 The most important and widely used application of the heterodyne technique is in the superheterodyne receiver ( <unk> ) , invented by U.S. engineer Edwin Howard Armstrong in 1918 . In this circuit , the incoming radio frequency signal from the antenna is mixed with a signal from a local oscillator ( LO ) and converted by the heterodyne technique to a lower fixed frequency signal called the intermediate frequency ( IF ) . This IF is amplified and filtered , before being applied to a detector which extracts the audio signal , which is sent to the loudspeaker . 
