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

Bob Camp kb8tq at n1k.org
Sat Dec 6 12:06:17 EST 2014


If you toss a Rb into the GPSDO “mix” things can get quite good. The Rb *should* be better than an OCXO in the > 1,000 second range. It’s crossover with the GPS ADEV will be further out than the OCXO’s. The gotcha with both the OCXO and Rb is their temperature dependance. Some / many / all of the lower cost Rb’s are not much better than a good double oven OCXO in terms of raw temperature performance. The approach they use to “correct” this does not help their ADEV at all. Yes, you can disable the correction and put the whole thing in a temperature controlled environment. 

Lots of details to take care of. If you get them all right, you’ll beat any / all of the older Cs standards. That *assumes* that GPS is not deliberately lying to you :) … (off to the Conspiracy Time Nuts mailing list). One very cute addition would be to pull down the NIST GPS data and use it to correct your system on an hourly / daily basis. If you do that with common view satellites, you most certainly will beat a surplus grade Cs standard. 


> On Dec 6, 2014, at 11:47 AM, Bert Kehren via time-nuts <time-nuts at febo.com> 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
> 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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