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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: Random thoughts on sinewave oscillators
Date: Sat, 19 Oct 2024 09:58:13 +0100
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Cursitor Doom <cd999666@notformail.com> wrote:

emiconductors.
> >>>>>
> >>>>> There are plenty of examples online, but some of them don't seem to
> >>>>> simulate.
> >>>>> Whether that's because they do work in reality but not in simulation
> >>>>> is hard to say.
> >>>>
> >>>> A real-world oscillator needs some kind of stimulus to start up.
> >>>
> >>> I don't seem to be having startup problems, startup is fine but then
> >>> it goes up to clipping or dies to nothing.
> >>> I don't find this surprising and I'm wondering whether a real circuit
> >>> would do the same.
> >>
> >> Yes, it's just what they do without some kind of dynamic gain control.
> >> The overall loop gain needs to be 1.
> > 
> > Yes. That's why I have a control loop which in theory should do that.
> > 
> > Any calculator will show that repeated multiplication of a number
> > slightly greater than 1 increases without limit.
> > In practice an amplifier will limit at or near the supply rail.
> > And if the number it slightly less than 1 it will reduce to 0.
> > 
> > A Wien bridge has an overall voltage gain of 1/3 so the circuit needs to
> > be held at a gain of 3.
> > 
> > As long as it starts up then the gain control loop should be able to
> > hold the gain at whatever is needed for a specific output level.
> > I'm still working on that.
> 
> Without doubt, it's the trickiest aspect of the design. Definitely do-able
> though. Let us know how you get on.
> 
> > 
> >  Fixed resistors might give you that for a
> >> few seconds, but when something warms up, you're either damped or
> >> through the rails.
> >>
> >>
> >>>> This could be a voltage 'shock' at switch-on or just inherent noise
> >>>> in the circuitry. JL informs me LTSpice doesn't have such a stimulus
> >>>> unless you provide it yourself. I suspect that's the main reason you
> >>>> will find oscillators difficult to get started in simulation.
> >>


-- 
~ Liz Tuddenham ~
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www.poppyrecords.co.uk