25 December 2011

Scope Sunday 21

Happy Holidays! Peace on Earth.

Merry Christmas. Happy Hanukkah. Joyous Kwanzaa. Happy Yule. Best Ramadan Wishes. Warm Winter Solstice. Happy Festivus. And, of course, Happy New Year!

23 December 2011

App Note 72 part 2

The applications section beings on page 21.

Again, as in App Note 13, he shows a wide variety of applications for this high-speed comparator. He starts with a crystal oscillator (Figure 47), also shown with switchable outputs (Figure 48), temperature compensation (varactor-biasing in Figure 49), and voltage control (more varactor-biasing in Figure 51). Figure 53 shows a voltage-control clock-skew generator with plus-and-minus 10-nanosecond skew. Voltage-to-frequency converters make an appearance in Figure 55 and 57. Figure 55 is a simple V-to-F, while Figure 57 is a precision topology, offering seven decades of frequency output (1 Hz to 10 MHz).

Several instrumentation applications are also included. Some of these circuits are related to the instrumentation circuits that he has used in recent (and will use in future) app notes. Trigger circuits with variable threshold (Figure 60) and adaptive threshold (Figure 63) are shown (we saw the adaptive-trigger circuit in Appendix C of App Note 70). A technique for increasing the comparator gain (using the venerable 733 amplifier, achieving 500-microvolt sensitivity) is shown in Figure 65.

A variable-controlled delay, up to 300 ns, is shown in Figure 69. Two circuits that use this delay are shown next: A high-speed sample-and-hold for repetitive signals (like Figure 29 in App Note 13) is shown in Figure 73. Figure 69's delay is also used in Figure 75 to add a programmable-delay trigger to his favorite pulse-generator topology (I think this circuit is the best circuit in the app note; I really need to build one for myself). Figure 77 shows these fast pulses using a 3.9-GHz sampling scope (a Tektronix 661 with a 4S2 plug-in).

Figure 78 shows a high-speed pulse stretcher that can trigger on pulses as small as 2-ns width input. The final circuit is a overload-protection circuit breaker, shown in Figure 83.

The app note ends with two appendices. The first one, "About level shifts", is from App Note 13. The second one, "Measuring probe-oscilloscope response", is a slightly modified reprint of Appendix D from App Note 47. This version includes Figure B2, taken with the 12GHz sampling oscilloscope that was alluded to in the footnote on page 93 of App Note 47 (seven years after writing that footnote, he finally gets to print the scope trace!).

The app note concludes with a cartoon that compares the LT1394 with other comparators, such as the LM311, the LM360/361, and the AD790.


20 December 2011

App Note 72 part 1

"A seven-nanosecond comparator for single supply operation: Guidance for putting civilized speed to work". 44 pages.

This app note is a major update to App Note 13, "High speed comparator techniques" from 1985. Jim admits this fact in the introduction, in his own humorous way...
This publication borrows shamelessly from earlier LTC efforts, while introducing new material. It approximates, affixes, appends, abridges, amends, abbreviates, abrogates, ameliorates and augments the previous work (an alliterative amalgamated assemblage).
Some of this material is also borrowed from App Note 47.

After a brief overview of the LT1394, Jim again starts with a tutorial section on probes, oscilloscopes, breadboarding, and bypass capacitors. The section begins with an updated "Rogue’s Gallery of High Speed Comparator Problems" updated with results using the new chip (Figures 3 through 15). The oscilloscope photos show the disastrous results of poor bypassing, improper probing, bad construction, and stray capacitance. Fuzz, ringing, overshoot, sluggish rise, and oscillations all rear their ugly heads. As before, this discussion is great reading.

The tutorial section discusses some cures for these common ailments. "Theory, techniques, prejudice and just plain gossip are offered as tools that may help avoid or deal with difficulties." After discussing some of his favorite pulse generators (HP-8110A, HP-8082A, and HP-215A), he discusses cables, connectors, and terminations. "Typically, inappropriate cable can introduce tailing, rise time degradation, aberrations following transitions, nonlinear impedance and other undesirable characteristics."

The next five sections, "About probes and probing techniques", "About oscilloscopes", "About ground planes", "About bypass capacitors", and "Breadboaring techniques" are borrowed directly from App Note 47, including the figures. It's still all good advice, though. Read it twice.

I'll cover the applications next time.


18 December 2011

Scope Sunday 20

I had an appointment with my optometrist last week, and when I was shown into the examination room, I noticed this box was tucked into the corner.

Pediatricians often have toys and stuffed animals in their offices to make their young patients feel more comfortable. It seems entirely appropriate that my doctor has an instrument built into a Tektronix 620 monitor on display. Nothing puts me at ease quite like a dose of Tektronix blue!

(Unfortunately, I didn't get a chance to ask the doctor what the instrument measured. Maybe next year!)

17 December 2011

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."


14 December 2011

App Note 70 part 2

Appendix A discusses the history of low-noise power conversion. He starts with the high-voltage CRT supplies in the Tek 454 and the Tek 7904 (Figures A1 and A2). As he says, "Designing a 10,000V output DC/DC converter that does not disrupt a 500MHz, high sensitivity vertical amplifier is challenging." These designs use sine-wave drive to the transformers to keep the harmonic energy low. He also revisits his low-noise designs from App Note 29 (Figures A3 and A4).

Appendix B is "Specifying and Measuring Something Called Noise". The appendix starts with a discussion of the definition of noise. As he says,
Actually switching regulator output "noise" isn’t really noise at all, but coherent, high frequency residue directly related to the regulator's switching. Unfortunately, it is almost universal practice to refer to these parasitics as "noise," and this publication maintains this common, albeit inaccurate, terminology. (Footnote: Less genteelly, "If you can't beat 'em, join 'em.")
It seems out-of-character for Jim to accept this inaccurate terminology. Usually, Jim would define the correct terms, and refuse to use any imprecise verbiage. I imagine that he felt that this battle was uphill too steep.

Personally, I am willing to fight this battle. The word "noise" should be reserved for aperiodic, structureless, random sources. True noise is the result of some random quantum-mechanical effect (like thermal noise, shot noise, flicker noise, popcorn noise, generation-recombination noise, avalanche noise, etc.). Other interfering signals (such as hum, ringing, pick-up, hash, oscillations, ripple, spikes, etc.) are undesirable, but they're not noise. Calling all unwanted signal components "noise" is like calling everything that falls out of the sky "snow". (As any Eskimo will tell you, there's a big difference between snow, sleet, hail, freezing rain, grapule, volcanic ash, etc.)

Perhaps he is just saving his wrath, not for bad terminology, but for bad measurements. The field of "noise" measurement is littered with bad practice. His quote from the second paragraph is worth repeating (with the footnotes inline),
It is common industrial practice to specify peak-to-peak noise in a 20MHz bandpass (One DC/DC converter manufacturer specifies RMS noise in a 20MHz bandwidth. This is beyond deviousness and unworthy of comment.). Realistically, electronic systems are readily upset by spectral energy beyond 20MHz, and this specification restriction benefits no one (except, of course, eager purveyors of power sources who specify them in this manner).
(I wish he named names here. Figures B6, B7, and B8 show how insidious some of the lies are!)

The rest of the appendix discusses instrumentation, calibration, and measurement of the noise floor. The preferred measurement chain is shown in Figures B1 and B2, and a table of useful preamplifiers is list in Figure B11.

Also, he again heaps significant praise upon his favorite instruments, "In our work we have found Tektronix types 454, 454A, 547 and 556 excellent choices. Their pristine trace presentation is ideal for discerning small signals of interest against a noise floor limited background." He concludes with a warning for people over-reliant on modern scopes: "The digitizing uncertainties and raster scan limitations of DSOs impose display resolution penalties. Many DSO displays will not even register the small levels of switching-based noise."


