Scope Sunday 27

I bought my tickets and made reservations for the Dayton Hamvention in May. I'm flying in on Wednesday to do a few random things, but I'll definitely be at the flea market on Friday. Anyone else going?

Jim told me that he attended the flea market once. He spent the whole day just walking around and laughing at the sheer enormity of it.

Here are a few pictures that I took last time I was there (in 2010):

Tektronix 7000 mainframes, priced at $1 per pound, which is a good deal for a working 7904A, but a terrible deal for a broken 7704A. Choose wisely!

A couple of Tektronix 7854 mainframes. Nice.

The consummate boat anchor: a Tek 551 dual-beam (but single-time-base) oscilloscope with the external power supply. I'll keep my 556, thankyouverymuch.

Vintage scopes are better part 2

Vintage scopes are better. (See the introduction in part one.)

Reason number 2: Sensitivity and bandwidth. With the appropriate plug-ins, analog oscilloscopes provide superior sensitivity compared to digital scopes. In discussing low-level noise measurements in App Note 70, Jim describes the oscilloscope requirements and laments,

Current generation oscilloscopes rarely have greater than 2mV/DIV sensitivity, although older instruments offer more capability. Figure B11 lists representative preamplifiers and oscilloscope plug-ins suitable for noise measurement. These units feature wideband, low noise performance. It is particularly significant that the majority of these instruments are no longer produced. This is in keeping with current instrumentation trends, which emphasize digital signal acquisition as opposed to analog measurement capability. (App Note 70, page 29)

While 2 millivolts-per-division is commonplace in digital oscilloscopes, plug-ins are available for 500-series and 7000-series scopes with sensitivity down to 10 microvolts-per-division. Yes, microvolts. In Appendix D of App Note 124, Jim lists the high-sensitivity, low-noise amplifiers of choice.

Of course, sensitivity and bandwidth are related (the wider the bandwidth, the higher the expected noise floor). However, in conjunction with superior noise floor, some vintage analog scopes also provide very large bandwidths. Some of Jim's favorites were

• Tektronix 556 with a 1S1 sampling plug-in, 1-GHz bandwidth (App Note 72, page 9, Figures 16 and 17)
• Tektronix 547 with a 1S2 sampling plug-in, 3.9-GHz bandwidth (App Note 79, page 19, Figure B4)
• Tektronix 661 with a 4S2 sampling plug-in, 3.9-GHz bandwidth (App Note 72, pages 34 and 35, Figure 77 to 82)
• Tektronix 7104 with 7A29 and 7B15 plug-ins, 1-GHz real-time bandwidth (App Note 94, page 4, particularly Figures 2, 11, 12, 13, 16, and 18)

There are modern digital scopes available today with wider bandwidths, but "vintage" and "analog" does not mean "slow." Of course, Jim would want me to point out that bandwidth isn't everything. In App Note 47, he explained,

Intimate familiarity with your oscilloscope is invaluable in getting the best possible results with it. In fact, it is possible to use seemingly inadequate equipment to get good results if the equipment’s limitations are well known and respected. All of the circuits in the Applications section involve rise times and delays well above the 100MHz-200MHz region, but 90% of the development work was done with a 50MHz oscilloscope. Familiarity with equipment and thoughtful measurement technique permit useful measurements seemingly beyond instrument specifications. A 50MHz oscilloscope cannot track a 5ns rise time pulse, but it can measure a 2ns delay between two such events. Using such techniques, it is often possible to deduce the desired information. (App Note 47, page 20)

To be honest, the first sentence of that quote applies no matter what oscilloscope you have.

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Footnote: One last comment while we're discussing plug-in oscilloscopes. The Tektronix 556 dual-beam instrument provides flexibility that is not found in modern instruments.

