[time-nuts] HP Cesium Standards in the International Atomic Time Scale, the legend of Felix Lazarus, and the "top cover

[Rice, Hugh (IPH Writing Systems)]

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 <magnus at rubidium.dyndns.org<mailto:magnus at rubidium.dyndns.org>>
To: time-nuts at lists.febo.com<mailto:time-nuts at lists.febo.com>
Cc: magnus at rubidium.se<mailto:magnus at 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-73b37cee913c at rubidium.dyndns.org<mailto:6626d228-ec95-f4c6-a91e-73b37cee913c at 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
>