This 1970 Electronic Design circuit uses a varactor or varicap. The circuit divides the frequency of a 150 MHz signal by 2. Similar versions appear on this site.
IDT ZSSC3230 Capacitive Sensor Signal Conditioner Available from Mouser Electronics stock, this IDT...
This circuit uses only two common transistors and provides good audio power in a speaker. Readers who...
This is the analog part of the previous circuit with the pressure sensor for the altimeter. The...