Scope Sunday 37

Remember how I mentioned last week that I hadn't bought any scopes this year, and I then I bought two 422 portables at the Boxboro flea market?  Well, funny thing: I found another deal on Craigslist this week...

The listing advertised two 547 scopes for $50 in western Massachusetts.  Good deal (I've paid a little more for a 547, but I've also paid less).  I called and said I'd take them.  Turns out the seller was a high-school science teacher who had gotten the scopes from a neighbor of his parents to give to his school.  Unfortunately, the school didn't want them because they didn't have the space, and now he needed to get rid of them. (Unfortunately, his parents live in Ohio, so my thoughts of meeting this excellent neighbor were quickly dashed.)  So, we made plans to meet up.  The seller was driving into Boston this weekend to help his son move, so we met at a gas station along Route 2.

What made this transaction a unexpectedly GREAT deal were the accessories that came with the scopes.  The scopes are nice, but the accessories (some of which are shown above) are fantastic:

1. A Model 500 scope cart, painted Tektronix grey instead of Tektronix blue.  This cart should match my Tek 511A.  It has a drawer, but no storage for plug-ins (of course, the early Tek scopes didn't have plug-ins, but that big blank panel still seems like a lot of wasted space).
2. A complete C-12 camera system.  This is my first scope camera, and I'm kind of excited about it. Anybody have a good source for Polaroid film?  (Gotta love that smell.)
3. A mint-condition vinyl cover, with the Tektronix logo on the side (very nice).
4. A pair of 1A4 plug-ins, in addition to the two 1A1 plug-ins that came installed in the scopes.
5. Original bound manuals for 547, 1A1, 1A2, the camera, and a 549 (for some reason).
6. A photocopied manual for the 1A4 plug-ins.
7. A 1971 Tektronix catalog.  So cool.
8. Three extra 154-0568-00 CRTs.  Unfortunately, they're all the same and two of them are labeled "gassy".  When I saw the box of extra CRTs, I had hoped for different phosphors, but, alas, no.

So I actually got all of this for $50 and a scope.  When the seller told me that he was a science teacher, who wanted a scope for his school, I offered him the smallest scope that I had (it was a "modern" 20-MHz, two-channel, no-name scope, but it works, and it's smaller and lighter than a 422 or 453).  I hope it's small enough that he can find room for it.  I wish my high-school science club had had an oscilloscope!

Scope Sunday 35

Last weekend, I finally got a chance to visit the vintageTEK Museum in Portland, Oregon.  I had a great time.  The museum is smaller than I expected; in fact, there are only a few dozen instruments on display.  Here is a picture of the whole museum taken from the front desk.

Despite being small, the museum has an impressive collection of rare instruments, including quite a few that I had never seen before in person.  First up, of course, is Tektronix's original oscilloscope from 1946, the 10-MHz-bandwidth 511. Only about 350 of these scopes were made (I have a 511A, which isn't nearly as rare).

On the other end of the frequency spectrum, they have a 519, the 1-GHz special-purpose monster from 1961. (There's also a 1-GHz 7104 in the museum, of course.)

Next up is the 945, which is the military ruggedized version of 545.  Heavy and heavy duty.

 They also have a 7704A mainframe, complete with the P7001 digitizer.

Of course, I thought the best part of the museum was the storage room and repair lab.  Here's the wall of letter-series plugins (check out the three different colors of type CA plugins in the bottom row; there's also two type O plugins and a type Q here, and a type N just out of frame).  I spent quite a bit of time poking around in the back room.

I actually spent most of the afternoon in the back, hanging around with two volunteers, who were busy sorting spare parts and fixing the horizontal sweep in a 547 (it was a busted tunnel diode, of course, part number 152-0125-00; luckily, there was a 547 parts donor on the shelf).  I don't know if I was a help or hinderance in the process, but I had fun.

After the museum closed, Ed Sinclair invited me to join the TERAC (formerly known as the Tektronix Radio Amateurs Club) for their weekly Friday night dinner at Round Table Pizza in Beaverton.  There I met another great group of (mostly former Tektronix) engineers, including Deane Kidd, and had a great time.

Vintage scopes are better part 4

Vintage scopes are better (also see part one, part two, and part three).

Reason number 4: Repairabililty and inspiration.

It is nearly impossible to get a useful service manual for a modern oscilloscope, and even if you could, the parts to repair it are generally unobtainable custom ICs. However, many vintage scopes are repairable, and complete service manuals are readily available for many models. While some vintage parts are getting hard to find (cough, tunnel diodes, cough), many repairs are possible with easily obtained parts. In his second book, Jim explains,

Older equipment offers another subtle economic advantage. It is far easier to repair than modern instruments. Discrete circuitry and standard-product ICs ease servicing and parts replacement problems. Contemporary processor-driven instruments are difficult to fix because their software control is "invisible", often convoluted, and almost impervious to standard troubleshooting techniques. Accurate diagnosis based on symptoms is extremely difficult... Additionally, the widespread usage of custom ICs presents a formidable barrier to home repair. [1]

In addition, vintage scopes are extremely well-designed and well-constructed. Much can be learned from examining and exploring the internals of classic oscilloscope.

