[time-nuts] GPSDO control loops and correcting quantization error

[Michael Tharp]

On 09/14/2012 05:31 PM, Chris Albertson wrote:
> Michael: Actually implementing a 16 bit DAC to its 1-bit minimum
> resolution will be headache enough. You will gain a real education in
> good grounding practice, shielding, power supply stability and noise,
> and other Murphy intrusion. A 32 bit DAC IMHO, is impossible, and that's
> the name of that tune.

Probably true, luckily as others have mentioned the long-term stability 
of the DAC and its voltage source are less important.

> I think the step size would be close to the same with the 32 bit DAC
> but the reason you use it is so you can control just about any OCXO,
> Rb or other things you drop in.  In other words I'd use the extra bits
> to extend the voltage range.  But while in use, I doubt you'd ever
> change the highest 16 or 18 bits
>
> Also if you are building a general purpose controller for OCXOs and
> Rb, remember that some Rb oscillators use RS-232 control to set the
> frequqecy.  It might be good to build in an RS232 port.   The firmware
> in the uP can always be changed but hard to add the DB9 connector
> later.
>
> One otherthing I was doing when I was working on a design like this
> was to discipline both an XO and an Rb from the same GPS.  Almost like
> building two controllers but you save some because you can use one uP
> and one GPS interface.   Now that you have a disciplined Rb it can be
> used for hold over in case the GPS goes away.   I thought it would be
> unlikely for GPS to actually fail but allowing for hold over would
> make the entire unit portable.

I had this idea as well, although not for disciplining the Rb (which 
unfortunately mine cannot, it's a less popular Efratom model clearly 
pulled from telecom application and has no external control that I can 
see) but just as a backup timing source for holdover. I've mentioned it 
here before, but the gist would be to estimate its frequency while GPS 
was working, then if GPS fails use the Rb instead either by dividing it 
by the last known frequency or by adding the error to the measurement 
loop. That said, I would like the holdover performance with just the 
OCXO to be as good as possible in its own right.

I'm planning to make a future version of this project available but this 
first revision is mainly an experimentation platform and wouldn't be of 
much use to someone who doesn't have the same equipment as me. Stay tuned...

-- m. tharp