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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jun 29 06:31:50 EDT 2010


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

EWKehren at aol.com wrote:
> Hi,
>   just a clarification, I did write 4066 it is a 4046 that I  replaced. Take
> a look at the MCP 4822 dual 12 bit D/A In the data  sheet they have an
> example using one for coarse, the other for fine steps, I  realize that the
> transition is not perfect but maybe code can compensate for the  transition.
>   Bert Kehren
>
> In a message dated 6/29/2010 5:10:39 A.M. Eastern Daylight Time,
> attila at kinali.ch writes:
>
> Moin,
>
> On Sat, 26 Jun 2010 21:14:02 EDT
> EWKehren at aol.com  wrote:
>
>    
>> What you want is basically a Shera Board. That design has  been around
>>      
> for
>    
>> quite some time and has served me very well.
>>      
> Yes. The Shera Board and similar designs serve as an example for  me.
>
>    
>> I have a total of six running
>> including two  controlling Rubidium. There are in my opinion a couple of
>>   problems: not every 4066 works on the design the 18 bit D/A is very hard
>>      
> to
>    
>> find  and now expensive and the single step of the D/A is  intended for a
>>      
> 1.7
>    
>> E-13  frequency step.
>>      
> Yes. My goal is  to update the venerable 4066 with something more
> modern and have components  that are easy to get trough farnell, digikey,
> mouser, and all the other  distributors. Yes, 16bit D/A seems to
> be the maximum that is currently  available. It crossed my mind
> to build a 24bit R-2R D/A using discrete  components, but this might
> have actually a worse performance than a off the  shelf 16bit D/A.
> (temperature drifft, resistor values missmatch, EMI,  etc)
>
>
> Attila Kinali
> --
> If you want to walk fast, walk alone.
> If you want to walk far, walk  together.
> -- African  proverb
>
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