Page 194 - The Art of Designing Embedded Systems

Troubleshooting 1 8 1

two vertical channels, most include two time bases as well. Seems odd,
doesn’t it? Double vertical channels intuitively make sense. since each
probe picks off a different sense point. Time, though, always flows in the
same direction at the same rate, so a single axis is all that makes sense.
Novice scope users understand the operation of time base A: crank
the time/division knob to the right and the signal on the screen expands in
size. Rotate it to the left and the signal shrinks, but much more history (Le..
more microseconds of data) appears.
Time base B is a bit more mysterious. If enabled, it doesn’t start until
sometime after time base A begins. Try it on your scope: select “Both” (or
“A intensified by B”) and select a sweep rate faster than that used by A.
You’ll see a highlighted section of the trace whose width is determined by
B’s sweep rate, and whose starting position is a function of the delay time
knob.
Switching from “Both” to “B” shows just the intensified part of the
sweep: the part controlled by time base B. In effect, you’ve picked out and
blown up a portion of the normal sweep. It’s like a zoom control-and you
can select the zoom factor using the sweep time. and the “pan position,” or
starting location, using the delay time adjustment.
Suppose you want to look at something that occurs a long time after
a trigger event. Using these zoom controls you can get a very high-
resolution view of that event-even when time base A is set to a very slow
rate.
Delayed sweep is always accompanied by a second trigger system.
Most of us have developed callouses twiddling the trigger level control in
an effort to obtain stable scope displays. Any instrument with dual time
bases will come with a second of these knobs to set the trigger point of the
B channel.
(Note: Newer scopes, like the MSO series from HP, remove most of
the uncertainty from setting trigger levels because they show an arrow on
the waveform indicating the exact voltage setting of the trigger level con-
trol. It’s a great time-saver.)
The second trigger is important when working on digital signals that
usually have unstable time relationships. Set the A trigger to start the
sweep (as always), position the intensified part of the sweep to some point
b<fire the section you’d like to zoom on, and then adjust trigger B until the
bright portion starts exactly on the event of interest.
This procedure guarantees that even though the second trigger event
moves around with relationship to trigger A, you’ll see a stable scope dis-
play after selecting the B time base. In effect you’ve qualified trigger B by
trigger A. and you can hope you’re zeroing in on the area needing study.