A Notch Filter is a filter on a radio receiver that is designed to eliminate a narrow, sharply-defined band, or "notch", of frequencies from reception. Usually, in shortwave receivers, the filter operates on the audio output; however, the concept of a notch filter can apply to any range of frequencies, including radio frequency signals.
Notch filters are useful for shortwave listening because of certain common kinds of interference. This sort of interference typically produces a fairly pure tone, or tones, in the audio output of the receiver. Since a pure tone is narrow in frequency range, and has sharp boundaries, a notch filter can eliminate it, with minimum impact on the rest of the audio signal. This makes the total signal more intelligible. The most common source of this sort of interference is an AM signal nearby in frequency to a signal being listened to; this produces a tone called a heterodyne, or het.
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