Forum: FPGA, VHDL & Verilog FM demodulation

von Milruwan P. (Company: Student) (milprasad)

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I want to demodulate the FM intermediate frequency (IF)signal using zero 
crossing detection. After several trials I was able to detect all the 
zero crossings. Now I need to generate the audio signal using the zero 
crossing data. Can any body suggest me a method to do the demodulation 
using zero crossings.

von W.S. (Guest)

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Zero crossing for FM demodulation?
In principle you need to memorize the vector (RE,IM) and calculate the 
angle difference between the memorized vector and the current vector. 
This is the integral of your current amplitude of the demodulated 
signal. BUT: you need to highpass this (differentiate), because the 
average would grow to infinity, when your FM source is not exactly 
aligned to the frequency of your local oscillator. The sampling speed 
depends on the bandwidth you want to have, so that the angle difference 
of the current vector to the memorized vector is small enough not to 
become confused by diffs of 180° or more.



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