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

Bob Camp kb8tq at n1k.org
Wed Dec 10 07:09:23 EST 2014 

Hi
> On Dec 9, 2014, at 9:46 PM, Angus <not.again at 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.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.
> 
> 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,  
>>>> 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
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