Radio Receiver Design - Superheterodyne

Superheterodyne

Practically all modern receivers are of the superheterodyne design. The RF signal from the antenna may have one stage of amplification to improve the receiver's noise figure, although at lower frequencies this is typically omitted. The RF signal enters a mixer, along with the output of the local oscillator, in order to produce a so-called intermediate frequency (IF) signal. The local oscillator is tuned to a frequency somewhat higher (or lower) than the intended reception frequency so that the IF signal will be at a particular frequency where it is further amplified in a narrow-band multistage amplifier. Tuning the receiver involves changing the frequency of the local oscillator, with further processing of the signal (especially in relation to increasing the receiver) conveniently done at a single frequency (the IF frequency) thus requiring no further tuning for different stations.

Here we show block diagrams for typical superheterodyne receivers for AM and FM broadcast respectively. This particular FM design uses a modern phase locked loop detector, unlike the frequency discriminator or ratio detector used in earlier FM receivers.

For single conversion superheterodyne AM receivers designed for medium wave (AM broadcast) the IF is commonly 455 kHz. Most superheterodyne receivers designed for broadcast FM (88 - 108 MHz) use an IF of 10.7 MHz. TV receivers often use intermediate frequencies of about 40 MHz. Some modern multiband receivers actually convert lower frequency bands first to a much higher frequency (VHF) after which a second mixer with a tunable local oscillator and a second IF stage process the signal as above.