26 August 2011

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.



zebonaut said...

There's another way how the picture with 11 traces could have been achieved: The 1A4 four-channel plugin unit has a connector labelled "signal output". He could have used a 1A4 in a 547 (or similar), fed this 1A4's output into one input of his 556's first 1A4, and used the remaining 3+4 inputs of the 556's two 1A4 units. 4+3+4=11.

My 556 quit working about four weeks ago. It blows the fuse of the mains outlet it is connected to. I hope that I will soon find the time to do a 'scope sunday and fix it. Maybe it's only a rectifier diode or cap right behind the transformer.

Darren Carter said...

If you look closely, you can see that this really just two photos of his 556 butted up against each other. (Notice the damaged graticule repeated in the middle, and near the bottom.)