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 is from a Radio Electronics issue of May 1972. It consists of a commercial heating...
Found in the American 73 magazine for radio amateurs, this circuit can test common quartz...
This circuit is from the 1995 National Semiuconductor Linear Application Handbook. The technique...