Found in 1980s Electronics Design, this circuit covers 4 decades of frequencies with a unique control. A multi-turn trimpot is used for fine adjustment. The frequency range depends on C1.
This circuit simulates a UJT, with two bipolar transistor. Frequency is given by C1 and P1...
This circuit eliminates hiss and other noise from audio sources by cutting frequencies above 10...
The frequency of this type is oscillator is given by the RC network. Calculations can be found in...