The zero beat method also makes use of a variable frequency, standard sinusoidal
source which is mixed with the unknown frequency signal. Recall that mixing, in its
purest form is multiplication. Mixing can also be accomplished by adding the two
signals together, and then passing them through a nonlinear amplitude transfer function,
such as y ¼ a þ bx þ cx2 þ dx3, etc. The output from the mixing process can be shown to
contain sine waves having frequencies which are the sum and difference of the two input
frequencies. Thus, as the variable known frequency is adjusted close to the unknown
frequency, the frequency of the output difference frequency term approaches zero.
This zero beat phenomenon can be detected by actually listening to the audio beat
frequency resulting from mixing the standard frequency source with the unknown.
The beat frequency method has an apparent dead zone, where the mixed difference
frequency lies below the human range of hearing. Visual observation of the beat
frequency sig