Page 163 - Troubleshooting Analog Circuits

I50 12. Roundup of “Floobydust”

gauge) leads-not when you use fine wire (26 or 28 gauge). The fine-wired leads
apparently do not draw as much heat from the front-panel binding posts. So, even the
best DVM auto-zero circuit cannot correct for drifts outside its domain.
Most engineers know that DVMs add a resistive (10 Ma) load to your circuit and
a capacitive load (50 to lo00 pF) that may cause your circuit to oscillate. But, what’s
not as well known is that even the better DVMs may pump noise back through their
input terminals and spray a little clock noise around your lab. So if you have a sensi-
tive circuit that seems to be picking up a lot of noise from somewhere, turn off your
DVM for a few seconds to see if the DVM is the culprit. If that’s not it, tum off the
function generator or the soldering iron. If it is the DVM’s fault, you may want to add
RC filters, RLC filters, or active filterbuffers with precision operational amplifiers,
to cut down on the noise being injected into your circuit. There is a little RC filter
shown in Figure 2.4 of Chapter 2, that is useful for keeping the noises of the DVM
from kicking back. Or, you might want to go to an analog meter, which-as we dis-
cussed on a previous pagedo not have any tendency to oscillate or put out noise.
An analog meter with a battery-powered preamplifier will not generate much noise at
all, by comparison to a DVM.. . .

Signal Sources
While I’m on the subject of instruments, I really enjoy using a good function gener-
ator to put out sines and triangle waves and square waves and pulses. I love my old
Wavetek 19 1. But I certainly don’t expect the signals to be absolutely undistorted-
all these waveforms will distort a little, especially at high frequencies. So if I want
my function generator to give me a clean sine wave, I put its output through an active
filter at low frequencies or an LC filter at high frequencies. If I want a clean, crisp
square wave, I will put the signal through a clipping amplifier or into a diode-limited
attenuator (Figure 12.3). If I want a cleaner triangle than the function generator will
give me, I just make a triangle generator from scratch.
But a function generator lets me down when some absent-minded person pushes
one button too many and the output stops. (Usually, that absent-minded person is me.)
It can take me five minutes to find what the problems are. I love all those powerful,
versatile functions when I need them, but they drive me nuts when the wrong button
gets pushed.
Similarly, a scope’s trace can get lost and hide in the comer and sulk for many
minutes on end if you don’t realize that somebody (maybe your very own errant fat
finger) pushed a treacherous button. When the digital scopes with their multiple layers
of menus and submenus start playing that game, I find I need a buddy system-some-
body to come and bail me out when I get hopelessly stuck. What menu is that dratted
beamfinder on, anyway?
But, scopes work awfully well these days. Just don’t expect precision results after
you drive the trace many centimeters off scale by turning up the gain to look at the
bottom of a tall square wave. Most scopes aren’t obligated to do that very well.
Similarly, be sure to keep the trimmers on your 10 X probes well adjusted, and run a
short ground path to your probes when you want to look at fast signals, as discussed
in Chapter 2.

Troubleshoot As You Go
Some people like to build up a big system and turn on the power; and, Voila, it
doesn’t work. Then they have to figure out what kind of things are wrong in the