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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: High purity 1kHz oscillator
Date: Fri, 25 Oct 2024 23:03:32 +1100
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On 25/10/2024 7:45 pm, JM wrote:
> On Fri, 25 Oct 2024 09:25:31 +0100, JM <sunaecoNoSpam@gmail.com> wrote:
> 
>> 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.
> 
> I still had the .raw file available.  I measure the 3rd harmonic at -118dB with a blackman-harris window with a 1s FFT.

Here's the .asc file that I ran

Version 4
SHEET 1 2964 1136
WIRE -896 -176 -960 -176
WIRE -800 -176 -816 -176
WIRE 288 -176 -640 -176
WIRE -960 -160 -960 -176
WIRE -896 -96 -960 -96
WIRE -800 -96 -800 -176
WIRE -800 -96 -832 -96
WIRE -224 -80 -272 -80
WIRE -112 -80 -160 -80
WIRE 176 -80 128 -80
WIRE 288 -80 288 -176
WIRE 288 -80 240 -80
WIRE -800 16 -800 -96
WIRE -800 16 -832 16
WIRE -768 16 -800 16
WIRE -640 16 -640 -176
WIRE -640 16 -688 16
WIRE -960 32 -960 -96
WIRE -896 32 -960 32
WIRE -272 32 -272 -80
WIRE -224 32 -272 32
WIRE -112 32 -112 -80
WIRE -112 32 -144 32
WIRE 128 32 128 -80
WIRE 160 32 128 32
WIRE 288 32 288 -80
WIRE 288 32 240 32
WIRE -800 48 -832 48
WIRE -640 48 -640 16
WIRE -800 96 -800 48
WIRE -640 144 -640 128
WIRE -528 144 -640 144
WIRE -272 144 -272 32
WIRE -272 144 -432 144
WIRE -240 144 -272 144
WIRE -1200 160 -1248 160
WIRE -1136 160 -1200 160
WIRE -960 160 -960 32
WIRE -800 160 -960 160
WIRE -112 160 -112 32
WIRE -112 160 -176 160
WIRE -80 160 -112 160
WIRE 16 160 -16 160
WIRE 128 160 128 32
WIRE 128 160 96 160
WIRE 160 160 128 160
WIRE -1248 176 -1248 160
WIRE -240 176 -272 176
WIRE 288 176 288 32
WIRE 288 176 224 176
WIRE 320 176 288 176
WIRE 464 176 400 176
WIRE -1136 192 -1136 160
WIRE -800 192 -800 160
WIRE 160 192 112 192
WIRE -464 208 -464 192
WIRE -416 208 -464 208
WIRE -272 208 -272 176
WIRE -272 208 -336 208
WIRE 464 208 464 176
WIRE -464 224 -464 208
WIRE -272 224 -272 208
WIRE 112 240 112 192
WIRE -960 256 -960 160
WIRE -800 288 -800 272
WIRE -800 288 -880 288
WIRE -1248 304 -1248 256
WIRE -1136 304 -1136 256
WIRE -1136 304 -1248 304
WIRE -1104 304 -1136 304
WIRE -800 304 -800 288
WIRE 464 304 464 288
WIRE -1248 320 -1248 304
WIRE -1136 320 -1136 304
WIRE -640 352 -640 144
WIRE -608 352 -640 352
WIRE -464 368 -464 304
WIRE -464 368 -544 368
WIRE -608 384 -640 384
WIRE -1248 416 -1248 400
WIRE -1136 416 -1136 384
WIRE -960 432 -960 352
WIRE -800 432 -800 384
WIRE -800 432 -960 432
WIRE -752 432 -800 432
WIRE -640 432 -640 384
WIRE -640 432 -672 432
WIRE -640 480 -640 432
WIRE -624 480 -640 480
WIRE -464 480 -464 368
WIRE -464 480 -480 480
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