Voltage Controlled Oscillators

What are voltage controlled oscillators?
The voltage controlled oscillator which is more commonly known, a vco, is an oscillator which works on the principal of variable capacitance or tuning with a varactor diode. The voltage controlled oscillator is tuned across its band by a "clean" dc voltage applied to the varactor diode to vary the net capacitance applied to the tuned circuit.

What are Varactor or Tuning Diodes
These types of diodes work on the principle that all diodes exhibit some capacitance. When the voltage applied across the diodes changes the capacitance developed across it also changes.

Temperature compensated voltage controlled oscillator
A VCO with temperature compensation is achieved using reverse biased diodes. The VCO includes an amplifier that provides the required signal gain, a resonator tank circuit that provides the required phase shift, and at least one frequency tuning circuit for tuning the frequency of the oscillator signal. Each frequency tuning circuit includes at least one tuning capacitor and at least one MOS pass transistor that connects or disconnects the tuning capacitor(s) to/from the resonator tank circuit. Each reverse biased diode may be a parasitic diode that is formed at a drain or source junction of a MOS transistor. The reverse biased diodes have capacitance that can be controlled by a reverse bias voltage to compensate for drift in the VCO oscillation frequency over temperature.