The inside of a broken, but well-designed piece of test equipment is an extraordinarily effective classroom... The clever, elegant, and often interdisciplinary approaches found in many instruments are eye-opening, and frequently directly applicable to your own design work. More importantly, they force self-examination, hopefully preventing rote approaches to problem solving... The specific circuit tricks you see are certainly adaptable and useful, but not nearly as valuable as studying the thought process that produced them. [2]

His love of vintage scopes was a continual source of inspiration. He studied, referred to, and borrowed extensively from classic instruments and their service manuals. For example,

• The high-speed ECL logic in the "King Kong V/F" in App Note 14 was inspired by the trigger circuitry of the Tektronix 2235.
• The adaptive threshold trigger circuit in Figure 130 of App Note 47 was also inspired by the Tektronix 2235.
• In his discussion of CCFL power supplies in [3], Jim explored the high-voltage resonant power supplies in the Tektronix 547 and the Tektronix 453.
• In his discussion of low-noise power conversion (App Note 70, Appendix A), he discusses circuits from the Tektronix 454 and 7904 (Figures A1 and A2).
• Figure 12 in App Note 98 was derived from the calibrator circuit in a Tektronix 485.

References:

[1] Jim Williams, "There's no place like home," in The Art and Science of Analog Circuit Design, Jim Williams, Ed. Boston: Butterworth-Heinemann, 1995, ch. 17, pp. 269–277.

[2] Jim Williams, "The importance of fixing," in The Art and Science of Analog Circuit Design, Jim Williams, Ed. Boston: Butterworth-Heinemann, 1995, ch. 1, pp. 3–7.

[3] Jim Williams, "Tripping the light fantastic," in The Art and Science of Analog Circuit Design, Jim Williams, Ed. Boston: Butterworth-Heinemann, 1995, ch. 11, pp. 139–193.

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.

---

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...

---

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."

App Note 79

"30 nanosecond settling time measurement for a precision wideband amplifier: Quantifying prompt certainty" 32 pages.

While technically the long-awaited update to App Note 10, to be honest, this app note is an abridged and modified version of App Note 74. Here the application is the measurement of amplifier settling time instead of DAC settling time, but the techniques (and the text) are significantly similar. The measurement problem tackled here is actually easier; amplifier settling is determined at the 0.1% point, while 16-bit DAC settling requires measurement of the 0.0015% point. (The amount of laboratory work involved was no less, however; Appendix E is proof of that fact.)

The settling-time measurement circuit is shown in Figure 6 (compare to App Note 74 Figure 6). The modifications for this application include a change to the input drive (to drive a voltage step to the op amp, instead of a digital command to the DAC), removal of the output amplifier, and removal of the temperature-control loop on the diodes (both because 0.0015% accuracy is no longer needed). Also, the sample delay and window generator is now implemented with LT1720 comparators instead of 74HC123 TTL logic.

Also, instead of three alternative measurement methods for comparison (as in App Note 74), he provides just one: the classical sampling oscilloscope. The "essentially identical" results of these two measurement are shown in Figures 18 and 19. Perhaps he felt he has less to prove this time?

The best quote is the app note's conclusion: "Examination of the photographs shows nearly identical settling times and settling waveform signatures. The shape of the settling waveform is essentially identical in both photos. This kind of agreement provides a high degree of credibility to the measured results."

Many of the appendices previously appeared in App Note 74. Appendix A is the same as App Note 74 Appendix B. Appendices C and D are similar to App Note 74 Appendices C and D (with light modifications for the new topology of the settling-time measurement circuit, and for op-amp settling instead of DAC-output-amp settling, respectively).

Appendix B is new, discussing one of his favorite topics, subnanosecond pulse generators. He complains about the prices of current production units ($10,000 to $30,000), discusses his favorite vintage units (HP-8082A, HP-215A, Tek 109, and Tek 111), and then shows his own design in Figure B1. This circuit, as he would likely say, "is the beneficiary of considerable attention over a protracted period of time." It is now loaded with features, including a fully adjustable pulse amplitude, an external input to determine repetition rate, and an output trigger pulse that is settable from before-to-after the main pulse. Figure B4 shows the high-speed pulse in all its glory, measured with a Tek 547 with 1S2 sampling plug-in.

Appendix E discusses breadboard construction (like App Note 74 Appendix G) and includes another photo essay on the construction of the settling-time measurement circuit (Figures E1 to E6), proving that the lab work was exhaustive.

The app note ends with a cartoon, of course. Thirty nanoseconds is hard!

App Note 74 part 2

This app note contains eight appendices (more than half of all the pages), and, as always, the appendices are great stuff.

Appendix A talks a little bit about the history of digital-to-analog conversion and includes a glamour shot of some items from his collection. The Kelvin-Varley divider is very nice. "What Lord Kelvin would have given for a credit card and LTC's phone number."