The Tektronix 556 offers an extraordinary array of features valuables in converter work. This dual beam instrument is essentially two full independent oscilloscopes sharing a single CRT. Independent vertical, horizontal and triggering permit detailed display of almost any converters operation. Equipped with two type 1A4 plug-ins, the 556 will display eight real time inputs. The independent triggering and time bases allow stable display of asynchronous events. Cross beam triggering is also available, and the CRT has exceptional trace clarity. (App Note 29, pages 43-44)

In App Note 65, he exploited these dual-beam advantages in a number of measurement. Figure 36 shows six waveforms, with independent triggering of the top two versus the bottom four traces. Figure 42 shows the ringing bursts at the resonant frequency of the Royer converter, with the explanatory footnote

The discontinuous energy delivery to the loop causes substantial jitter in the burst repetition rate, although the high voltage section maintains resonance. Unfortunately, circuit operation is in the "chop" mode region of most oscilloscopes, precluding a detailed display. "Alternate" mode operation causes waveform phasing errors, producing an inaccurate display. As such, waveform observation requires special techniques. Figure 42 was taken with a dual-beam instrument (Tektronix 556) with both beams slaved to one time base. Single sweep triggering eliminated jitter artifacts. Most oscilloscopes, whether analog or digital, will have trouble reproducing this display. (App Note 65, page 38)

Finally, the flexibility of the Tektronix 556 allows for some great measurement displays. In Figure 34 of App Note 35, he showed a 115-volt sine wave, its distortion products, and its frequency spectrum all in one shot.

In the accompanying footnote, Jim teased,

Test equipment aficionados may wish to consider how this picture was taken. Hint: Double exposure techniques were not used. This photograph is a real time, simultaneous display of frequency and time domain information. (App Note 35, page 16)

This picture was (most probably) produced with his trusty Tektronix 556 with a vertical-amplifier plug-in in one bay (perhaps a 1A2 or 1A4), and a spectrum-analyzer plug-in in the other bay (perhaps the 1L5 50Hz-to-1MHz spectrum analyzer).

Vintage scopes are better part 1

I was discussing these application notes with a colleague, and he commented, "Jim sure did love his vintage oscilloscopes. I wonder, is there anything that you can do with a vintage scope that you can't do with a modern digital one?"

"Yes!" I cried, and I listed off several things, but I don't think I convinced him. Over the next four Sundays, I'm going to enumerate and explain the list of reasons why vintage scopes are better than modern digital abominations, including

1. Trace clarity (resolution and spot size)
2. Sensitivity and bandwidth (and noise floor)
3. Overdrive resilience (of sampling plug-ins)
4. Repairability and inspiration

Mostly, I'll just be quoting the relevant passages in Jim's app notes. Truly, they don't make them like they used to...

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Reason number 1: Trace clarity. The low-level-measurement resolution of a oscilloscope is limited, in part, by the minimum size of the trace on the screen. A well-designed (and well-calibrated) vintage scope can have a vanishingly small spot size on the CRT. With a digital scope, the resolution of the input analog-to-digital converter often becomes apparent as you increase the vertical gain on a digital scope (or you are limited by the size of the LCD pixels).

Discussing oscilloscope selection for low-level noise measurements in Appendix B of App Note 70, Jim commented,

The monitoring oscilloscope should have adequate bandwidth and exceptional trace clarity. In the latter regard high quality analog oscilloscopes are unmatched. The exceptionally small spot size of these instruments is well-suited to low level noise measurement. (Footnote: 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.) (App Note 70, page 29)

He continues this train of thought to say,

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. (App Note 70, page 29)

Again discussing noise measurements in Appendix C of App Note 90, Jim says,

Diehard curmudgeons still using high quality analog oscillscopes routinely discern noise presence due to trace thickening. Those stuck with modern instruments routinely view thick, noisy traces. (App Note 90, page 22)

The detail that is visible with well-focused oscilloscope trace is evident on Jim's Tektronix 556 in Figures B9 and B10 in App Note 70.

I'd like to see these plots replicated on a modern all-digital scope.

App Note 85

"Low noise varactor biasing with switching regulators: Vanquishing villainous vitiators vis-à-vis vital varactors." 24 pages.

