[time-nuts] Disciplining Rubidium

SAIDJACK at aol.com SAIDJACK at aol.com
Thu Apr 24 01:57:33 EDT 2008

Hi Tom,
by the way, are Cesium and Rb isotopes used in these clocks  radioactive to 
any degree? I remember on your website you mentioned that Cesiums  have to be 
shipped as hazardous material..
In a message dated 4/23/2008 19:53:13 Pacific Daylight Time,  
tvb at LeapSecond.com writes:

>  More precisely, if I had two black boxes, one containing
> a GPS-Rb-XTAL  setup and another containing GPS-XTAL,
> what measurement would you make  from outside the boxes
> to distinguish from one another?

Ah,  clever question. Here's ten ways to distinguish them:

1) Use a scale or  ruler -- the GPS-Rb-XO is likely heavier
and larger.

2) A wattmeter  -- the GPS-XTAL will probably use less power.

3) Thermometer -- the Rb  version will likely get much warmer,
maybe even requiring a heat  sink.

4) Use a frequency counter -- until you connect the  antenna
the GPS-Rb-XO will be much more accurate. It will also warm
up  quicker; take less time to be on-frequency, something you
can see by  plotting a series of frequency readings.

5) Time interval counter or  phase meter -- the GPS-Rb-XO
will also be more stable, mid- to long-term.  However, when
you connect the antenna, the long-term stability will be  about
the same for both.

6) Turn-over -- if you flip the black box  upside down, the Rb
version will show little or no deviation. The GPS-XO  version,
on the other hand, will probably show a sudden deviation  in
phase and frequency, lasting seconds to minutes. Portable
use, shock,  and vibration might also reveal differences.

7) Hold-over -- remove the  antenna and watch the frequency
drift over hours or days. The Rb version  should be ten to a
thousand times better at timekeeping than the XO  version.

8) Magnetic field -- (this is a guess; I've not tried it)  place the
black box near a strong magnetic field. The XO version  won't
care much but the Rb version will start to suffer.

9) Patience  -- run them both for the rest of your life. The one
that fails first is  likely the Rb.

10) Compare the credit card receipts.  ;-)


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