[time-nuts] Did a member of time-nuts buy this?

Bob Camp kb8tq at n1k.org
Sun Dec 7 19:34:02 EST 2014


Hi


> On Dec 7, 2014, at 7:15 PM, Angus <not.again at 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.

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,  
>> kb8tq at n1k.org writes:
>> 
>> Hi
>>> On  Dec 6, 2014, at 10:35 AM, Magnus Danielson 
>> <magnus at 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) 
>> <drkirkby at 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 at 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 at 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 at 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 at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.



More information about the time-nuts mailing list