Re: 80dB now but still needs improvement at 1KHz

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.