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

[Matthias Welwarsky]

On Mittwoch, 7. Oktober 2020 22:25:40 CEST John Ackermann N8UR wrote:
> It would be possible to lower the noise slightly by using a 16 MHz clock
> rather than 10 MHz (but if you look at Figure 15, the improvement
> wouldn't be very great).  That would require reprogramming the PIC
> divider chip, and may some Arduino code changes as well.  (I *think* the
> clock speed is set as a constant in the code that could be changed at
> compile time, but I never tested to see if that would work without
> breaking anything.)

Besides giving only a marginal improvement, requiring a 16MHz reference clock 
would be rather unpractical...

> 
> John
> ----
> 
> On 10/7/20 2:29 PM, Attila Kinali wrote:
> > On Sat, 03 Oct 2020 10:37:59 +0200
> > 
> > Matthias Welwarsky <time-nuts at welwarsky.de> wrote:
> >> 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).
> > 
> > That is to be expected. There are two resons for this:
> > 
> > First, the major limit to the measurement is the noise within
> > the TDC7200. If you want to get lower, then you have to reduce
> > this noise. If you look at Figure 17 in the TDC7200 manual, you
> > will see that the noise of the TDC is highly dependent on the
> > length of the measurement. Shortening the measurement will
> > decrease the noise. For this you need to use a higher clock
> > of the stop signal to measure against, than the 1ms that the TICC
> > does. But that will not work with the Arduino. You can get around
> > this if you use a faster µC like an STM32F4. See Tobias Pluess GPSDO
> > design for an example how to do this.
> > 
> > Second, both inputs of the TICC measure against the same divided
> > 1kHz clock with a modified half-Nutt interpolator. I.e. most of
> > the measurement time will be common to both input signals and thus
> > most of the noise seen due to the TDC and the reference clock are
> > common.
> > 
> > 
> > On Wed, 07 Oct 2020 18:34:00 +0200
> > 
> > Matthias Welwarsky <time-nuts at welwarsky.de> wrote:
> >> 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?
> > 
> > Adding noise will not break any correlation. It will only mask it.
> > I.e., the correlation will pop up once again, when you start
> > using methods to remove the added noise.
> > 
> > Adding noise helps only if your noise is mostly quantization noise,
> > then it acts as a dithering mechanism which allows you to average
> > over the quantisation (and added) noise, which wouldn't be possible
> > otherwise.
> > 
> > 			Attila Kinali
> 
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