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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 13:50:46 -0500
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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