I've noticed that if I build a Colpitts oscillator (audio range) using a transistor in the common base configuration (similar to the circuit here) I can vary the voltage on the base and get a wide range of frequencies. This is a very simple circuit, so what's wrong with it? All of the VCO schematics I see are substantially more complicated. I can think of several potential reasons for this. Can you identify which are the case and elaborate on why VCO designs are often so complex?
- In an RF design, this might suffer from a small bandwidth - perhaps the frequency can't be modulated enough to be useful
- Perhaps the frequency stability is poor, or the phase noise is bad
- The frequency vs voltage could be highly nonlinear (although it seemed fairly linear to my ear - I do not have fancy instruments to measure these things)
- It could be temperature dependent
- The amplitude could change too much with changing base voltage
- Some other reason?