12 December 2011

App Note 70 part 1

"A monolithic switching regulator with 100μV output noise: Silence is the perfectest herald of joy..." 72 pages.

Almost two years have elapsed since App Note 65. Jim returns from his short absence with a great app note. This app note is one of the all-time classics, and it is filled with great advice, great circuits, and great quotes.

The primary topic of this note is low-noise switching regulators. The main trick is one that he previously discussed in App Note 29: limiting the slew rate of the transformer drive to limit the high-frequency harmonics at the output. Sharp edges on the switching waveforms give good efficiency (up to 95%), but cause wideband spikes in the output (see Figure 1). The LT1533 regulator, as discussed here, uses slew-rate control on the integrated switches to implement low-noise power supplies.

Of course, Jim spends about one-third of the app note talking about applications and two-thirds talking about measurements and instrumentation. Building a low-noise power supply is easy; verifying the results is the hard part!

The best parts of this note are the quotes. On page 1 he laments the usual difficultly of the task, "Unconscionable amounts of bypass capacitors, ferrite beads, shields, Mu-metal and aspirin have been expended in attempts to ameliorate noise-induced effects."

Most of the circuits (such as Figure 5) are straightforward applications. The meat of the app note begins on page 4 with "Measuring Output Noise". Jim discusses his instrumentation, and he places his usual emphasis on calibration of the measurement equipment. Footnote 7 tweaks the competition for their measurement techniques, "It is common industry practice to specify switching regulator noise in a 20MHz bandpass. There can be only one reason for this, and it is a disservice to users." Note the oscilloscope cartoons in Figures 6 and 8. Although the figures aren't labeled, these "artist's renditions" bear a striking resemblance to Jim's Tek 547.

He also extols the virtues of lab work. In footnote 10 he says, "The noise and efficiency characteristics appearing in Figures 20 to 23 were generated at the bench in about ten minutes. All you CAD modeling types out there might want to think about that."

The second half of the main text discusses a few more application circuits. A negative supply is shown in Figure 24, and isolated supplies are shown in Figures 25 and 26. Battery powered circuits are shown in Figures 34, 35, and 36.

High-voltage and high-power circuits are shown in Figures 38, 40, 42, and 44. In adapting the LT1533 for high-voltage inputs, cascode transistors must be used to protect the 30-volt integrated switches. Jim defines the cascode in footnote 14,
The term "cascode," derived from "cascade to cathode," is applied to a configuration that places active devices in series. The benefit may be higher breakdown voltage, decreased input capacitance, bandwidth improvement, etc. Cascoding has been employed in op amps, power supplies, oscilloscopes and other areas to obtain performance enhancement. The origin of the term is clouded and the author will mail a magnum of champagne to the first reader correctly identifying the original author and publication.

(For those of you who can't stand the suspense, this footnote was answered in App Note 75, footnote 12.)

I'll discuss the appendices next time.

Footnote: Along the same lines of footnote 14, Jim and I tried to find the earliest reference to a cascode topology using just transistors. I have a web page dedicated to the search. Long story short: 1960.


11 December 2011

Scope Sunday 19

My friend Will sent me an email last week with the subject line "Sorry." Inside, he said, "I know you are out of space but..." and he linked to a Craigslist posting with the headline "Tektronic Scopes $50" [sic] and the following picture.

How could I resist? These orphans needed rescue!

Saturday morning, I drove an hour to meet the retired gentlemen who was selling them. Long story short, he was cleaning out his barn, where he had stored them for many years after his employer discarded them. In the lot was a 585 (the fastest 500-series mainframe), a 547 (which I'm happy to have as a spare), and a 180 OCXO time-mark generator. There was also a 504 (to complement my 503), a 535A (seems like there's a 535 in every lot), a 561B (not a great scope, but I wasn't going to say "no"), a few spare plug-ins, and a box of original manuals.

The scopes were dirty, but in pretty good shape considering they had been stored in a barn. I don't think I brought home any mice this time, but there were definitely some spiders. Unfortunately, the box of manuals had extensive water damage, but nonetheless, it was a good day.

My list of projects for 2012 is getting too long. I should stop now.

UPDATE: I spent some time carefully inspecting the scopes Sunday evening. All of the scopes had all of their tubes, which was more than I had hoped for. Clearly, they were stored in a DAMP barn, as there was a fair bit of rust and corrosion. I didn't find any mice, and only found one spider infestation.

The 535A is in the worst shape, with rust on the laminations of the power transformer and on the back of the front-panel controls. There is also some strange corrosion around the neck of the CRT. I really didn't need another 535A anyway.

The 585 is in pretty good shape, except the top-right-back corner is smushed in a little bit (it was clearly dropped). The insides are clean and complete, without any obvious corrosion (but it did include the spiders). I hope that it can be restored to full operation, as I would really like to have a working 100-MHz 585.

The 547 has the worst-looking exterior but the best-looking interior. I don't know if it's worth trying to restore the chassis, but this one will make a good parts donor.

The 504 and 561B are in fair shape, no better or worse than the others. They are lowest on my "restoration and repair" list. I'll probably fix up the 504 (to go with my 503), but I'm not really a fan of the 560 series. However, the 561B is an all-solid-state design, so if I run out of other things to do, it might be fun to poke around in it... but I'm trying to avoid collecting another series of plug-ins.

09 December 2011

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.

04 December 2011

Scope Sunday 18

Christmas came early!

As I've mentioned before (for example, here and here), I have been looking to acquire a Tektronix 556 for some time. The Tek 556 was one of Jim's favorite scopes. Its damaged graticle was a trademark touch in many of his application notes, for both National and Linear. He used it in many of his otherwise-impossible measurements (such as simultaneous time and frequency domains and multiple asynchronous waveforms), he used it as a prop for pictures of his son (here and here), and he used it for comic relief.

Last week, one appeared on Craigslist in New Hampshire.

It even has two 1A4 four-channel plug-ins! Sweet. I crafted a slightly boastful email to the seller, promising to take great care of it and warning against the parasites who would strip it for tubes. (I also pointed to this blog as proof of my good intentions.) Unfortunately, I had to "bid" against some guy in Texas who was willing to pay for shipping, but after a brief negotiation, I prevailed. I drove up to New Hampshire and picked it up this morning.

My wife made me open it up and check it for mice before I was allowed to bring it into the house.

No mice. It's beautiful, inside and out.

02 December 2011

App Note 65 part 5

The rest of the appendices are either copies or updates of the appendices from App Note 55 (with the exception of Appendix G).

Appendix E is an updated version of Appendix E from App Note 55. Open-lamp protection is built into the LT1182 family of parts, so the solution is much easier now than it was in App Note 55. See the connection to the BULB pin in Figure E1.

Appendix F is an updated version of Appendix F from App Note 55. Again, the LT1182 family provides significant advancements over the simple schemes shown in the previous app note. The precision PWM generator in Figure F6 allows for careful calibration (of course).

Appendix G is wholly new, discussing layout and component issues. Careful consideration is given to the current flow in various traces. The proper layout of the high-voltage section is also discussed and illustrated in the figures. Slits and routed voids in the board (Figures G5 and G6) prevent leakage. Additional good board construction is shown in Figures G7 through G10. Figures G11 and G12 are suboptimal, and Figures G13 and G14 are a disaster ("Board failed spectacularly at turn-on."). I think the best quote in the whole app note is in the caption of Figure G14, "Board needs complete re-layout. Computer layout software package needs E and M course."

