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You'd avoid having this problem by detecting whether FM is present before demodulating it. The reason is simple: FM demodulation throws away a lot of the info that's still in the RF signal – most prominently, the actual received power – and that info is what you'd want to use to squelch. That can (unless there's other types of transmissions than audio FM ...


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SDR is not applying a lowpass (baseband) sampling on its RF input, instead it effectively employs a band-pass signal sampling. According to Shannon-Nyquist baseband sampling criteria, the bandwidth of the signal to be sampled should be less than half the sampling frequency. That's what you are talking about. However for narrowband modulated signals bandbass ...


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A negative frequency is given by the complex signal $e^{-j2 \pi f_c t}$ for which the physical implementation requires two real signal paths, which is made clearer from Euler's formula: $$e^{-j2 \pi f_c t} = \cos(2 \pi f_c t) + j \sin(2 \pi f_c t)$$ So if the Local Oscillator is generated with sine and cosine outputs, and the multiplier is a complex ...


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