Re: 1kHz 130dB distortion sinewave oscillator.

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.
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.
>
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.
>
If you believe that a real version of the circuit below could not be adjusted to better than -130dB (with suitable test equipment)
then please let me know.
It might take an hour to complete the adjustments but that's ok with me because as previously stated it's not intended as a
production design which just works after assembly.
>
It takes my computer about 10 minutes (LTSpice 24.1.1) to complete 20 seconds of simulation for the following circuit.
I usually watch the output and the current in each of the four diodes.
>
When it's done, take a sample of the output near 20s and FFT with Blackman-Harris window.
>
Version 4.1
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WIRE 480 -672 400 -672
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WIRE 1872 48 1872 -96
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WIRE 160 112 160 48
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FLAG 960 96 V+
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WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
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SYMATTR Value 8.2k
SYMBOL res 1136 -16 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R16
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SYMBOL OpAmps\LT1058 -80 -304 R0
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SYMBOL OpAmps\LT1058 384 -496 M0
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SYMBOL res 816 -624 R0
SYMATTR InstName R20
SYMATTR Value 120k
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WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
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SYMATTR Value 10n
SYMATTR SpiceLine V=50 Irms=291m Rser=0.34258 Lser=0 mfg="KEMET" pn="C0805F103K5RAC" type="X7R"
SYMBOL OpAmps\LT1115 464 64 R0
SYMATTR InstName U4
SYMBOL OpAmps\LT1115 960 80 R0
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SYMBOL schottky 560 -224 M180
WINDOW 3 24 0 Left 2
WINDOW 0 24 64 Left 2
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SYMATTR Type diode
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WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
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SYMATTR Value 1.5k
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WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
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SYMATTR Value 100n
SYMBOL schottky 1504 -224 M180
WINDOW 3 24 0 Left 2
WINDOW 0 24 64 Left 2
SYMATTR Value BAS40HY
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SYMATTR Type diode
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WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
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WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
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SYMATTR Value 100k
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WINDOW 3 32 32 VTop 2
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SYMATTR Value 100n
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WINDOW 0 24 64 Left 2
SYMATTR Value BAS40HY
SYMATTR InstName D4
SYMATTR Description Diode
SYMATTR Type diode
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WINDOW 0 36 76 Left 2
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SYMATTR InstName R21
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WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
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SYMATTR Value 10000n
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TEXT -496 136 Left 2 !.tran 0 20 1m 1u uic
TEXT -496 88 Left 2 !.options plotwinsize=0 numdgt=7 method=trap
TEXT -424 -648 Left 2 ;1kHz low distortion sinewave oscillator.
TEXT -424 -616 Left 2 ;133dB. Edward Rawde 31 Jan 2025.
>

I you're using polyphase sampling you may as well use a svo.  The
LT115 is not a good choice - it's phase margin is close to zero and it
is liable to sing at about 25MHz.

Version 4
SHEET 1 3020 1316
WIRE 432 -160 64 -160
WIRE 1152 -160 512 -160
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WIRE 432 -48 64 -48
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WIRE 320 32 256 32
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WIRE 176 144 64 144
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WIRE 608 176 576 176
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WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
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WINDOW 3 32 32 VTop 2
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WINDOW 3 32 32 VTop 2
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WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
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SYMATTR Value 10K
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WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
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WINDOW 3 32 56 VTop 2
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SYMATTR Value 470
SYMBOL voltage -224 288 R0
SYMATTR InstName V1
SYMATTR Value 15
SYMBOL voltage -224 400 R0
SYMATTR InstName V2
SYMATTR Value 15
SYMBOL njf 16 992 R0
SYMATTR InstName J1
SYMATTR Value 2N4391
SYMBOL res -160 1040 M90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R8
SYMATTR Value 100k
SYMBOL res 0 1040 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R9
SYMATTR Value 100k
SYMBOL cap -320 832 M90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C3
SYMATTR Value 1u
SYMBOL res 384 1200 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R10
SYMATTR Value 330K
SYMBOL diode 528 1200 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D1
SYMATTR Value 1N914
SYMBOL res 304 960 R0
SYMATTR InstName R11
SYMATTR Value 47K
SYMBOL OpAmps\LT1679 -288 1056 R0
SYMATTR InstName U7
SYMBOL cap -288 912 M90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C4
SYMATTR Value 10U
SYMBOL res -400 912 M90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R12
SYMATTR Value 10K
SYMBOL diode -256 768 M270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D3
SYMATTR Value 1N914
SYMBOL res 880 592 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R18
SYMATTR Value 10K
SYMBOL res 768 432 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R19
SYMATTR Value 10k
SYMBOL diode 336 656 M0
SYMATTR InstName D2
SYMATTR Value 1N914
SYMBOL diode 576 656 M0
SYMATTR InstName D12
SYMATTR Value 1N914
SYMBOL diode 720 656 R0
SYMATTR InstName D13
SYMATTR Value 1N914
SYMBOL diode 1168 656 M0
SYMATTR InstName D14
SYMATTR Value 1N914
SYMBOL res 80 672 M0
SYMATTR InstName R29
SYMATTR Value 95
SYMBOL OpAmps\opamp 208 96 R0
SYMATTR InstName U5
SYMATTR SpiceLine Aol=5Meg
SYMATTR SpiceLine2 GBW=60Meg
SYMBOL OpAmps\opamp 640 96 R0
SYMATTR InstName U6
SYMATTR SpiceLine Aol=5Meg
SYMATTR SpiceLine2 GBW=60Meg
SYMBOL OpAmps\opamp 1040 96 R0
SYMATTR InstName U8
SYMATTR SpiceLine Aol=5Meg
SYMATTR SpiceLine2 GBW=60Meg
SYMBOL res 304 736 R0
SYMATTR InstName R13
SYMATTR Value 68k
SYMBOL res 544 736 R0
SYMATTR InstName R14
SYMATTR Value 68k
SYMBOL res 752 736 M0
SYMATTR InstName R15
SYMATTR Value 68k
SYMBOL res 1136 736 R0
SYMATTR InstName R16
SYMATTR Value 68k
SYMBOL OpAmps\opamp 800 528 R90
SYMATTR InstName U1
SYMATTR SpiceLine Aol=5Meg
SYMATTR SpiceLine2 GBW=60Meg
TEXT -408 40 Left 2 !.ic v(vp3)=5 V(VG)=-6.8
TEXT -408 72 Left 2 !.tran 0 10 0 1u uic
TEXT -408 8 Left 2 !.options plotwinsize=0 numdgt=7
TEXT -408 104 Left 2 !.lib opamp.sub
TEXT -408 136 Left 2 !.ic v(vout)=2 v(vx)=-4