The rest of Appendix G discusses the discrete capacitors, transistors, and magnetics. "Substitution of standard devices can degrade efficiency by 10% to 20% and in some cases cause catastrophic failures (Don't say we didn't warn you)." A excerpt from Zetex App Note 14 discusses transistor operating conditions and requirements.

Appendix H is copied from Appendix G of App Note 55.

Appendix I, "Additional Circuits" is an updated version of Appendix H from App Note 55. Figure I1 is a high-power CCFL supply, and Figure I2 utilizes two (physically smaller) transformers for space savings. Figure I3 is the laser power supply that we've seen before (Figure H2 in App Note 55).

Appendix J is mostly new, with some figures and text borrowed from the main body of App Note 55 (Figures J2 and J3 originally appeared as Figure 13 and 15 in App Note 55). Figures J4 through J7 outline the use of the built-in LCD contrast control in the LT1184 family.

Appendix K is copied from Appendix I of App Note 55.

Appendix L is copied from Appendix J of App Note 55, with two changes that I was able to find: (1) the last line on page 121 questions the effect waveform crest factor on lamp lifetime, and (2) Figure L9 and the accompanying text is new.

Of course, the app note concludes with a cartoon. The second best quote in this app note is the last line of the cartoon: "Edison got it right the first time."


01 December 2011

App Note 65 part 4

Sixty-four total pages of appendices!

Appendices A and B are reprinted directly from App Note 55 (and App Note 49). Appendices C and D both have significant updates.

Appendix C discusses instrumentation for electrical measurements. In App Note 55, this discussion was six pages long, but here it has been expanded to twenty-four pages. His instrumentation suggestion are much more detailed. Figures C1 and C2 illustrate the problem: the necessary measurements need to be made with wide-band tools. For example, a new 10-MHz current-probe amplifier is detailed in Figures C3 through C7, and Figures C8 and C9 show a Wien-bridge oscillator used for its calibration and demonstrate its use.

Voltage probes and RMS voltmeters are discussed starting with Figure C10, with a calibration circuit shown in Figure C13. With the new emphasis on floating-lamp circuits in the main text, a new section on "Voltage Probes for Floating Lamp Circuits" starts on page 72. "Measuring voltage of floating lamp circuits requires a nearly heroic effort." Figure C14 shows a wide-band differential amplifier for use with high-voltage probes and a single-ended RMS voltmeter. Calibration of this fully differential RMS voltmeter apparatus is a significant challenge. Jim explains the difficulties, starting with first principles and a detailed schematic of a high-voltage probe in Figure C15. "No less than seven user adjustments are required to compensate the probe to any individual instrument input." And the warning in footnote 10, "Allow at least six hours for the entire session. You’ll need it."

A fully floating, differential, high-voltage calibrator is shown in Figure C16 (schematic in Figure C19). Photos of the details and internals are shown in Figures C17 through C23. The calibrator produces a 500-VRMS waveform to check the calibration of the probes. "Those who construct and trim the differential probe and calibrator will experience the unmitigated joy that breaks loose when they agree within 1%."

RMS voltmeters are discussed starting on page 82. Much of this discussion is copied from App Note 55, with the addition of Figures C25 and C26 (which are mostly borrowed from App Note 61 Figures 22 and 23). The appendix ends with the discussion of calorimetric measurements from App Note 55.

Appendix D discusses instrumentation for photometric measurements. In App Note 55, this discussion was less than two pages long, but here it has been expanded to six pages. He again starts with a picture of his "glometer" tube in Figure D1, which has been modified to allow variable-frequency drive to test individual lamps. The drive schematic is shown in Figure D2, and the measurement schematic is shown in Figure D5.

Finally, Figure D7 shows his complete "CCFL test set", including current probe, differential RMS voltmeter, calibrators, and photometer. Compare to the CCFL test set in App Note 55 (Figure C6). He has also upgraded his computer to a slide rule!

I'll discuss the rest of the appendices next time.


28 November 2011

App Note 65 part 3

Even after the introduction of the LT1182 family of CCFL Switching Regulators on page 36, several of the following circuits in the app note are still borrowed from App Note 55. In other words, in many of these cases, Jim got it right the first time (or is it the third time?).

For example, the low-power CCFL supplies (Figures 40, 41, and 42) are quite similar to App Note 55 Figures 10, 11, and 12. However, both the low-power circuit in Figure 43 and the high-power circuit in Figure 44 are new. Figure 44 produces up to 25W of power!

Floating-lamp circuits are discussed starting on page 40 with Figures 45 and 46. Previously, floating lamps were used to extend the illumination range (without the thermometer effect). Here, floating lamps are explained to be (potentially) more efficient than lamps that are grounded on one end. The circuits in Figures 47 and 48 are mostly the same as App Note 55 Figures 27 and 28.

The following figures are new, utilizing the LT1182 family of CCFL controllers. Using the ROYER pin (discussed previously!) to sense the current in the primary, there is no need for direct feedback from the secondary (lamp) side of the transformer. Thus, the lamp is isolated and floating. Figures 49, 50, and 51 use this approach. Figures 52 and 53 outline a microcontroller interface and the necessary software (not Jim's fault!).

Figure 54 shows a floating-lamp supply that can provide 30W (which is more power than the LT1182 family can deliver). The feedback is implemented with a current-sense transformer.

In the next-to-last section (starting on page 46), Jim outlines the selection criteria for CCFL circuits and discusses the numerous trade-offs involved. He discusses a laundry list of topics on pages 47 through 49. Some of the trade-offs are summarized in the tables in Figures 55 and 56, but clearly, Jim was "asked" to include these summary tables over his objections. Two related quotes betray his true feelings: "There is simply no intellectually responsible way to streamline the selection and design process if optimum results are desired" and the follow-up footnote, "Readers detecting author ambivalence about inclusion of Figures 55 and 56 are not hallucinating." Even the caption is revealing, "Chart makes simplistic assumptions and is intended as a guide only." The word "ambivalence" is clearly the polite version!

The last section, starting with "General Optimization and Measurement Considerations", has a new introduction (and new Figures 57 and 58 displaying the wave shapes of the lamp drive), but the rest of the section comes from App Note 55. Again, the main emphasis is on understanding and optimizing the efficiency of the system design, not just a single piece of it. Figures 60 through 67 (and the accompanying text on "Electrical Efficiency Measurement" and "Feedback Loop Stability Issues") are copied from App Note 55 Figures 19 through 26.

I'll discuss the appendices (64 pages of them!) next time.


27 November 2011

Scope Sunday 17

Sometimes you HAVE to look a gift horse in the mouth.

Not long ago, I bought some oscilloscope plug-ins from a local collector. While I was picking them up, he pointed at a piece of equipment in the corner and asked "You want that? I have no use for it."

It was a Bruel and Kjaer Type 1019 Automatic Vibration Exciter Control. I'd never seen one before, but it looked really cool (since then, I have seen another in a surplus shop). To be honest, I have no use for it either, but I'm not one to say "no" to free vintage equipment, especially such a beautiful piece of art. I just love the giant center knob.

I brought it home and stored it in my basement for a while. When I did finally get around to taking the back panel off and looking at it, this is what I found:

It is a mouse nest the size of a volleyball. There's also significant damage to the rest of the electronics. If I had seen this damage before I brought it home, I would have refused to accept it. (I am certain this nest wasn't built by local rodents because the construction materials are not "native" to my basement.) However, now that I have it, I feel some obligation to clean, fix, and restore it.

Just what I need! Another crazy project!

25 November 2011

IC Pins Named after Persons

This topic deserves its own post!

Jim Williams, in discussing the ROYER pin on the CCFL power-supply chips that Linear Technology produces, commented in a footnote [1] that "Local historians can't be certain, but this may be the only IC pin ever named after a person." After I posed this question in a previous blog post, some faithful correspondents came up with two other examples: the ZENER pin on the CD4046 PLL chip and the KELVIN pin on the CAT2300 switch controller.

