Hi

Since the physics package in the small Rb’s is different than the stuff in the large units,
you have some basic limits on what you can do to improve them. The main things people
have done are to modify them to turn off the temperature compensation and replace it
with some sort of precision controlled thermal enclosure. Pressure compensation is a good
idea on any of these parts (large or small). How much your particular unit benefits is a
“that depends” sort of thing.

Bob

Hi
I am wondering if anyone tried to put a Rb unit into a vacuum container. And how much the performance is improved? Someone told me that's why the Rb clocks are more stable than Cs clocks on the GPS satellites.

LiAng

---Original---
From: "Bob Camp"kb8tq@n1k.org
Date: 2017/1/17 21:20:23
To: "Discussion of precise time and frequency measurement"time-nuts@febo.com;"Perry Sandeen"sandeenpa@yahoo.com;
Subject: Re: [time-nuts] How to create a super Rb standard

Hi

Since the physics package in the small Rb’s is different than the stuff in the large units,
you have some basic limits on what you can do to improve them. The main things people
have done are to modify them to turn off the temperature compensation and replace it
with some sort of precision controlled thermal enclosure. Pressure compensation is a good
idea on any of these parts (large or small). How much your particular unit benefits is a
“that depends” sort of thing.

Bob

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Hi

You would need to redesign the Rb to work in a vacuum. There is a lot involved
in doing that. They depend on the air inside the package to properly operate the
heating in the physics package. Different parts of the package are at different
temperatures. Without the air cooling the temperature offsets could not be properly
maintained.

There are a lot of differences between the GPS Rb’s and the ones we buy on
eBay. The fact that they operate in pretty far down the list of significant differences.
The most simple answer to “why” the Rb is better is that the design requirements
on the two standards were different, as were the design teams. Coming up with
an Rb with better short term stability is the way that all worked out. That short
term stability is better than other large cell Rb designs, but not by a crazy amount.
How much better it is depends a lot on which large cell Rb design you compare to.
It also depends a bit on which specific unit you are looking at.

All that said, space benign is indeed a pretty quiet environment. It certainly does
help a bit if you operate there. There is also data on the GPS Rb’s that show them
doing quite well on the ground. So no, it’s not all space, but space does not hurt
their performance.

Bob

Hello,

And about temperature, in this article
http://www.dtic.mil/dtic/tr/fulltext/u2/a495520.pdf there is an
interesting point, in the last page: Riley answer to the question "You
mentioned larger cells. Where there any other things done go get these
fantastic results?

Regards,

Javier

On 18/01/2017 13:12, Bob Camp wrote:

On Tue, Jan 17, 2017 at 9:23 PM, Li Ang 379998@qq.com wrote:

Vacuum is a VERY good thermal insulator.  If the electronics used even
just a few watts of power you'd need some way to cool it, other then
convection.    Almost all electronics depends on air for cooling.  You'd
need to redesign it for conductive or radiative cooling.  That is not easy.

I did something very simple that works.  I placed the Rb unit in a metal
box and used a thermostatically controlled fan. The fan speed is controlled
by a PID controller.  It all runs on one 8-pin AVR chip,  I use two analog
inputs for two temperature sensors (one epoxied to a heat sink and one in
free air.  Another pin is analog output to control the fan speed.  In
improvement would be to use a three wire fan with a tachometer.

I tried before to make an analog fan controller but without using a dozen
op amps it is hard to implement any kind of decent algorithm.

But BEFORE you try and improve the Rb performance you need to have some way
to MEASURE its performance.  This is likely much harder.

Chris Albertson
Redondo Beach, California

Hi,

I'm always a bit surprised that it's not done more.  Adding

temperature control, air pressure compensation and drift compensation
transforms the medium to long(ish) term performance of a lot of
rubidiums.

A typical Adev plot for a rubidium starts increasing again

after a few 1000's of seconds as environmental factors take their
toll, but with even a simple version of these mods it's more like
days. This makes it a great reference for testing GPS receivers and
GPSDO's among other things.

A basic setup could be something like an LPRO in a temp

controlled enclosure, with an analogue pressure sensor feeding into an
EFC circuit, both temp controlled with the LPRO. Log it against GPS
and you get the long term (like weeks+) ageing which can then be
removed. Even simpler, you could just log the air pressure and correct
for that too, but that's more of a pain if it's being used as a
general purpose reference.

Of course, the problem then is testing just how good it is....

Angus.