This 80's Exar circuit has a carrier detector for frequencies up to 500 kHz. The circuit uses a company 567 which is still a common component.
This circuit, which appears in this section elsewhere, is from an American 1980s documentation....
August 2017 - Mouser Electronics is making available the Gen III generation of Vishay P Channel...
This variable filter that lets frequencies from 500 Hz to 1500 Hz pass through was found in a...