Here is the list so far:

1. The ROYER pin on the LT1182 family of CCFL Switching Regulators (and other CCFL chips from Linear Technology):

2. The ZENER pin on the CD4046 Micropower Phase-Locked Loop:

3. The KELVIN pin on the CAT2300 Current Mirror and Switch Controller for SENSEFETs:

Please let me know if you find any more!

[1] Jim Williams, "SMBus controlled CCFL power supply," Linear Technology Magazine, vol. 9, no. 3, p. 35, September 1999.

Footnote: Even with these examples, Jim's statement is still true if you consider that the ROYER pin was probably named directly in honor of George H. Royer, while the other pins were named after functions that were, in turn, named after persons...

23 November 2011

App Note 65 part 2

The discussion of actual power-supply circuits in this app note begins on page 32, with the cautionary introduction:
Choosing an approach for a general purpose CCFL power supply is difficult. A variety of disparate considerations make determining the "best" approach a thoughtful exercise. Above all, the architecture must be extraordinarily flexible. The sheer number and diversity of applications demands this. The considerations take many degrees of freedom.
I think this introduction is partly an inoculation to the "summary" tables that are included later in the app note. See footnote 18 on page 49.

Figures 35 through 38 are quite similar to App Note 55 Figures 6 through 9 (although a newer, higher-frequency switching regulator is used in the last two figures).

Figure 39 is a brand new circuit, using a dedicated CCFL integrated circuit, the LT1183. This single-chip solution is the major advancement in this "fourth generation" treatment of CCFL power supplies, and it includes many of the features that Jim has discussed in previous app notes, such as open-lamp protection and the variable LCD-contrast supply voltage.

Note the name of pin 13 on the LT1183, "Royer". Although he doesn't mention it here, Jim commented in a later publication [1] that "Local historians can't be certain, but this may be the only IC pin ever named after a person."

That's a very good question. Dear loyal readers, is this the only IC pin named after a person?

[1] Jim Williams, "SMBus controlled CCFL power supply," Linear Technology Magazine, vol. 9, no. 3, p. 35, September 1999.

Tutorial footnote (for new readers): The above power-converter topology is a resonant Royer converter. The LT1183 is a controller chip that drives a Royer converter for this specific application: driving the cold-cathode fluorescent lamp (CCFL) for the backlight in a laptop screen.

The so-called ROYER pin connects to the center-tapped primary of the transformer (which is the unregulated-power input in this topology). In the circuit shown, the pin isn't doing much. However, if the BAT pin and the ROYER pin aren't connected together, then all of the input current flows in the BAT pin and out the ROYER pin, and the chip can sense the primary-side input current in the transformer. Thus, the LT1183 can drive the lamp in a floating (isolated) topology, without any direct feedback to the chip from the high-voltage side.

Here's the explanation from the LT1183 datasheet:
ROYER (Pin 13): This pin connects to the center-tapped primary of the Royer converter and is used with the BAT pin in a floating lamp configuration where lamp current is controlled by sensing Royer primary side converter current. This pin is the inverting terminal of a high-side current sense amplifier. The typical quiescent current is 50μA into the pin. If the CCFL regulator is not used in a floating lamp configuration, tie the Royer and BAT pins together. This pin is only available on the LT1182/LT1183/LT1184F.
For more details, see Linear Technology App Note 65: "A fourth generation of LCD backlight technology: Component and measurement improvements refine performance." Appendix K is titled "Who was Royer and what did he design?"

Update: A commenter on another website suggested that an argument could be made for the ZENER pin on the CD4046 Phase-Locked Loop chip. That's a good one. Any others?

Update 2: Further discussion on this topic has been moved to a dedicated post: IC pins named after persons.


21 November 2011

App Note 65 part 1

"A fourth generation of LCD backlight technology: Component and measurement improvements refine performance." 124 pages!

This one-hundred-twenty-four-page app note is Part 4 (as the title says, "fourth generation") in the grand saga of cold-cathode fluorescent lamps (CCFLs). Just to remind you:
Being a major, major update to the material published previously, this app note is over twice as long as App Note 55, and it is the second-longest one he ever wrote (App Note 47 is still the king of length). The Preface on page 1 sets the stage and summarizes the issues. The Introduction on page 4 acknowledges the length of this effort, "the longest sustained LTC application engineering effort to date." He also acknowledges the very beginning, "a single circuit in a 1991 publication" (Figure 36 in App Note 45), which he didn't acknowledge in Chapter 11 of his second book.

The organization of this app note, as compared to App Note 55, shows a distinct change in emphasis. "Perspectives on display efficiency" and Figure 1 are first up in the main text. This section is an updated version of Appendix K in App Note 55, but it now takes the stage front-and-center. Clearly, as the circuits got better, display efficiency became a more important issue. Figures 2 through 9 summarize typical characteristics of CCFLs, some of which is above and beyond the material presented in the introduction of App Note 55 (particularly the plots of on-time and lamp emission).

Next up is display losses (illustrated schematically by Figures 10 and 11), and the gallery of "display situations" in Figures 12 through 32 (thirty pages of photos!). "The deleterious effects of display parasitics dominate practical backlight design." The examples shown demonstrate losses from 1.5% (Figure 14) to 31% (Figure 31). I assume that Jim had spent a lot of time talking to customers and saying, "No, don't do that."

The final display-loss-related section is "Considerations for multilamp designs" on page 31. It is funny to examine how his opinion of multilamp designs has changed over time. They were suggested in App Note 49 (page 4), with a little discussion of "different transformer rating" and the "differences in lamp wiring", but, "Practically, these differences are small, and the lamps appear to emit equal amounts of light." In App Note 55 (pages 12 and 13), he writes, "Systems using two lamps have some unique layout problems," but he concludes, "imbalanced illumination causes fewer problems than might be supposed." Here in App Note 65, "The text's tone is intended to convey our distaste for multilamp displays. They are the very soul of heartache."


I'll talk about the circuits next time.


18 November 2011

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.



17 November 2011

Book 2 Chapter 11

Chapter 11, "Tripping the light fantastic"

This fifty-five-page chapter discusses "how the best circuit I ever designed came to be". (I'm not sure if I agree with that assessment.) This chapter is Part 3b in the grand saga of cold-cathode fluorescent lamps (CCFLs). Just to remind you:
I'm calling this chapter "Part 3b" because it is nearly identical to Part 3a. The only really difference between this chapter and App Note 55 is the introduction on pages 139 to 153 (and the epilogue on page 174).

The introduction discusses the genesis of this project, starting with his postpartum blues after the publication of App Note 47. He claims that Bob Dobkin asked him to look into backlights around Christmas 1991, and then an engineer from Apple called him a few days later. (This narrative, while it makes a good story, seems to omit the CCFL circuit in App Note 45 from June 1991.) The engineer from Apple, Steve Young, had seen the cartoon in App Note 35 ("Call me and lets discuss your switching regulator requirements"), and was looking for some applications help with the power converters in the Powerbook, including the backlight.

The description of taking apart portable computers ("The Luddite approach to learning", page 142), sets the stage for Jim's significant research and exploration of CCFLs. He explains, "almost all of them utilized a purchased, board-level solution to backlight driving." No vendors were optimizing their designs (likely because nobody really understood the lamps). Jim was traveling into unexplored territory. This fact perhaps explains the quote at the beginning of App Note 55, "One notable aspect of [the publication of App Note 49] is that it generated more response than all previous LTC applications notes combined."

