This circuit was obtained from an 80's Radio Electronics. The circuit decodes “scrambled” or sinusoidal encoded signals by providing an audio signal at the output.
According to the author of this project, from a documentation on espionage from the 1980s, this...
This circuit suggested by Siemens can provide time intervals for triggering values that are given...
This circuit generates a good-intensity 1 kHz rectangular signal. The circuit is from Electronics Now...