A circuit which changes the frequency of alternating current. Alternating current with a frequency of 1,000 hertz, for example, can be changed to 200 hertz by combining it with an AC signal having a frequency of 1,200 hertz.

When alternating currents having these two frequencies are combined, or mixed, two more alternating currents are produced: one having a frequency which is the sum of the two, 2,200 hertz, and the other a frequency which is the difference between the two — in this case, 200 hertz, the desired frequency.