Can someone explain to me / point me to a good explanation of the theory behind an electronic oscillator? The Wikipedia entry explains what, but not how, which is what I need to know.
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Oscillators are just amplifiers with feedback greater than or equal to one, at the frequency of oscillation, so you can generally analyze a given oscillator circuit as an amplifier with a feedback network.
There may be a few components for biasing active element(s), or compensating for the effects of changes in temperature on the circuit elements, but at the heart of it, some active device amplifies, while a certain other components form a feedback path.
The amplifier aspect of an oscillator circuit will generally be capable of over-unity amplification across a relatively broad band of frequencies, and it's mainly the feedback network that will control the frequency of oscillation within that band, so you will almost always find reactive elements in the feedback network, almost always involving capacitors, and sometimes inductors.
Even the classic two-transistor, two-resistor, two-capacitor multivibrator can be viewed as an amplifier with feedback; you have two stages, each of which has high gain and produces 180 degree phase shift, but with the two stages together, the resulting phase shift is zero.
Some people find the description at "Circuit Idea/How do We Create Sinusoidal Oscillations?" useful.
Others like Circuit Idea/ring oscillators.
Once you have figured out what is missing from these explanations, please add a few words about that missing piece -- it's a wiki.
The oscillator is an amplifier with filter and loop back connection. In theory with no noise it will never work. In practice there is always some thermal/mechanical/electromagnetic noise. So noise is amplified, filtered and fed back.
For this reason some modeling software fails to model oscillation. For the same reason oscillators with very high quality of signal (narrow band) have difficulty to startup (slow startup time).