In ASK demodulation, the amplitude of received baseband signal is varying, the fluctuation is caused by various factor such as path loss, obstacle in the path and multipath fading. In such way, the threshold of 1/0 decision must be adaptive to the dynamic changing amplitude, a digital lowpass filter is needed. and the bandwidth needed to be narrow compare with the symbol rate, because the symbol is random. for the computational efficiency reason, power of 2 coefficient is chosen.
The proposed filter is consist of two stage, first stage is a recursive filter running at 8x of symbol rate
y = (31y + x)/32
x is input and y is output
The second stage is another recursive filter running at 2x of symbol rate
g = (31g+y)/32
Green trace is the output of first filter and red trace is the output of the second filter. It produce a quite smooth amplitude smoothing despite the symbol is random with equal chance of 1 and 0.
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