Enhanced selectivity can be obtained by tailoring the frequency response in the AF amplifier stages following the detector, in addition to the RF selectivity provided in the IF amplifier stages preceding the detector.
A voice signal requires a band of frequencies ranging from about 300 Hz to 3000 Hz. An audio bandpass filter, with a passband of 300 Hz to 3000 Hz, can improve the quality of reception with some voice receivers. An ideal voice audio filter has little or no attenuation within the passband range, and high attenuation outside the range, with a near-rectangular response.
A CW or FSK signal requires only a few hundred hertz of bandwidth to be clearly read. Audio CW filters can narrow the response bandwidth to 100 Hz or less, but passbands narrower than about 100 Hz produce ringing, degrading the quality of reception. With FSK, the bandwidth of the filter must be at least as large as the difference (shift) between mark and space, but it need not, and should not, be much greater.
An audio notch filter is a band-rejection filter with a sharp, narrow response. An interfering carrier that produces a tone of constant frequency in the receiver output can be greatly attenuated with this type of filter. Audio notch filters are tunable from at least 300 Hz to 3000 Hz. Some sophisticated AF notch filters can tune themselves automatically. When an interfering carrier appears and remains for a few tenths of a second, the notch is activated and centers itself on the audio frequency of the detected offending signal.
A squelch silences a receiver when no signal is present, allowing reception of signals when they appear. Most FM communications receivers use squelching systems. The squelch is normally closed, allowing no audio output, when no signal is present. The squelch opens, allowing everything to be heard, if the signal amplitude exceeds the squelch threshold, which can be adjusted by the operator.
In some systems, the squelch does not open unless the signal has certain characteristics. This is known as selective squelching. The most common way to achieve this is the use of subaudible (below 300 Hz) tone generators, or AF tone-burst generators. The squelch opens only when an incoming signal is modulated by a tone, or sequence of tones, having the proper characteristics. This can prevent unauthorized transmissions from accessing repeaters or being picked up by receivers.