Found in a 1964 Electronic Design this circuit compares two signals by providing the inverse of the algebraic sum of their frequencies. The circuit can be made with modern equivalent transistors.
In the absence of a modulation signal, the carrier of this simple short-range transmitter is cut...
This circuit was obtained from an 80's Electronic Engineering. The circuit detects ice formation...
Other operational amplifiers series operatives, such as MC34072, MC34074, MC35071, MC35072,...