This app note discusses low-noise bias-voltage generators for varactor diodes. The main application, as shown in Figure 1, is VCO tuning for phase-locked loops. This application is extremely sensitive to power-supply ripple, as any corruption of the bias voltage will cause spurs in the VCO output. In the circuit shown in Figure 6, careful power-supply design and appropriate filtering produce the varactor-bias voltage shown in Figure 11, which exhibits only 20 microvolts of ripple and noise.

However, the major emphasis of this app note isn't the circuitry, but the instrumentation (this topic is well-worn territory for Jim, of course). Measuring 20 microvolts of ripple and noise is HARD, and Figures 12 through 17 show several ways that improper measurement technique ruin the measurement. Figures 19 through 24 show frequency-domain measurements of the VCO output using a spectrum analyzer. Again, it is shown that improper measurement technique, or careless construction and layout, degrade performance significantly.

Appendix A is a primer on varactor diodes written by Neil Chadderton of Zetex.

Appendix B discusses amplifier and oscilloscope selection in order to facilitate the 20-microvolt sensitivity needed for Figure 11. This appendix is borrowed from Appendix B in App Note 70. I can't resist quoting the footnote again:

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.

Appendix C is copied from App Note 70 Appendix C (as he explains in the footnote).

The app note ends with a varactor cartoon. Probably the only varactor cartoon in the world. "I never had it so good."

Top Five Posts 2011

Since all the cool kids are doing it, I thought I'd jump on the bandwagon. I started this blog in July, so I really only have six months of traffic, but that doesn't stop me from declaring:

The Top Five Most Popular Posts in 2011

1. Scope Sunday 3

My airline adventure with a Tektronix 453, along with a letter from Jim.

2. App Note 65 part 2

Commentary about CCFL power supplies, and the "Royer" pin on the LT1183. This post generated a lot of traffic and spawned a discussion and the dedicated post on "IC pins named after persons".

3. Scope Sunday 13

Photos from my trip to the Computer History Museum to see the opening night of Jim's Linear Technology laboratory bench on display (with my business card in the mess).

4. Introduction

The entry point to the blog, often posted on other web sites (and listed here under Jim's picture).

5. App Note 35 part 1

Jim's amazing simultaneous time-domain and frequency-domain measurement (using a Tek 556 scope from the 1960s), that puts Tektronix's new MDO4000 to shame.

Of course, listing these links gives these posts a head start in the race for "Most Popular Posts in 2012" (it's an interesting positive-feedback loop, isn't it?), but I hope that I will produce even more interesting posts in the coming months!

Which post was your favorite?

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Postscript: Coincidentally, this post is my one-hundredth post to this blog. Yikes!

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."

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Related:

• App Note 70 part 1
• App Note 70 part 3

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.

Book 2 Chapter 17

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

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

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

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

Beautiful.

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Related:

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

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.

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Related:

• App Note 55 part 1

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.

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.

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Related:

• App Note 47 part 1
• App Note 47 part 2
• App Note 47 part 3
• App Note 47 part 4
• App Note 47 part 6

App Note 47 part 2

The first sections of App Note 47 are "Mr. Murphy's Gallery of High Speed Amplifier Problems" and the "Tutorial Section". Mr. Murphy's Gallery is a expanded and improved version of "The Rogue's Gallery of High Speed Comparator Problems" from App Note 13. The best part of this gallery is the rating system: the average number of phone calls they received per month due to each problem. The winner, of course, with 165 calls per month, is "Excessive Capacitive Load".

The "Tutorial Section" begins with a discussion of termination quality and high-speed pulses (using his new 350ps-rise-time pulse generator from App Note 45: compare Figure 29 with App Note 45 Figure 28). A discussion of probe and probing technique follows, with Figure 31 demonstrating the (quite good) advice that "Sometimes the best probe is no probe" (demonstrated with his trusty Tektronix 556). Much of this section is borrowed from Appendix B of App Note 13.

