[time-nuts] PCB layout question for GPSDO

[Matthias Welwarsky]

On Samstag, 29. Februar 2020 17:51:15 CET Bob kb8tq wrote:
> 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 ….

Attached is the mdev graph of a recent test run of my GPSDO. Based on roughly 
100.000 samples of DAC movements, drift removed. The LPRO has a short-term 
stability of I guess around 1e-11 (probably slightly worse), 1e-14 is well 
below that.

> 
> Bob
> 
> > On Feb 29, 2020, at 11:45 AM, Matthias Welwarsky <time-nuts at 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 be outdated when the next measurement starts.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
> >> 
> >> 
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