This voltage to frequency converter circuit has an oscillator that is voltage controlled and has a small, 0.5% deviation. IC1 function as a multivibrator and produces rectangular impulses with equal width.
The width of the impulses depend on R4, P1 and C1. With P1 we can do fine adjustments of the output frequency.
The output frequency can be easily adjusted with the help of U1 voltage. D3 diode is required because we want to eliminate R4 and P1 influence. D1 and D2 diodes produce a small flow of temperature. With P2 we adjust the offset voltage. Because of its high quality, this voltage-frequency converter (VCO) can be used in a large field of applications.