Page 163 - The Art of Designing Embedded Systems

150 THE ART OF DESIGNING EMBEDDED SYSTEMS

Unless the sweep starts at the same point on the input signal each
time, the display will look like a meaningless jumble. In the bad old days
before trigger circuits, people tried to tune the sweep frequency to exactly
match the input, but this is hard to do at best, and is pretty much impossi-
ble with digital circuits.
The modern solution is the third component of any decent scope.
The “Trigger” controls let you pick the sweep starting point.
Generally, selector switches let you pick AC or DC coupling, trigger
level, holdoff, slope, and trigger source selection. The correct procedure
is to select a reasonable source (channel 1 or 2: which one do you want to
use to start the sweep?), and then start twiddling knobs until the display
stabilizes.
Sure, it makes sense to follow some semblance of a procedure. Select
a (+) slope if you want to see the upgoing edge of the input at the very left
side of the screen. Select (-) slope to position the downgoing edge there.
Start twiddling with the holdoff control set to OFF (usually all the
way counterclockwise). Most of the magic will be in the Trigger knob,
which requires a delicacy of touch that takes some practice to develop.
Triggering on any repetitive signal is pretty easy, because the differ-
ences from sweep to sweep are small. Digital signals are more challenging.
A constantly changing pulse stream is all but impossible to capture on a
scope.

Scoping Tricks
One of the worst mistakes we make is neglecting probes. Crummy
probes will turn that wonderful 1-GHz instrument into junk. Managers
hate to spend a lot on probes when they see them drooling onto the floor,
mixed with all of the other debris. Worse, we always immediately lose the
tips and other accessories acquired at great expense, and so connect to a
node using a 12-inch clip lead hastily purchased at Radio Shack.
Then. after destroying a couple of chips by accidentally shorting
things to ground with that nice alligator ground clip mounted on the probe,
we tear it off in frustration, losing it as well. Tip: If you really don’t intend
to use the ground connection, clip that alligator lead to itself, keeping it out
of harm’s way but instantly available for use.
Take care of your probes. Keep them off the floor; don’t let your chair
roll over the leads, squishing the coax and changing its impedance. Buy de-
cent ones before every probe in the shop falls apart. After trying all of the
cheap varieties found in general electronic catalogs, I now swallow hard and
spend the $150 needed to get high-quality probes from Tektronix or HP.