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

[Magnus Danielson]

Hi Erik,

There is a strong benefit in sampling the waveform over using the 
original TIC approach, and that is that the decorrelation that the later 
can experience is almost completely avoided. The slow variations you see 
is quite typical and synthesizers have a variating degree of phase noise 
to them. They can be just fine for some applications but not very useful 
for others. Using a couple of low noise sources helps to establish the 
measurement setup noise floor. A cheat-setup is to use one source and 
split the signal into both inputs. Difference in cable length will cause 
similar decorrelation, but unless you really get into longer lengths, it 
is not significant to bother about.

I've been found to use cross-correlation setups to measure the noise of 
sources and then using my quietest sources to measure the other ones 
with good speed of convergence and trust in result.

It would be interesting to see just how far one could get the affordable 
nanoVNA setup to run.

Cheers,
Magnus

On 9/29/22 19:38, Erik Kaashoek via time-nuts wrote:
> 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
>
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe send an email to time-nuts-leave at lists.febo.com