[time-nuts] Re: What phase variations to expect in a DMTD due to temperature fluctuations?

[Erik Kaashoek]

Hi Carsten

Thanks for the very interesting article. As my DMTD uses an audio ADC, 
the 10MHz inputs first have to be down mixed to the selected audio IF. 
This probably will add to long term instability.
Although in the article "Measurement of Phase Difference Using DSP 
Algorithms by Non-Coherent Sampling" by Michal Krumpholc and Milos 
Sedlacek it is explained that DFT is not an optimal choice for a DMTD, I 
decided to use DFT. This because a bit true simulation showed the impact 
of the spectral leakage to be below the performance ambition and using 
all samples reduced noise. Also a PLL is used to sync the sampling with 
the reference input to reduce the worst impact of the non-coherent 
sampling.
In your "DSP for Sine Wave Reference Signals " method you only have one 
NCO, how do you avoid the non-coherent sampling and spectral leakage 
problem with two non coherent inputs?
In my measurements there is a clear difference in noise between using 
coherent inputs (e.g. single clock send to both inputs) and non coherent 
inputs, even if the frequency difference is in the order of 10 micro 
Hertz, but this could be due to the noise from the DDS used to generate 
the dual signals with very small frequency differences. As I have no 
access to very stable clocks the DDS is the only tool I can use for 
stability measurements.
Erik.

On 24-10-2022 10:10, Carsten Andrich wrote:
> Hi Erik,
>
> only the ADC clock should matter and the used ADC should be of the 
> simultaneous sampling type. If it's not, its multiplexer may have a 
> detrimental temperature-dependent effect on the phase measurement.
>
> I've implemented a fully digital DMTD using USRP N210 with LFRX 
> daughterboards [1]. To analyze stability of the system itself, I 
> compared a split 10 MHz signal. Over the course of 4 days, the 
> measured standard deviation was 359 fs [1, Fig. 11]. I don't have 
> temperature measurements available, but the lab wasn't air 
> conditioned, populated, and diurnal difference between two SRS FS725 
> was clearly observable (another measurement not in the paper).
>
> The high stability could be explained by the N210's dual-channel ADC 
> that directly sampled both 10 MHz signals. I believe, temperature 
> differences between the preceding analog components (most importantly 
> the LFRX daughterboard) probably have a very limited effect on account 
> of the negligible relative bandwidth of the measured 10 MHz signals' 
> true frequency (a few (dozen) mHz vs. 10 MHz). If the 10 MHz were 
> down-mixed to a few Hz in the analog domain, the relative bandwidth 
> would increase substantially. Of course, that's just an educated 
> guess. I did not investigate temperature stability when I wrote the 
> paper.
>
> Best regards,
> Carsten