Re: Random thoughts on sinewave oscillators

On 19/10/2024 2:03 pm, Edward Rawde wrote:

"john larkin" <JL@gct.com> wrote in message news:j656hjp1rq659uh61k3q75bipaf386qqh1@4ax.com...

On Fri, 18 Oct 2024 18:58:43 -0400, "Edward Rawde"
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"Jeroen Belleman" <jeroen@nospam.please> wrote in message news:veumn5$3fbqu$1@dont-email.me...

On 10/18/24 23:19, Edward Rawde wrote:

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On Fri, 18 Oct 2024 15:59:09 -0400, Edward Rawde wrote:
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On Fri, 18 Oct 2024 14:20:48 -0400, Edward Rawde wrote:
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On Fri, 18 Oct 2024 13:47:02 -0400, Edward Rawde wrote:
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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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...

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Without doubt, it's the trickiest aspect of the design. Definitely do-able
though. Let us know how you get on.
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Ok. This simple circuit is based on the circuit you can find here.
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https://electronics.stackexchange.com/questions/697687/how-to-control-the-amplitude-of-a-wien-bridge-oscillator
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It produces a something wave.
I wouldn't call it sine but at least it's not clipping.
What's going on here?
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Version 4

[Snip...]
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You're hitting the flat portion of the Id vs. Vds curve around the
top of the wave. In that region the dynamic resistance of the FET
is very large, and therefore the gain of the opamp drops to about
one. As a result, the positive tip of the output gets sort-of
squashed.
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There are probably ways to fix this, for example by feeding a
portion of the output signal to the FET gate, but a quick
attempt I made didn't work very well. This is why FETs aren't so
great as gain setting elements.
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Using a lightbulb --or more generally a PTC resistor-- for R7 is
really hard to beat.

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Ok thanks Jeroen.
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It looks like the best approach for the gain control is either a filament or something like that used in the document Bill Sloman
posted.
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https://www.analog.com/media/en/technical-documentation/application-notes/AN132f.pdf

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Note that the LDR has a very small influence range on the loop gain.
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 That's why I added R3 in this circuit.
It does not seem to be safe to reduce R3 below 1k.
 R4 helps a lot too for reasons I don't fully understand.
It may be moving the FET to a better part of its operating characteristics.
 A single rail version also works with another op amp producing 6V for R4 and two 20k resistors for R2 between 12V and 0V.
As expected, this produces twice the output voltage and I've not found a way to reduce it.
 This will probably be my final offering for a 1KHz sinewave oscillator unless anyone can suggest improvements without using light
dependent resistors.
 From the LTSpice plot, I can't discern any impurity in the signal this circuit produces.
It would be interesting to see what a real circuit and a spectrum analyzer says but I probably won't be building it.
 I haven't used an LDR since playing with an ORP12 around age 10.
I seem to remember that they can degrade over time but maybe that only happens in sunlight.

I got your earlier circuit to work a lot better simply by increasing R7 to 5.6k. If you use the View option on the trace viewing panel to pull out an FFT of the output (I use Blackmann-Harris windowing) from 10sec to 20 sec, you can see that second harmonic distortion is about 20dB below the primary - not great but better than it was.
And the waveform looks like a sine wave.
The less influence the FET has on the gain of the circuit, the better the sine wave.
The AD734 would do a lot better - or at least it does in my simulations - but it isn't cheap.
--
Bill Sloman, Sydney