The section "About Oscilloscopes" includes two pieces: a piece on bandwidth, and a piece on overload performance. The first piece begins with an interesting statement and footnote: "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)." (Note that Jim wrote this statement almost five years before the publication of Dava Sobel's popular "Longitude" book.) An interesting gallery of oscilloscope pulse responses are shown in Figures 38 through 44. The fastest scope shown (Figure 38) is the Tek 556 with the 1S1 1-GHz sampling plug-in. A Tek 485 is used in Figures 39 through 42, showing the 350-MHz (50-ohm) and 250-MHz (1-megaohm) bandwidths, with several different probes. A 150-MHz Tek 454A (fits under an airline seat!) is used in Figure 43, and, finally, a 50-MHz measurement is shown in Figure 44, using a Tek 556 with a 1A4 plug-in.

The piece on overload performance (Figures 45 through 50, with text beginning on page AN47-22) is borrowed from Box Section A of App Note 10. The next two sections "About Ground Planes" and "About Bypass Capacitors" (Figures 55 through 60) are also borrowed, from Appendix C and Appendix A of App Note 13 (respectively).

Finally, the "Breadboarding Techniques" and "Oscillation" sections include a pictorial tutorial on construction for high-speed circuits in Figures 62 through 65. "More than anything else, breadboarding is an iterative procedure, an odd amalgam of experience guiding an innocent, ignorant, explorative spirit." I often have a hard time convincing people that such a construction technique really works, but it really does! Nice, neat layouts, with lots of straight wiring, often have too much parasitic capacitance, parasitic inductance, and parasitic feedback loops. "Despite the breadboard’s seemingly haphazard construction, the circuit worked well."

Best quote (page AN47-5): "Like all engineering endeavors, high speed circuits can only work if negotiated compromises with nature are arranged. Ignorance of, or contempt for, physical law is a direct route to frustration. Mother Nature laughs at dilettantism and crushes arrogance without even knowing she did it."

I'll discuss the first application sections next time.

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Related:

• App Note 47 part 1
• App Note 47 part 3
• App Note 47 part 4
• App Note 47 part 5
• App Note 47 part 6

App Note 45 part 2

Again, we have a collection of circuits here that captured Jim's imagination in some way or are improvements of circuits from previous app notes.

Figure 18 shows a quartz-stabilized oscillator, which is a different approach from the Hewlett-Packard-inspired oscillators in App Note 43. This circuit achieves 9 ppm distortion (Figure 48 in App Note 43 achieved 3 ppm distortion), but it requires a 4-kHz J-cut crystal.

Figure 19 is a single-cell-powered temperature-compensated crystal oscillator, similar to App Note 15, Figure 9. A boost converter is used to drive the varactor diode with bias voltages up to 4V.

Figure 21 appears to be an improved version of the "Zoo Circuit" V-to-F converter (App Note 23 Figure 16) with even lower power consumption (maximum 90 microamps). Figure 24 is another V-to-F converter, this one with a bipolar input (and a start-up circuit adapted from the Tek 547 trigger circuit).

Figure 27 is a 350-ps rise-time pulse generator. This circuit will be very useful in App Note 47 and other upcoming app notes (and it is much better than the 1-ns pulse generator in App Note 13 Figure D1). The pulse in Figure 28 is very clean, shown on his Tek 556 with the 1S1 sampling plugin. "I'm sorry, but 1GHz is the fastest scope in my house." (See Reference 7.)

Figure 30 is a low-dropout regulator using the LT1123 and the specially design (and now unavailable?) MJE1123 transistor. A germanium 2N4276 is explored as a replacement (but is no easier to obtain!).

Figure 36 is a power supply for a cold-cathode fluorescent lamp. Look at all the bottles! I count 48 of them. Yikes. Although Jim may not know it yet, this application is the beginning of a long-term obsession (or was it an assignment?). More praise for the Tektronix 556 and 547 on the bottom of page AN45-22.

Best quote (from Figure 36, a harbinger of future difficulties): "Do not substitute components."

More than half of the references on page AN45-23 (References 9 to 17) have to do with fetal heart monitoring (as shown in Figure 1). I feel a little sorry for Jim's wife and unborn son at this point. The App Note concludes with a great picture of Michael, perched atop a Tektronix 556.

