Pulse Amplitude Modulation - Pulse Modulation Techniques - PAM Modulation and Waveform

In this lecture on Pulse Modulation Techniques, basics of Pulse Amplitude Modulation (PAM) and PAM waveform are explained. Pulse Amplitude Modulation (PAM Modulation) is a Analog pulse modulation technique. Other techniques of analog pulse modulation are- Pulse Width Modulation (PWM) and Pulse Position Modulation (PPM). But before discussing pulse amplitude modulation, it is important to know, what is pulse modulation and how it is different from continuous wave modulation.
In pulse modulation, some parameter (amplitude, width or position) of the pulsed carrier wave is varied as per the instantaneous value of the modulating signal (message signal). Types of analog pulse modulation techniques are- Pulse Amplitude Modulation (PAM), Pulse Width Modulation (PWM) and Pulse Position Modulation (PPM).
While continuous wave modulation is a technique of modulation where some parameter (amplitude, frequency or phase) of a sinusoidal carrier is varied in accordance with the instantaneous value of the modulating signal. Types of continuous wave modulation are- Amplitude modulation (AM), Frequency Modulation (FM) and Phase Modulation (PM).

Pulse Amplitude Modulation (PAM)
In pulse amplitude modulation, the amplitude of pulsed carrier wave is varied according to the modulating signal.

Pulse Time Modulation (PTM)
In pulse time modulation, the timing of pulses of the pulsed carrier is varied. Therefore the variations in the pulse are on the time axis.
Pulse Time Modulation (PTM) is of two types-
Pulse Width Modulation (PWM)/ Pulse Duration modulation (PDM) and
Pulse Position Modulation (PPM)

Pulse Width Modulation (PWM)

In PWM, the width of pulsed carrier wave is varied as per the instantaneous value of the modulating signal.

Pulse Position Modulation (PPM)

In PPM, the position of pulsed carrier is varied as per the modulating signal (message signal).

Now we understand the basic difference between PAM, PWM and PPM. Now it's time to discuss pulse Amplitude Modulation in some detail-

Pulse Amplitude Modulation (PAM)

Definition:-
The modulation technique in which the instantaneous amplitude of the pulsed carrier is varied according to the modulating signal (message signal) is called as pulse amplitude modulation.

Waveform of pulse amplitude modulation
The message signal can have multiple frequencies and variable amplitude. Here we have used a sinusoidal message signal. The carrier wave that we have used here, is a train of pulses having high frequency. Since we know that in Pulse modulation, the carrier wave is in the form of pulses.
Now we will understand how can we make pulse amplitude modulated wave (PAM).
As we have discussed that in Pulse amplitude modulation, the instantaneous amplitude of the pulsed carrier is varied according to the modulating signal (message signal). Now look at the image shown above very carefully, observe that, as the amplitude of the modulating signal (message signal or baseband signal) increases, the amplitude of the pulsed carrier wave increases and becomes maximum when the amplitude of the modulating signal reaches at its maximum. Now the amplitude of the carrier wave starts decreasing with decrease in amplitude of the modulating signal. So it is clear from the image that the amplitude of the pulsed carrier wave varies according to the instantaneous value of the message signal.

Important Observations (PAM)

#As it is Pulse Amplitude Modulation (PAM), therefore only the amplitude of the pulsed carrier changes.
#There is no change in the width or position of the pulsed carrier wave. So the width and position of the carrier wave pulse is constant here.
#Therefore all the information of the modulating signal is contained in the amplitude variations of the pulses. No information is present in the width or position of the pulses.

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