buddypoor
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Hi Eugene ! Thanks for your reply. However, your answer doesnīt meet exactly the subject of my question. Most probably I have missed to express myself clearly.
Quote: For sustained oscillation, I think you also need a nonlinearity in the loop that reduces the gain as the signal amplitude increases.
I am aware of this, and in my inquiry I have tried to exclude this additional requirement from the formulation of the problem. I know that such a nonlinearity always is necesssary ! Ohh no ! There is one exception: the double-integrator loop, I have discussed this specific problem in this forum some weeks ago. The topic (theory of harmonic oscillators) has startet on Jan 15th. I fight for the fact, that this circuit does not need any nonlinearity for producing sinusoidal signals. But thatīs another point. Quote: You might try reading up on describing functions. They do not cover all possible unstable nonlinear systems but they may give you some insight into the sufficient conditions you seek.
No, I donīt think that describing functions can answer the question. The reason: (1) The DF rely on non-linearities and cannot give cause to start oscillation, (2) I think, the DF principle requires a lowpass behaviour of the frequency dependent part of the circuit. There are enough oscillator counter examples without lowpass characteristics (notch and allpass oscillators). Thanks again. Buddypoor
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