[time-nuts] Re: Rb settling time after C-field adjust?

[jim_johnson at agilent.com]

Nice work, Tom.  My source here says that for
a small change in C-field you wouldn't even see
the effect unless you're looking at the level of
1E-12 or smaller.  His recommendation is to make
the adjustment as follows ... for a one-turn
adjustment of the C-field pot, turn it slowly, 1.5 turns,
then slowly turn it back to about 0.5 turns, followed
by a slow turn to 1.25 turns, then to 0.75 turns, then
to 1.1 turns, back to 0.9 turns, then to 1.0 turns.  This
technique tends to remove the effects of any residual
magnetization due to hysteresis in the mag shields.
Granted, you won't even see this effect unless you're
in the "metrology world" of 1E-12, but that's the procedure
to use.  He says that what happens by just turning the
knob 1.0 turns, is an initial step (seen in your plots),
followed by a slow drift at the 1E-12 level for as much
as a few hours, depending upon the status of the magnetic
shields around the physics package.  When the instrument
is (was) new, the mag shields have been (were) annealed 
(degaussed) and should be in excellent shape with very
little hysteresis.  However, as the years go by, the
instrument is subject to unknown environmental conditions,
including stray mag fields, bumping, dropping, etc., so
the mag shields can take on a memory (residual mag fields)
and can exhibit various amounts of hysteresis.  That is
what you're fighting when you turn the C-field knob.

So that's my interpretation of what he said.  If there
is anything wrong with my explanation it's most likely
due to my own ignorance or misunderstanding of what he said (!).

Regards,
Jim
Agilent Laboratories
Palo Alto, CA



>-----Original Message-----
>From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
>Behalf Of Tom Van Baak
>Sent: Friday, November 12, 2004 12:08 AM
>To: Discussion of precise time and frequency measurement
>Subject: [time-nuts] Re: Rb settling time after C-field adjust?
>
>
>An interesting topic about 5065A C-field setting.
>
>Since this is something I can measure, here's
>the detailed result:
>
>http://www.leapsecond.com/pages/hp-5065a-cfield/
>
>/tvb
>
>
>
>
>
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