Intermediate Frequency
In the Superhet oder Superheterodyne receiver, also known as single conversion receiver, the RF signal coming from the antenna is mixed with the signal of an oscillator in a mixer stage to generate an „intermediate frequency“ on a fixed frequency higher or lower then the original RF signal. As this frequency is fixed, filters and amplifier stages for this single frequency or easier to design without the risk of distortion or unwanted signals.
The intermediate frequency usually is amplified in several IF amplifier stages and finally fed to the demodulator, which generates the audible signal.
The engineers choose the intermediate frequency depending on the frequency range of the receiver and the selection of IF filters. In a VLF or longwave receiver a low intermediate frequency is more efficient, as cheaper LC (coil - capacitor) filters can be used with good results and the risk for mirrors (the tuned signal can be heard a second time on the dial with a distance of the double of the intermediate frequency) is small. On shortwaves, this is more a problem, as coil filters offer not such a good selectivity and disturbing „mirror signals“ are appearing near the wanted signal on the dial. So crystal or mechanical IF filters have to be used for good selectivity.
To prevent „mirror signals“ or „ghost stations“ appearing on the dial just 900 kHz away (twice the standard IF for domestic radio receivers), high intermediate frequencies (higher then the upper end of the shortwave bands) habe been used, e.g.IF around 40 - 50 MHz. To prevent the necessity of the development of expensive filters on high IF, often a double conversion design is used: a first step, the signal is mixed to a high first IF and in a second mixer stage converted to a low second intermediate frequency, for which cheaper and standard IF filters can be used.
A typical intermediate frequency used in AM broadcasting receivers is 455 kHz.
In a double conversion receiver, the antenna signal is mixed with a much higher oscillator signal, so that a first high intermediate frequency, for exemple at 5 MHz, is generated. This IF has to pass through a bandpass filter, then it's amplified and converted in a second mixer stage to generate the second intermediate frequency. The final filtering and amplification, which determines receiver selectivity, can now be done with standard filters which are easier and cheaper to be constructed for standard IF frequencies.
In FM broadcasting receivers, a typical intermediate frequency of 10,7 MHz is used.