sentences:  In this an electronic oscillator generated a radio signal at a frequency f"o offset from the incoming radio wave carrier f"C. This was applied to the rectifying detector with the radio carrier.In the detector the two signals mixed, creating a heterodyne (beat) signal at the difference f"C - f"C between these frequencies, which was in the audio frequency range.The heterodyne provided the audible tone in the earphone whenever the carrier was present.