Hi Magnus –
To resolve the cliff hangers: 😊
There really wasn’t a way to “fix” the top cover effect. Looking back now as
a career engineer, rather than from my inexperienced view from 1986, what I
realize is that in any precise application, there will always be something
holding you a step away from perfection. You can’t tune this out in every
individual product. One challenge with the 5061B is that it was a *primary*
standard, meaning that it didn’t need calibration, by definition. If you
spent a bunch of effort calibrating a unit, is it still a primary standard?
With careful enough measurements, you could detect that all were not identical.
(We would do this by comparing the phase drift of the 10MHz signal against
our house standard. Over a few hours, an instrument would drift a few
nano-seconds. That is, the unit under test was different than the house
standard.
But it then begs the question, which one is right? Time keeping labs, like
the one with NBS in Colorado, or the west coast standard in our building in
Santa Clara CA, or the one in Geneva, would determine the “best” time by
averaging a bunch of primary standards. The unit that was closest to the
average would be declared the house standard. (At least that is how I remember
it worked.) Essentially establishing truth by taking a vote. (As many on
this list know so well.)
For a specific instrument shipping to an isolated customer, where they didn’t
have another standard to compare against, you had to take your cesium standard
as perfect. It was the primary standard after all. As the instrument would
get handled or power cycled, it could shift a touch each time. And God forbid
if you took the top cover off for some reason! All of Felix’s fine tuning,
screw by screw, would be lost. (I don’t think he actually hand tuned each
screw on every product. More likely, he pointed out that if the screws were
removed or tightened differently, the frequency offset would change. Not an
ideal behavior. )
From a practical “factory” approach to the 5061 products, we guaranteed that
every cesium standard was within specs. We couldn’t guarantee that they would
be exactly centered (if we could, we would tighten up the specs), or would
always remain centered (or even the same) if you messed around it with it.
Just comfortably within specs.
So, with no disrespect to Felix and his efforts to make things better, at some
point, work to perfect each unit becomes silly. You could fool yourself into
thinking it was better, but then only if the instrument was never touched,
moved, power cycled, etc. And even then, would it still be exactly the same
months later?
Fortunately, the managers and senior engineers had a realistic perspective on
how perfect was achievable. Felix was good in that he kept us from being
sloppy, and sometimes would find real things that we screwed up. But
torqueing screws carefully (and uniquely for each product), or twisting cables
left vs. right was a step too far. To really make the cesium standards
better, they needed a better design. And that was what the 5071A was all
about. But that is a story for another day.
Hugh Rice
Date: Fri, 21 Dec 2018 08:22:05 +0100
From: Magnus Danielson
<mag...@rubidium.dyndns.org<mailto:mag...@rubidium.dyndns.org>>
To: time-nuts@lists.febo.com<mailto:time-nuts@lists.febo.com>
Cc: mag...@rubidium.se<mailto:mag...@rubidium.se>
Subject: Re: [time-nuts] HP Cesium Standards in the International
Atomic Time Scale, the legend of Felix Lazarus, and the "top cover
effect".
Message-ID:
<6626d228-ec95-f4c6-a91e-73b37cee9...@rubidium.dyndns.org<mailto:6626d228-ec95-f4c6-a91e-73b37cee9...@rubidium.dyndns.org>>
Content-Type: text/plain; charset=windows-1252
Dear Hugh,
I really enjoyed reading this! You have several cliff-hangers in there:
Did you (HP) fix/reduce the top cover issue? Did you alter the setup to
meet tighter specs? Did you fix the oven controller cable offset?
What else war-stories do you got?
It is by war-stories one shares knowledge, lessons learned is not
without its background and at least you have a great story.
Cheers,
Magnus
On 12/20/18 12:36 AM, Rice, Hugh (IPH Writing Systems) wrote:
> Hello Time Nuts,
> I found this HP Application note in my archives, and attached a scanned copy:
>
> Application Note 52-4. Contribution of HP clocks to the BIH's International
> Atomic Time Scale (IATS).
> I also found a couple of archives for HP application notes for anyone who may
> be interested:
> http://hparchive.com/appnotes<http://hparchive.com/appnotes>
> https://www.keysight.com/main/editorial.jspx?cc=US&lc=eng&ckey=1127547&id=1127547&cmpid=zzfindclassic-app-notes<https://www.keysight.com/main/editorial.jspx?cc=US&lc=eng&ckey=1127547&id=1127547&cmpid=zzfindclassic-app-notes>
>
>
> It is an interesting snapshot at the method of keeping the official IATS
> time, and how HP Cesium standards are a major part of it, published in 1986.
>
> The author, Felix Lazarus, was a legendary Field Application Engineer (or
> something like that) for HP in Europe, based in Geneva Switzerland. He was
> obsessively fussy, and insisted that any Cesium Standard shipped to key
> customers in Europe were first shipped to him, so he could verify acceptable
> performance before the customer received the instrument.
>
> He would fire up the product, re-tune and re-align all the settings, and then
> compare it to his house standard. If it wasn't up to his exacting standards,
> he would keep tuning and testing until it was acceptable - to him. He was
> looking for performance several times better than our published
> specifications, which were 5 x 10e-12. He wasn't satisfied until is was less
> than 2, or something like that. It drove us factory guys crazy. He was a
> well-respected figure in the time keeping world, and would bash us for
> shipping product that were not beating the specs by enough margin.
>
> I think he is the one that discovered the "top cover effect". If you removed
> the top sheet metal cover from the instrument, the offset would shift by a
> part in 10 to the 12th or so. If you put the cover on, and changed how tight
> the screws were tightened, it would shift differently. I recall he wanted us
> to fix this.
>
> I was the "Production Engineer" on the Cesium standards, a young BSEE college
> graduate. I barely knew how a basic op-amp amplifier worked, and was
> completely overwhelmed by the complexity of the Cesium Standards. "Go fix the
> problem on the most accurate commercial atomic standard for sale in the
> world, where if you change how tight a screw is, the performance shifts a
> touch." It is safe to say that I didn't make this my highest priority. There
> were theories that the root cause was subtle changes to the ground loops with
> a change like this. The whole product used all the sheet metal as a common
> ground, meaning that the ground return paths were not designed at all, just
> left to chance.
>
> A related issue that I didn't work on was the "oven controller cable offset."
> There was a big multi wire cable o the cesium oven heater controller, and if
> you twisted it left vs. right before plugging it in, the offset of the
> standard would change.
>
>
> Working on the 5061B destroyed my confidence in my engineering abilities. I
> didn't think I could solve "real" engineering problems, because of issues
> like this. After working on the 5061B product for several years, I applied
> for a job as an engineering manager over the frequency counter production
> product line. During the interviews, my low technical self-confidence came
> through, and the R&D management partners to this position were worried I
> couldn't provide technical leadership to the other engineers. So, in true HP
> fashion, my they sent me through the full scale HP R&D engineering interview
> -about a half dozen deep 1:1 technical interviews with EE experts in the lab.
> Turns out that I wasn't a dunce after all, just scarred from my experience
> working on the cesium standards. I got the job.
>
>
> I have a handful of other stories like this from my days inside HP frequency
> and time division. Let me know if you want to hear more. Maybe Rick Karlquist
> will tell some stories of developing the 5071!
>
> Hugh Rice
>
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