Hi The TDC has a range dimensioned in ms. It has a resolution dimensioned in ps. As long as your “reference” is stable in the ~ ppm range, moving it around will not impact the calibration. Hopefully any change you make to a stabilized GPSDO will be in the < ppb range …. ( so roughly 1000X lower than what might have an impact ….
Bob > On Feb 29, 2020, at 11:45 AM, Matthias Welwarsky <time-n...@welwarsky.de> > wrote: > > On Samstag, 29. Februar 2020 16:48:30 CET John Ackermann N8UR wrote: >> On 2/29/20 7:40 AM, Matthias Welwarsk wrote: >>> For example, there's the TDC7200 itself. When I tested my design first, >>> the >>> measurements had a distinct relation to temperature, in the order of maybe >>> 300ps/°C, but there's other stuff as well, for example, the 74ALVC74 in >>> yours, the whole analog output section etc. >> >> The TDC7200 should compensate for temperature changes, though in >> practice I'm not sure how quickly. >> >> Because the ring oscillator in the chip that provides the >> picosecond-level clock is free running and violently temperature >> sensitive, it needs to be calibrated and the TDC7200 does that >> automatically after each measurement. >> >> After the STOP pulse is received, the chip measures the time interval of >> several cycles of the external 10 MHz clock, and works that result >> backward to determine the actual frequency of the ring oscillator. The >> TI calculation uses that measured frequency as an input and thus should >> take into account any tempco. >> >> Now, I've tried sticking my finger on the chip and that causes a wild >> tempco for at least some seconds, so there is some inertia in the >> correction, but the calibration process should take care of more typical >> slowly changing temperatures. Isolating the chip from moving air should >> be helpful. > > There's two interesting implications here, one general, one in the context of > a GPSDO: > - If the temperature gradient is sufficiently steep, the calibration will be > outdated when the next measurement starts. > - Mine as well as Tobias GPSDO design use the disciplined LO as the reference > clock for the TDC7200. The control loop perturbs the LO as a consequence of > the measurement. If the response is very fast, this perturbation might fall > into the calibration window. This window is 40 cycles of the reference clock > at max. So, a total of 4000ns worst case. For my own software I know that the > response will not happen faster than 1ms, because everything is driven from a > mainloop with 1ms ticker granularity so it will not be a problem. > > BR, > Matthias > >> >> John >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow >> the instructions there. > > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
