Page 179 - Troubleshooting Analog Circuits

I66 13. Letters to Bob

Dear Mr. Sturgeon:

Thanks for your tip. Most of the guys in our lab don’t use Tempilaq, but it’s a good
tip. We use thermocouples on the can or diodes in the chip.

RAP

Dear Bob,

So, what do you think about spreadsheets?

Anonymous

Dear Anonymous,

On beds, they are fine, but for linear-circuit design, they can be Bad News. Nor-
mally, I hate, despise, and detest them because they give you an answer, but they do
not give you a feel for what is important. Also, sometimes spreadsheets lie. When
they lie, most people still trust them and never check on them. We have found several
cases of a spreadsheet with an error in it, and the error went uncorrected, unsuspected,
and even undoubted for a long time. Finally we ran a sanity check and the answer
was so silly that we realized nobody had ever checked to see if it even made sense.
Like any other form of computer output, you should not trust spreadsheets (and their
results) blindly.

RAP

Dear Bob:

You’re right. Most people, even technical people who should know better, tend to
treat any numeric display or computer readout as if it were engraved in stone, ig-
noring whatever imperfect mechanism generated digits. They’re even in awe of num-
bers scratched in Jello.
Back in the sixties, there was a story circulating about the Apprentice Engineer
who had been experimenting with the plant’s new analog computer. He ran up to the
Chief Engineer waving a sheaf of printouts. “Look,” he said excitedly, “I’ve come up
with a simulation of our power-plant heating and air-conditioning system that will
double the plant’s efficiency!” The Chief Engineer studied the printout for a few
moments. “Yes,” he said, “but look here.” And he pointed to the flow diagram, “This
17 OF water is going to be awfully hard on the pumps.”
Your frustration with the menu burden of these wonderful new instruments is right
on target. One thing I detest intensely and that seems to occur with increasing fre-
quency is staring blankly at a screen or cursor, knowing full well that the reason it