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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Random thoughts on sinewave oscillators
Date: Sat, 19 Oct 2024 19:27:52 +1100
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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"
>> <invalid@invalid.invalid> wrote:
>>
>>> "Jeroen Belleman" <jeroen@nospam.please> wrote in message news:veumn5$3fbqu$1@dont-email.me...
>>>> On 10/18/24 23:19, Edward Rawde wrote:
>>>>> "Cursitor Doom" <cd999666@notformail.com> wrote in message news:veuirv$3cmo3$10@dont-email.me...
>>>>>> On Fri, 18 Oct 2024 15:59:09 -0400, Edward Rawde wrote:
>>>>>>
>>>>>>> "Cursitor Doom" <cd999666@notformail.com> wrote in message
>>>>>>> news:veucs2$3cmo3$9@dont-email.me...
>>>>>>>> On Fri, 18 Oct 2024 14:20:48 -0400, Edward Rawde wrote:
>>>>>>>>
>>>>>>>>> "Cursitor Doom" <cd999666@notformail.com> wrote in message
>>>>>>>>> news:veu7kt$3cmo3$8@dont-email.me...
>>>>>>>>>> On Fri, 18 Oct 2024 13:47:02 -0400, Edward Rawde wrote:
>>>>>>>>>>
>>>>>>>>>>> "Cursitor Doom" <cd999666@notformail.com> wrote in message
>>>>>>>>>>> news:veu45s$3cmo3$5@dont-email.me...
>>>>>>>>>>>> On Fri, 18 Oct 2024 11:25:19 -0400, Edward Rawde wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> "piglet" <erichpwagner@hotmail.com> wrote in message
>>>>>>>>>>>>> news:vetde5$38sbk$1@dont-email.me...
>>>>>>>>>>>>>> Edward Rawde <invalid@invalid.invalid> wrote:
>>>>>>>>>>>>>>>
>>>>> ...
>>>>>>
>>>>>> Without doubt, it's the trickiest aspect of the design. Definitely do-able
>>>>>> though. Let us know how you get on.
>>>>>>
>>>>>
>>>>> Ok. This simple circuit is based on the circuit you can find here.
>>>>>
>>>>> https://electronics.stackexchange.com/questions/697687/how-to-control-the-amplitude-of-a-wien-bridge-oscillator
>>>>>
>>>>> It produces a something wave.
>>>>> I wouldn't call it sine but at least it's not clipping.
>>>>> What's going on here?
>>>>>
>>>>> Version 4
>>>> [Snip...]
>>>>
>>>> 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.
>>>>
>>>> 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.
>>>>
>>>> Using a lightbulb --or more generally a PTC resistor-- for R7 is
>>>> really hard to beat.
>>>
>>> Ok thanks Jeroen.
>>>
>>> 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.
>>>
>>> https://www.analog.com/media/en/technical-documentation/application-notes/AN132f.pdf
>>
>> Note that the LDR has a very small influence range on the loop gain.
>>
> 
> 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