Hi > On Dec 9, 2014, at 9:46 PM, Angus <not.ag...@btinternet.com> wrote: > > On Sun, 7 Dec 2014 19:34:02 -0500, you wrote: > >> Hi >> >> >>> On Dec 7, 2014, at 7:15 PM, Angus <not.ag...@btinternet.com> wrote: >>> >>> On Sat, 6 Dec 2014 11:47:10 -0500, you wrote: >>> >>>> I am looking forward to long term data on the Lucent unit. GPSDO's are >>>> getting closer and closer to Cesium. Having worked for 18 month on two >>>> GPSDO >>>> projects we find that the limiting factors are the Cesium Standards. >>>> Working >>>> presently on a Cesium GPSDO. Short term OCXO, medium Rb and long term >>>> Cesium. With Cesium may be able to use 14 day filter. Will find out. If >>>> we do >>>> not see an improvement we will most likely retire our Cesium units. >>>> Bert Kehren >>> >>> Hi Bert, >>> >>> Out of curiosity, what Rb are you using, and how does it respond to >>> air pressure changes? >> >> Properly identifying / measuring pressure induced drift is not as easy as >> one might think. The tweak and see approach seems to be the best bet. Hmm > I wonder who originally suggested that > . oh, yea it was Angus. > > That depends a lot on the Rb. With a temperature controlled LPRO it's > easy - just logging air pressure against frequency get you most of the > way. With the LPRO's I've tested that gets the variation with pressure > to down under +/- 2E-13, some well under, over 60mbars pressure range. > I have one that had almost no residual left after correction, though > the others had a little. Getting past that is harder. > > There is also some time lag - looked like something around 3/4Hr, but > with GPS as my reference and air pressure moving so slowly, it was > hard to tell. > > Of course the lower the numbers, the more error sources start to > become significant, but since the LPRO's I've test are all around > 8E-14/mbar, it's not exactly hard to measure. It is time consuming > though, since it normally took 3-6 weeks to do each test, depending on > the weather. > > Interestingly, the both the FE5680A's I tested had similar responses > to pressure variations - very variable compared to the LPRO's, so > impossible to correct simply and well. Seeing the comments about the > temp correction in FE5680A's causing problems, I did wonder if that > might be part of the problem, but have not got around to testing that > yet.
….. and I’ve been playing with FE5680’s. They (for what ever reason) do not correct easily. > > Angus. > >> Bob >> >>> Combining temp control, air pressure compensation and drift >>> compensation can give very good results with the right Rb. >>> >>> Angus. >>> >>>> >>>> In a message dated 12/6/2014 10:46:57 A.M. Eastern Standard Time, >>>> kb...@n1k.org writes: >>>> >>>> Hi >>>>> On Dec 6, 2014, at 10:35 AM, Magnus Danielson >>>> <mag...@rubidium.dyndns.org> wrote: >>>>> >>>>> Bob, >>>>> >>>>> On 12/06/2014 04:16 PM, Bob Camp wrote: >>>>>> Hi >>>>>> >>>>>>> On Dec 6, 2014, at 9:54 AM, Dr. David Kirkby (Kirkby Microwave Ltd) >>>> <drkir...@kirkbymicrowave.co.uk> wrote: >>>>>>> >>>>>>> I see this cesium reference on eBay, where apparently someone returned >>>>>>> it due to the fact it had a bad tube. >>>>>>> >>>>>>> >>>> http://www.ebay.com/itm/HP-Agilent-5061A-Cesium-Beam-Frequency-Standard-FOR-PARTS-REPAIR-/141483787108 >>>>>>> >>>>>>> I'm wondering if it was someone on this list. It is likely to be >>>>>>> practical to replace the tube? >>>>>>> >>>>>> >>>>>> New tubes for Cs standards are in the >$20K range. Getting a modern one >>>> re-tubed with a high performance tube is > $32K. >>>>>> >>>>>> The stock of ?new old stock? tubes is long gone. About the only tubes >>>> you see are pulls from used gear. The question with them (as with any Cs) >>>> is just how many years (or months) is left on the tube. You physically >>>> move >>>> Cs from one end of the tube to the other when you operate the device. One >>>> you have exhausted the pre-loaded stock, the tube is dead. It?s also >>>> coated >>>> all over the inside with surplus Cs. Since signal to noise ratio is very >>>> important, the drop in Cs at end of life and crud on the inside leads to >>>> degradation in the performance towards the end of the tube life. Even if >>>> the >>>> tube works, it may (or may not) be useful in a given application. >>>>>> >>>>>> For many applications, GPSDO?s are the more useful device. Their >>>> performance rivals that of most of the older Cs standards. They are way >>>> cheaper, >>>> and they don?t wear out. Indeed, if you have a 5071A with a high >>>> performance tube in it, a GPSDO is not going to match it?s performance. >>>> I?ve >>>> replaced two tubes in one of those, so they are correct when they talk >>>> about the >>>> projected life of the tube. >>>>>> >>>>>> The other subtle issue with Cs standards is shipping. If you are going >>>> to do it ?right? it?s a major pain. Sending one back for re-tube does >>>> require you to do all the formal shipping nuttiness. That may or may not >>>> be an >>>> issue on the surplus market >>> . >>>>> >>>>> Well, there is one use-case for a cesium, which is the validation of GPS >>>> receivers. Rubidiums do help to some degree. Comparing two GPS clocks >>>> with >>>> their highly systematic sources, so you can't get useful differences that >>>> way for the stability of the produced signal. >>>> >>>> Unless you are making a GPS receiver from scratch (which you might be), >>>> there is a certain ?trust factor? that comes into using a GPS for timing. >>>> Since you can?t play with the firmware, you trust that the guy who wrote >>>> it >>>> did a good job. >>>> >>>> In making a GPSDO, yes on a commercial basis verification against primary >>>> standards is likely to be required by this or that customer. In a >>>> basement >>>> lab, I?m not so sure that?s true. Simply comparing things against an >>>> ensemble of ?known good? designs (and cross checking the results) should >>>> be >>>> good enough. If your design passes the performance of the ensemble, >>>> building >>>> several of your design is likely to be cheaper than keeping a Cs running >>>> long >>>> term. That?s even more true if you need a fully functional 5071A to do the >>>> comparison. Let?s see .. new BMW or rebuild the 5071 >>> hmmm :) >>>> >>>> Bob >>>> >>>>> >>>>> Cheers, >>>>> Magnus >>>>> _______________________________________________ >>>>> time-nuts mailing list -- time-nuts@febo.com >>>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>> and follow the instructions there. >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.