undersampling

Calculation

For a 2 kHz message, the Nyquist condition requires

\[f_s\ge2f_m=2(2\,\text{kHz})=4\,\text{kHz}.\]

The applied sampling frequency is 10 kHz, so the 2 kHz signal is sampled faithfully. For the 6 kHz message,

\[f_{s,\min}=2(6\,\text{kHz})=12\,\text{kHz}.\]

Since $10<12$ kHz, spectral overlap produces the observed aliasing distortion.

Maxima Code

Download the PAM sampling check.

Result

The pulse amplitudes follow the message signal, and the low-pass filter recovers the message when the sampling frequency is sufficiently high.

Viva Questions

1. What is sampled in PAM? The amplitude of the message at discrete instants.
2. What is aliasing? Misrepresentation of the message caused by insufficient sampling frequency.
3. Why is a low-pass filter used? To remove the sampling-frequency components.