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.
