Re: Random thoughts on sinewave oscillators

On Fri, 18 Oct 2024 15:59:09 -0400, Edward Rawde wrote:

"Cursitor Doom" <cd999666@notformail.com> wrote in message
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On Fri, 18 Oct 2024 14:20:48 -0400, Edward Rawde wrote:
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"Cursitor Doom" <cd999666@notformail.com> wrote in message
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On Fri, 18 Oct 2024 13:47:02 -0400, Edward Rawde wrote:
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"Cursitor Doom" <cd999666@notformail.com> wrote in message
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On Fri, 18 Oct 2024 11:25:19 -0400, Edward Rawde wrote:
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"piglet" <erichpwagner@hotmail.com> wrote in message
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Edward Rawde <invalid@invalid.invalid> wrote:

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The circuit below produces a reasonable looking sinewave but the
rise time still seems to be slower than the fall time. It may be
that the amplifier in use is not ideal for this.
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Could that just be second harmonic distortion? You could test the
amplifier by uncoupling the Wien network and injecting test
inputs.
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Elsewhere I think your amplitude control problems could be simply
due to too much gain.

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Perhaps, but I've not so far been able to get the circuit I posted
in response to Bill to produce a sine wave no matter what I do
with the control loop gain.
It either grows to clipping or dies.

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That's the main purpose behind having a thermistor or filament bulb
in the f/b path.

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Sure, but why use thermistors or filaments if you don't have to?
Filaments don't last forever, particularly not if you drop your
equipment, and filaments make me think of something like a 5U4.
I forget when I last saw a filament. House lamps don't even have
them now.
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I'm trying to make a low cost oscillator which produces the cleanest
1K Hz sinewave I can get, using only resistors, capacitors and
semiconductors.
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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.

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A real-world oscillator needs some kind of stimulus to start up.

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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.

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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.
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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.

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