Very few people really "NEED" a Rb or Cs as they are not really doing
anything that requires that level of precision in frequency or accuracy in
time (me included). These days a BC-221 or WWB is not good enough, but 1
Hz at X-Band is for almost all practical uses.

It's a hobby. Just enjoy it!

FWIW,
-John

================

I know I don't "need" any of this stuff. I was just wondering what I
could do with a rubidium vs what I already have.

On Mon, Oct 5, 2009 at 10:58 AM, J. Forster jfor@quik.com wrote:

Here's a reason why you might "want" (as opposed to "need"!) another
standard beyond the GPSDO.

An Rb or Cs is self-contained.  The GPSDO relies on an external factor,
the presence of GPS.  Holdover mode may be good to keep thing close for
a while, but over the long term the GPSDO without GPS is no better than
its crystal oscillator.

The Rb is a secondary standard and therefore isn't "correct" by
definition, but it has a low aging rate and gives you something
independent of GPS to use for measurements.  Monitor it against GPS for
a while to learn its offset and drift characteristics, and then you can
extrapolate its performance out over a much longer time than you could
with an OCXO.

You may not require that independence, but it gives you additional
measurement capability.  For example, comparing the output of two GPSDO
may not be meaningful because their frequencies could be correlated by
their common view of the GPS constellation.  Using an Rb reference would
eliminate that common mode error and reveal information about the
GPSDO's short and medium term stability that would otherwise be hidden.

John

Joseph Gray wrote:

Thanks, John. That was the type of information I was looking for.

Joe Gray
KA5ZEC

On Mon, Oct 5, 2009 at 11:34 AM, John Ackermann N8UR jra@febo.com wrote:

A GOOD crystal oscillator will stay w/in parts in 10E10 for days at least.
And I don't mean a DO. Just run it with a continuous comparison to LORAN
or WWVB or GPS and MANUALLY tweek it as needed. A benefit is that you KNOW
when you made an adjustment and not have to worry about random changes
from the oscillator. That's a reason I stopped using mine.

frequency in absolute terms. Being very stable is often enough. As with
voltage and current, often what really matters is the ratio, not the
absolute value. Only if you are doing fundamental physics research or
tracking crustal movements is such ultra high precision really needed.

Why do you really care if your clock is off a few nanoseconds, or your
position off a few feet? If you are going to put up a fence, the legally
defined property line is the issue, not what GPS says.

My main gripe w/ Rb and especially Cs is they have very limited lives.
IMO, it's just not worth paying extra money for stability I don't need. If
I ever do need their capabilities, there are several on the shelf, but
until then they can sit unpowered.

FWIW,
-John

==============

On Mon, Oct 05, 2009 at 12:35:17PM -0700, J. Forster wrote:

The telcom grade (LPRO and similar) Rb's that show up cheap are

supposedly designed for 20 years life continuously operating... and some
have only a few hundred hours on them or less when surplussed.  They
should typically last 10-15 years at least in continuous home lab
service and given their price these days the electricity to power them
for that time interval is much more money than the hardware.

Cesiums have REALLY expensive tubes with limited life, but on the

other hand I understand the tubes don't like to be kept turned off... they
deteriorate when sitting on a shelf and may not recover.

--
Dave Emery N1PRE/AE, die@dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."