This tone recognizer circuit that relies on R5 and C1 was found in the December 1994 issue of Electronics Now. The circuit operates on voltages from 6 to 12 V typically.
This circuit is indicated for the PX or 11-meter band, having been found in a 2003 documentation. The...
This configuration from a 1976 technical documentation shows how to use a varicap in a direct...
Other operational amplifiers can be used in this circuit obtained from the manufacturer's own...