Re: Very simple 1kHz sinewave circuit is more than 133dB down on harmonics

On Wed, 05 Feb 2025 14:06:47 -0800, john larkin <jl@glen--canyon.com>
wrote:

On Wed, 5 Feb 2025 19:08:25 +0000, John R Walliker
<jrwalliker@gmail.com> wrote:
>

On 05/02/2025 17:51, Edward Rawde wrote:

<albert@spenarnc.xs4all.nl> wrote in message news:nnd$01c0b887$041e6dfb@960b25520c3d6ac2...

In article <vnk0ul$2b1o$1@nnrp.usenet.blueworldhosting.com>,
Edward Rawde <invalid@invalid.invalid> wrote:

Elaborate DC stabilization isn't needed to get below 130dB.

>
Is this a measurement or a simulation?

 
Simulation only at present.
There has been debate about what meaningful measurments may or may not be possible below about -90dB.
In a simulation I can get down to -135dB but I make no claim to be able to achieve or measure that in practice.

>
I needed a very low distortion audio oscillator in 1997 in order to
characterise a 16-bit codec for an application report.
>
The simplest approach was to use a 20-bit sigma-delta DAC with
a combination of an active low-pass filter followed by a passive
single-pole output filter.
The output frequency was 1.1kHz.  The third harmonic, measured by
the device under test was about -82dB below the fundamental.  All
other harmonics were considerably lower.
The signal source may well have been better than that, but I had
no easy way of measuring it at the time.
>
The results are in figure 4-2 (page 37) of:
https://www.ti.com/lit/ug/slau039/slau039.pdf
>
John
>

>
TLC2272 is a 4.5 MHz opamp. So at 3 KHz, it only has a gain of 1500.
That might dominate distortion.
>
AD5791 is a 20-bit sub-1-lsb accurate fast voltage out DAC, which
should make a -120 db distortion synthesizer.  If there was any way to
measure sub-PPM distortion, one could sum it with, say, a fine-trim 16
bit DAC and tune the pair for really low distortion.
>
One might subtract two such synthesizers to measure their distortion.
>
Or three, sort of like the idea of rubbing three pieces of glass
against one another to make them flat.

In the Time World, this is called the three-cornered hat algorithm:

.<https://rubiola.org/pdf-articles/conference/2016-IFCS-Three-cornered.pdf>

This is actually used for more than three references, but the name is
still of three.

Joe Gwinn