This circuit was obtained from a Radio Electronics issue from January 1989. The indicator is a ceramic transducer, and the frequency of the emitted sound is given by C3.
This VCA was found in the July 1988 issue of Radio Electronics magazine. It uses an OTA op amp and...
Found in US documentation, this circuit changes the waveform and phase of a sine signal. The...
Found in an American amateur radio documentation by the name of “antennascope” this circuit is...