Selectivity refers to the ability of a receiver to separate one signal from another signal on an adjacent frequency without interference or with acceptable intelligibility.
The selectivity is specified in the technical data of a receiver as the bandwidth outside of which a signal on an adjacent frquency drops by -6 dB and sometimes by -60 dB. If there is a low value even for -60 dB given, this will indicate a high-quality filter with steep filter skirts.
The broadcasting stations in the shortwave broadcast bands are relatively close to each other with a channel spacing of 5 kHz. A station transmitting on the adjacent channel 5 kHz up or down from the desired frequency can heavily impair the signal, an unpleasant whistling tone occurs with a pitch of 5 kHz in this example. On mediumwaves, this phenomenon is less pronounced because a channel spacing of 9 kHz (internationally) or 10 kHz (in the USA) is used. On VHF with a channel spacing of 50/100 kHz, adjacent channel interference is only a problem with weak signals in the vicinity of a very strong transmitter.
In every shortwave receiver, only a very narrow section of the entire frequency spectrum is filtered out and demodulated. The task of letting pass through only the desired signal is performed by the bandwidth filters or IF filters.
In cheap receivers, only one usually wide cheap filter is used; with these receivers it can happen that a mix of several stations can be heard from speaker at the same time, e.g. the reception of a weak signal from Peru right next to the strong signal from Radio Moscow is impossible.
In better receivers, two or more IF filters are built in. With a NARROW / WIDE switch or a Bandwidth selector, the most favourable filter can be selected.
The narrower an IF filter, the better it is at blocking out interference from an adjacent channel, but the audio becomes bassy and muffled. A second important parameter is the filter skirt; with cheaper filters with wide filter skirts, the interference cannot be cut off as effectively as with high-quality filters with steep skirts.
It is also important to choose the optimum bandwidth according to the reception mode: IF filters with a width of 4 - 6 kHz are useful for AM reception of broadcasting stations, 1.8 - 3 kHz for SSB reception, and narrow-band filters with a width of 250 - 500 Hz are used for CW reception. LC filters are inexpensive to produce, but they only have a high quality or steep skirts at low intermediate frequencies. Ceramic filters are also inexpensive. More expensive are crystal filters with steep skirts and the best results are achieved with mechanical filters.
In some receivers, a continuously variable bandwidth (BWC) has been realised; with modern DSP technology (digital signal processing), the IF bandwidth is set digitally with programmable filter curves.