App Note 70 part 3

Appendices C and D discuss circuit-probing and breadboard-construction techniques. We've seen a lot of this advice before (in App Note 47), but here the emphasis is on low-level sensitivity, rather than high-frequency fidelity. In short, it's all about the experimental measurements. I love the quote,

When results seem optimal, design an experiment to test them. When results seem poor, design an experiment to test them. When results are as expected, design an experiment to test them. When results are unexpected, design an experiment to test them.

The stars of Appendix C are the isolated trigger probe (Figure C15) and the adaptive amplifier (Figure C18). Jim sure did love his probes! Back in Chapter 17 of his second book, he said, "It's too embarrassing to print how many probes I own", and here he is inventing and building new ones.

Appendix E briefly discusses selection criteria for linear regulators. Really, I think that it was included as an excuse to show off his HP200A oscillator in Figure E2.

This photograph is perhaps a hint as to what he did in the two years since the publication of App Note 65. In Figure E1, the oscillator is label "High voltage, floating output sine wave generator, HP200A typical". Typical. Ha!

Appendix F discusses transformers and inductors. Appendix G was guest-written by Carl Nelson, discussing the advantages of slew control. Appendix H discusses hints for low-noise performance, including slew-rate adjustment, component selection, and another admonition to read Appendices B and C. The warning in the footnote on page 50 is my favorite: "I do not wax pedantic here. My guilt in this offense runs deep."

Appendices I and J are two more guest-written contributions. Appendix I (from Jon Roman of Coiltronics) discusses radiated noise from transformers and inductors. Appendix J (from Bruce Carsten) discusses a clever EMI sniffer probe. (Another probe!)

Appendix K shows the results of using an LT1533 low-noise power supply with a 16-bit analog-to-digital converter. Perhaps a preview of things to come? Is Jim thinking about A/D conversion?

The app note concludes with a (hastily drawn?) cartoon. "I can't believe it's not a battery."

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Related:

• App Note 70 part 1
• App Note 70 part 2

Last Day of Classes

Today, I teach my last class of the term for MIT 6.331 (Advanced Circuit Techniques). This term has been particularly bittersweet, since every other time that I've taught this class over the past twenty years, Jim has flown out and given a guest lecture. These lectures were always interesting, entertaining, and well attended.

The last time I taught this class (two years ago), Jim came out and gave a lecture on the history of the Hewlett Packard 200A oscillator. He also told the tale of the acquisition of his own (very early model) HP 200A, which is a great story (see Figure E2 in App Note 70). One of the attendees, my friend Anson Whealler, took this photo right before the lecture.

We miss you, Jim.

Book 2 Chapter 17

Chapter 17, "There’s no place like home"

Jim's final chapter in the second book discusses the importance of having a home laboratory. "I estimate that about 90% of my work output has occurred in a home lab... A lot of work time is spent on unplanned and parasitic activities. Phone calls, interruptions, meetings, and just plain gossiping eat up obscene amounts of time."

The chapter is filled with great advice. In the following pages, he discusses the requirements for a home lab, including oscilloscopes ("Types 547 and 556 are magnificent machines"), probes ("It's too embarrassing to print how many probes I own."), power supplies, signal sources ("The Hewlett-Packard 200 series sine wave oscillators are excellent, cheap, and easily repaired."), voltmeters (Fluke handhelds and the HP3400A, of course), and other instruments.

The chapter is peppered with glamor shots of his home lab, such as the one below.

Beautiful.

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Related:

• Book 2 Introduction
• Book 2 Chapter 1
• Book 2 Chapter 11

Book 1 Chapter 7

Chapter 7, "Max Wien, Mr. Hewlett, and a rainy Sunday afternoon."

The first time I read this chapter (back in grad school), I was quite intimidated. It would take me two weeks to design, build, test, and document all of the circuits in this discussion, and Jim did it in one Sunday afternoon?!? I'll never be that good of an engineer.

But this chapter title is a bit of a fib, because it didn't all happen on a single rainy Sunday afternoon. He had clearly been thinking about Hewlett's oscillator since App Note 5 (Figures 12, 13, and 14), and most of this chapter is based on circuits that appeared in App Note 43 (Figures 39 through 49).

On the first page, he writes "No lab is complete without an HP series 200 oscillator." We should all be so lucky! However, on page 47, he seems to admit that (at the time he wrote this chapter), he owned an HP 201 oscillator. His HP 200A must have come later.

