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)
<[email protected]> 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.
Cheers,
Magnus
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