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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: 1kHz 130dB distortion sinewave oscillator.
Date: Mon, 3 Feb 2025 03:38:25 +1100
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On 2/02/2025 8:23 pm, Cursitor Doom wrote:
> On Sun, 02 Feb 2025 01:02:22 +0000, JM
> <sunaecoNoChoppedPork@gmail.com> wrote:
> 
>> On Sat, 1 Feb 2025 12:26:28 -0500, "Edward Rawde"
>> <invalid@invalid.invalid> wrote:
>>
>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vnl3gl$30im$1@dont-email.me...
>>>> On 1/02/2025 4:37 pm, Edward Rawde wrote:
>>>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vnka4g$3ut8i$1@dont-email.me...
>>>>>> On 1/02/2025 1:34 pm, Edward Rawde wrote:
>>>>>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vnim5i$3hnrj$1@dont-email.me...
>>>>>>>> On 31/01/2025 5:47 am, Edward Rawde wrote:
>>>>>>>>> Approaching 130dB now. Any suggestions for improvement?
>>>>>>>>> It wouldn't be hard to add another four rectifier phases but then I'd have more components than Bill.
>>>>>>>> But at least I know what mine are doing.
>>>>>>>>
>>>>>>>>> This circuit was simulated in LTSpice 24.1.1 with all component updates as of 30th January 2025.
>>>>>>>>>
>>>>>>>>> Take a sample near 20s and FFT on current zoom extent with Blackman-Harris window.
>>>>>>>>> It's approaching 130dB at 2kHz and approaching 140dB everywhere else.
>>>>>>>>>
>>>>>>>>> I do not know why C13/R28 and similar are needed but without them the simulation speed goes down to us/s
>>>>>>>>> It appears that LT1115 doesn't like being simulated with very little load on its output.
>>>>>>>>> Whether or not that's true in reality I've no idea and it may not be the case in earlier versions of LTSpice.
>>>>>>>>>
>>>>>>>>> Simulation speed in 24.1.1 appears to be about twice as fast as earlier versions for this circuit.
>>>>>>>>
>>>>>>>> This is a strange circuit.
>>>>>>>
>>>>>>> It's also way more complex than it need be. Please see my most recent post.
>>>>>>> I think you'll find the FET is operating conventionally. Roughly at the centre of its range when I last checked.
>>>>>>>
>>>>>>>> One problem with using an N-channel junction fet like J113 is that if you bias the gate at a higher voltage than the source or
>>>>>>>> drain, it acts as a diode and feeds current into the channel.
>>>>>>>>
>>>>>>>> I've downloaded and run the second version of the simulation, and the gate doesn't seem to end up more negative than the
>>>>>>>> source
>>>>>>>> or
>>>>>>>> drain.
>>>>>>>>
>>>>>>>> It takes quite a lot of simulation time before the circuit starts acting as if were an amplitude controlled oscillator, and
>>>>>>>> I'm
>>>>>>>> not interested enough to try and work how it is actually working.
>>>>>>
>>>>>> I've modified the circuit to make sure that FET gate doesn't get forward-biased.
>>>>>
>>>>> Why does it matter if the FET gate is forward biased during the first 10ms Bill?
>>>>> That isn't going to make the FET explode is it?
>>>>>
>>>>>>
>>>>>> As John May pointed out a long time ago, the J113 isn't a great choice for the application.
>>>>>
>>>>> Did he say why?
>>>>>
>>>>>    I've swapped in a J111. The integrator around U6
>>>>>> isn't well designed, and I've deleted a redundant resistor and added a huge damping resistor (R4). It means that there's a lot
>>>>>> of
>>>>>> 1kHz ripple on the gate voltage, and loads of harmonic content on the output - 2kHz is only 25dB below the fundamental.
>>>>>
>>>>> Sorry bill but I don't get why you would sabotage the circuit like that.
>>>>
>>>> You aren't a quick study.
>>>>
>>>>>> Getting a low ripple rectified output to feed into the integrator isn't a trivial task. It's part of a negative feedback control
>>>>>> problem which you seem to be reluctant to recognise.
>>>>>
>>>>> Please see the most recent circuit I posted.
>>>>> It doesn't bother with DC stabilization circuits but it does do 133dB down on harmonics with pretty much nothing at 2kHz at all.
>>>>
>>>> Perhaps if you measure the distortion only when the stabilisation circuit isn't feeding anything into the integrator wrapped
>>>> around U6.
>>>>
>>>> The current feed to set the output level by balancing out  the inputs from the rectifiers was feeding in current when it should
>>>> have been sucking it out, so it wasn't a well thought-out design.
>>>>
>>>> Here's a version of that circuit  which does work sensibly, even if the harmonic content of the output is only about 60DB below
>>>> the fundamental.
>>>
>>> But that's completely nuts Bill.
> 
> And so is Bill! Sadly, Bill stopped making useful contributions to
> this group many years ago and now spends his days attempting to appear
> superior to everyone else.
> 
>>> If I really do want 1kHz with -133dB harmonic distortion in reality then I'm prepared to wait 20 seconds for it.
>>> In fact I'm prepared to wait a minute or two if that's what it takes for the harmonic distortion to go down to -133dB.
> 
> Of course. Nothing unreasonable about that.
> 
> 
>>> If there's a problem with the design around U6 then why doesn't that also apply to U7 and U9?
>>>
>>> It's not intended as a volume production design. I might make two or three boards and test it.
>>> But I don't have equipment capable of measuring -133dB harmonic distortion.
>>>
>>> I would also be prepared to make adjustments to the circuit for minimum distortion.
>>> This circuit might have up to six adjustments if built for real so it can be adjusted for correct output level and minimum
>>> distortion.
>>>
>>> The circuit included below might be my final offering on this matter.
>>> There's nothing visible at 2kHz and 3kHz is barely visible.
>>> It requires only a cheap quad op amp package in addition to the two LT1115 devices.
>>>
>>> If you can show me a circuit which has -133dB harmonic distortion and also faster settling time then I'd like to see it but if
>>> you're going to insist that it has to have only -60 dB harmonic distortion performance (which interestingly is about that of your
>>> own circuit) just because the settling time can be made shorter then I'm sorry but all that's going to do is make me wonder about
>>> your mental health.

I didn't insist on anything. I debugged Edward's simulation to the point 
where it started up properly and settled down to a stable state fairly 
quickly, and reported that the changes that I'd made had upped the 
harmonic content. After I'd found another bug in  his design I finally 
got the harmonic content closer to about 75dB below the fundamental.

My current mirror variation is stuck at a -60dB. Posting it here didn't 
get me any helpful suggestions about making it better.

> Many of us here have been wondering about Bill's mental health for the
> past 2 decades. Please try to remember you're dealing with SED's
> biggest troll here (and he hasn't been fed well lately).

Cursitor Doom - who actually is an anonymous troll - does like to claim 
that people who object to his habit of posting fatuous nonsense aren't 
to be taken seriously. One can understand why - it's just one more 
tranche of fatuous nonsense, and he gets some kind of bizarre 
satisfaction from posting all sorts of different kinds of utter nonsense.

I really shouldn't go to the trouble of objecting one more example of 
his malicious fantasising, but I do find it irritating when he starts 
sticking his oar into serious discussions.

-- 
Bill Sloman, Sydney