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The term "beat frequency" has its origins in acoustics. When two sound waves are of different frequency, they 'beat' against each other, producing a third (or intermediate) note which is the difference between the two. But if the two are identical in frequency and phase, there is "zero beat".
The same thing occurs with RF signals. The beat is sometimes called a heterodyne, as in superheterodyne receivers; the incoming signal 'beats' with a local oscillator, producing the intermediate frequency (or IF) signal which is then amplified and demodulated.
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