Here is a schematic of a simple FM transmitter. I understand that this work in the following way: the sound picked up by the mic is translated into a voltage at the base of the Q1 transistor. As the internal base-collector capacitance of a BJT depends on the base-collector voltage, the B-C capacitance changes with the sound signal. This internal capacitance forms a resonant circuit with C1, C3 and L1. The resonant frequency of this circuit changes as B-C capacitance changes, and so the signal is FM modulated with the sound signal.
But how do I work out the frequency of the FM signal by reading this schematic? Let's say there is no sound signal. I set VC1 to a known capacitance. I know that C1 is large enough so it offers no impedance for the FM signal and therefore the base is effectively grounded for the FM signal. C3 is even bigger. So therefore we have, in effect, L1 directly parallel with VC1 and B-C capacitance. To get the total capacitance in parallel with L1, I need to add VC1 and B-C capacitance.
But how do I know the capacitance of the B-C junction? Let's say I want to know the base frequency (no sound signal), how much capacitance is there? I don't see C-B capacitance as a function of C-B voltage on datasheets.