Control of frequency in VCOs
A voltage-controlled capacitor is one method of making an LC oscillator vary its frequency in response to a control voltage.
Any reverse-biased semiconductor diode displays a measure of voltage-dependent capacitance and can be used to change the frequency of an oscillator by varying a control voltage applied to the diode.
Special-purpose variable capacitance varactor diodes are available with well-characterized wide-ranging values of capacitance.
Such devices are very convenient in the manufacture of voltage-controlled oscillators.
A voltage-controlled inductor would be in principle as useful, but such devices are unsatisfactory at the frequencies usually desired.
The frequency of a ring oscillator is controlled by varying either the supply voltage, the current available to each inverter stage, or the capacitive loading on each stage.