[time-nuts] PCB layout question for GPSDO

[John Ackermann N8UR]

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.

John