Broadcast radio & TV transmitters, police & fire repeaters, and others have potent
signals. If your system does not attenuate them, by using low-pass, high-pass, or band-pass
filters, the peak voltage at your analog to digital converter may at times exceed "full
scale". The resulting non-linear response produces many spurious signals, that may mask the
typically weak signals you are wanting to recieve. An initial solution is to use broad-band
attenuators to keep the total power reaching the ADC below full scale. Then observe the spectral
peaks and insert appropriate filters to knock them down. The attenuation may then be reduced.
Frequently AM broadcasters are the worst. A high-pass with cutoff around 1.6 MHz may be all you
need.