The 'mixing' or 'heterodyne' process is one in which two signals are mixed to produce two new signals. The new signals produced are one equal in frequency to the sum of the original frequencies, and the other equal to the difference between them. The undesired product and the two original frequencies are rejected by some form of filter which is generally a tuned circuit. This process is also called 'frequency changing', 'frequency conversion' or 'frequency translation' and is shown in Fig 2.18.
Fig 2.18. Process of frequency mixing
The mixer has many uses in communication engineering. In particular, it is the basis of the superheterodyne receiver and the single-sideband (SSB) transmitter.
Owing to non-optimum characteristics in the mixer element, spurious products harmonically related to the two original frequencies (f1 and f2) are often produced. The suppression of these is particularly important in an SSB transmitter. They may be reduced by careful mechanical layout of the actual circuit and the use of a push-pull, ie a balanced circuit.
