Re: 1kHz 130dB distortion sinewave oscillator.

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