[time-nuts] Experiment in lowering the TAPR TICC noise floor

[Matthias Welwarsky]

Hi John,

the noise is likely not white, but it really depends on what is the dominant 
noise source in the system. I guess there is some correlation but still enough 
entropy to make a difference. I'll try with different cable lengths next to 
see if it makes a measureable difference, but ideally you'd use two TICCs and 
two non-coherent reference clocks. But they'd need to be somehow frequency 
locked.. You'd need some mechanism that causes enough jitter to break the 
correlation. A delay line controlled by some noise source?

On Dienstag, 6. Oktober 2020 21:24:07 CEST John Ackermann N8UR wrote:
> That's very interesting, Matthias!  The lower noise floor intuitively
> and the sqrt(2) improvement comes to mind, except that would apply only
> if the two measurements were uncorrelated.  Here, you have a common 10
> MHz reference, so there's correlation.
> 
> But... the high-speed ring counters inside the two TDC7200 chips are
> free-running and independent, so when looking at picoseconds, maybe the
> non-correlation requirement is met, at least to some extent.

For the next run I'll have a look at the individual result registers. The 
entropy source is maybe identifiable this way. Either the time1 results is 
already random enough, or the cal1 and cal2. A cross-correlation between the 
raw register readings between the two channels might reveal something.

I expect to find a high correlation between the time1 results of the two 
channels, and no correlation for cal1/cal2.

> 
> John
> ----
> 
> On 10/3/20 4:37 AM, Matthias Welwarsky wrote:
> > Dear all,
> > 
> > When I started to look more into the software side of the TICC and
> > especially the ominous "time dilation" parameter, I set up an experiment
> > where I feed the same event into both channels of the TICC, for
> > evaluating the sensitivity of the measurements to this parameter
> > (spoiler: there is a measurable influence but it's not as critical as I
> > originally thought).
> > 
> > While looking at the phase measurements the idea evolved to see if the
> > noise floor could be lowered by combining the measurements of the two
> > channels.
> > 
> > I have attached the resulting ADEV and the raw channel timestamps. The red
> > trace is one individual channel, the blue trace is the combination of both
> > channels. I used Octave to evaluate the measurements.
> > 
> > I used the following commands to get from timestamps to phase data:
> > 
> > A=detrend(cumsum(1-diff(load("chan-a.txt"))));
> > B=detrend(cumsum(1-diff(load("chan-b.txt"))));
> > 
> > The combination of both channels is the an element-wise arithmetic mean:
> > 
> > V=(A+B)/2;
> > 
> > As you can see in the ADEV chart, there is indeed a slight improvement of
> > the noise floor from 7.8e-11 @1s to 6.8e-11 at 1s.
> > 
> > Of course this combining doesn't work too well if the noise of the two
> > channels is correlated, and there's plenty opportunity in this setup for
> > this to happen. For one, both channels are driven by the same clock
> > source, they share the same power supply, connected to the same Arduino
> > base board and the cables from the event source to the channel inputs are
> > of the same length. It would be interesting to repeat the experiment with
> > a more elaborate setup, for example using two TICCs with individual clock
> > sources (locked to each other but not coherent), using different lengths
> > of cables to feed the channels etc.
> > 
> > Regards,
> > Matthias
> > 
> > 
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