[time-nuts] Phase noise measurement experiment by Andrew Holme

Attila Kinali attila at kinali.ch
Sun Nov 12 09:57:15 EST 2017

On Fri, 27 Oct 2017 09:35:08 +0800
"Li Ang" <379998 at qq.com> wrote:

> I just found Andrew recently post a phase noise measruement page on www.aholme.co.uk/PhaseNoise/Main.htm .

Indeed a nice post, as usual for Andrew.

> He uses 4-channel 14bit ADC to do the sampling work. -170dBc noise floor 
> seems not bad for me. 

Which is close to the theoretical limit.

But be aware, that measurements close to the limit of thermal noise
will make your measurement go sour. There the noise of your splitter
will cause an anti-correlation effect and the measured noise will
suddenly drop way below thermal noise. Craig Nelson and Archita Hati
from NIST, Enrico Rubiola from FEMTO, Magnus from time-nuts and several
others have been discussing this for a couple of years now at PTTI,
(e.g. https://www.nist.gov/publications/cross-spectral-collapse-anti-correlated-thermal-noise-power-splitters )

> Since the cross correlation could reduce noise a lot, I am wondering what 
> the differences between 14 bits and 16 bits ADC are.

Beside the price? :-)
There are two things that limit the measurement here. One is the noise
of the ADC itself and the other is the apperture jitter. The former
is almost the same for both 14 and 16bit high speed ADCs. Unless you
go well below 30Msps, you will not gain much in noise performance
from going to 16bit.

The aperture jitter is more or less unrelated to sampling rate and
bit width but faster ADCs usually have lower aperture jitter,
as the designers take more care (because it becomes more important
for "normal" applications). This means, that if you are limited by
aperture jitter (which is the case for close in measurements below
1-100Hz probably), then the additional bits of a 16bit ADC will not
help you at all.

That said, it is probably worth trying what actually happens when
using a 16bit ADC instead of 14bit. If there are any students here
looking for a bachelor or master thesis project doing noise measurement,
feel free to contact me :-)

			Attila Kinali
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

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