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Question about frequency modulation system operation in Simulink

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rvdp

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Not really digital but since it'a a Simulink question I thought it fitted best in here...

I have a very basic frequency modulation system in Simulink, consting of a sine wave input, FM modulator passband block, FM demodulator passband block and a scope. When executing the simulation you can see that the modulating and demodulated signal don't exactly match. The demodulated signal is shifted in phase and has a lower amplitude.

If I change the values of the modulation constant for both the modulator and demodulator from 3e3 to 15e3, both signal do seem to match a lot better.

Then, when further increased to 50e3 for example, they are way off.

Does anyone know why this is the case and what the math is behind it?

I attached the Simulink file. You need execute [num,den] = butter(2,30000*.005e-3); in the Matlab window for it to work. These are the coefficients of the low pass filter in the demodulator.

Thanks!
 

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  • fm simulink.rar
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