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.


1 comment:

Robby Beard said...

I read the references on the Murphys law paper and reference number 2 is Paul Birman from Kepco. He was there from 61-98 and wrote one of my favorite electronics books, "The Kepco Power Supply Handbook," printed in 1965. Contains tons of still useful information all about linear power supplies.

Im kinda curious as to what all he contributed to the murphys law paper.