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

Magnus Danielson magnus at rubidium.dyndns.org
Sun Nov 12 10:34:35 EST 2017

On 11/12/2017 03:57 PM, Attila Kinali wrote:
> 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.

Quite respectable. Averaging may help some but...

> 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,
> IFCS and EFTF.
> (e.g. https://www.nist.gov/publications/cross-spectral-collapse-anti-correlated-thermal-noise-power-splitters )

This is a hard problem. Turns out that beyond the anti-correlation noise 
from the splitter, isolation breaks in many interesting ways, creating 
cross-talk which can or can not cause disruptions. My approach to this 
is to steer the effects of the cross-talk away from the measurement. 
This is however a hard balance to maintain in practice, but hopefully 
will inspire new approaches.

>> 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.

Well, that is not completely true. As you average cross-correlations you 
sense a thad more of the actual noise, so there is a small gain there. 
Having too few bits will acts as a discriminator for how low levels 
become visible, even if covered in noise. We've seen people make this 
mistake before. Just because it's hidden in noise does not prohibit it 
from being detected in a correlation process, and cross-correlation is 
one such process.

I saw one such spectrum this summer that had exactly this fault, there 
is a limit to how deep a cheap SDR goes, and the mistake was quickly 
pointed out by someone with the bitter experience.

Noise tend to smooth over quantization steps, and hum, and I think you 
recall my poor excuse of a poster-presentation on that topic, but to 
make benefit it takes lots of samples.

So, anyway, there is some benefits in that extra bits. To make good use 
of them one need to be a bit careful in processing.

> 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 :-)

Just do it. I'm sure I can contribute some to that if it would happen.


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