[time-nuts] Disciplining Rubidium

Tom Van Baak tvb at LeapSecond.com
Wed Apr 23 22:51:20 EDT 2008

> 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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