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.
The figure shows how to implement an alarm buzzer in an LM3914 to warn about crossing the scale...
This circuit can act as a shield being controlled by a microcontroller by pin 1 or 2. The...
This circuit was obtained from a 1980's QST magazine. The circuit can generate 1 to 15 MHz signals...