[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. Its 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, GPSDOs are the more useful device. Their
>> performance rivals that of most of the older Cs standards. They are way cheaper,
>> and they dont wear out. Indeed, if you have a 5071A with a high
>> performance tube in it, a GPSDO is not going to match its performance. Ive
>> 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 its 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 cant 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, Im not so sure thats 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. Thats even more true if you need a fully functional 5071A to do the
>> comparison. Lets see .. new BMW or rebuild the 5071
> hmmm :)
>>
>> Bob
>>
>>>
>>> Cheers,
>>> Magnus
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