Heterodyne

Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden where high frequency signals are converted to lower frequencies by combining two frequencies.^[1] Heterodyning is useful for frequency shifting information of interest into a useful frequency range following modulation or prior to demodulation. The two frequencies are combined in a vacuum tube, transistor, diode, or other non-linear signal processing device. Heterodyning 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 mixer.