On 11/27/21 3:20 PM, Magnus Danielson via time-nuts wrote:
Hi,
There is an overemphasis on the atom being used, and especially on
cesium as that is what is used for SI definition. However, actual
implementation means actual physical devices, and the physical devices
have a physics package, for which details will be important to the
actual performance. Various atoms have been more or less well adapted
to different types of physical package types. The beam type of device
can be made to have very little perturbation, and cesium was well
suited for that, while rubidium ended up being very well suited for
the gas cell type. The CSAC is really a cesium based gas cell, but the
original benefit of rubidium filtered optical pumping has been
replaced with semiconductor lasers for pumping. Today both cesium and
rubidium gas cells with the same mechanism exists. With gas cell you
get wall shift from atoms banging around the wall, but also gas shift
as buffer gas makes the atoms hit the buffer gas most of the times.
With a bit of selection of gas mixture, these can be made to balance
each other.
So, what is the claim to fame for CSAC? It's actually not being
cesium, but for the stability it provides for the small amount of
power it consumes. That's also where it finds actual applications. If
you can afford more power, there is cheaper alternatives available.
Not only is the CSAC low power (~ 100 mW) it's physically small, which
is attractive for some applications (inside a cubesat, for instance).
It used to be price competitive with a Rb, too ($1000-1500, as I recall)
but now they're about $5k. Microsemi also has the NAC (which is more a
conventional Rb, but small)
Parameter NAC1 CSAC
Aging 3E-10/mo 9E-10
1E-9/yr 10E-9
ADEV 2E-11 @ 100sec 2.5E-11
8E-11 @ 10 8E-11
2E-10 @ 1 sec 2.5E-10
phase noise
-86 @ 10 -70
-120 @ 100 -113
-138 @ 1000 -128
-143 @ 10k -135
-148 @ 100k -140
-150 floor
max chg 1E-9 (-20 to 65) 5E-10 (-10 to 35)
pwr 1.2W op 0.12
1.8W warm 0.14
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