[time-nuts] Re: Project Great

[Lux, Jim]

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
 Â Â  Â Â Â Â  8E-11 @ 10Â Â Â  8E-11
         2E-10 @ 1 sec    2.5E-10

phase noise
 Â Â  Â Â Â Â  -86 @ 10Â Â Â  -70
 Â Â  Â Â Â Â  -120 @ 100Â Â Â  -113
 Â Â  Â Â Â Â  -138 @ 1000Â Â Â  -128
         -143 @ 10k    -135
         -148 @ 100k    -140
 Â Â  Â Â Â Â  -150Â Â Â  floor

max chg        1E-9 (-20 to 65) 5E-10  (-10 to 35)

pwr        1.2W op        0.12
         1.8W warm    0.14