PAM

Modulate the amplitude of a high speed pulse train using a baseband message. This serves as the critical first step before digitizing audio into PCM.

Core Concept

Pulse amplitude modulation maps the continuous message voltage directly to the height of discrete pulses. The signal remains continuous in amplitude but discrete in time. A simple analog switch like the CD4016 easily achieves this operation.

Because the information sits entirely in the vertical height of the pulse it is highly vulnerable to channel noise. Static and interference immediately distort the perceived amplitude. Additive noise destroys signal integrity.

Due to this poor noise immunity engineers rarely use PAM for direct radio transmission. Instead it serves as an intermediate stepping stone inside analog to digital converters.

Key Equations

The transmission bandwidth relates directly to the pulse width. BT >= 1 / (2 * tau) determines the minimum channel requirement.

Here tau represents the precise duration of a single pulse. Narrower pulses require significantly wider frequency bandwidths.

Given:\nPulse width (tau) = 10 us (0.00001 s)\n\nWhy this formula:\nThe Nyquist criterion states minimum transmission bandwidth depends inversely on pulse width.\n\nFormula:\nBT = 1 / (2 * tau)\n\nSubstitution:\nBT = 1 / (2 * 0.00001)\n\nCalculation:\nBT = 1 / 0.00002\nBT = 50000\n\nFinal Answer:\n50 kHz

Exam Tip: PAM is highly sensitive to additive channel noise compared to other pulse modulation schemes. Never select it as a good option for noisy long distance radio links.

Key Properties

• Circuitry for generation and demodulation remains extremely simple.
• Transmitted power varies constantly depending on the message signal amplitude.
• It suffers from severe amplitude degradation over long coaxial lines.
• A simple low pass filter can recover the baseband signal perfectly.
• It functions as the core mechanism for flat top sampling.

Quick Revision

• Amplitude carries all the information.
• Time intervals between pulses remain strictly constant.
• Pulse width remains strictly constant.
• Noise immunity is the worst among all pulse schemes.
• Bandwidth depends entirely on pulse duration.
• Exam trap: Confusing PAM with PCM. PAM pulses still have infinite possible amplitude values until they undergo quantization.