[time-nuts] HP5065A reversing the C-field ?

[Bob kb8tq]

Hi

A few possible complications:

I suspect you would run into bits and pieces of “stuff” that had
slowly been magnetized inside the C field ( think of things like
component leads). Just how they would impact things depends a
bit on how fast they respond to the change. ( = you have both external 
and internal stray fields).

If you look at the C-field reversing telecom Rb’s, they pay a penalty 
for doing it. They get a noise hump at the reversing frequency. Is this
a “must have” or simply the best they could do? No way to know 
without a lot of work trying to do better.

Finally there is a really tiny effect due to the C field not being linear.
The frequency shifts more per ma of change at 6 ma than at 2 ma. 
As you did the reversal, there would be a ever so slightly greater 
sensitivity in the direction the fields added. 

http://www.wriley.com/Rubidium%20Frequency%20Standard%20Primer%20102211.pdf <http://www.wriley.com/Rubidium%20Frequency%20Standard%20Primer%20102211.pdf>

Has the details of how and why this happens. 

=======

All that said, nope, never done it. However I’m about to tear into the C 
fields of the ensemble here in the next couple days. I can put it on the 
“to do” list. 

There is … ummm …. anecdotal evidence that the mag shield on the 
5065 is less than “the best of the best of the best”. That’s based on 
various conversations back in the mid 1980’s. 

In a fixed location, I’m not sure how much this matters. If you move your
5065 around, then indeed it would matter. (along with a bunch of thermal
issues …). 

Bob


> On May 2, 2020, at 12:09 PM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> 
> In physical terms, the direction the C-field does not matter in a
> atomic frequency standard, it just has to be of constant magnitude.
> 
> The ambient magnetic fields adds to the C-field, most notably the
> geomagnetic field, and any (strong) nearby DC currents.
> 
> Some of the "telecom" rubidiums reverses the C-field periodically,
> presumably to average out any stray magnetic fields which get past
> their puny, if any, mu-metal shielding.
> 
> The HP5065A is stationary and shielded, so the stray fields should
> be small and almost constant, and implementing periodc reversal would
> likely convert it from a frequency normal to geomagnetometer.
> 
> But if one reversed the C-field, by swapping the wires to the solenoid,
> and measured the frequency-shift, wouldn't that give a measurement of
> how good the magnetic shield works ?
> 
> Has anybody ever tried this ?
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.
> 
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