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

Chris Albertson albertson.chris at gmail.com
Fri Sep 14 21:31:27 UTC 2012


---------- Forwarded message ----------
From: Don Latham <djl at montana.com>
Date: Fri, Sep 14, 2012 at 2:01 PM
Subject: Re: [time-nuts] GPSDO control loops and correcting quantization error
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>


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.

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.


Chris Albertson
Redondo Beach, California



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