Hi 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 centered. 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. Bob -----Original Message----- From: [email protected] [mailto:[email protected]] 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 <[email protected]> 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
