Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: 1kHz 130dB distortion sinewave oscillator.
Date: Sat, 1 Feb 2025 23:18:57 +1100
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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.

-- 
Bill Sloman, Sydney

Version 4
SHEET 1 3828 932
WIRE 976 -816 816 -816
WIRE 1056 -816 1056 -848
WIRE 880 -752 880 -768
WIRE 976 -736 976 -816
WIRE 976 -736 912 -736
WIRE 816 -720 816 -816
WIRE 816 -720 720 -720
WIRE 848 -720 816 -720
WIRE 1056 -704 1056 -736
WIRE 1056 -704 912 -704
WIRE 288 -672 256 -672
WIRE 384 -672 352 -672
WIRE 480 -672 464 -672
WIRE 720 -672 720 -720
WIRE 1056 -672 1056 -704
WIRE 1168 -672 1056 -672
WIRE 880 -656 880 -688
WIRE 1056 -656 1056 -672
WIRE 1168 -640 1168 -672
WIRE 1056 -544 1056 -576
WIRE 1168 -544 1168 -576
WIRE -192 -480 -192 -512
WIRE 384 -464 384 -480
WIRE 480 -448 480 -672
WIRE 480 -448 416 -448
WIRE 576 -448 480 -448
WIRE 640 -448 576 -448
WIRE 720 -448 720 -592
WIRE 720 -448 640 -448
WIRE 1056 -448 720 -448
WIRE 1168 -448 1056 -448
WIRE 1296 -448 1168 -448
WIRE 1872 -448 1296 -448
WIRE 256 -432 256 -672
WIRE 352 -432 256 -432
WIRE 448 -416 416 -416
WIRE 464 -416 448 -416
WIRE 576 -400 576 -448
WIRE 1056 -400 1056 -448
WIRE 1168 -400 1168 -448
WIRE 1872 -400 1872 -448
WIRE -416 -384 -416 -480
WIRE -336 -368 -336 -400
WIRE 384 -368 384 -400
WIRE 1296 -368 1296 -448
WIRE 1328 -368 1296 -368
WIRE 1424 -368 1408 -368
WIRE 1520 -368 1504 -368
WIRE 16 -352 -144 -352
WIRE 1056 -304 1056 -320
WIRE 1168 -304 1168 -320
WIRE 1168 -304 1056 -304
WIRE 576 -288 576 -320
WIRE 1168 -288 1168 -304
WIRE 1520 -288 1520 -368
WIRE 1872 -288 1872 -320
WIRE -336 -272 -336 -304
WIRE -80 -272 -80 -304
WIRE -144 -256 -144 -352
WIRE -112 -256 -144 -256
WIRE 16 -240 16 -352
WIRE 16 -240 -48 -240
WIRE 48 -240 16 -240
WIRE 64 -240 48 -240
WIRE 256 -240 256 -432
WIRE -192 -224 -192 -400
WIRE -112 -224 -192 -224
WIRE -416 -208 -416 -304
WIRE 976 -192 880 -192
WIRE 1168 -192 1168 -224
WIRE 1168 -192 1040 -192
WIRE -192 -176 -192 -224
WIRE -192 -176 -256 -176
WIRE -80 -176 -80 -208
WIRE -192 -144 -192 -176
WIRE -256 -128 -256 -176
WIRE 1376 -112 1328 -112
WIRE 1520 -112 1520 -224
WIRE 1520 -112 1456 -112
WIRE 448 -96 384 -96
WIRE 576 -96 576 -224
WIRE 576 -96 512 -96
WIRE 880 -96 880 -192
WIRE 976 -96 880 -96
WIRE 1168 -96 1168 -192
WIRE 1168 -96 1056 -96
WIRE 1728 -96 1680 -96
WIRE 1872 -96 1872 -224
WIRE 1872 -96 1808 -96
WIRE 208 -80 160 -80
WIRE 256 -80 256 -160
WIRE 256 -80 208 -80
WIRE -256 -32 -256 -64
WIRE -192 -32 -192 -64
WIRE 160 -32 160 -80
WIRE 1328 -16 1328 -112
WIRE 1376 -16 1328 -16
WIRE 1520 -16 1520 -112
WIRE 1520 -16 1440 -16
WIRE 384 0 384 -96
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