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

EWKehren at aol.com EWKehren at aol.com
Sat Dec 6 11:47:10 EST 2014

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:

> 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.
>>> 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  :)


> Cheers,
> Magnus
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