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.