In the next part of the story, Jim explores the high-voltage resonant power supplies in oscilloscopes, including the Tektronix 547 (Figure 11-4) and the Tektronix 453 (Figure 11-6, also shown below).

Finally, he introduces the Royer topology on page 148 and discusses the start of his work on page 152. The best quote: "But there comes this time when your gut tells you to put down the pencil and pick up the soldering iron... Build and raft and start paddling."

The rest of the chapter (pages 154 to 193) is basically a reprint of App Note 55.


16 November 2011

Book 2 Chapter 1

Chapter 1, "The importance of fixing"

In this chapter, Jim tells the story of his start at M.I.T., and how his mentor, Prof. Jerrold Zacharias ("the father of atomic time"), shaped his early career. The key development was Zacharias's refusal to pay for instrument repair and calibration. He simply demanded, "You fix everything." This moment was Jim's introduction to fixing (and stealing good ideas from) Tektronix oscilloscopes, starting with a 1A7 plug-in. As the saying goes, "Good artists copy, great artists steal."

Jim goes on to discuss his educational philosophy with regard to vintage test equipment. "The inside of a broken, but well-designed piece of test equipment is an extraordinarily effective classroom."

He concludes with a quote I love (and have cited before; see Scope Sunday 1): "It just seems sacrilege to let a good piece of equipment die... fixing is simply a lot of fun. I may be the only person at an electronics flea market who will pay more for the busted stuff!"

Best quote (in the caption of Figure 1-1 (above) on page 4): "Oh boy, it's broken! Life doesn't get any better than this."


15 November 2011

Book 2

Another detour for the rest of this week: App Note 61 was published in August 1994. The next app note (App Note 65) was written over a year later in November 1995. In the intervening 15 months, Jim published a second book, "The Art and Science of Analog Circuit Design". So for the rest of this week, I will blog about the book. As I recommended for the first book, if you don't own a copy, you owe it to yourself to buy one.

This book contains twenty chapters, again written by a wide variety of authors, including Barrie Gilbert (again), Greg Kovacs, Carl Battjes, Carl Nelson, and many others. Jim wrote three of the chapters himself. I'll discuss them each in turn. One interesting note at the beginning of the book is in the biographies of the contributors (starting on page xi). In Jim's bio, it says "He lives in Palo Alto, California, with... 28 Tektronix oscilloscopes", twice as many as he claimed last time!


14 November 2011

App Note 61 part 2

Continuing on, Figure 26 shows an improved high-speed pulse generator. Compared to Appendix D in App Note 47, this pulse generator has a distinct advantage in that it is triggered. "This feature permits synchronization to a clock or other event." The 2N2369 is biased just below avalanche breakdown, and the incoming trigger input causes breakdown (due to the difference between Vceo and Vces). Note the comment about the Tektronix 661 with the 4S2 sampling plug-in ("I'm sorry, but 3.9GHz is the fastest 'scope in my house"). Nice upgrade from the 1-GHz 1S2 sampler that he has used previously. "Ground plane type construction with high speed layout, connection and termination techniques are essential for good results from this circuit." Yep. (I think this circuit is the best circuit of the app note.)

Figure 29 shows a special voltage regulator for flash-memory programming. Unfortunately, he does not compare and contrast this design with the designs in App Note 31. Is this circuit just as good? (It probably is, but why?). What are the features of the LT1109 that make it a good fit for this (touchy and sensitive) application?

Figure 31 shows a low-voltage voltage-to-frequency converter (operating from a 3.3-volt supply). The next two circuits are broadband noise sources. Figure 33 uses a Noisecom NC201 noise diode with a selectable filter to produce the noise, and a RMS AGC loop to set the amplitude. Figure 37 uses a standard zener diode as the noise source, but requires a trim to set the initial noise level. Figure 38 is a switchable-output crystal oscillator, which uses diodes to select which crystal is active in the feedback path of the comparator. Cute.

Appendix A includes significant reprints from the manual of an HP3400A True RMS Voltmeter. Jim compares the approach used here to his approach in Figure 22. The instrument he discusses here is a classic, with an impressively creative solutions to several design challenges. The input buffer in Figure A1 uses Nuvistors (because JFETs weren't good enough in 1965). Figure A2 shows a photograph of the input-buffer circuit board. The "video amplifier" in Figures A3 and A4 is an impressive design with DC and AC feedback loops and clever bootstrapping. The chopper amplifier in Figures A5 and A6 uses neon bulbs and photocells for the chopping action! (As Jim says, "Hewlett-Packard has a long and successful history of using lamps for unintended purposes.") Figure A7 shows the circuit board for Jim's RMS-to-DC converter from Figure 22.

The topic of RMS-to-DC conversion was near and dear to Jim's heart. He covered these circuits in detail in App Note 22, and he used them in his CCFL explorations. He helped to design the LT1088 IC and was its main evangelist. In the footnote on page 28, Jim tries to explain the context: "We are all constantly harangued about the advances made in computers since the days of the IBM360. This section gives analog aficionados a stage for their own bragging rights. Of course, an HP3400A was much more interesting than an IBM360 in 1965. Similarly, Figure 22's capabilities are more impressive than any contemporary computer I'm aware of."

Do I detect some frustration in Jim's voice here? Compare all of the circuitry of the HP3400A with the circuit in Figure 22. In effect, I think Jim is saying, "Look how easy I'm making it for you! Why aren't you buying more LT1088 chips?"

Best quote (page AN61-38): "Incidentally, what were you doing in 1965?"

The app note concludes with a great cartoon (perhaps the best cartoon so far).


11 November 2011

App Note 61 part 1

Practical circuitry for measurement and control problems: Circuits designed for a cruel and unyielding world. 40 pages

Another collection of random circuits, like App Note 45 (from June 1991). In fact, the introduction says this circuit collection includes circuits from June 1991 to July 1994. Unfortunately, there is no baby-bottle-rating system, however, there are plenty of great circuits.

Figure 1 shows a switching regulator where the switching frequency is synchronized to the system clock. (Synchronization was also discussed in App Note 55.) This trick is a great one. I once designed a "noise-less" power supply for a sensitive RF receiver with this method... I simply "hid" all of the EMI from the switching regulator underneath the EMI from the digital hardware (I admit that this approach is cheating, but it works).

Some of the circuits were supplied by Steve Pietkiewicz, and many of them were "inspired" by Jim's recent work on CCFL power supplies in App Note 55. Two methods for 1.5V-to-5V conversion are shown in Figure 3 and 6 (high power and low power, respectively). Figure 9 is a low-power CCFL supply (an alternative to App Note 55 Figure 11). Figure 10 is a LCD contrast supply (compare to App Note 55 Figure 14). Figure 11 is a power supply for a HeNe laser (the same as App Note 55 Figure H2), and Figure 12 is an electroluminescent-panel supply.

Jim's interest in instrumentation also appears. Several barometer circuits are shown in Figures 13, 14, 15, and 16, and a quartz-based thermometer appears in Figure 17. A FET-input instrumentation amplifier is shown in Figure 18.

There are also some improvements on circuits from App Note 47. The high-speed adaptive trigger makes another appearance in Figure 21 (compare this circuit to Figure 131 in App Note 47). Another wideband thermally based RMS-to-DC converter is shown in Figure 22 (compare to Figure 137 in App Note 47 and Figure 8 in App Note 22).

The best circuit (so far) is Figure 24, not because it's a great circuit in isolation, but because it is true to his "fixer mentality." Although he doesn't admit it here, I imagine that he was elbow deep into fixing a Tektronix P-6042 current probe when he realized that he couldn't get the replacement parts that he needed (see Figure 25: is M18 still available?). So, he designed his own replacement, and Figure 24 was born.

I'll discuss the rest of the circuits and the appendices next time.

