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 portable transceiver circuit for the 11 meter or 27 MHz band was found in a 1991...
This circuit was found in an American 1980s documentation. It makes use of an unusual MOSFET today...
This circuit was found in an old Linear Technology documentation. The working principle can be...