superheterodyne

Superheterodyne refers to a method of designing and building wireless communications or broadcast equipment, particularly radio receivers. Sometimes a receiver employing this technology is called a "superheterodyne" or"superhet."

In many wireless applications, the equipment must function over a range,or band, of frequencies.  But it is easier to process a modulated signal at a single frequency than over a band, especially if the highest frequency in the band is much different than the lowest frequency. Toovercome this inherent difficulty when engineering variable-frequency wireless equipment,the desired data-carrying signal can be combined with the output of an unmodulated,variable-frequency oscillator (VFO) in a circuit called a mixer. When this is done,output is produced at a fixed frequency representing the difference between the input frequencies.

When the correct range of frequencies is chosen for the VFO, a receiver can be designed that will intercept incoming signals over a specific band. For example, if the desired input frequency range is 20 MHz to 25 MHz, a VFO can be built that generates an unmodulated carrier at 29 to 34 MHz.  When the incoming signal is mixed with the VFO output, the resultant has a constant frequency of 9 MHz, representing the difference between the frequencies of the inputs. The 9-MHz output retains the modulation characteristics of the incoming signal. In this case, the receiver is said to have an intermediate frequency (IF) of 9 MHz. This IF signal can be amplified and filtered more easily than signals having frequencies that vary from 20 to 25MHz.

The theory and design of superheterodyne equipment is a rather sophisticated business. Numerous books are devoted to this topic; some engineers make their entire living designing oscillators, mixers, and amplifiers that use this technology. For more information about superheterodyne engineering practice, a professional-level textbook or a formal course is recommended.