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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Random thoughts on sinewave oscillators
Date: Sun, 20 Oct 2024 15:27:59 +1100
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On 20/10/2024 3:36 am, Edward Rawde wrote:
> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vevqip$3q3dn$1@dont-email.me...
>> 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.
> 
> If you run this circuit then View, FFT, Use current zoom extent, Ok
> It implies that unwanted harmonics are 40dB down.
> I'm not sure I believe that but if true then it's not bad for a very low cost circuit.

<snipped .asc file>

The revised .asc file is a bit of a mess. You've added R11 to get the 
output frequency close to 1KHz.

What you should have done is to have used 0.1% 10.5k resistors -it's an 
E96 value and you can buy them off the shelf - at R1 and R2.

That got me to 1.001kHz.

Since the capacitors at C1 and C2 can at best only be +/-1% tolerance 
parts - you can't buy anything better off the shelf - this is quite 
close enough.

You can use a trimming potentiometer to get closer to the target 
frequency, but that does have its downsides.

R4 certainly does make the circuit settle faster - how is a bit of a 
mystery - but you've created a total mess with R5,R7, R8, and R10. It's 
not clear what you were trying to do.

I found that I could get by without R4 provided that I stuck with 
sensible resistance values at R7 and R8 - R7 went up to 6.2k and I put 
330R across J1.

The cheapest plastic +/-1% film capacitors I could buy from Element-14 
in Australia cost $A1.53 each so it isn't a particularly low cost circuit.

-- 
Bill Sloman. Sydney