Best quote (page 18, discussing figure 22): "It is worth considering that this circuit performs the same function as instruments costing thousands of dollars."


09 November 2011

App Note 55 part 2

As always, the appendices (up to K!) are great.

Appendices A, B, and G are copied from App Note 49.

The first half of Appendix C, "Achieving meaningful efficiency measurements", is also borrowed from App Note 49, but the second half (starting with the calibration source in Figure C3) shows that considerable effort has gone into making the measurements more meaningful. Jim always devotes great effort to instrumentation, and it shows here. Again, precision RMS-to-DC conversion plays a large role. (Although Figure C6 is captioned "Typical efficiency measurement instrumentation", there's nothing typical about it.) The appendix concludes with a discussion of using calorimetric measures as a efficiency double-check (Figures C7, C8, and C9). As the man says, "Calorimetric measurements are not recommended for readers who are short on time or sanity."

Appendix D discusses more instrumentation, now focusing on photometric measurement. How much light is the CCFL producing? This question raises a key issue, as he explains in the footnote, "It is possible to build highly electrically efficient circuits that emit less light than "less efficient" designs." (See Figure J3.)

The next four appendices are short. Appendix E discusses protection circuitry (important!) for broken lamps. Appendix F discusses shutdown control, and a calibration source (Figure F2) for intensity control. Appendix G is copied from App Note 49, and Appendix H (HeNe laser power supplies) is mostly copied from App Note 49.

Appendix I is a brief (too brief!) discussion of the history and operation of the Royer topology.

Appendix J is my favorite. Titled "A lot of cut-off ears and no Van Goghs", it discusses some not-so-great ideas. On the whole, we engineers don't spend enough time talking about engineering failures, and there is often a lot to learn. Jim relates, "Backlight circuits are one of the deadliest places for theoretically interesting circuits the author has ever encountered." Figures J1, J2, and J3 attempt to increase efficiency by removing the losses in the LT1172. Unfortunately, the resulting lamp drive waveforms are undesirable. Figures J5 through J8 show suboptimal sensing schemes for measuring the output current. It is very instructive to consider why these circuits didn't work: I wish Jim had written more appendices like this one!

Appendix K is a brief discussion of the various sources of inefficiency in a backlight application, including the electrical-to-electrical step, the electrical-to-light step, and the light-to-light step (see Figure K1). Importantly, the electrical-to-electrical efficiency stays high under a variety of operating conditions (see Figure K2), which extends battery runtime.

The app note concludes with a cartoon of his son Michael sitting in front of a 556.


07 November 2011

App Note 55 part 1

"Techniques for 92% efficient LCD illumination: Waste not, want not..." 44 pages.

This app note is Part 3 in the grand saga of cold-cathode fluorescent lamps (CCFL). Part 1 was Figure 36 back in App Note 45, and Part 2 was App Note 49. This app note begins with an unbelievable quote: "One notable aspect of [the publication of App Note 49] is that it generated more response than all previous LTC applications notes combined." It is hard to imagine that this topic is so much more popular than all his other (prolific) work. On the other hand, reading through this app note, you get the sense that CCFLs are poorly understood, and that Jim's efforts were perhaps the first attempt to truly explore this under-appreciated topic.

This app note is effectively a (significant) update to App Note 49. "The partial repetition is a small penalty compared to the benefits of text flow, completeness and time efficient communication." It is dated one year after App Note 49 (August 1992 to August 1993), so it represents a considerable amount of time and effort. "Getting the lamp to light is just the beginning!"

Figures 1 through 4 show some of the additional investigation and research done since App Note 49, summarizing some typical characteristics of CCFLs. The schematic in Figure 6 is mostly copied from Figure 2 in App Note 49 (with new transistors and a new value for C1, which increase the efficiency from 82% to 88%), however, Figures 8 and 9 (with 91% and 92% efficiency, respectively) are new. Low voltage operation (shown in Figure 10) is also new. Figures 11, 12, and 13 are copied from App Note 49, while the more-in-depth discussion of LCD bias (with Figures 14 and 15) is new.

A considerable discussion of mechanical layout begins on page 11 with Figure 16. "Poor layout can easily degrade efficiency by 25%, and higher layout induced losses have been observed."

Feedback-loop stability is discussed starting on page 13. Figures 20 through 26 show some of the problems and cures thereof in the feedback loop. (Footnote 10, "The high priests of feedback..." seems to be mocking me.) The compensation approach used in these circuits is very conservative (see the 2-uF capacitors on the VC pins in Figures 6, 8, 9, and 10). "Isn't a day of loop and layout optimization worth a field recall?" (Is experience speaking here?)

The final two sections discuss dimming ("extending illumination range") and synchronization. Figures 28 and 29 improve the dimming capability of the lamp by using isolated drives (a 40:1 dimming ratio is achieved), maintaining high effeciency. The symmetric electric field around the lamp reduces the occurrence of the "thermometer" effect. Figures 30 through 36 discuss synchronization (a topic that he originally discussed, and dismissed, in Appendix A of App Note 29). "In particular, pen based computers may be especially sensitive to asynchronous components." Figures 32 and 34 abandon the Royer base drive for a flip-flop-based scheme. This approach reduces jitter at the expense of efficiency.

The best quote is the caption of Figure 20: "Destructive high voltage overshoot and ring-off due to poor loop compensation. Transformer failure and field recall are nearly certain. Job loss may also occur."

I'll discuss the appendices next time.


30 October 2011

Scope Sunday 15

In the App Note 47 List of References, Jim cited three books in the "Tektronix Concepts" series. These books discuss circuit theory from basic introductory concepts to detailed, specific design examples (from Tektronix products, of course). They were authored by a variety of engineers at Tektronix. Reading through them, you wonder how many of Tektronix's competitors bought and devoured these books. Scans of some of the volumes in the series can be found online in PDF format.

A friend of mine has a nearly complete set.

I am green with envy. My bookshelf needs a set of these.

28 October 2011

Halfway there!

With Wednesday's post about App Note 49, I have now covered 31 of Jim's 62 app notes (but only 40% by page count). I thought that it would be a good time to take a quick look back.

Top five app notes (so far):
  1. App Note 47, High speed amplifier techniques: A designer's companion for wideband circuitry (the crown jewel, of course: absolutely required reading).
  2. App Note 25, Switching regulators for poets: A gentle guide for the trepidatious (the advice, the cartoon, and the lack of equations).
  3. App Note 43, Bridge circuits: Marrying gain and balance (mostly for the low-distortion sine-wave oscillator development).
  4. App Note 28, Thermocouple measurement (the history lessons and discussions of errors).
  5. App Note 45, Measurement and control circuit collection: Diapers
    and designs on the night shift (for the baby-bottle rating system and Figure 36).
Two other app notes that would have been contenders are App Note 10 and App Note 13, but the good parts of those notes were updated and incorporated into App Note 47. App Note 49 would also get an honorable mention, but it's quickly going to be buried by its successors, App Notes 55 and 65.

