[time-nuts] PCB layout question for GPSDO

[Magnus Danielson]

Hi,

On 2020-02-29 13:40, Matthias Welwarsky wrote:
> On Donnerstag, 27. Februar 2020 16:17:56 CET Tobias Pluess wrote:
>
>> Attached 2 photos and the new schematic!
> I'm always wondering about the merit of placing the temperature sensor close 
> to the OCXO. It's certainly not the only, or maybe not even the dominant 
> source of temperature related disturbance.
>
> 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.
>
> Wouldn't placing the temperature sensor close to a heat source make it less 
> suitable for compensating those effects?

You discuss as if there would only be one temperature sensor. If you
have only one temperature sensor, then one would place it where it would
measure the dominant effect. Temperature sensing the TIC would help to
stabilize phase variations, but the frequency variations of the
oscillator integrates over time and becomes the dominant pretty fast if
it wasn't already. So, for most reasonable cases, putting the sensor
near the oscillator makes very good sense.

If it makes sense to use another sensor is really a matter of
diminishing returns, as you tend to sense similar enough temperature as
it is, even if not measured at the place of effect for that feature.
Trying to measure for best compensation of dominant shifts is reasonable.

So, rather than just showing there is temperature effects in one part,
show which will be dominant, reason about what will be indirectly
measured and part compensated.

Cheers,
Magnus