Basically, the old pre-superhet designs used a tuned microwave antenna/cavity containing a modulator diode and a detector diode. The detector diode basically converted the received signal to DC. A signal was applied to the modulator diode, basically shorting and unshorting the cavity.
So, when a received signal was present, the detector diode would produce an output signal that was pulsed/modulated at the same rate that was applied to the modulator diode. This was either pumped through directly as the receive audio, or fed through a separate circuit to produce an alert.
Just about all pre-superhetrodyne detectors used some variation of this method.
Bookmarks