Demodulation of Amplitude Shift Keying Signal

Theory

It is apparent that the ASK signal has a well defined envelope. Thus it is amenable to demodulation by an envelope detector. With bandlimiting of the transmitted ASK neither of these demodulation methods, envelope detection or synchronous demodulation would recover the original binary sequence; instead, their outputs would be a bandlimited version. Thus further processing- by some sort of decision-making circuitry for example - would be necessary.

Ths demodulation is a two-stage process:

  1. recovery of the bandlimited bit stream
  2. regeneration of the binary bit stream

Having a very definite envelope, an envelope detector can be used as the first step in recovering the original sequence. Further processing can be employed to regenerate the true binary waveform.



Fig.1 Showing Demodulated wave with distortion.

The output from the above demodulators will not be a copy of the binary sequence TTL waveform. Bandlimiting will have shaped it If the ASK has been bandlimited before or during transmission (or even by the receiver itself) then the recovered message, in the demodulator, will need restoration (‘cleaning up’) to its original bi-polar format.

Some sort of decision device is then required to regenerate the original binary sequence. This could be done with a COMPARATOR.