[time-nuts] Hybrid analog digital dual stage DMTD based on nanoVNA HW. First actual measurement results

[Erik Kaashoek]

Not sure if there are people on this list interested in experimenting 
with phase and frequency measurement technologies but maybe this 
experiment can make someone smile.
Regular DMTD (dual mixer time difference) use a double mixer to convert 
the two input signals to a low frequency and then uses regular time 
interval measurement technology to measure the phase difference between 
the two signals.
Using a nanoVNA HW with its dual mixer input and a new embedded SW one 
can use the two input ports to input two signals into the dual mixer. 
The output of the mixers is at 5kHz IF and send into a dual 16 bit ADC. 
The 96kHz samples are processed in realtime in an STM32 MCU by doing an 
I/Q down mix to DC for both inputs and the phase of the down mixed 
signal is measured and the phase difference is calculated.
As test signal 10MHz outputs from a Rb and a DOCXO are used, roughly set 
to 0.01Hz difference.
Phase and ADEV over a 100 second period where measurements and compared 
with a similar measurement using a Picotest U6200A that claims to have a 
12 digit per second resolution. [1]
The ADEV measured by the Picotest U6200A is roughly a factor 2 better
Looking at the phase showed there is some leakage causing extra phase 
rotation during a 100 seconds cycle [2]
Further experiments using a RigolDG812 to generate the two test signals 
0.01Hz apart showed it is possible to reduce the impact of the phase 
leakage with at least a factor 3 but this also showed why you should not 
use a DSS based generator for these measurements [3]
The slow fluctuation over a 100Hz period is similar but there are 
additional fast phase fluctuations (not present with the Rb/DOCXO) that 
are caused by the DSS trying to generate both 10MHz and 10.00000001Hz 
using a 1GHz sample rate
Setting the DG812 frequency difference to 0.001Hz and measuring the 
actual output frequency difference clearly showed the up and down jumps 
that cause the fast phase fluctuations [4]
The nanoVNA HW seems to be able to measure phase and frequency 
differences. Performance still needs to be determined.

[1] http://athome.kaashoek.com/time-nuts/PNA/nanoVNA_vs_Picotest_U6200A.png
[2] 
http://athome.kaashoek.com/time-nuts/PNA/nanoVNA_vs_Picotest_U6200A_phase.png
[3] http://athome.kaashoek.com/time-nuts/PNA/DG812_0.01Hz_diff_phase.png
[4] http://athome.kaashoek.com/time-nuts/PNA/DG812_0.001Hz_diff_freq.png