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

John Miles john at miles.io
Sun Nov 12 18:30:04 EST 2017

> 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 )

Cross-spectral collapse due to thermal noise is basically Mother Nature complaining that we're trying to do something that's physically ill-advised, by trying to measure the noise in a system at levels near the thermal floor of the termination resistance that's necessary to define such a system in the first place.  Unfortunately the issue can affect measurements at levels several dB higher than the thermal floor, making it a genuine occupational hazard.

On the bright side, splitter topologies that use a single resistive load with no differential-mode termination have little or no vulnerability to this phenomenon.  Of course the common-mode termination imposes its own noise floor, but that's life in this particular universe.   

That being said, it's still a bit of an awkward situation for designers of future test sets.  We can't use Wilkinson splitters anymore -- or at least we shouldn't, given what's been learned over the past couple of years.  As a result, some customers' best devices may suddenly start looking noisier than they did when measured on earlier instruments, or (worse) when measured on competing ones.  Those customers may not respond agreeably when advised to dunk the termination in liquid helium...

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

None to speak of.  The original TSC 5120A used 14-bit parts while the later test sets have used 16-bit parts, and the performance limits are essentially identical.  Once you go below a certain point, generally well below -170 dBc/Hz, you tend to run into a correlation floor that arises from one or more different causes unrelated to the quantization precision itself.  These effects seem to arise in and near the S/H stage and aren't improved by going beyond 14-bit precision. 

-- john, KE5FX
Miles Design LLC

More information about the time-nuts mailing list