Re: Final final 1kHz oscillator

On Fri, 8 Nov 2024 15:43:41 -0500, "Edward Rawde"
<invalid@invalid.invalid> wrote:

This is the simplest circuit I can come up with.
Harmonics are more than 80dB down in simulation.
FFT the last 30 seconds.
There is only one harmonic stopping me claiming 90dB but the exact FFT result can depend
on exactly how much output is selected for the FFT. You may see different results.
>
If a prototype is ever built, attention to the type and quality of the capacitors used will be needed.
Electrolytics are not intended. Put C1 and C2 near U1 supply pins.
Also pay attention to the stability of +/- 6V.
It would be nice to have a single 12V rail version but so far I can't get it to work.
>
R1 and R2 and probably R7 should be easily changeable.
R3 should be 220 ohm ten turn. R9 should be 1M ohm ten turn in series with 470k.
>
I don't fully understand why the DC stabilization circuit through U2B achieves what it does
and this makes me concerned about whether a real circuit would have the same behaviour.
It was added when I noticed that the diodes D1 and D2 weren't contributing equally so I
decided to try to derive a DC correction signal. This was originally connected to the FET source
and then to R13. Since this is a DC correction signal I decided to try out the effect of
connecting it to U1 Vocm and was amazed that this works as well as it does.
If you disconnect R5, harmonics will not be more than 50dB down because D1 and D2 are no
longer providing stable and equal contributions to the level control feedback.
>
If you need even better performance then the circuit referenced by Bill Sloman can be used.
https://www.google.com/search?q=AN132+purity
NSL-32SR3 is not expensive and simulation models exist but LT1468 is expensive.
>
Other references
>
https://www.google.com/search?&q=The+field+effect+transistor+as+a+voltage+controlled+resistor
>
https://www.google.com/search?&q=sine+wave+oscillator&udm=2
>
>
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TEXT -624 272 Left 2 !.tran 0 300s 0s startup
TEXT -1568 -728 Left 2 ;Edward Rawde's high purity 1kHz sinewave oscillator. 8 Nov 2024\n \nHarmonics are more than 80dB down in
simulation.\nFFT the last 30 seconds.\nThere is only one harmonic stopping me claiming 90dB but the exact FFT result can depend\non
exactly how much output is selected for the FFT. You may see different results.\n \nIf a prototype is ever built, pay attention to
the type and quality of the capacitors used.\nElectrolytics are not intended. Put C1 and C2 near U1 supply pins.\nAlso pay attention
to the stability of +/- 6V.\nIt would be nice to have a single 12V rail version but so far I can't get it to work.\n \nR1 and R2 and
probably R7 should be easily changeable.\nR3 should be 220 ohm ten turn. R9 should be 1M ohm ten turn in series with 470k.\n \nI
don't fully understand why the DC stabilization circuit through U2B achieves what it does\nand this makes me concerned about whether
a real circuit would have the same behaviour.\nIt was added when I noticed that the diodes D1 and D2 weren't contributing equally so
I \ndecided to try to derive a DC correction signal. This was originally connected to the FET source\nand then to R13. Since this is
a DC correction signal I decided to try out the effect of\nconnecting it to U1 Vocm and was amazed that this works as well as it
does. \nIf you disconnect R5, harmonics will not be more than 50dB down because D1 and D2 are no\nlonger providing stable and equal
contributions to the level control feedback.\n \nIf you need even better performance then the circuit referenced by Bill Sloman can
be used.\nhttps://www.google.com/search?q=AN132+purity\nNSL-32SR3 is not expensive and simulation models exist but LT1468 is
expensive.\n \nOther references\n \nhttps://www.google.com/search?&q=The+field+effect+transistor+as+a+voltage+controlled+resistor\n
\nhttps://www.google.com/search?&q=sine+wave+oscillator&udm=2
>

Using an NSL-32SR3 doesn't really gain you anything over a FET.  If
you keep |vds| much less than |Vt-vgs| in the circuit and compensate
for the vds/2 variation on the gate drive the distortion levels
produced will be lower than almost all opamps out there.  Here's a
link to a sim using an LDR.

https://1drv.ms/f/c/1af24d72a509cd48/EkjNCaVyTfIggBrrAQAAAAABHXNKs52BoVHi7zaxKmdMMQ?e=oVD0JS

I've added some 3rd harmonic distortion to a linear model to give
distortion figures roughly in the ballpark of Fig 6 in the attached
application note (for small signal swings across the device).  In the
oscillator o/p the 3rd is about 150dB down.  If you use the linear LDR
model, or a FET, it will be much the same. (The opamps used in the sim
actually have distortions specs of about 130dB in reallity.)