“Show phase of harmonics” changes calculated distortion in case of coherent averaging

[tombo56]

There is, what seems to me, an error with coherent averaging.
I first noticed that with certain sampling rate and FFT length ratios, when coherent averaging is turned on, measured signal and harmonics would start to jump down and up by several dB and up to 10 dB or more in some combinations. As a result, measured distortion would with averaging go down considerably.
However, if option “Show phase of harmonics” is checked, then signal and harmonics remain stable with any sampling rate/FFT length combination and only noise floor goes down reveling low level signals.
I wonder which is correct application behavior, suspecting it to be the latter.
Measuring across distortion range from -120 dBc to -80 dBc, produced 4-6 dB worse results with harmonics phase turned on.

Regards,
Boris

 

[John Mulcahy]

with certain sampling rate and FFT length ratios

Such as?

What is the source? Is its frequency stable?

 

[tombo56]

Source is Victor’s oscillator which is very stable and low distortion source. Observed behavior is present with both ADC’s I have, Focusrite Solo 3rd gen. and Cosmos ADC. It is enough to tick “Show phase of harmonics” and distortion numbers will go up by some 6 dB in most cases. Example measurements with Focusrite:

ASIO driver -> 44.1 kHz/64K – carrier jumps 6dB, H2 jumps 12 dB
Java EXCL driver -> 44.1 kHz/64K – carrier jumps 6 dB, H2 jumps 12 dB
Java EXCL driver -> 44.1 kHz/128K – carrier jumps 12 dB, H2 stable

To easily spot that behavior, best is to put averages to 4-8 and zoom in carrier or some harmonic peak. Turning on phase of harmonics immediately makes carrier and harmonics stable.

 

[tombo56]

There is possible interaction between external generator jitter and REW algorithms. If using internal REW generator, there is as well some carrier jumping if option “Lock frequency to RTA FFT” is not being used.
However, using “Show phase of harmonics” immediately makes carrier stable in RTA.

 

[phofman]

This is interesting. I thought the "Show phase of harmonics" just enables displaying the phases on the distortions panel.

 

[John Mulcahy]

I have fixed the behaviour of coherent averaging for tones that are not FFT-locked for the next build.

 

[tombo56]

Thank you very much.
How will that fix influence calculated distortion? I just wonder in which case calculated distortion was correct.

 

[John Mulcahy]

The data when "Show phase..." was selected is correct.