Electronics Handbook/Devices/Oscillator/Active Oscillator/Variable Frequency Oscillator or VFO/Colpitt Oscillator

Configuration

Oscillation frequency

The ideal frequency of oscillation for the circuits in Figures 1 and 2 are given by the equation:

$f_{0}={1 \over 2\pi {\sqrt {L\cdot \left({C_{1}\cdot C_{2} \over C_{1}+C_{2}}\right)}}}$

where the series combination of C1 and C2 creates the effective capacitance of the LC tank.

Real circuits will oscillate at a slightly lower frequency due to junction capacitances of the transistor and possibly other stray capacitances.