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

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.