[time-nuts] FE-5680A Question

Bob Camp lists at rtty.us
Wed Feb 15 17:49:21 UTC 2012


They do indeed need to know that they start low and go high. They do need to
sweep to either side of the resonance. Every Rb I've ever torn into does
those things. 

Why sweep *way* low, and almost not sweep high enough? Normally the sweep is

Why spend most of the sweep time not sweeping and then sweep real fast? I've
never seen one that stops at the ends.

Wouldn't it be better to spend the same time sweeping slowly? That's what
the other designs all do. 

We're not going to do much about the dead time, but the centering of the
sweep is something you will impact if you re-tune the VCXO. When you do
retune it, it would be nice to know if the centering was dead on when
manufactured and it's drifted,  or if it needs to be off center in order to
lock properly under all conditions. 

What could happen? There are multiple transitions in the Rb atom. Some of
them are pretty close to the one we use. If you lock onto the wrong one, you
are on the wrong frequency. Different designs get around the problem
different ways.  


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Chris Albertson
Sent: Wednesday, February 15, 2012 12:36 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] FE-5680A Question

On Wed, Feb 15, 2012 at 9:12 AM, Bob Camp <lists at rtty.us> wrote:
> Hi
> Which still gets us back to - why the really odd sweep on the FE's? and
> should you center the VCXO as a matter of routine maintenance?

I think a very asymmetric sweep makes the most sense.

First some history of sailing ships.   Back in the 1600's navigation
was not perfect and you never knew your exact position on the open
ocean.   Knowing within 30 miles was hard to do.  So to get to a given
location in North America from Europe they typically would aim about
50 or more miles to the north of their intended destination and then
when they reached land would sail south until they found the
destination.   This added a day or more to the trip.   If they tried
to hit the target dead-on they would likely miss but then they'd have
to literally guess wetter to go North or South and if they guessed
wrong it could really be bad so they always headed for enough north of
the target so there was no guessing about which way to turn.

I think the sweep is done the same way.  If you start way low you know
100% which way to go, the lock has to be up.  Seeing as we know they
are using software in the loop this makes sense.

So on pwr up the uP starts looking with bottom up sweeps.   The sweeps
fail to find lock because the temperatures of the crystal and Rb are
to low but after some minutes, finally a sweep finds the lock

Chris Albertson
Redondo Beach, California

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