28 September 2011

Book 1 Chapter 7

Chapter 7, "Max Wien, Mr. Hewlett, and a rainy Sunday afternoon."

The first time I read this chapter (back in grad school), I was quite intimidated. It would take me two weeks to design, build, test, and document all of the circuits in this discussion, and Jim did it in one Sunday afternoon?!? I'll never be that good of an engineer.

But this chapter title is a bit of a fib, because it didn't all happen on a single rainy Sunday afternoon. He had clearly been thinking about Hewlett's oscillator since App Note 5 (Figures 12, 13, and 14), and most of this chapter is based on circuits that appeared in App Note 43 (Figures 39 through 49).

On the first page, he writes "No lab is complete without an HP series 200 oscillator." We should all be so lucky! However, on page 47, he seems to admit that (at the time he wrote this chapter), he owned an HP 201 oscillator. His HP 200A must have come later.

This chapter is worth reading, even if you have memorized App Note 43. His conversational tone gives a window into his thought processes (even if the "rainy afternoon" is a work of fiction). This chapter is also noteworthy for his clear exposition of Williams's Rule (his spelling), "a little known tenet of precision op amp circuits... always invert (except when you can't)."

It is curious that all of the schematics in this chapter are drawn by hand:

For App Note 43 all of these schematics had been neatly drawn by someone at Linear Technology. Why did Jim redraw everything by hand? Are these the original schematics from the lab notebook before the Art Department got a hold of them? Did he feel challenged by Bob Pease (see Chapter 9) for the crown of "Messiest Schematics in the Book"?

There is a typo on the top of page 44. He writes "DeForest hadn't even dreamed of adding a third element to Edison's Effect in 1891." He misspelled "DeForest hadn't even dreamed of adding a third element to the valves he stole from John Fleming."



lightning bulb (aka ngvrnd) said...

I am the proud owner of an HP200CD wide range oscillator, courtesy the MIT Flea, which I recently got out and tested as part of re-activating my workshop (dormant since the birth of my first child in 2000!).
I snagged it pretty much as a result of having read that chapter.

It still works, and looking at the output (on an Agilent MSO!) I was amazed at how it held the output amplitude to within a few tenths of a volt from the lowest to the highest frequency range. I was also amused to watch the feedback response -- it seems that the filaments of the light bulbs respond slowly enough that you can see it happening.

Anonymous said...

Little historic tidbit (worked for HP T&M, love tech history and friend with the then-HP, later-Agilent corporate archivist): the 200A and 200B were discontinued only because the bulb used became unavailable so a tweak was made to allow a different, available one. The nonlinear characteristic of the bulb (and its temperature coefficient) was pretty essential and not just any incandescent lamp apparently would do.