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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: 80dB now but still needs improvement at 1KHz
Date: Tue, 19 Nov 2024 13:21:36 -0500
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"Jeroen Belleman" <jeroen@nospam.please> wrote in message news:vhik6t$1v4ua$1@dont-email.me...
> 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.

It would be nice to detect zero crossings with positive slope as the start and end points but I'm not sure whether there's a way to 
do that in LTSpice.

>
> Jeroen Belleman
>