[time-nuts] Question for my new GPSDO

[Hal Murray]

attila at kinali.ch said:
> Err.. correction: 1e-12 frequency control resolution with 1ppm EFC tuning
> range results in 1e-6 required EFC voltage control. Which in turn is 20bits.
> This is much saner requirement. 

You can get a few bits by reducing the tuning range.  Suppose you follow the 
DAC with an OpAmp with a gain of 1/8 while the reference is adjustable.  That 
adds 3 bits of resolution by decreasing the output range.  The downside is 
that you have to manually set the adjustment.

That's crappy for production gear but might be interesting for a one-off 
project.  You may have to adjust the reference as the crystal ages or the 
temperature changes from summer to winter.

If the output range is too narrow, you will have to set the adjustment too 
often.  I don't know how to calculate things, but I think I have years of 
pack-rat data from logging a Z3801.

That adds the op-amp to the temperature consideration list.


> While temperature coefficient is important, it is not as important as you
> think. Unless you want to operate your GPSDO outside or where you expect
> large temperature swings. Instead, what you do is give your whole GPSDO
> enough thermal mass, such that you don't see a significant temperature change
> at time scales shorter than the loop time constant.  ...

How big a box of whatever do I need to get the thermal time constant large 
relative to 100 or 1000 seconds?


> I'd rather heat up the PCB using some simple resistive heater (ie have a thin
> meander line on one layer, coupled with an NTC thermistor and some cheap,
> opamp based PI control loop) 

I was going to ask if anybody had any experience/data on ovenizing the DAC and 
friends.

Has anybody tried mounting it on the side of the OXCO and putting a blanket 
over it?




-- 
These are my opinions.  I hate spam.