30 October 2011

Scope Sunday 15

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

A friend of mine has a nearly complete set.

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

28 October 2011

Halfway there!

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

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

Top five best circuits (so far):

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 (p.103 in Chapter 13, "Should Ohm's Law be repealed" in Jim's first book)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

26 October 2011

App Note 49

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

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

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

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

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

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

The next app note significantly expands on this topic.

24 October 2011

App Note 47 part 6

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

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

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

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

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


23 October 2011

Scope Sunday 14

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

I started at HSC Electronic Supply in Santa Clara.

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

After striking out at HSC, I headed to WeirdStuff.

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

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

That was a good day.

21 October 2011

App Note 47 part 5

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

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

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

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

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

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

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

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

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

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

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


17 October 2011

App Note 47 part 4

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

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

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

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

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

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

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

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

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

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

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


16 October 2011

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.

14 October 2011

App Note 47 part 3

The first two application sections (pages AN47-32 to AN47-51) cover amplifiers and oscillators, well-worn and familiar territory.

I admit that Figure 66 confused me (doesn't the LT1220 current-to-voltage converter require a feedback resistor?) until I realized that the 5k resistor is the feedback resistor that is integrated on the AD565A die (see the AD565A datasheet for a clearer schematic).

Five DC-stabilization schemes for wideband amplifiers are shown in Figures 71, 72, 73, 74, and 76. Some of these circuits follow up where App Note 21 left off. I think Figures 72 and 73 (the feedback approaches) are best, of course. Figures 74 and 76 require too much tweaking and "select at test" for my taste (although the results that Jim achieves here are impressive).

The next two circuits (Figures 86 and 88) are differential comparators with offset. It seems like he was thinking about variations on the settling-time-measurement theme. See Appendix B for instrumentation and discussion of settling-time measurement (and there is an upcoming app note dedicated to the topic).

The next group of circuits are amplifiers for fast photodiodes. Starting from the simple (Figure 90) to the adaptively triggered (Figure 97). (The latter circuit is an improved version of Figure 38 in App Note 13, employing wideband op amps instead of the previously used discrete stages.)

Figure 99 is another high-speed current-to-voltage converter application. The AD834 is a 500MHz multiplier, with differential-current outputs. Here, the LT1193 converts the differential current into a single-ended voltage with a 50MHz bandwidth.

The next two circuits (Figure 101 and 104) are improved versions of the power gain stages from App Note 18. Figure 106, 108, and 109 show circuits using piezoceramic and crystal filters.

The next section revisits some oscillator topologies. Figure 111 is a quartz-stabilized oscillator with a lamp for amplitude control. Figure 112 uses a feedback loop with a FET as the variable-resistor control element. Figure 114 adds frequency control, using varactor tuning.

The best circuit is the Complete AM Radio Station in Figure 116. I'm a sucker for clever uses of op-amp offset pins. Be sure to fill out the forms in Appendix G.


10 October 2011

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.


09 October 2011

Scope Sunday 12

My 575 works! Here it is tracing a 2N2219.

I still need to test the 175 high-current adapter. Does anyone have a really big transistor that I can borrow? I mean really, really big?

07 October 2011

App Note 47 part 1

"High speed amplifier techniques: A designer's companion for wideband circuitry." 132 pages.

App Note 47 is the longest (and in my opinion, the best) application note that Linear Technology has ever produced. I whole-heartedly recommend it as required reading for circuit geeks. It contains a wealth of information (some of it borrowed from previous app notes, see footnote 15) about high-speed measurement techniques, test equipment, oscilloscope probes, and, of course, applications.

Some of the highlights include:
  • Mr. Murphy's gallery of high speed amplifier problems, pages 7-15
  • About oscilloscopes (a gallery of scope and probe responses), pages 20-24
  • Breadboarding techniques (a pictorial tutorial), Figures 62-65
  • Appendix A, The ABC's of Probes, by Tektronix, pages 69-81
  • Appendix F, Additional comments on breadboading, pages 98-122
  • Appendix J, The contributions of Edsel Murphy, pages 130-131
  • And, of course, all the applications
It's going to take several days to get through the whole thing.


05 October 2011

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.


03 October 2011

App Note 45 part 1

"Measurement and control circuit collection: Diapers and designs on the night shift." 24 pages.

This app note is another classic. It is the first with a rating system; here the circuits are rated by how long they took to complete, measured by the number of feedings that his infant son required. "I decided to introduce Michael to the glories of late night circuit hacking... We loaded up on formula, diapers, and bottles and went into the lab." The number of bottles is shown in each figure.

The selection of circuits here is very interesting. I believe that Jim took a leave of absence from Linear Tech after his son was born, and that the circuits here represent his own interests, rather than the the requirements of customers or the assignments of his bosses ("the Captains of this corporation", as he called them in App Note 25). Thus, we have a collection of circuits that captured Jim's imagination in some way: low-noise amplifiers, thermometers, a hygroscope, a barometer, oscillators, V-to-F converters, a pulse generator, and a fluorescent-lamp power supply. Many of these circuits are improvements of circuits from previous app notes. It's always fun to go back to previous designs and see if you can improve them!

Figures 2 and 4 show extremely low-noise amplifiers using chopper stabilization. Figure 2 is especially noteworthy, with only 40 nV of noise in a 10-Hz bandwidth. Figure 6 is a 20-MHz cable driver, using current feedback (for the original circuits see App Note 21 Figures 6 and 11, the latter for the current-feedback gain stage and the former for the JFET front end). Figure 8 is a simple, programmable current source. Figure 10 is a floating current-loop transmitter (an improved version of App Note 11, Figures 10 and 11).

The next few circuits are instrumentation projects (some of which, I imagine, turned up in his art projects; for example, see the cover of his 1995 book). Figures 11 and 13 are thermometers (a previous version appeared in App Note 7 Figure 2). Figure 15 is a battery-powered hygroscope (a previous version appeared in App Note 3 Figure 8). Figure 16 is a barometer using a low-cost sensor, and Figure 17 is a battery-powered cosmic-ray detector.

I'll cover the second half of the app note (starting with the oscillator in Figure 18) next time.


02 October 2011

Scope Sunday 11

How many oscilloscopes did Jim own?

Jim wrote Chapter 18 in Bob Pease's book, "Analog Circuits: World Class Designs" (this chapter is actually just a reprint of "The Zoo Circuit" from Jim's 1991 book). I recently added this chapter to the bibliography. While I was thumbing through the book for the bibliographic data, I noticed that the biography for Jim said that he owned 84 Tektronix oscilloscopes.

It is interesting to see how this number changed over time. In his 1991 book, he claimed 14 oscilloscopes. In his 1995 book, he claimed 28 oscilloscopes. In Bob's 2008 book, he claimed 84 oscilloscopes. Such is the trajectory of an obsessive collector!

However, in a short bio that he gave me in 2009 (prior to speaking to my classes at M.I.T. and Olin College), he wrote that he owned 62 oscilloscopes. Was this number out-of-date? Did he sell some? (I shudder at the thought.)

Does anyone have any other published data points?