The capability of a receiver to separate one signal from another on an adjacent frequency channel is referred to as selectivity.
In the technical documentation, the selectivity is often referred to as bandwidth; the frequency difference which will cause asignal on an adjacent channel a signal level drop of -6 dB, partly also -60 dB is states. A lower value for the - 6 dB 8and the - 60 db) will stand for a more narrow bandwidth filter.
In the shortwave range, the radio stations appear relatively close to each other with a channel spacing of 5 kHz. A station a channel only 5 kHz away from the operation frequency can severely affect the signal, causing an unpleasant whistling tone in this example with a pitch of 5 kHz. On medium waves, this phenomenon with a channel spacing of 9 kHz (international) resp. 10 kHz (in the US) is less pronounced, on FM with a channel spacing of 50/100 kHz, adjacent channel interference is only a problem with very weak signals in the vicinity of a „boomong strong signal“.
It is important to choose the optimum bandwidth for reception: For AM reception of broadcasting stations, IF filters with a width of 4 - 6 kHz perform well, for SSB reception 1.8 - 3 kHz, for CW reception are very narrow filters of 250 - 500 Hz used.
Coil filters can be produced cheaply, but they perform well only on low intermediate frequencies, higher quality filters which are also inexpensive to manufacture are ceramic filters. The narrow and steep flank quartz filters become more expensive and the best results are achieved in short-wave receivers with mechanical filters.
In some receivers, a continuously variable bandwidth (BWC) has been realized; with the modern DSP technology (digital signal processing), the IF bandwidth is set digitally with preprogrammed filter curves.