John Miles wrote:
-----Original Message-----
From: [email protected] [mailto:[email protected]]on
Behalf Of Bob Voelker
Sent: Sunday, March 21, 2010 1:59 PM
To: [email protected]
Subject: [time-nuts] HP 5065A performance vs. others
As several postings have indicated, the performance of the HP
5065A is better
than many of the other rubidium standards. What enables the HP 5065A to
achieve better performance?--Is it the physics package or the particular
control system implemented in electronics? Would it be possible to
achieve the HP 5065A's performance by modifying a more commonly
available rubidium such as the LPRO? Would an ensemble of LPROs
match a single HP 5065A in performance?
Bob
I think those are all open questions, because it's not immediately clear
what limits the performance of the smaller telecom-grade physics packages,
or what low-hanging fruit might be left on the tree.
For instance, how important is the length of the path the light takes
through the filter cell and/or resonance cell? It's obviously a lot longer
in a 5065A.
I think you would find this article of interest:
http://tycho.usno.navy.mil/ptti/1990/Vol%2022_39.pdf
Especially page 446-447.
Another mentioning of optical path length is here:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19830027087_1983027087.pdf
This Litton contribution goes into more detail about ligth-shift and
path length:
http://tycho.usno.navy.mil/ptti/1983/Vol%2015_34.pdf
Read the Ligth shift chapter starting at page 679, then check figure 4.
These is more if you care to search for Rubidium and light shift.
There is considerable amount of efforts spent on reducing light shift.
It has many sources, including lamp temperature, filter cell
temperature, filter cell length etc. etc. Filter cell
temperature-stability is a key issue from what I understand.
the difference? Is there something special about HP's lamp? Is their
microwave synthesizer that much better?
Could be part of it.
HP's temperature stabilization is better than the LPro's -- so maybe it
would help if you just moved the LPro's Rb assembly into an outer oven,
separate from the rest of the electronics.
Temperature stability is a key issue as it connects via filter cell
light shift, lamp intensity (which spectrally needs to match that of the
filter cell... they do not match very well actually and the filter is a
bit steep), then buffert gas frequency pulling alongside of the
wall-shift. Then toss in the cavity detuning and the frequency pulling
effect that it causes.
How important is all that
mu-metal shielding on the 5065A, given that most people these days would
care more about stability than absolute accuracy (thanks to GPS)?
The uniformity of the C-field is an issue, and if it is allowed to
change (due to externally shifting fields) well...
Someone with more free time should tackle these questions. :) F. G. Major's
book would be a good starting point, and this paper on laser-pumped Rb
clocks also has a lot of hints about what limits the performance of ordinary
sources: http://tf.nist.gov/general/pdf/1219.pdf . They used the cell from
a commercial Rb standard in their experiment, although they didn't say which
one. If nothing else, you can infer from this paper that the path length
through the resonance cell isn't a huge deal.
Depends on how it balance with other things. See above links.
Cheers,
Magnus
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