[time-nuts] Question for my new GPSDO

[Magnus Danielson]

Hi,

On 2019-10-08 14:35, Tom Van Baak wrote:
> Hi Tobias,
>
> 2) I suspect one can spend a great deal of time picking and testing
> optimal parts in a GPSDO based on tempco specs. But at what point is
> it cheaper to just control the environment of the GPSDO board itself?
>
> You mention opening lab doors and windows and such. Wouldn't it be
> simpler to spent time to design a box that is wind-proof, or
> fan-controlled; maybe even oven controlled? That way you can relax all
> your worries about exotic passive and active components and just build
> a controlled enclosure.
>
I had exactly this problem a long time ago, and the real problem is that
any wind-drag is coupled directly to the oscillator. This will also be
true if you have forced air convection of the box, i.e. fans. The
air-flow couples very well to the metal shield of the oscillator. For
sure you want your oscillator in a winddrag free environment, so you
want a barrier to stop direct flow onto the oscillator. You do not want
to think insulation, as the OCXO needs to dump excess heat to a cooler
environment. On the lab-bench I use a small box, but Bob's approach is
to throw a towel over it. In designs we put plastic caps over the
oscillators. In the HP5370A/B the OCXO sits in a quiet corner but not
isolated.
> That could not only take care of temperature, but humidity and
> pressure as well if they were found to be sources of instability. One
> advantage of this approach is that the same box design could then be
> applied to any other T&F projects like distribution amps or phase /
> frequency counters that you design in the future.
>
> 3) About sensors.
>
> > many GPSDOs I have tested (e.g. Trimble Thunderbolt, STAR4) show
> info about the OCXO temperature
>
> The Trimble Thunderbolt (TBolt) uses an onboard DS1620 temperature
> sensor. I don't recall it ever being called "OCXO temperature"; it's
> ambient board temperature. Perhaps because the board is inside a
> larger aluminum enclosure the board temperature is related to OCXO
> case temperature. But as far as I know the reported temperature value
> from a TBolt is definitely not the temperature of the crystal
> resonator itself.

The ambient temperature is not perfect as measured on a distance, but
helps. What imperfections there is in the setup, the big difference will
be in how transients is handled, not the stable conditions.

Cheers,
Magnus