 In telecommunication networks , <unk> is used to move frequencies of individual signals to different channels which may share a single physical transmission line . This is called frequency division multiplexing ( FDM ) . For example , a coaxial cable used by a cable television system can carry 500 television channels at the same time because each one is given a different frequency , so they don 't interfere with one another . Continuous wave ( CW ) doppler radar detectors are basically heterodyne detection devices that compare transmitted and reflected beams . 