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Related:

• App Note 45 part 1

Book 1 Chapter 4

Chapter 4, "Is analog circuit design dead?"

Is analog circuit design dead? All of these publications, all of these great circuits, and Jim's whole career are evidence to the contrary. Clearly, the answer is "no", and Jim writes a great polemic. (Did James Solomon really say, "all classical analog techniques are dead"? That's terrible.) Twenty years later, we don't hear much from the "analog is dead" choir. The "war" of which Jim speaks here is long over. Few doubt the value and necessity of the analog interfaces. Analog may not be king, but without analog, digital would have nothing to rule over.

On the other hand, I do have to relate that I was recently asked why I was bothering with designing low-noise RF amplifiers: "Can't you just use an analog-to-digital converter?" (Um, no.)

"Do all you bit pushers out there get the message?" Yes, Jim, I think they finally did.

I love the quotes from George Philbrick (from an article that is also reprinted in this book, as Chapter 2), and the shout-out to Korn and Korn and Henry Paynter's "Palimpsest on the Electronic Analog Art". Good reading, all around. Best quote (page 17): "Analog computers did not die out because analog simulation are no loner useful or do not approximate truth; rather, the rise of digital machines made it enticingly easy to use digital fakery to simulate the simulation."

Finally, consider Figure 4-2:

If you ever, ever, ever see a Tektronix 556 in somebody's trash, please call me.

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Related:

• Book 1 Introduction
• Book 1 Chapter 7
• Book 1 Chapter 13
• Book 1 Chapter 23

App Note 35 part 1

"Step down switching regulators." 32 pages.

This app note is another great work that will take several days to cover. Again, I want to start by talking about some of the instrumentation. One measurement that is worthy of note is Figure 30, showing a 500-volt square wave! As the caption says,"DANGER! Lethal potentials present..."

However, the best measurement is clearly Figure 34, that shows a 115-volt sine wave, its distortion products, and its frequency spectrum all in one shot.

Jim teases us, but gives away no secrets, in the footnote:

Test equipment aficionados may wish to consider how this picture was taken. Hint: Double exposure techniques were not used. This photograph is a real time, simultaneous display of frequency and time domain information.

How did he do that? I assume he's using his trusty Tektronix 556 with a vertical-amplifier plug-in in one bay (perhaps a 1A2 or 1A4), and a spectrum-analyzer plug-in in the other bay (perhaps the 1L5 50Hz-to-1MHz spectrum analyzer). As for the distortion products, perhaps a HP 339A Distortion Analyzer?

This picture actually pretty funny at present. Tektronix is currently touting their new "Scope Revolution", the "world's first and only" mixed-domain oscilloscope (which they call the MDO4000) that has a built-in spectrum analyzer. Their ad copy says,"See both time and frequency domains in one glance. View the RF spectrum at any point in time to see how it changes." Take a look at http://www.scoperevolution.com/ and then take another look at Figure 34. Jim Williams beat them to the punch 22 years ago with technology from the 1960s.

(OK, OK, I admit that the MDO4000 does a lot more than I suggest above (for one thing, there is no logic analyzer here, nor would Jim use one, and the MDO4000 time-correlation functions are really cool); however, the superficial similarities are striking. And hilarious.)

I once heard that Tektronix offered Jim a brand new oscilloscope of his choosing, anything he wanted, if he promised to stop using vintage instruments in his app notes. No deal.

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Related:

• App Note 35 part 2
• App Note 35 part 3

Scope Sunday 8

Here's a quick family photo of my 500-series oscilloscopes. From left to right, the scopes are a 535A (with a 503 underneath), a 545A, a 547 (which I purchased especially in Jim's honor), and a 575 curve tracer (with the 175 high-current adapter underneath).

Missing from this picture is my 511A, which is still in storage in California.

I'm still looking for other 500-series scopes, especially a 556 dual beam.

(I've also updated "Scope Sunday 6" with a great letter from Bill Hewlett.)

