A quick intro for those who haven't seen past "HP Stories": I work for Hewlett Packard, and my first job at HP in 1984 was working on the HP 5061B Cesium Beam Frequency Standard. I was part of the two man team that did the 5061A to 5061B development, and then I was the "production engineer" on the 5061B for a few years. I have been writing a series of stories from the old days for the entertainment of the time-nuts mailing list.
---------------------------- The 5061A "A4 Harmonic Generator", at first glance, appears to be an odd mechanical device attached to the top of the Cesium Beam Tube in the 5061A. I have faint memories of technicians using it as a carrying handle for the CBT, when removing it from the instrument. But it has that suspicious shape of a microwave wave guide, where it attaches to the CBT, that tips off trained eyes that this is much more than an oddly shaped carrying handle. As my knowledge of the 5061B system grew (under the tutelage of Chuck Little, Lou Mueller and Robert Montesi), the Harmonic Generator innocence transformed into an intimidating presence. A quick read of the theory of operation from the service manual seems simple enough. "The phase modulated 5MHz signal is multiplied by 18 in the A3 Multiplier, and then multiplied by 102 in the A4 Harmonic Generator Assembly." No big deal. Easy math. "Just" multiply the 5MHz signal a couple times. But later in the details of the Harmonic Generator: "The A4 Harmonic Generator generates a microwave signal of 9192.63... MHz (phase-modulated microwave by 137 Hz) to be applied to the cesium beam tube. A4 receives a phase-modulated 90 MHz signal from the A3 Multiplier and a 12.63... MHz signal from the A1 Synthesizer. Step-recover diode CR2 is the heart of the harmonic generator. The conductivity of the p-n junction diode during reverse recovery is nearly a step function. The transition from reverse conduction to cutoff occurs in about 0.1 nanoseconds. This produces high-order harmonics with greater efficiency than conventional non-linear harmonic generators." It continues on for three more paragraphs in more obscure RF circuit nomenclature detail. The old guys, when talking about the harmonic generator and frightening me about the black magic it contained, often concluded with: "But it works really well, so just don't touch it." Rick Karlquist in a recent posting talked about the RF design work he did on the 5071A, replacing the A2 Synthesizer, A3 Multiplier, and A4 Harmonic generator with modern (1990) techniques to generate the 9,192,631,772.5 magic Cesium frequency, without the use of wave-guides and step recovery diodes. Rick mentioned the Korean visiting professor who invented the A4 in about 1964. Lou Mueller once told me a story about this same professor (who I will call Professor K), when he was working at HP back then: Professor K had an elaborate microwave test bench, full of wave guides, attenuators, tuning circuits and RF instrumentation that he used in development of the A4. One day someone thought it would be fun to play a joke on him, and put a honey-bee inside the wave guide in his test setup. Professor K returns from lunch, and starts noticing anomalies in his measurements. He carefully measures and calculates, scratching his head to figure out what was wrong. After a bit of analysis, he finally unbolts on section of the wave guide, which according to his calculations was the problem. And sure enough, that was were the bee was. Looking back, the A4 was an brilliant design. It was in production from 1965 to 1990, 25 years, essentially untouched and working flawlessly the whole time. As you can see from the attached picture, there are a number of adjustments that can be made to the A4, which leads to Vic Olsen. Vic was in his early 60's when I met him, and the epitome of a curmudgeonly old school tech. A bit course around the edges, wore the same sloppy jeans and simple plaid shirt to work every day, but knew how to do his job, and begrudgingly endured ignorant young engineers like me. Vic was one of the two technicians that worked on the 5061A in production, and was the one who did the adjustments on the A4 as it was assembled into a complete instrument. I have faint memories of the ancient test setup he used to "tune" the A4 before attaching it to the CBT. There was some sort of oscilloscope like instrument with a small round CRT, with a few wave guide like things. I'm sure they all dated back to the original 5060A setup from 1965. Vic would fiddle with all the adjustments on the A4 for a few minutes, in a seemingly random manner, and then declare it done. He then would really crank down on all the lock nuts for each tuning element, and the attachment for the step-recovery-diode. "Gotta make these things tight, or the tuning will drift off" This concluded with him bashing the A4 on all sides on the table top, and then doing a final check that was all OK. And then re-checking that all threaded devices on the A4 were really tight. One of my jobs was to update all the production documentation for the 5061A to 5061B. I'm sure there was a procedure for tuning the A4, but I have zero memory of the details. Perhaps I looked at what was written, and then watched what Vic did, and noticed virtually no correlation. I suspect that I then concluded: "Well, what he is doing seems to work. Maybe I should just move on to another task." And then hope Vic never retired or got really sick. In about 1987, disaster struck the A4. The casting vendor that had been making the main body of the Harmonic Generator decided that obscure, hard to cast parts, in very low volume (~15 per month) wasn't business they wanted any more, and told us to find a new supplier. They agreed to mail the molds back to us to pass along to our next supplier. A couple of weeks goes by, and the mold tooling never showed up at our shipping dock. We all panicked. I have vivid memories of a meeting called to figure out what do. We had production engineering managers, production managers, and the buyers (people who interfaced with our suppliers) in the meeting. There was an eager young buyer, maybe 30 years old, who exclaimed with glee: "These kind of challenges are where buyers really get to earn their keep!" He saw this as a fun opportunity to save the day! I remember thinking: "This is an unmitigated disaster. This part is the very definition of unobainium. No one who works at HP has any idea what the critical elements are of the harmonic generator design, and we've not seen Professor K who invented it in 20 years, if he is even still living. When we run out of parts in inventory, HP is done making Cesium Beam Frequency Standards. The end of legendary product." And then during the meeting, someone pulled me out of the meeting and said I should go to the receiving area. The guy who oversaw incoming deliveries in found a poorly labeled box, and kinda remembered me from when I had been down hunting for treasure the week before, and maybe this is what we were looking for. It was the mold tooling. I rushed back to the meeting with the news, to the huge relieve of all that were there. While I don't remember the reaction of the buyer, I'll bet he was like: "See, we figured it out! I knew we could do it!" Meanwhile, all the engineers realized just how close to disaster we had just been. And everyone lived happily ever after. With the new tooling, the A4 Harmonic generator still worked, and we kept building the 5061B for a few more years until it was replaced by the 5071A. Hopefully some of you will find this interesting. If you would like to see more HP stories, keeps sending me notes of encouragement. Hugh Rice
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