[time-nuts] Lars GPSDO

[Bob kb8tq]

Hi

One thing that may well help in terms of internal bias resistors on an OCXO is that
they often are inside the ovenized section of the device. There’s no guarantee of
course. 

If you have an effective thermal gain “in the hundreds”, a common 50 ppm/C resistor
set would get down to a fraction of a ppm. Effectively it would be better than almost
anything else you might last up outside the device.

Again - no guarantees ….. 

The device needs to hit it’s temperature spec at a range of EFC voltages, whatever is 
in there does have some constraints on it. So maybe not totally unlikely. 

Bob

> On Apr 5, 2019, at 8:44 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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