 An intermediate frequency was first used in the superheterodyne radio receiver, invented by American scientist Major Edwin Armstrong in 1918, during World War I.A member of the Signal Corps, Armstrong was building radio direction finding equipment to track German military signals at the then-very high frequencies of 500 to 3500 kHz.The triode vacuum tube amplifiers of the day would not amplify stably above 500 kHz, however, it was easy to get them to oscillate above that frequency.