14 November 2011

App Note 61 part 2

Continuing on, Figure 26 shows an improved high-speed pulse generator. Compared to Appendix D in App Note 47, this pulse generator has a distinct advantage in that it is triggered. "This feature permits synchronization to a clock or other event." The 2N2369 is biased just below avalanche breakdown, and the incoming trigger input causes breakdown (due to the difference between Vceo and Vces). Note the comment about the Tektronix 661 with the 4S2 sampling plug-in ("I'm sorry, but 3.9GHz is the fastest 'scope in my house"). Nice upgrade from the 1-GHz 1S2 sampler that he has used previously. "Ground plane type construction with high speed layout, connection and termination techniques are essential for good results from this circuit." Yep. (I think this circuit is the best circuit of the app note.)

Figure 29 shows a special voltage regulator for flash-memory programming. Unfortunately, he does not compare and contrast this design with the designs in App Note 31. Is this circuit just as good? (It probably is, but why?). What are the features of the LT1109 that make it a good fit for this (touchy and sensitive) application?

Figure 31 shows a low-voltage voltage-to-frequency converter (operating from a 3.3-volt supply). The next two circuits are broadband noise sources. Figure 33 uses a Noisecom NC201 noise diode with a selectable filter to produce the noise, and a RMS AGC loop to set the amplitude. Figure 37 uses a standard zener diode as the noise source, but requires a trim to set the initial noise level. Figure 38 is a switchable-output crystal oscillator, which uses diodes to select which crystal is active in the feedback path of the comparator. Cute.

Appendix A includes significant reprints from the manual of an HP3400A True RMS Voltmeter. Jim compares the approach used here to his approach in Figure 22. The instrument he discusses here is a classic, with an impressively creative solutions to several design challenges. The input buffer in Figure A1 uses Nuvistors (because JFETs weren't good enough in 1965). Figure A2 shows a photograph of the input-buffer circuit board. The "video amplifier" in Figures A3 and A4 is an impressive design with DC and AC feedback loops and clever bootstrapping. The chopper amplifier in Figures A5 and A6 uses neon bulbs and photocells for the chopping action! (As Jim says, "Hewlett-Packard has a long and successful history of using lamps for unintended purposes.") Figure A7 shows the circuit board for Jim's RMS-to-DC converter from Figure 22.

The topic of RMS-to-DC conversion was near and dear to Jim's heart. He covered these circuits in detail in App Note 22, and he used them in his CCFL explorations. He helped to design the LT1088 IC and was its main evangelist. In the footnote on page 28, Jim tries to explain the context: "We are all constantly harangued about the advances made in computers since the days of the IBM360. This section gives analog aficionados a stage for their own bragging rights. Of course, an HP3400A was much more interesting than an IBM360 in 1965. Similarly, Figure 22's capabilities are more impressive than any contemporary computer I'm aware of."

Do I detect some frustration in Jim's voice here? Compare all of the circuitry of the HP3400A with the circuit in Figure 22. In effect, I think Jim is saying, "Look how easy I'm making it for you! Why aren't you buying more LT1088 chips?"

Best quote (page AN61-38): "Incidentally, what were you doing in 1965?"

The app note concludes with a great cartoon (perhaps the best cartoon so far).


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