[time-nuts] Phase measurement of my GPSDO

[John Miles]

> b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution does
> my counter need? If the above was true, I would expect that a 1ps
> resolution (and an even better stability!) was required to measure ADEV of
> 1e-12, The fact that the (as far as I know) world's most recent,
> rocket-science grade counter (some Keysight stuff) has "only" 20ps of
> resolution, but people are still able to measure even 1e-14 shows that my
> assumption is wrong. So how are the measurement resolution and the ADEV
> related to each other? I plan to build my own TIC based on a TDC7200, which
> would offer some 55ps of resolution, but how low could I go with that?

That sounds like a simple question but it's not.  There are a few different approaches to look into:

1) Use averaging with your existing counter.  Some counters can yield readings in the 1E-12 region at t=1s even though their single-shot jitter is much worse than that.  They do this by averaging  hundreds or thousands of samples for each reading they report.  Whether (and when) this is acceptable is a complex topic in itself, too much so to explain quickly.  Search for information on the effects of averaging and dead time on Allan deviation to find the entrance to this fork of the rabbit hole.

2) Search for the term 'DMTD' and read about that.

3) Search for 'direct digital phase measurement' and read about that.

4) Search for 'tight PLL' and read about that.

Basically, while some counters can perform averaging on a post-detection basis, that's like using the tone control on a radio to reduce static and QRM.  It works, sort of, but it's too late in the signal chain at that point to do the job right.  You really want to limit the bandwidth before the signal is captured, but since that's almost never practical at RF, the next best thing to do is limit the bandwidth before the signal is "demodulated" (i.e., counted.)   

Hence items 2, 3, and 4 above.  They either limit the measurement bandwidth prior to detection, lower the frequency itself to keep the counter's inherent jitter from dominating the measurement, or both.  You'll have to use one of these methods, or another technique along the same lines, if you want to measure the short-term stability of a good oscillator or GPSDO.

-- john, KE5FX