App Note 29 part 1

"Some thoughts on DC-DC converters." 44 pages.

At 44 pages, this app note is the longest one so far (but not the longest one ever, by far). It's also the first one with a coauthor (Brian Huffman). Given the length, I'll cover this one over the next few days. Today, I can't help but to talk about the oscilloscopes!

First, there are eleven traces in Figure 7! How did he do that? I don't know of any Tektronix mainframe that allows for 11 traces on a single display. You can get 8 traces using a 556 with two 1A4 plugins (which we've seen before, for example, see App Note 3 Figure 16), but I don't know how to get eleven. I suspect a double exposure with the camera.

Second, there are some great measurements here. Figure 5 has a trace at 20 microvolts per division. Figure 33 has three traces with current probes at 2 amps per division. There are some high-voltage measurements, too (20 volts per division in Figure 2), but we've seen higher (for example, 200 volts per division in Figure D7 of App Note 25).

Appendix F contains some more sage advice on instrumentation. Again he starts by talking about probes, but after that discussion, we finally, finally have some explicit oscilloscope recommendations. After dismissing the more modern Tektronix scopes (the 2445 and 2446 were modern at the time), he recommends his favorite, the 547 with a four-trace type 1A4 plugin. I'm surprised that he suggested the three-bay 7603 (with two 75-MHz 7A18 plugins) as an equivalent mainframe. There are much nicer (and four-bay!) 7000-series mainframes available. You certainly don't need the bandwidth of the specialty 7104 here, but I definitely prefer the 7704A and 7904A mainframes to the sluggish 100-MHz 7603. (However: in most applications, Jim preferred low-bandwidth scopes. It's actually good advice.)

He also heaps significant praise on the 556 dual-beam scope, and Figure F2 contains the first actual photograph of a scope (a type 556 dual-beam oscilloscope with 1A7 plugin). He also discusses some specialty low-level and differential plugins, including the 1A7 and 7A22 plugins (with 10 microvolt sensitivity) and the differential comparator plugins W, 1A5, and 7A13.

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Related:

• App Note 29 part 2
• App Note 29 part 3

App Note 25

"Switching regulators for poets: A gentle guide for the trepidatious." 24 pages.

This application note is a classic. I think it's the first one where Jim's personality and sense of humor really shine through his writing. There are a lot of gems here, even just on the front page: the title is a classic, along with the discussion of Everyman and the poets, and "my poetry ain't very good." I also like his acknowledgment of the "encouragement" from the Captains of his corporation. Page AN25-24 sports his first cartoon, which will become something of a trademark.

The app note itself is pretty short: only twelve pages of text (and twelve pages of appendices). Discounting Figure 1, there are really only five application circuits here: Figures 4, 6, 9, 17, and 18. The best circuit is the monster in Figure 9, the 100W off-line switching regulator, an impressive achievement.

The appendices contain some great advice. Appendix B is a very practical treatment of compensation for switching converters. It's not as analytical as I would like (what's new?), but it is practical and exhaustive for this application (and it isn't the mess that App Note 18 contains).

Appendix C is worthy of particular note: Figure C1 shows the first oscilloscope labeled as a Type 547 with a four-channel Type 1A4 plug in (which we've known all along). However, I believe that the pictures in Figures C2, C3, and C4 are from Jim's Tek 556 (note the damaged gradicule). If you look back at Figure 16 in App Note 18, you'll see the same damaged gradicule with six traces on the screen, which requires a 556 with two 1A4 plug ins. Figures C5 and C6 show some nonideal turn-off and turn-on effects in diodes, which is the topic of a future app note.

The evolutionary design approach in Appendix D is very good method. I have seen more than one engineer attempt to power up a design such as Figure 9 all at once, and the fireworks are often worthy of the 1812 Overture.

The best (funniest) quote appears on page AN25-13: "The author acknowledges Carl Nelson's abundance of commentary, some of which was useful, during preparation of this work", although the parenthetical statement on page AN25-4, "(ground as I say, not as I do)", is a close second.

