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 <[email protected]> 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 <[email protected]> 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 > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow > the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
