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überarbeitet am 21.10.2010

In the shortwave band, the frequencies of broadcasting stations are much nearer then in the FM band. Channel spacing is usually 5 kHz, so a signal of a station in the adjacent 5 kHz channel will cause an unpleasant 5 kHz interference noise.

A receiver will precess only a very small selection of a large high frequency spectrum, this very narrow bit of the spactrum will be filtered and finally be demodulated. The intermediate filters have the task to act like a narrow gate, through which the desired signal has to pass through.
In cheap receivers usually only one cheap filter with a large bandwidth is used, in these "world radios" it can occur that You can hear several stations roaring at out of the speaker the same time: the reception of a weak signal of a Peruvian local station near to the strong transmitter of Radio Moscow is impossible.
In high-quality receivers, You will find two or more IF - filters, a NARROW / WIDE - switch or a Bandwidth switch allows to select them according to the reception situation and mode.

The narrower the IF - filter, the better interferences by adjacent stations can be faded out, but the tone becomes more bassy and dull.
A second important parameter is the skirt selectivity, the steepness of the edges of the filter passband. A filter with steep edges is more suitable to "cut" interfering signals off.

The choice of the optimal bandwidth is also important: For reception of AM - signals of broadcast stations, IF-filters of 4 - 6 kHz are usually found suitable, for SSB reception 1.8 - 3 kHz and for CW - reception of Morse code signals switch the 1 kHz, 500 or 250 Hz filter, if available.
In the NRD-535, variable bandwidth (BWC) was realized, receivers using DSP technology technology for intermediate signal processing may have a nearly infinite number of IF filters, You can even program one to Your own requirements.