Found in an American 1980s documentation, this circuit compares signals from two TTL sources, providing an output that depends on the frequency difference. The circuit can be used as a shield.
The basis of this circuit is the 723, an old integrated circuit not easier to find these days. The...
This circuit was found in a May 1981 Radio Electronics magazine article. The circuit is offset...
The circuit presented was found in a Japanese magazine in 1973. The circuit uses transistors of...