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

[hugen at outlook.com]

Hi Erik, it looks like a separate circuit could be designed for this device to further improve performance.
I can provide some nanovna for you to test.
Is it feasible to use two centsdr-like switches to obtain IQ signals instead of using a mixer?


hugen at outlook.com
 
From: Erik Kaashoek
Date: 2022-10-05 17:54
To: time-nuts; Bob kb8tq
Subject: Re: Hybrid analog digital dual stage DMTD based on nanoVNA HW. Getting closer to 1e-13 frequency resolution
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.
 
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.
 
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.
 
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
 
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