Appendix B is an updated discussion of oscilloscope-overdrive performance (see App Note 72 Figures 32 to 37 and App Note 47 Figures 45 to 50). Here he has added Figure B1, which compares the topology of various oscilloscopes, and he sings the praises of vintage analog instruments. "Unfortunately, classical sampling oscilloscopes are no longer manufactured, so if you have one, take care of it!" Indeed.

Several of the appendices follow-up on and expand on some issues from the main text. Appendix C discusses calibration of the amplifier delay in the settling-time measurement circuit. Appendix D discusses amplifier compensation (in his usual, seat-of-the-pants way), and the moral of the story seems to be "build it and see." Appendix F shows the circuitry necessary to interface DACs with serial-data interfaces to the settling-time measurement circuit.

Appendix E discusses the special case of using a chopper-stabilized amplifiers and the possible dangers involved therein. The scope trace in Figure E3 is especially frightening, although he admits,

This is admittedly worst case. It can only happen if the DAC slewing interval coincides with the amplifier's internal clock cycle, but it can happen. (Footnote: Readers are invited to speculate on the instrumentation requirements for obtaining Figure E3's photo.)

Such a tease! It'd be nice if he occasionally exposed the trickery behind some of these displays.

Appendix G discusses breadboard construction, and in particular talks about proper steering of the ground currents. Wise advice! As the footnote says, "I do not wax pendantic here. My abuse of this postulate runs deep." This appendix also includes a very nice photo essay on the construction of the settling-time measurement circuit (Figures G1 to G10). I'd love to see some high-resolution color versions of these pictures.

Finally, Appendix H shows some power-gain stages, some of them borrowed from App Note 18 and App Note 47 Appendix C.

The app note concludes with a picture of his work bench instead of a hand-drawn cartoon.

On the left is his Tek 661 sampling scope, with a Tek 454 on top. Next to that is his Tek 547, and in front of the 547 is the General Radio 1422-CL variable capacitor, propped up by a data book. Classic stuff.

(Yes, I really want a 661.)

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Related:

• App Note 74 part 1

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

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.

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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.

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Related:

• App Note 70 part 2
• App Note 70 part 3

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.

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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.

Book 2 Chapter 17

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

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

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

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

Beautiful.

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Related:

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

Book 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:

• Part 1 is Figure 36 in App Note 45
• Part 2 is App Note 49
• Part 3a is App Note 55 (part 1 and part 2)

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.

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Related:

• Book 2 Introduction
• Book 2 Chapter 1
• Book 2 Chapter 17

Scope Sunday 13

Jim's lab bench from Linear Technology is on display at The Computer History Museum until April 2012. They moved his primary work bench, trademark mess and all, to the museum (apparently, they just shrink-wrapped the whole thing for transport). It's part of a exhibit dedicated to Jim called "An Analog Life", and they have a very nice write-up about it on their web site.

Last night, there was a reception and panel discussion to officially "open" the exhibit. I flew out to California (and back!) yesterday in order to attend the festivities, talk to some attendees, and take a few pictures. It was a great evening, with a large crowd that included many of Jim's coworkers and friends.

A large picture of Jim looms over the bench.

Here's a view of the whole thing. Not shown in this picture is the Tektronix 575 curve tracer that is sitting behind the phone in the foreground (you can just see a tiny piece of the transistor socket behind the handset).

His trusty Tek 547 and 1A4, with camera attachment and a jungle of probes and cables, sits at one end of the bench.

His soldering iron sits between a pile of components and a giant pile of his project boards. One long assembly of boards forms a bridge from voltmeter to voltmeter in the background.

Another close-up of the circuit-board jumble, showing some recent(?) power-converter work, a large stack of polaroid scope photos, and a pile of production CCFL boards (lower right).

One piece of the exhibit made my heart stop. My business card is on the top of one of his piles in front of his Tek 454. It's dirty, smudged, and crinkled, but it's right there. On top of some circuit boards, on Jim's bench, on display at the Computer History Museum.

This is the highest honor I have ever received.

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

A brief detour this week: App Note 43 was published in June 1990. The next app note (App Note 45) was written a year later. In the intervening 12 months, Jim had a son, Michael, and published a book, "Analog Circuit Design; Art, Science, and Personalities". So this week, I want to blog about the book. If you don't own a copy, you owe it to yourself to buy one. It's a fun, fun read.

Yes, his workbench really did look like that. Note the presence of this trusty Tektronix 547.

This book contains thirty chapters, written by a wide variety of "personalities", including Barrie Gilbert, Paul Brokaw, Bob Pease, James Roberge, John Addis, and many others. Jim wrote four 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 xiii). In Jim's bio, it says "He lives in Belmont, California, with... 14 Tektronix oscilloscopes." Such a small collection! (In the following years, he acquired many, many more.)

I have several copies of this book (I hate seeing them in used-book stores; I buy them when I see them). I even got Jim to sign one of them.

You can find some excerpts of the book on Google Books.

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Related:

• Book 1 Chapter 4
• Book 1 Chapter 7
• Book 1 Chapter 13
• Book 1 Chapter 23

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.

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