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.
I found this circuit in a 1980 documentation, but it can be easily assembled with BC548...
Originally this circuit was designed to operate with microcomputers from 1988, but it can be...
This circuit was found in an article about the 555 in the November 1992 Radio Electronics...