National App Notes

Last Thursday, I mentioned the National Semiconductor Application Note 294 that Jim wrote. On Friday, a commenter referred to a circuit in NSC App Note 299 (which Jim also wrote).  Jim worked for National in the Linear Integrated Circuits Group from 1979 to 1982. During this period, he wrote many app notes, but getting a complete list of his notes is a bit of a mystery hunt. There are several unfortunate reasons for this difficulty:

1. National doesn't always print bylines with author's names on their app notes.
2. National regularly deletes old app notes from their archives.
3. National sometimes updates the publication date of their app notes upon revision.

However, I spent some time digging, and I think I now have a complete list. To find which application notes he wrote, I had to infer Jim's authorship based on the right time period and other clues. One reliable clue was the inclusion of photographs of Jim's Tektronix 556 oscilloscope with the damaged graticule. In other cases, I made educated guesses based on his use of references, footnotes, or subject matter. A reference to one of Jim's past publications is a good hint, a footnote discussing the Hewlett Packard HP200 oscillator is a dead giveaway!

Based on this research, there are (at least?) twenty-one application notes that he wrote.  They are App Notes 256, 260, 262, 263, 264, 265, 266, 269, 272, 285, 286, 288, 289, 292, 293, 294, 295, 298, 299, 301, and 311. Not bad for three years' work!

You can find most of the app notes on National's master list.  For more details on the frustrations of this mystery hunt, see http://web.mit.edu/klund/www/jw/jw-nsc.html.  (After I finish reading all of the Linear Tech app notes, it will be interesting to go back and reread all of his National app notes.  I should have done that first.)

App Note 13 part 3

The appendices of App Note 13 include a wealth of practical information. Appendix A talks about bypass capacitors and includes five scope traces that warn of potential troubles. Figure A7 is particularly horrifying. (I wish he named some names here; I'd like to know what specific combination of capacitors caused that shameful ringing. I guess I'll have to experiment myself... Personally, I've been using a combination of tantalum and X7R for bypassing. I really should check it out, as Jim suggests.)

Appendix B further discusses probes and oscilloscopes. Again, he doesn't name any specific makes and models of oscilloscopes, but we can guess what he's using (a Tek 547 and a Tek 556). It's funny how he suggests that the oscilloscope should have 150 MHz of bandwidth, after admitting that "90% of the development work was done with a 50MHz oscilloscope." More space is devoted to discussing probes, FET probes, current probes, and (of course) grounding. I think that I will steal the test circuit in Figure B1 to use at the basis of a lecture demo and/or lab project. It is simple, yet instructive. The picture in Figure B5 shows a wide variety of probe types ("Note the ground strap on the third finger."). 

Appendix C discusses some suggestions for ground planes. In short, use them and love them. 

All three of the above appendices will appear again (in one form or another) in App Note 47.

Appendix D shows an interesting and strange circuit for producing very fast pulses. First comment: the LM301A is only specified for a maximum voltage of 36V. The military-grade version, the LM101A, is specified to 44V. I wonder why he didn't suggest the LM101A? Second comment: the circuit uses a TD-263B tunnel diode! That's cool (it's the right tool for the job), but I don't think that Germanium Power Devices even makes tunnel diodes any more. Does anyone? In Figure D2 and the accompanying caption, we learn that the heretical HP scope that we occasionally see is a 275-MHz unit. (I don't know my HP scopes very well. Can anyone identify this model? Is it an HP 1725A?)

Appendix E discusses high-speed level shifters. Figure E2 shows a TTL-inspired level shift with a 15-volt output. I like figure E3 with the speed-up capacitor and the Baker clamp. I really do have a soft spot in my heart for old logic-circuit topologies. I'm curious about what application requires that power FET switching one amp(!) in 9 nanoseconds in Figure E4.

Best quote (page AN13-27): "Probes are the most overlooked cause of oscilloscope mismeasurement." Yep.

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Related:

• App Note 13 part 1
• App Note 13 part 2