Page 171 - Troubleshooting Analog Circuits

I58 13. Letters to Bob

1. Noise from a video terminal? I have not seen that problem, but such noise may be a
serious problem in some cases. I don’t have a digital computer near my workbench,
but other people may. As I mentioned in Chapter 2, if you bring an AM radio near a
computer or keyboard, the radio will detect various amounts of graunchy RF noises.
2. I, too, recommend RF bypassing. But you point out that bypassing should be against
ambient RF. I rarely think of bypassing in those terms, but you are right.
3. Yeah, I believe that. Some people are capable of building circuits that are the exact
opposite of “fail-safe.”
4. Yeah, I believe that too. But data sheets for digital as well as linear ICs do not help
because they only indicate quiescent power drain, and give no clue about what will
change when the output is swinging slow or fast. Even TTL draws more current when
running fast.
5. I’m sure that most Vishay resistors are quite stable; I myself keep a group of old
wire-wounds for that purpose. Some of them are so old, they are older than the oldest
Vishays. Consequently, I have confidence that they have good long-term stability,
which I find more important than temperature coefficient.
6. I agree that spiriting away equipment for calibration is a serious issue. In our group,
we used to have equipment disappear for calibration just when we needed the equip-
ment desperately. We finally resolved the problem by requiring that engineers put
their equipment on a calibration shelf when it was due to be calibrated. If they don’t
put it there, the calibration guys won’t steal it.

7. Scope probe parts and ground leads do tend to disappear. We have a budget for those
items every year, so we avoid running out.
8. No argument. Mr. Heisenberg is not the only one who can affect a measurement by
looking at it.
9. You are right that adding a chunk of coaxial hose on an op-amp’s output can sure
make it unhappy, more often than doing so makes it happy. You are right to complain
that foolish people pull such stunts. “Show me where it says I can’t do it,” they
protest. (Ed. Note: The Sufis have a saying, “There’s no point in putting up signs in
the desert saying, ‘Thou shalt not eat rocks.”’)
10. Good statement about probe compatibility. Sometimes you turn the adjustment all the
way to the end, and the probe still won’t neutralize out.
1 1. Good point. I often check my soldering iron by running the tip past my nose-about
one inch away-to see if it’s good and hot. (Note: Humans have infrared receptors in
their lips. If you close your eyes and slowly raise the back of your hand past your
lips, you should be able to sense the presence of your hand. Use the back of your
hand because the callouses on the palm of your hand block heat radiation.)
13. You point out that “. . .drafting departments tend to lose notes.” Well, where I work,
drafting departments do a task, and if that includes adding notes, then they add what-
ever the engineer requires. Actually, I usually do my own drafting. Some people
gripe, but the information is all there.

14. I don’t usually get too enthusiastic about using incandescent lamps for amplitude
control. It’s true that most oscillators don’t have distortion as low as -80 dB, but you
can put the output of a mediocre oscillator through a filter and get distortion lower
than -80 dB.
16. Yes, I believe that. We have found that our lowest input current amplifiers such as