In radio and signal processing, heterodyning is the generation of new
frequencies by mixing (multiplying) two
oscillating waveforms. It is useful for placing information of interest into
a useful frequency range following modulation or prior to demodulation . The two frequencies are mixed in a vacuum tube ,transistor , diode, or other
signal processing device. Mixing two frequencies creates two new frequencies, according to the properties of the sine function; one is the sum
of the two frequencies mixed, the other is their difference. These new frequencies are called heterodynes . Typically only one of the new
frequencies is desired—the higher one after modulation and the lower one after demodulation. The other signal is filtered out of the output of the