This chapter is worth reading, even if you have memorized App Note 43. His conversational tone gives a window into his thought processes (even if the "rainy afternoon" is a work of fiction). This chapter is also noteworthy for his clear exposition of Williams's Rule (his spelling), "a little known tenet of precision op amp circuits... always invert (except when you can't)."

It is curious that all of the schematics in this chapter are drawn by hand:

For App Note 43 all of these schematics had been neatly drawn by someone at Linear Technology. Why did Jim redraw everything by hand? Are these the original schematics from the lab notebook before the Art Department got a hold of them? Did he feel challenged by Bob Pease (see Chapter 9) for the crown of "Messiest Schematics in the Book"?

There is a typo on the top of page 44. He writes "DeForest hadn't even dreamed of adding a third element to Edison's Effect in 1891." He misspelled "DeForest hadn't even dreamed of adding a third element to the valves he stole from John Fleming."

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Related:

• Book 1 Introduction
• Book 1 Chapter 4
• Book 1 Chapter 13
• Book 1 Chapter 23
• App Note 5
• App Note 43 part 2

App Note 43 part 3

Just a quick post to finish up App Note 43: This note ends with five appendices, two of them with guest authors. In Appendix A, Jim writes a little about the history and construction of strain gauges, and introduces the first guest authors from Motorola Semiconductor (since spun off into ON Semiconductor and Freescale Semiconductor), who write about semiconductor-based strain gauges, which they spell "gage" (Jim explains the alternative spelling in the first footnote on page AN43-36). I wonder if this foray into semiconductor gauges was part of the original impetus of this app note?

Appendix B discusses historical bridge-readout techniques, starting with nineteenth-century galvanometers, and continuing up through modern chopper-stabilized amplifiers and analog-to-digital converters. Again, he gives unwarranted credit to Lee DeForest, which is disappointing. Note the captions in Figures B1 and B2, "Courtesy the J.M. Williams Collection."

Appendix C (briefly) discusses Bill Hewlett's oscillator design and includes the schematic (Figure C2) from his Stanford thesis.

Appendix D presents some theory behind distortion measurements, and is written by guest author Bruce Hofer. There is some good discussion here. Unfortunately, he starts the appendix by defining "linear distortion" and "nonlinear distortion", the former of which I consider a oxymoron.

Appendix E briefly mentions some cabling and filtering considerations for bridge circuits, and the final page is another of his famous cartoons, this one starring the Golden Gate Bridge.

Best quote (page AN43-42): "The Hewlett-Packard Company and light bulbs have had a long and successful association."

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Related:

• App Note 43 part 1
• App Note 43 part 2

App Note 43 part 2

The second major part of this app note discusses low-distortion sine-wave oscillators, inspired by his obsession with the HP200A oscillator.

He has discussed this topic before, of course. In 1981, he wrote National Semiconductor App Note 263, "Sine Wave Generation Techniques", which includes a wide variety of schemes, including the Wien-bridge approach (see NSC App Note 263, Figures 2 and 3). He uses a Wien-bridge oscillator, without much comment, in Figure 10 of LT App Note 3 (compare LT-AN3 Figure 10 with NSC-AN263 Figure 2b). In App Note 5, Figures 12, 13, and 14, he discusses a Wien-bridge oscillator with the light bulb for amplitude control (these figures are reproduced in the present app note). He also references Bill Hewlett's thesis (see App Note 5, Reference 5).

In this app note, the discussion of oscillators starts on page AN43-27, with a simple multivibrator in Figure 32, and a quartz-stabilized version in Figure 33 (quartz-stabilized oscillators are discussed in App Note 12, but not using bridge circuits). A #327 lamp is added in Figure 35 for amplitude control, and a common-mode-suppression loop is added in Figure 37.

The Wien bridge is introduced in Figure 39 (compare to Figure 2a in NSC App Note 263). Figures 40, 41, and 42 and copied from App Note 5, Figures 12, 13, and 14, using multiple lamps to increase the thermal time constant and reduce the distortion at low frequencies. This approach, as explained in Appendix C, is the basis of the HP200A oscillator.

