[time-nuts] Disciplining Rubidium

Tom Duckworth tomduck at comcast.net
Thu Apr 24 00:14:36 EDT 2008


Great list! Your number 8 (magnetic field) will effect both. We tried this
when I was with XL Microwave. The Rb's noticeable effect depends on how
strong the magnetic field in the physics package is compared to the external
magnetic field. In the Rb, the external field would need to be quite strong
to see much change in the short term. The magnetic effect, long term, would
be swamped by thermal stability issues.

For the xtal, orienting the xtal 90°, 180°, upside down, etc. will have an
immediate noticeable effect that can bee seen on a scope (Earth's magnetic
field is responsible). This effect settles rapidly though.

Tom Duckworth

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Tom Van Baak
Sent: Wednesday, April 23, 2008 7:51 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Disciplining Rubidium

> 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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