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

[John Ackermann N8UR]

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.

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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