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 for cars with mechanical ignition systems was found in the Electronics Experimenter handbook...
This code practice circuit was found in a 1990s issue of Radio Amaterur Today magazine. It...
The circuit presented was found in the Linear Applications Handbook of the National Semiconductoir of 1994....