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.
This circuit responds to frequencies from 100 Hz to 3 MHz. We find it in a 90s Radio Electronics....
Found in a 90s Linear Technology documentation, this circuit uses an operating system of the time....
Found in an old Electronic Design, this circuit provides an output voltage that is the absolute...