[time-nuts] Lars GPSDO

[Jim Harman]

Thanks for your ideas, Ed. In my actual implementation I am using
reasonably low tempco 1% fixed resistors. In the photo they are the blue
resistors between the DAC and the OCXO.

I have noticed a significant temperature sensitivity in the system,
especially if I run it with the cover off.so the ambient temperature can
change quickly. Moving air has a short term effect on the OCXO and to a
lesser extent the GPS. Sustained ambient temperature changes cause an
offset in the DAC value when the loop is closed, so the loop is doing its
job to compensate for them. Putting the system in a small picnic cooler
with several water bottles makes any temperature changes slower than the
loop time constant, which should minimize temperature effects on the
overall system.

I calculate the open loop temperature sensitivity at about 7e-11 / deg C.
The C-MAC oscillator spec for what it's worth is  "-10 C to +70 C / ref.at
+30 C  < +/- 3.0 e-9". If the variation was linear, which I realize it is
probably not, that would be 7.5e-11 / deg C so I am in the same ballpark.

On Fri, Apr 5, 2019 at 8:21 PM ed breya <eb at telight.com> wrote:

> I'd recommend that once you get things figured out and tuned up to
> nominal running conditions, you should narrow the offset pot range, and
> use good low-TC resistors to make up most of the network R, with the pot
> having as small an effect as practical. BTW I don't see the pot in the
> pictures, but I do in the schematic.
>
> You may want to consider lowering the entire network resistance by
> scaling everything down, say ten times lower or more. This would reduce
> noise, and lessen effects from the varicap bias (leakage) current in the
> OCXO. Also, I have seen a number of OCXOs with an internal termination
> resistor (like 50-100 k) on the tuning line - that has spoiled a lot of
> fun for me, having to worry about the characteristics of that resistor,
> and including it in the deal. With an unknown or unspecified OCXO, it's
> good to check for any unwanted extra parts.
>
> It may help the stability to put some insulation around the tuning
> resistor network and maybe the DAC too, especially if the waste heat
> from the OCXO is significant.
>
> Ed
>
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-- 

--Jim Harman