This circuit was found in the August 1996 Electronics Now magazine. The circuit is based on an operational circuit whose type determines the gains and limits for the frequency.
Varactor diodes are our varicaps of today. This circuit for switching signals in an RF circuit was found...
This Hughes circuit is from 1968. Equivalent modern components can be used. The input impedance is...
This circuit digitally counts input pulses. The circuit from a November 1970 Radio Electronics uses...