[time-nuts] PCB layout question for GPSDO

[Bob kb8tq]

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-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 outdated when the next measurement starts.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
>> 
>> 
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at 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 at lists.febo.com
> To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.