This 1/2 hertz high selectivity filter was found in the December 1990 issue of Radio Electronics magazine. The source must be symmetrical. The capacitors can be changed for other frequencies.
This circuit uses three inverters to generate a rectangular signal whose frequency depends on R1...
This configuration presents a negative resistance caracteristic such as the one found in...
Found in a magazine 73 for radio amateurs, this circuit makes use of general purpose transistors and...