[time-nuts] RE: phase locking Rb to GPS (was time-nuts Digest, Vol 14, Issue 30)

Tom Van Baak tvb at leapsecond.com
Sun Oct 23 21:52:15 EDT 2005

> Be aware that Brooks can modify the sensitivity of his PIC for a 
> rubidium.  Brooks has made me several custom PICS for my FRS-C. 

See, this makes much more sense to me. His architecture
is fine and when you swap an OCXO for an Rb things like
EFC gain, range, and span are just part of the problem. I
would guess TIC and PLL time constants are a bigger part
of the equation. You need to match the Rb ADEV. This is
simple with modified source code; way harder (impossible?)
with external hardware hacks.

> I used his controller without any of its offsets circuits.  I built a 
> summing amp and summed the output of the controller, with a precision 
> variable voltage reference circuit based on the LM199.  I set the 
> precision reference to the voltage it needs to put the rubidium on UTC, 
> and then let the controller step around to control it.

Yup, this makes sense.

> I had Brooks to modify the PIC for an different update rate.  The stock 
> units are 30 seconds.  We have tested a unit at 120 seconds, and I am 
> testing a unit at 60 second update rates.  Some of the data is pointing 
> to optimized design around 81 seconds.

Can't wait to see that data. Given Shera's TIC resolution
and GPS noise, I would have guessed much longer than
that. See the PRS10 plot, for example:


The other thing I'm wondering is what you're trying to
optimize. A high update rate or short time constant
may be good for a GPS time standard but bad for a
GPS frequency standard. I don't think you can have
it both ways.

> I plan to publish the results in the future - but the long term test 
> take about 5 days for each test - and the unit is being tested against 
> several reference oscillators.  Doing all combinations of timebases, 
> results in 20 some test that have to be performed.
> Brian - N4FMN

Have you considered doing all of this with some sort of
software simulation? If you take 5 days of real GPS data
and 5 days of real Rb data (or OCXO data, etc.) my gut
tells me it shouldn't be that hard to then wrote code to
simulate the entire thing and get answers in a matter of
minutes. Then change the parameters and run it again.

As a by-product you'll get ADEV plots of GPS, the LO,
and the virtual GPSDO.


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