To reduce a receivers selectivity, so that a receiver can be tuned to a single desired frequency and only transmissions on this frequency and not interfering signals from adjacent channels are demodulated, IF filters are used.
In simple sets, only one IF filter is provided. In more sophisticated recivers a choice of two ore more filters is available; with a NARROW / WIDE - switch or a rotary switch, the IF bandwidth can be changed.
In a superhet receiver, an incoming radio signal from the antenna is mixed once or several times to generate an intermediate frequency. The IF signal has to pass a more or less narrow filter that will only allow frequencies very near the intermediate frequency to pass. This will make up the selectivity of a receiver, the capability to select one out of a few signals on very nearby frequencies in a shortwave band.
Using a narrow IF-filter allows to eliminate unwanted signals from interfering signals on a nearby frequency more effectively. But when a narrow IF bandwidth / filter is in use, audio becomes bassy or dull.
A second important technical parameter of an IF filter is it's skirt selectivity: With very steep shoulders or „skirts“, interfering signals can be cut off easier, wile the information - bearing parts of the audio signal still can pass.
For standard broadcast signal, i.e. AM reception, an IF filter with a bandwidth of 4 - 6 kHz will do a fine job, for SSB reception, you need a 1.8 - 3 kHz filter, which will be wide enough to let the single sideband pass through. For CW reception of Morse code signals, sometimes filters with a bandwidth of 250 - 500 Hz are provided, these allow to virtually cut the signals from one station out of a mix of several interfering transmitters. For audio transmissions, these filters are not suited, you won't understand the spoken word nor music.
Some sets like the NRD-535 provide a variable bandwidth, this technology is nowadays replaced by digital processing of the RF signal, with a virtually unlimited number of IF filter bandwidths using DSP technology.
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 and are available for the common intermediate frequencies.
More expensive are crystal filters with steep skirts, these are available for different commonly used intermediate frequencies. For sets with an uncommon IF used, the original filters for that IF require special crystals and often are difficult to get as a replacement part.
The best results are achieved with mechanical filters with a mechanically resonating structure, they are used in professional or military equipment and in some amteur radio gear (or at least, they could be installed as an option).
Receivers with DSP (Digital signal processing) rely on digital sampling of the radio frequency or the intermediate frequency. After this is done, digitally programmed filters can be used to process the signal, as additional filters can be programmed at virtually no extra cost, a big selection of filters with different skirts is available.