In all superheterodyne receivers, an intermediate
frequency (IF) is obtained from the modulated input frequency by mixing it with an oscillator frequency. This technique has the advantage that the intermediate frequency is lower than the input frequency, has a fixed value and can therefore be amplified more selectively than a wideband input signal. The level of the IF depends on the receiver technology and is uniform worldwide. For radio receivers for long wave, medium wave and short wave it is 455 kHz, for VHF 10.7 MHz
and for TV receivers 27 MHz.amplifier with a filter, whereby, in order to make the reception technology more selective, several RF amplifiers with filter functions are often connected in series. The filters are usually LC elements; in the case of higher selection, ceramic filters or crystal filters are used instead of LC elements. An IF amplifier of this type operates as a bandpass
filterwith the appropriate bandwidth, high selectivity and suppression of adjacent frequencies
. It amplifies only the intermediate frequencies.To enable IF amplifiers to adapt to different reception conditions and thus amplify signals over as wide a dynamic range as possible, they are equipped with gain control
. In this process, a control voltage is obtained from the amplified IF output signal, which further controls the IF amplifier at a low output level. The bandpass curves of IF amplifiers cannot be steepened at will, since group delay problems occur in television with steeper filters.