[time-nuts] yet another GPSDO design, or so
EWKehren at aol.com
EWKehren at aol.com
Tue Jun 29 11:29:36 UTC 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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