Voltage controlled oscillator

The voltage controlled oscillator or VCO, is an oscillator of which the frequency of its output signal is a function of an input voltage.

Sawtooth core VCOs work by converting the control voltage to a current and using this to charge a small capacitor. A comparator compares the voltage across this capacitor with some fixed voltage (E.G. 10 V). When that threshold is reached, a FET shorts the capacitor, bringing the voltage across back to 0 and starting a new cycle.

A VCO can go out of tune because:
 * The control voltage drifts with temperature. This is easily taken care of by using op amps with low offset and low temperature coefficient.
 * The exponential voltage to current converter drifts with temperature. This is by far the worst offender. The fix is usually to use a tempco (a positive temperature coefficient thermistor), with a temperature coefficient of about +3350 PPM/K. Or doing the exponential conversion with an SSM2164/V2164 (David Dixon's speciality).
 * The value of the capacitor drifts with temperature. For this (and for other reasons) use a polystyrene or NP0/C0G ceramic cap.
 * The threshold voltage drifts with temperature or power supply load. This is avoided by using a proper voltage reference instead of a resistive divider.