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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Final final 1kHz oscillator
Date: Sun, 17 Nov 2024 22:58:58 -0500
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"JM" <sunaecoNoChoppedPork@gmail.com> wrote in message news:n7iijjdeqecl0kmub0bq5in0dbm60m7qam@4ax.com...
> On Thu, 14 Nov 2024 11:14:28 -0500, "Edward Rawde"
> <invalid@invalid.invalid> wrote:
>
>>"JM" <sunaecoNoChoppedPork@gmail.com> wrote in message news:t5fajjdteskfftvkf84iqsp2vc4b9ta5kj@4ax.com...
>>> 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.
....
>>>>
>>>
>>> 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.)
>>
>>Thanks for that. I left it overnight for 100 seconds of simulation. It looks like it has better harmonic performance than my
>>circuit.
>>Why use two op amps in the oscillator instead of one like my circuit?
>
> Because of common mode (non)linearity.  In most op-amps (especially
> older designs) the distortion generated by the common mode signal is
> typically one to two orders of magnitude larger than the transfer
> distortion.  If it is not clear from the data sheet, it is best to
> measure the CM performance or just assume that it's 40dB worse than
> the specified figure.
>
> Note that it is seldom modeled in a (free) spice model.
>
> This is a link to an extract discussing it's measurement.
>
> https://1drv.ms/u/c/1af24d72a509cd48/ESErQT4Elb5FsgcgsOp1J3wBWZSYHvooHblQJPA3DmydbA?e=wwqtSy

Thank you very much for that information.

>
>>What happens without the initial conditions? Does it take a long time to settle?
>>I'll be away until next week.
>>
>
> No idea.  LTSpice is useless for handling large simulation data files
> so I don't bother trying to use it to check for that sort of thing.