In Pulse Amplitude Modulation, amplitude of pulses is varied in accordance with the instantaneous value of modulating signal.
Pulse Amplitude Modulation Generation:
The carrier which is in the form of narrow pulses has frequency fc. The uniform sampling takes place in the multiplier to generate PAM signal. Samples are placed Ts sec away from each other.
- Here we see a simple emitter follower circuit.
- The output will follow the input in the absence of the clock signal.
- The input signal is the modulating signal.
- The base of transistor has another input as the clock signal.
- The desired carrier pulse train frequency is kept equal to the frequency of the clock signal.
- At some negative voltage sufficient to bring the transistor in the cutoff region the amplitude of the clock signal is chosen at a low level.
- The circuit will operate as an emitter follower when the clock is high and the output follows in the input modulating signal.
- The output is zero when the transistor is cut off and the clock signal is low.
- This gives the required output of the PAM signal.
PAM Demodulator:
The PAM demodulator circuit is simply an envelope detector. This is then followed by the low pass filter circuit as we see here. The low pass filter is also removes the noise.
Advantages:
- It is easy to modulate as well as demodulate.
- The PAM technique is the most basic technique which we need for all digital modulations.
- Then the generation and detection in this technique is easy and simple.
- This technique can carry the message or information at the same time. It simultaneously generates other pulse modulated signals also.
- The circuit required for transmission as well as for reception is very simple.
Disadvantages:
- It is easy to remove the added noise because it has an impact on amplitude which carries information.
- There is more noise interference.
- The bandwidth for transmission is also too large.
- The power requirement for transmission is also more.
- There are variations in the instantaneous power of the transmitter.
- The Higher bandwidth is require to fulfil the Nyquist criterion.
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