how to seperate mixed signal

My circuit is:
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In the transmitter:

audio signal (500Hz - 40KHz) -> FM Modulation (around 400KHz, square wave) -> mixed with low frequency signal(20Hz or 50Hz, square wave) by using opamp summing circuit

In the receiver:
mixed signal -> low pass filter to get low frequency signal (OK)
mixed signal -> high pass filter to get FM modulation signal (X)
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My question is:
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How to get the FM modulation signal from the mixed signals? I tried RC High pass filter. There was sharp edge of low frequence signal passed through the HPF.

THANKS.