Hi > On Oct 7, 2020, at 4:25 PM, John Ackermann N8UR <[email protected]> wrote: > > Hi Attila -- > > Just a couple of corrections -- the "coarse clock" in the TICC runs at 10 kHz > (100 us), not 1 kHz, and therefore the TDC never sees a measurement interval > longer than 100 us, not 1 ms. > > More importantly, the chart in Figure 17 of the datasheet is for operation in > "Mode 1" of the TDC, which is recommended for time intervals of 500 > nanoseconds or less. But the TICC uses "Mode 2" which doesn't have that > limitation, and Figure 17 doesn't apply. > > 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.)
……. it would also require some sort of PLL / synth to come up with the 16 MHz *and* do so in a fashion that does not add more jitter. Not trivial … Bob > > 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.