Figure 43 begins the hunt to replace the light bulb. Using a FET as a simple voltage-controlled resistor increases the distortion by fifty times. As he says in the footnote, "What else should be expected when trying to replace a single light bulb with a bunch of electronic components? I can hear Figure 39's #327 lamp laughing." Figure 45 corrects the error (due to channel-length modulation), and Figure 47 replaces the FET with an optically driven CdS cell. Figure 48 further improves the distortion with a common-mode-suppression loop (a similar improvement was seen between Figures 35 and 37). The distortion is now below 3 ppm, below the uncertainty floor of his distortion analyzer. (This section is the basis of one of his book chapters, so we'll see this material again, soon.)

The app note concludes with a synchronous-rectifier AC-to-DC converter in Figure 50 and 51, which are copied from App Note 13, Figure 36 and 37.

Best quote (from page AN43-29): "History records that Hewlett and his friend David Packard made a number of these type oscillators. Then they built some other kinds of instruments."

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Related:

• App Note 43 part 1
• App Note 43 part 3

Scope Sunday 6

Speaking of test equipment (it's not all Tektronix around here), I am dumbfounded by the announcement this week that Hewlett-Packard was going to shed its computer business. Wasn't getting into the computer business the whole reason HP spun off Agilent and bought Compaq ten years ago? I predict that in ten years, HP will be a management consulting company, and ten years after that, it will be a zucchini farm. Despite the incomprehensibility of the logic behind this announcement, it does make a recent acquisition of mine even funnier.

On the left is the 1999 HP Test and Measurement Catalog, and on the right is the 2000 Agilent Test and Measurement Catalog (also known as the transition of the coming apocalypse). One good sign was a picture of the HP 200A audio oscillator on the inside front cover of the Agilent catalog, but I'm afraid the other omens were bad (at least bad for HP).

I am reminded of a picture and caption of David Packard, Bill Hewlett, and Bill Gates that Jim had on his bulletin board at work. I've tried to recreate it (and paraphrase it) below.

Packard: "What's that young man talking about?" Hewlett: "Hell if I know, but I know I don't like it."

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UPDATE: My friend Eugene sent me a link to this letter, which is an appropriate addition to this conversation:

Dear Fred:

I have no personal knowledge of computers nor does anyone in our organization have any appreciable knowledge.

Sorry we can't help you out in this regard.

Sincerely yours,

William R. Hewlett

Letter from Steve Blank, Elephants Can Dance – Reinventing HP.

App Note 5

"Thermal techniques in measurement and control circuitry." 8 pages.

This app note has two best circuits: first, Figure 6 shows a wideband thermal RMS-to-DC converter. This circuit uses the old servo-multiplier (or light-bulb multiplier) technique of using a feedback loop to force two matched parameters (in this case, the heat produced in the two composite thermistors) to have equal value. We'll see this circuit again. The flowmeter (Figure 8) and anemometer (Figure 11) are related tricks.

The second celebrity circuit is on the final page: Jim's obsession with the HP200 makes an early appearance. Figure 12 is an op-amp recreation of the Hewlett-Packard HP200 audio oscillator circuit (and the references include William Hewlett's thesis). While the flowmeter and anemometer are useful applications, I wonder if they weren't just a set up so that he could justify spending time at work playing with Hewlett's light-bulb circuit.

The other applications are also interesting and instructive. Figure 1 shows an interesting temperature controller (but the LT3525A is now discontinued). The explanation includes a simplified discussion of the various delays and time constants in a heat flow problem, but the full story is much worse: it's not just a multiple-time-constant system, it's a diffusion-equation system! Also, his discussion of insulation needs a caveat about the trade-off: you want to keep the losses small to save power, but large enough to allow the system to quickly recover from an over-temperature condition. Regardless, using the 50-ohm resistor and switch to check the loop response is a great idea, but the loop response needs to be checked for positive steps (when the heater heats) and negative steps (when the losses cool). He only shows the former. (Also, I don't think he needs the 100M resistor in the integrator, but that's another discussion.)

The circuit in Figure 4 uses a CA3096 array to thermally stabilize the feedback transistor in a log amp. This neat trick is also used to thermally stabilize a VCO in National Semiconductor's App Note 286. I'll comment here: Yikes! That LM301A has a huge compensation capacitor! A 33nF capacitor gives a unity-gain frequency of 1 kHz and a slew rate of 0.5 V/ms (yes, volts per millisecond). Replace the LM301A and 33nF with an integrator, and I think you'll get better (and more repeatable) performance. Looking back at Figure 1, is Jim afraid of integrators? I'm guessing Bob Widlar didn't consult on this circuit.

Best quote (page AN5-1): "The close relationship between temperature and electronic devices is the source of more design headaches than any other consideration." Word.