Thanks for the informative replies, all.
Much appreciated.
Interested to see your comments (TomC) ref the methods of fine tuning
the Rb cell-that in fact was where this train of thought all started
as I was wondering about applying an EFT arrangement to an existing Rb
unit for GPS locking. Got sidetracked on the 'integer Hz' issue
though!
Regards
Dave Brown, NZ
----- Original Message -----
From: "Tom Clark, W3IWI" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Sunday, September 18, 2005 11:22 AM
Subject: [time-nuts] Re: time-nuts Digest, Vol 14, Issue 28
Tom van Baak, responding to Dave Brown noted:
But what about the Rb resonance frequency? It is always quoted as
being an integer number of Hz as well- viz 6834 682 608 Hz. Is
there
some aspect of the physics of these transition frequencies that
means
the Rb resonance has to be an integer number of Hz, based on the
Cs133
definition?
Or is the Rb resonance frequency, which is generally quoted as
above,
actually rounded to be an integer number of Hz for convenience?
Perhaps a set of environmental conditions need to be specified as
well?
Regards,
Dave Brown, NZ
Yes, depending on where you see the value it will be
rounded. The most accurate measurement that I've
seen for the Rb resonance is 6 834 682 612.904324 Hz.
But it is common to see 6.8 GHz, 6 834 MHz, and
6 834 682 612 Hz as well. Not sure about the ...608
vs. ...612 thing.
The hyperfine transition that defines the frequencies of all the
atomic standards are subject to the magnetic field in the atomic
"cloud"/RF interaction region. Cesium is a good reproducible
reference
standard because the magnetic interaction is small.
In the case of Rubidium, the "C-field" magnetic field is used to
tune
the Rb cell slightly off frequency to make it possible to lock a
clean
crystal signal to the atomic signal. In most Rb standards I know
of,
the electronics are configured so that the tail-end digits of the
frequency are equal to 5/16 MHz (xxx.3125 MHz) so that the RF
frequency can be easily generated. The frequency knob that you
tweak
to correct the Rb frequency passes some tens of ma thru a coil
surrounding the RF interaction region. If you try to phase lock a
Rb
to GPS, you need to develop a current source error signal.
Another Rb FYI item. In earlier Rb designs, the xtal oscillator
was
given a slight frequency modulation, so that the frequency wobbles
over a few Hz around the "proper" frequency. The optical detector
would the look for the phase of optical level modulation and
center
the xtal on the Rb transition. This causes many of the older Rb
standards (including 5061s) to have a lot of RF phase noise when
the
5/10 MHz signal is multiplied up into the microwaves. Amateur
microwave enthusiasts have learned that they need to phase-lock
their
radios with time constants ~1 second in order to improve the ratty
signal.
In some later standards (including even the inexpensive SRS unit),
the
tail-end frequency and FM are generated by a DDS driven from a
"clean"
unmodulated xtal and are not subject to as much noise.
Regards, Tom Clark
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