Top five best circuits (so far):

  1. App Note 43 Figure 48 (Sine wave oscillator with 3ppm distortion)
  2. App Note 14 Figure 1 (King-Kong V-to-F)
  3. App Note 23 Figure 16 (The Zoo Circuit V-to-F)
  4. App Note 21 Figure 12 (Son of Godzilla Amplifier)
  5. App Note 47 Figure 116 (Complete AM Radio Station)
Top five best quotes (so far):
  1. "Mother Nature loves throwing a surprise party. Technologically driven arrogance is a dangerous brew, as any Titanic passenger will assure you." (p.103 in Chapter 13, "Should Ohm's Law be repealed" in Jim's first book)
  2. "The author acknowledges Carl Nelson's abundance of commentary, some of which was useful, during preparation of this work" (p.13, App Note 25)
  3. "While I certainly wouldn't wish lifetime employment on a digital circuit board to anyone, the reality is that the need exists. (Footnote: I suppose it's not all that bad. Some of my best friends are digital circuits. If I had a daughter, I'd even consider letting her go out with one.)" (p.1, App Note 31)
  4. "History records that Hewlett and his friend David Packard made a number of these type oscillators. Then they built some other kinds of instruments." (p.29, App Note 43)
  5. "The translation of this statement is to hide the probe when you are not using it. If anyone wants to borrow it, look straight at them, shrug your shoulders and say you don't know where it is. This is decidedly dishonest, but eminently practical. Those finding this morally questionable may wish to re-examine their attitude after producing a day's worth of worthless data with a probe that was unknowingly readjusted." (p.12, App Note 49)
Also, I've updated the bibliography again. Onward!

26 October 2011

App Note 49

"Illumination circuitry for liquid crystal displays: Tripping the light fantastic..." 16 pages.

This app note is Part 2 in the grand saga of cold-cathode fluorescent lamps (CCFL). Part 1 was a single schematic (Figure 36 back in App Note 45, remember the 48 baby bottles?), reproduced here with minor changes as Figure 2. This problem is an important one in portable computers, since "The CCFL and its power supply are responsible for almost 50% of the battery drain."

The converters here use the Royer switching topology. Back in Appendix A of App Note 29, Jim discussed the disadvantages of the Royer topology at length. Mostly, he mocked it for being noisy. Here, he uses it as a high-voltage step-up circuit in a current-control loop. Footnote 3 explains, "Controlling a non-linear load's current, instead of its voltage, permits applying this circuit technique to a wide variety of nominally evil loads." Evil loads.

Of course, instrumentation plays a large role in this effort. High-voltage oscilloscope probes are recommended and discussed ("Don't say we didn't warn you!"). The dual-beam dual-time-base Tektronix 556 get considerable praise: Figures 3 and 6 would be nearly impossible without it ("Most oscilloscopes, whether analog or digital, will have trouble reproducing this display."). True RMS voltmeters ("the meter must employ a thermal type RMS converter") are also discussed on page 4 and in Appendix C. Of course, proper instrumentation is critical, as the footnote on page 11 explains, "It is worth considering that various constructors of Figure 2 have reported efficiency ranging from 8% to 115%."

Other design considerations are mentioned. Parasitic capacitance from mechanical layout is discussed. Transformer leakage due to silkscreen ink is considered. System designs employing two tubes are discussed on page 4 (this advice will be retracted in App Note 65). LCD-contrast-bias sources are shown in Figures 7 and 8.

Best quote (footnote 3 on page AN49-12): "The translation of this statement is to hide the probe when you are not using it. If anyone wants to borrow it, look straight at them, shrug your shoulders and say you don't know where it is. This is decidedly dishonest, but eminently practical. Those finding this morally questionable may wish to re-examine their attitude after producing a day's worth of worthless data with a probe that was unknowingly readjusted."

The next app note significantly expands on this topic.

24 October 2011

App Note 47 part 6

The last piece of App Note 47 that I want to discuss is the references. This app note has 46 interesting and useful references. (You know you're a hopeless academic when you spend time studying other people's references.) The list of references can actually be broken down into a dozen broad categories:

  • The excellent Tektronix Concept Series books [1,8,42]
  • Linear Technology App Notes [2,9-11,14,19,35,40]
  • Chapters from Jim's first book [3,12,13,46]
  • Some general history references [4-6,41]
  • Tektronix service manuals (always good for inspiration!) [7,25,29]
  • Articles about settling time [15-17] and scope measurement [34]
  • A private communication with John Addis [18]
  • Articles about bridge oscillators [20-24,30]
  • Analog multipliers [26,27], op amps [31,32,38], and converters [33]
  • Hewlett Packard Schottky diodes [28] and photodiodes [37]
  • Shielding and noise reduction techniques [36,39]
  • Some subtleties of transistor circuits [43,44,45]

I found it interesting and entertaining to actually go through the main text and look at the places that cited the references. (Curiously, ten of the references are not cited in the text of the app note... I just can't find references [30-39] mentioned anywhere.) Here are the quotes and excerpts (some of them paraphrased), and the references that go with them:

  • "Similarly, mismatches in almost all adaptors, and even in "identical" adaptors of different manufacture, are readily measured on a high-frequency network analyzer such as the Hewlett-Packard 4195A (for additional wisdom and terror along these lines see Reference [1])." (p.16)
  • "Current probes are useful and convenient... See Reference [2]." (p.17)
  • "For an enjoyable stroll through the history of oscilloscope vertical amplifiers, see Reference [3]. See also Reference [41]." (p.20)
  • "See Reference [3] for history and wisdom about vertical amplifiers." (p.82)
  • "The protracted and intense development effort put toward [oscilloscopes] is perhaps equaled only by the fanaticism devoted to timekeeping. In particular, the marine chronometer received ferocious and abundant amounts of attention. See References [4,5,6]." (p.20)
  • "While the oscilloscope provides remarkable capability, its limitations must be well understood when interpreting results. Additional discourse on oscilloscopes will be found in References [1] and [7-11]." (p.24)
  • "By nature of its operation, a sampling scope in proper working order is inherently immune to input overload, providing essentially instantaneous recovery between samples. See Reference [8] for additional details." (p.24)
  • "Reticence to try things is probably the number one cause of breadboards that "don’t work". A much more eloquently stated version of this approach is found in Reference [12]." (p.27)
  • "There is no substitute for intimate familiarity with your tool's capabilities and limitations. Further exposition and kvetching on this point is given in Reference [13]." (p.31)
  • "The use of the lamp to control amplifier gain is a classic technique, first described by Meacham in 1938. See References [19,20,21]." (p.49)
  • "This method of generating fast pulses borrows heavily from the Tektronix type 111 Pretrigger Pulse Generator. See References [8] and [25]." (p.93)
  • "Figure 124 shows a simple, very fast sample-hold circuit... The Schottky bridge, similar to types used in sampling oscilloscopes (see References [7,8,28]) gives 1ns switching and eliminates the charge pump-through that a FET switch would contribute." (p.56)
  • "Figure 130 is an extremely versatile trigger circuit... [which] allows any point on the input waveform edge to be selected as the actual trigger point. This technique is borrowed from oscilloscope trigger circuitry, see Reference [29]." (p.58)
  • "Figure 137's economical wideband thermally based voltmeter is based on a monolithic thermal converter. The LT1223 provides gain, and drives the LT1088 RMS-DC thermal converter (complete details on this device and a discussion on thermal conversion considerations are found in Reference [40])." (p.61)
  • "This is why oscilloscope probes were developed, and why so much effort has been put into their development (Reference 42 is excellent)." (p.16)
  • "Emitter followers are notorious sources of oscillation and should never be directly driven from low impedance sources (see References [43] and [44])." (p.87)
  • "The circuit of Figure 132 allows very short pulsewidths (in this case 250ns full-scale) to be determined to a typical accuracy of 1%... Q3, aided by Baker (see Reference [45]) clamping, capacitive feedforward and optimized DC base biasing, turns off in a few nanoseconds." (pp.58-59)

A few other citations are scattered through the text: Figure 99 (on p.44) shows a high-speed analog multiplier, using the AD824, whose data sheet appears in Reference [26]. Settling-time measurements are discussed in Appendix B (see p.83). Jim's previous approach is described in [14]. (Also, did he mean to cite References [15,16] here?) The "Harvey Method" is described in [17]. The use of sampling oscilloscopes is described in [7,8,18]. Appendix H, in its discussion of current feedback, again touches on the Hewlett sine wave oscillator, which leads to References [19-24] and [46] (see p.124). He probably meant to cite [30] here, too.


