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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: 80dB now but still needs improvement at 1KHz
Date: Tue, 19 Nov 2024 16:55:11 -0500
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On Tue, 19 Nov 2024 12:16:05 -0800, john larkin <jl@glen--canyon.com>
wrote:

>On Tue, 19 Nov 2024 13:50:46 -0500, Joe Gwinn <joegwinn@comcast.net>
>wrote:
>
>>On Tue, 19 Nov 2024 19:12:18 +0100, Jeroen Belleman
>><jeroen@nospam.please> wrote:
>>
>>>On 11/19/24 17:47, Edward Rawde wrote:
>>>> "Liz Tuddenham" <liz@poppyrecords.invalid.invalid> wrote in message news:1r3a1m9.1lg1mngftnegwN%liz@poppyrecords.invalid.invalid...
>>>[...]
>>>>>
>>>>> The startup transient of the FFT would have to be included, so that may
>>>>> set a limit to the accuracy of the simulation measurements.  Even if you
>>>>> start on a zero-axis crossing, you will get spurious results because of
>>>>> the finite length of the sample.
>>>> 
>>>> Yes I understand that. It's actually a sinewave multiplied by a rectangular function.
>>>> That's why I try to use a sample length of many tens of seconds.
>>>> A window function might help but then I'd have to decide which window to use.
>>>> 
>>>>>
>>>>> This can be demonstrated by doing FFT on samples of fewer and fewer
>>>>> cycles from the middle of an apparently pure sinewave.  As the sample
>>>>> length decreases, spurious harmonics begin to appear, even though they
>>>>> don't exist in reality.
>>>
>>>An FFT implicitly connects the end of the recording back to the
>>>beginning. It's sufficient that the number of periods is integer.
>>
>>If testing with sine waves that all fit with an integral number of
>>cycles per window, the FFT noise floor will be determined by the
>>precision of the arithmetic used. 
>>
>>But there is often a splice error at that join because the fit into
>>the window is approximate, which is a good reason to use a window
>>function.  The default choice in the radar world is Taylor.
>>
>>Joe Gwinn
>
>I wouldn't trust LT Spice for simulating a low-distortion oscillator.
>
>Run a sim and zoom up on the top of some sine wave. It looks all
>segmented and ratty.
>
>I was just simulating something and had a KHz range loop oscillation
>that refused to be compensated. Turns out I had a BI behavioral
>current source driving an inductor, and that oscillated all by itself.
>A G current source does the same thing.
>
>At really low distortion, the opamp models can getcha too.

Oh yeah.  I don't use spice simulations unless I'm designing a
circuit, which is rare these days.

Mostly, I use Wolfram Mathematica, often with Bessel window functions
(for phase linearity across the passband).

Joe Gwinn