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

EWKehren at aol.com EWKehren at aol.com
Tue Jun 29 07:29:36 EDT 2010


Not being able to write code and not understanding what is required it is  
easy for me to suggest a fix. In the case of a good Rub. the annual aging 
rate  is 1 E-10 which would mean the lower 12 bits would cover one month. 
During  transition the PIC could modify the time constant and at the same time 
turn on a  red LED warning that the unit is in transition.
16 bits will also work, how ever an annual manual adjustment will be  
needed, depending on what the smallest step would be.
The Shera unit has been around for 12 years now and served many of us well  
an update would in my opinion make sense, specially since the total cost 
could  be well below $50. Done right it could also be used as a digital filter.
Bert Kehren
 
 
In a message dated 6/29/2010 6:38:57 A.M. Eastern Daylight Time,  
bruce.griffiths at xtra.co.nz writes:

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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