[time-nuts] Did a member of time-nuts buy this?
Bob Camp
kb8tq at n1k.org
Wed Dec 10 12:09:23 UTC 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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