[time-nuts] Re: Hybrid analog digital dual stage DMTD based on nanoVNA HW. Getting closer to 1e-13 frequency resolution

[Bob kb8tq]

Hi

I’d second everything Tom posted in his last email. 

Name wise, I’d come up with something unique for the product. I would not
worry a lot about a name for the process it uses. 

(see below)

> On Oct 5, 2022, at 5:54 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> After some more work it became possible to measure the coherent noise floor in a similar way as was described in "A Small Dual Mixer Time Difference (DMTD) Clock Measuring System" (Small DMTD) by W.J. Rely
> 
> Both inputs where connected to separate outputs of a two channel DDS. This allowed checking the frequency and phase measurement by testing with various frequency difference and phase offsets. The frequency and phase measured  for all frequency and phase variations as expected and it was fairly simple to measure the minimum phase step of the DDS (0.006 degrees)
> For the coherent noise floor tests the frequency difference was set to zero.
> 
> Coherent phase data [1]
> Over a roughly 45 minutes period the relative phase slowly varied over a +2ps/-5ps range, comparable to the Small DMTD.
> The measurement is susceptible to temperature variations as the prototype does not yet have an enclosure. Draft from opening doors had an observable impact, most easily spotted in the phase data.

Typically test gear gets spec’d in a very stable environment. The real world 
is never stable, but the still do it that way. Even if you still have a coupe of 
ps due to a few degrees C variation, it’s not a big deal. Your 45 minute period
is pretty representative of a typical HVAC system. Lots of stuff shows up at 
that same sort of period. 

> 
> Frequency data [2]
> The frequency appeared to be stable, no linear trend was observed. At first sight the frequency noise appeared to be less then the Small DMTD but the Small DMTD did measure the frequency at a lower tau of 0.1 second.

While short tau is a “cool feature”, I have never seen a commercial ( or DIY ) 
use for it. The world seems to like ADEV ( and the other DEV’s ) for long tau
stuff. We all go to phase noise for the “faster” side of things. If it costs anything
to put in < 0.1 seconds, I’m not sure I’d vote for doing that. 

> 
> Frequency stability [3]
> Although no long term drift was present the slope of the stability plot suggests there is still some hidden issue. The frequency stability of the Small DMTD is clearly better.

Welcome to the club.
There always will be a next layer of issues. That’s been true on every
similar device I’ve ever built. Note: I’ve done a lot of single mixer setups ….

> 
> Frequency histogram [4]
> The histogram also hints to some none Gaussian behavior of the noise. The total width of the histogram seems comparable to the Small DMTD
> 
> Auto correlation of the frequency [5]
> The frequency auto correlation clearly shows there is some repetitive signal hidden in the frequency data.
> The frequency of the signal, about 1/120 Hz, suggests this could be due to the 0.01 Hz resolution of the digital PLL that locks the internal reference to one of the inputs. As the PLL is frozen during measurements the frequency difference between input and internal reference could easily be 1/120 Hz

As many have noted, I have a “thing” about anything other than a pure
sine signal as the mix down source. Yes it’s a bias. Between spurs and
steps things like digital PLL’s and DDS’s ( I believe ) can create issues. 
I don’t have an closed form proof, just observations. There are other points
of view and they could be right. 

If you listen to Sam Stein and John Miles in their public posts, they both
hint very strongly that the 32 bit math in their down conversion processing
is a really big deal. It suggests that both ran into issues there and came
up with a “better way”. Not exactly a DDS, but very close. 

Any errors in the above will be blamed on the coffee pot being a bit slow
to warm up this morning ….

Bob

> 
> Erik.
> 
> [1] http://athome.kaashoek.com/time-nuts/DMTD/phase.PNG
> [2] http://athome.kaashoek.com/time-nuts/DMTD/freq.PNG
> [3] http://athome.kaashoek.com/time-nuts/DMTD/stability.PNG
> [4] http://athome.kaashoek.com/time-nuts/DMTD/histo.PNG
> [5] http://athome.kaashoek.com/time-nuts/DMTD/autoc.PNG
> 
>