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Why is this simple RC oscillating?

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ejeffrey:

--- Quote from: T3sl4co1l on December 18, 2021, 01:32:34 pm ---
--- Quote from: Neper on December 18, 2021, 12:52:18 pm ---Maybe it' the virtual equivalent of the old adage that homebrew oscillators, on first power-up, never oscillate but homebrew amplifiers always do.  ;)

--- End quote ---

You don't usually have to screw up an amplifier very much to make an oscillator, though.  How badly do you have to screw up a passive filter to make an oscillator or amplifier? :-DD :-DD

(Obligatory Simpsons reference)

--- End quote ---

In this house we obey the laws of thermodynamics!

NiHaoMike:

--- Quote from: Neper on December 18, 2021, 12:52:18 pm ---Maybe it's the virtual equivalent of the old adage that homebrew oscillators, on first power-up, never oscillate but homebrew amplifiers always do.  ;)

--- End quote ---
I wonder if it's the result of a workaround that disturbs the circuit slightly if it senses things settling down, as a workaround to allow oscillators to start.

mawyatt:
Remember back in undergrad electronics the Prof telling us, "If you want an Amplifier design an Oscillator, if you want an Oscillator design an Amplifier" :o

Best,

mawyatt:
On a more serious note. A steady state sinusoidal electronic oscillator meets the Barkhausen Criteria, however the nature of a simulator is a sampled system which subtly removes a tiny amount of energy from the oscillator's fundamental waveform and displaces it well outside the oscillator loop bandwidth inherent in the sampling process. Thus the very fact of a simulation subtly violates the Barkhausen Criterial for steady state of exactly unity loop gain.

In high "Q" oscillator designs which don't have excessive loop gain this can create an extremely long simulation startup until the oscillator reaches an acceptable steady state (it actually never does reach steady state). A solution we developed a few decades ago was to estimate the oscillator active device steady state dynamic semi-sinusoidal waveform current and replicate this with a PWL bipolar current "doublet" source strapped around the tuned portion of the circuit, usually the inductor. This PWL current doublet source would "ping" the oscillator at startup with a waveform that closely resembled the steady state waveform and quickly achieve a semi-steady state solution, since the doublet had no offset (average value is 0) this had little effect on the bias conditions which usually have a very long time constant. The closer the doublet replicated the final semi-steady state condition the quicker the simulation converged on such. Using this technique proved highly useful with very high "Q" oscillators of various types, and much later Cadence implemented a special feature for quickly achieving steady state circuit operation. If you use this concept start the transient simulation with the biasing conditions preconditioned so the simulation doesn't have to deal with the bias ramp up and the doublet will kick start the oscillator very close to a semi-steady state condition.

Best,

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