23 October 2011

Scope Sunday 14

Last weekend, when I was in California to attend the event at the Computer History Museum, I also hit two of Jim's favorite surplus stores.

I started at HSC Electronic Supply in Santa Clara.

They had a selection of Tektronix 7000-series plug-ins (always good to see!), but nothing that I really needed (they were mostly slow time bases and a few low-bandwidth vertical amplifiers).

After striking out at HSC, I headed to WeirdStuff.

In the very back, they had a Tektronix 7904 for $250. No thank you.

The last time I was at WeirdStuff with Jim, we found an ancient custom-built rack mount ten-channel amplifier box on the floor near the back wall. Inside the chassis, there were ten Philbrick K2-X op amps and ten K2-P chopper stages. We bought it, took it back to LTC, and stripped all the parts out of it.

That was a good day.

21 October 2011

App Note 47 part 5

The appendices of App Note 47 are numerous, voluminous, and excellent.

Appendix A is an abridged version of Tektronix's excellent introduction to oscilloscope probes, "The ABCs of Probes". It wasn't written by Jim, of course, but it's still essential reading for the uninitiated. The most recent version is 60 pages long, and can be found on the Tektronix website.

Appendix B is a treatise on measuring settling time, a topic originally discussed in App Note 10. In this treatment, an improved version of the circuits from App Note 10 is shown in Figure B2. Jim's superb attention to instrument calibration shine through here. The operation of the circuit in Figure B2 is explained, and then compared to a single trace sampling oscilloscope (a 556 with a 1S1 plug-in) and the "Harvey Method" (discussed in reference 17). The resultant measurement traces are shown in Figures B3, B4, and B5. A single sentence summarizes the work, "All methods agree on 280ns to 0.01% settling (1mV on a 10V step)." This sentence probably represents months of intense effort. (The "Harvey Method" is several times more complex than Figure B2!)

Appendix C is a discussion of frequency compensation without tears, which was first discussed in box section of App Note 18. This treatment includes significant new material that didn't appear in App Note 18, starting with Figure C7, which discusses several of the application circuits from the main text. As I said back in App Note 18 part 2, I'm not a fan of this treatment. I think the the analytical approaches to feedback systems are superior (the "large body of complex mathematics", as Jim dismisses it). See Reference [38].

Appendix D talks about measuring probe and oscilloscope response, continuing Jim's careful attention to the proper calibration and specification of his instrumentation. The approach here uses the avalanche pulse generator that originally appeared in Figure 27 in App Note 45 (repeated here in Figure D1). Note the effort expended in finding a workable approach here: "A sample of 50 Motorola 2N2369s, spread over a 12 year date code span, yielded 82%." Take a long look at the tight construction in Figure D3 (well, probably Figure F5, to be honest) and imagine building that fifty times!

Appendix E discusses a high-impedance probe circuit, based on the Elantec EL2004 350-MHz FET-input buffer amplifier. The resulting probe has a input capacitance of about 4 pF. Again, the tight construction in Figure E2 is impressive.

Appendix F is a brilliant pictorial essay on construction techniques. Figures F1 through F3 (all captioned "No") display a variety of sins. Figure F2 is of historical interest (I admit to feeling old when I have to first describe wirewrap to my students before I can make fun of it). I had never thought of the clip-lead construction in Figure F3 (a creative disaster). Figures F4 (another 556 picture!) and F5 show the prototype avalanche pulser from Appendix D, constructed in Jim's trademark style. Figure F6 shows the settling-time-measurement circuit from Appendix B. Figures F7 to F24 show various high-speed circuits from the main text, demonstrating the attention to shielding and stray capacitance, and the inattention to layout. Smaller and tighter is better. Figure F23 again shows that sometimes the best cable is no cable. Figure F25 shows the good life.

Appendix G contains the FCC forms appropriate for the circuit in Figure 116. See also the contributions of Prof. C. Berry in Figure 117.

Appendix H contains a brief history of "current feedback" (it's older than you think) and an introduction to "Current Feedback Basics" written by William Gross. "So, while the technique is not new, marketing claims notwithstanding, the opportunity is." (There's also a very good discussion of current-feedback amplifiers in Chapter 25 of Jim's first book, written by Sergio Franco.)

Appendix I is documentation for the "enticing" LTC high-frequency amplifier demo board, that is, the good life as suggested in Figure F25.

Finally, Appendix J ends the publication on a humorous note, if the observations contained therein doesn't strike too close to home. Some days, I just don't think Murphy's Law is all that funny.


17 October 2011

App Note 47 part 4

The final two application sections (pages AN47-51 to AN47-66) cover data-conversion circuits and some miscellaneous things. As footnote 15 on page AN47-51 says, "Seasoned readers of LTC literature, a hardened corps, may recognize this and other circuits in this publication as updated versions of previous LTC applications. The partial repetition is justified based on improved specifications and/or simplification of the original circuit." Jim undersells the application circuits here; some of the improvements (utilizing these high-speed op amps) are a big deal.

Figure 118 shows a sine-wave VCO, again using a AD639 as a triangle-wave-to-sine-wave converter (see Figure 21 in App Note 13). As I've said before, the AD639 was a Barrie Gilbert's brilliant Universal Trigonometric Function Converter, and using it as a triangle-to-sinusoid converter is like using a Lamborghini to go get your groceries.

Figure 121 is a 1Hz-to-10MHz V-to-F converter. Ten megahertz is pretty fast, but remember that App Note 14 includes the 100-MHz "King Kong" V-to-F converter. Of course, the advantage here is that with the high-speed op amps, there's no more need for the scope-trigger circuits and exotic 10H ECL parts. I especially like the caption of Figure 123 "(Whoosh!)".

Figure 124 is a high-speed (100 ns!) sample and hold, using a four-diode gate with transformer drive. This topology is an elegant solution for a high-speed S&H. Compare to the discrete 200-ns sample-and-hold circuit in Figure 23 of App Note 13.

Figure 131 is a trigger circuit with adaptive threshold, basically the adaptive subcircuit from Figure 97 (I wonder why he didn't present these two circuits in the opposite order?).

Figure 132 is a simple pulse-width-to-voltage converter, using a current source charging up a capacitor, like the single-slope converter in Figure 33 of App Note 13. Despite (or, perhaps, due to) being simple, the performance is very fast: able to resolve 1% accuracy on 250-ns pulses. The drive circuitry for Q3, including the Baker clamp and the speed-up capacitor, is especially instructive.

Figure 137 shows another application circuit for his LT1088 RMS-to-DC converter. This circuit is the same as Figure 8 in App Note 22, except the LT1223 is used instead of the discrete buffer suggested in App Note 22. Figure 139A shows a RF-leveling loop (from App Note 22 Figure 27), using the RMS-to-DC converter from Figure 137. (Figure 139B show a much simpler RF-leveling loop.)

Figure 140 shows a voltage-controlled current source (basically, it's Figure 8 from App Note 45 with much faster op amps). Figure 142 shows a higher-power version of the current source, using a discrete output stage.

Figure 144 shows a high-speed (18 ns!) circuit breaker (compare to the 12-ns version in Figure 40 of App Note 13, which required a floating load).

The best circuit in these sections is a toss up between the elegant sample-and-hold circuit in Figure 124 or the high-speed pulse measurement in Figure 132. (I still think the best circuit in the whole app note is the AM radio station in Figure 116.)

Best quote (page AN47-58): "Digital methods of achieving similar results dictate clock speeds of 1GHz, which is cumbersome." Understatement?