[time-nuts] Symmetricom CSAC is Cs acting like a Rb unit

Magnus Danielson magnus at rubidium.dyndns.org
Wed Jan 19 20:14:33 UTC 2011

On 01/19/2011 04:17 PM, Brooke Clarke wrote:
> Hi:
> Here's a response I got from Symmetricom:
> · The SA.45s actually uses cesium, not rubidium. But it is a vapor-cell
> clock just like our Rb clocks.

Which is apparent from papers published over time.

> · The Tx and Rx pins are for an RS-232 interface. Using the interface,
> the clock can be programmed, and the clock’s functions can be monitored.
> I have attached a User’s Guide that describes all the commands in detail.

It's didn't make it through. Toss it my way and I toss it on the web.

> · The SA.45s does not attempt to correct the 1 PPS input, nor does it
> use a sawtooth to correct itself to the 1 PPS input. Instead, it can be
> commanded (see User’s Guide) to discipline itself to the 1 PPS input.
> How fast it does this is a function of user-selected bandwidth inputs.

Slap a GPS receiver on top of it and a small micro-controller to 
interface to them both and you have one compact solution and really nice 
hold-over properties for its size and power.

> I get the feeling that a vapor cell is not an absolute standard but
> rather works like a Rb vapor cell, i.e. the frequency has some drift and
> the starting frequency depends on how the cell was made.

You *will* have wall-pulling effects, to some degree compensated by the 
buffer gas and also the laser amplitude will cause a pulling effect.

This is a secondary standard and not a primary standard. Compare it up 
against telecom rubidiums for a more suitable size, power and 
performance matchup. This is not to discredit it, but rather put it in 
the right context.

> The 1 PPS output can be synchronized (using the 1 PPS input) to the
> nearest 10 MHz zero crossing (i.e. to within 50 ns). With a continuously
> applied 1 PPS input after some time it will adjust both the frequency
> and 1 PPS output to about 5E-13.

Not bad.


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