[time-nuts] Re: Hybrid analog digital dual stage DMTD based on nanoVNA HW. First problems solved

[Erik Kaashoek]

Its a bit embarrassing but almost everything in my previous post was wrong.
After improving the FW the performance improved a lot.
To test the performance, two signals, either 0.01Hz apart or at 
identical frequencies (from the same source) where used
Measurement of the signals with 0.01Hz difference was done with a 
Picotest U6200A and with the nanoVNA based DMTD (further referred to as 
tinyDMTD).
First the raw phase over time was measured to confirm the measured phase 
values where correct [1]
Keep in mind the picotest, in time difference mode, measures between 0 
and 100 ns with 100 ps resolution where the tinyDMTD measures the 
instantaneous phase between -50 ns and +50 ns and about 1 ps resolution
Then the phase was unwrapped and the linear phase residue was calculated [2]
As the variations of the phase measured by the picotest where an order 
higher than what the tinyDMTD measured a second plot shows only the 
tinyDMTD measurements [3]
The leakage in the tinyDMTD between the two channels clearly has an 
impact but the amount is limited to 30 ps and the impact goes down to 
about 10 ps when more samples are used to calculate the phase
The ADEV of these three measurements confirms the resolution of the 
tinyDMTD versus the picotest [4]
The ADEV and the phase measurement of the two identical inputs by the 
tinyDMTD shows there is a fast fluctuation in the order of 3 ps and the 
actual phase noise to be in the order of 1 ps.
Next step is to identify and remove the cause of the fast fluctuation in 
order to see the true noise floor of the tinyDMTD
Erik.

[1] http://athome.kaashoek.com/time-nuts/PNA/tinyDMTD_U6200A_raw_phase.png
[2] 
http://athome.kaashoek.com/time-nuts/PNA/tinyDMTD_U6200A_phase_residue.png
[3] 
http://athome.kaashoek.com/time-nuts/PNA/tinyDMTD_U6200A_phase_residue_DMTD.png
[4] http://athome.kaashoek.com/time-nuts/PNA/tinyDMTD_U6200A_ADEV.png