[time-nuts] a link to a explanation of Rb vs Cs?

Bob kb8tq kb8tq at n1k.org
Tue Apr 11 15:59:23 EDT 2017


Assuming you are doing a “conventional” Rb and Cs there are a number of differences. There is the sub set of doing a gas cell based on Cs which is 
a lot more similar to Rb. With the Cs, you are building a very complicated vacuum tube that plays with a focused beam of ions traveling in space. With the
Rb you have essentially a couple of odd (Rb based) "neon bulbs" that do some cute interactions optically. Making light bulbs is easier than making 
microwave vacuum tubes. Temperature and magnetic filed mess with both systems so they need to be controlled in both cases. 

The more subtle issue is that we have an implicit expectation of performance when we talk about an Rb or a Cs. An ADEV of 1x10^-13 at 1,000 seconds is 
interesting. An ADEV of 1x10^-9 at 1,000 seconds is not as interesting. Going from a level of “it works” to the point that it works reasonably well is a 
long road with lots of zigs and zags. Teams of people do indeed spend a lot of time learning where those twists in the road are. In the case of an Rb, a half 
dozen people working full time for a decade is probably in the range. For a Cs both the group size and time frame would be longer. In both cases there is 
a “right” mix of skills for the team members, it’s not just a body count. 


> On Apr 11, 2017, at 10:30 AM, jimlux <jimlux at earthlink.net> wrote:
> I'm looking for a link to point to an explanation (at a basic level) of the difference between Rb and Cs references, and what the tradeoffs are.
> I googled a bit, but all I got were some explanations of the differences in things like vapor pressure, etc.
> Wikipedia  (Atomic_clock) has some nice words, but no block diagram of the innards, which would be nice.
> And there's plenty of charts showing the relative performance of Rb and Cs, but not with any accompanying explanation of why.
> I'm looking for something a bit more detailed, but not a 20 page tutorial on atomic clocks.
> Maybe someone has seen something on a manufacturer website or something?
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