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From: JM <sunaecoNoSpam@gmail.com>
Newsgroups: sci.electronics.design
Subject: Re: High purity 1kHz oscillator
Date: Fri, 25 Oct 2024 09:25:31 +0100
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On Fri, 25 Oct 2024 18:16:26 +1100, Bill Sloman <bill.sloman@ieee.org> wrote:

>On 25/10/2024 3:25 pm, Bill Sloman wrote:
>> On 25/10/2024 4:48 am, JM wrote:
>>> On Thu, 24 Oct 2024 16:16:49 +0100, JM <sunaecoNoSpam@gmail.com> wrote:
>>>
>>>> On Wed, 23 Oct 2024 02:05:52 +0100, JM <sunaecoNoSpam@gmail.com> wrote:
>>>>
>>>>> On Tue, 22 Oct 2024 01:10:41 -0400, "Edward Rawde" 
>>>>> <invalid@invalid.invalid> wrote:
>>>>>
>>>>>> But I suspect that component tolerances and mismatched FETs will 
>>>>>> ruin it.
>>>>>>
>>>>>> Otherwise it should be easy to get 60dB down on unwanted harmonics 
>>>>>> with a better filter.
>>>>>>
>>>>>> FWIW I likely won't be here for the next week.
>>>>>>
>>>>>
>>>>> If you actually need a low distortion oscillator look for Viktor 
>>>>> Mickevic's designs on diyaudio.   I've attached a schematic.
>>>>>
>>>>> PK
>>>> 
>>>>
>>>> It seems that I have a problem posting encoded binariesto this group 
>>>> with my provider.  Attached is a link instead.
>>>>
>>>> https://1drv.ms/u/c/1af24d72a509cd48/EakMPPRi-pdLgaAKtJ2rrwwBNMGZZsy84MV2QoH1dPcZJQ?e=o59V9a
>>>
>>> And that links to an incorrect schematic - the rectified output tap is 
>>> connected to the wrong node.  Might be other errors..
>> 
>> Even so, the third harmonic is 78dB below the fundamental. The 
>> simulation runs slowly on my computer, so it may take me a while to get 
>> the schematic to where the designer intended it to be.
>> 
>> The choice of op amp is what I'd expect from an audio group - it does 
>> seem to be a low distortion part.
>> 
>> I just seem to have wrecked the .raw file from the simulation, so it may 
>> be a while before I have more to say.
>
>The file wasn't wrecked, just huge (4.063 GB) and slow to load, despite 
>the fact that I've a solid state disk on my computer put in to hold 
>LTSpice .raw files and load them tolerably quickly.
>
>The circuit uses a half-wave rectifier, then runs the error signal into 
>an integrator wrapped around U4. C4 -at 6.8u - is a biggish integrating 
>capacitor. R7 - at 120k - has the main purpose of stabilising the 
>feeback loop controlling the amplitude and also delivers around 6mV of 
>1kHz sinusoidal ripple into the gate of the FET. There about 12mV of 
>1kHz sine wave across the FET channel so this minimises any channel 
>modulation. Ingenious.
>
>It would have been even move impressive if he'd intended to do that from 
>the start, but R5 and R6 make look like it was an afterthought.
>
>My feeling is that an AD734 could do better. Jim Williams did better 
>with his FET controlled version, but I've no idea how.
>
>78dB below the fundamental is respectable, but not impressive.

On the bench the THD in the audio band is approx. -140dB (I think - it's years since I looked into the performance of this).  Not measured by myself, but there are hundreds of measurements documented on diyaudio and other audio forums.  Viktor used to (possibly still does) sell these oscillators on ebay for a few pounds.  I have a couple and borrowed an audio precision analyser to test them - I think the AP measured to about -115dB, and the oscillators performed better than that.  That level of distortion was much better than I required so I didn't attempt the find the true value.

I doubt if LTSpice will give accurate distortion figures with the simulation models I provided, they have not been verified in isolation.  Very few opamp macro models provide realistic distortion results.  

There may be a more recent schematic available as this circuit has been tweaked over the years.

I think when I simulated this only the 2nd or 3rd harmonic was visable in a 1 second FFT at about -120dB.  However, I didn't spend any time on it, just enough to see that it did actually oscillate, and posted it only because it's real performance is very well documented, and it may be of use to the OP.  I could spend some time validating the models and simulating in spectre but real measurments trump simulations.