[time-nuts] yet another GPSDO design, or so

[Stanley Reynolds]

If we lower the size of each step to over lap more would this lower the error ?  Software would adjust both converters at the cross over point so neither would change it's full range at this point. Two 12 bit converters would form one 18 or 20 bit converter.

I guess taken to the extreme we could interweave any number of converters so hopefully the errors would average out :-)  64 times 12 bit converters making a 20 bit converter each with an equal contribution at each step. 

Stanley



----- Original Message ----
From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Tue, June 29, 2010 5:31:50 AM
Subject: Re: [time-nuts] yet another GPSDO design, or so

The problem is that the gain and offset of the 2 DACs changes with time 
and temperature so that the required corrections also change.
Ideally an autocalibration technique would be used to dynamically track 
such changes.

Since changes in the coarse DAC are only required infrequently and the 
mismatch only affects the region around coarse DAC transitions which are 
relatively infrequent (or should be) most designers choose to live with 
the increased loop settling time at such transitions.
With sufficient overlap between the coarse and fine DACs only small fine 
DAC changes should be required to compensate for mismatch between the 
coarse and fine DACs after a change in the coarse DAC output.

The coarse + fine DAC approach is used in some GPSDOs and in particle 
accelerator control systems.

Bruce

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