Page 162 - The Art of Designing Embedded Systems

Troubleshooting Tools 149

wrong, generally there is a hardware problem. I set up the vertical controls
just to get a decent-sized waveform and then mostly ignore them.
Timing, though, is always crucial. The horizontal system doesn’t just
randomly move the beam back and forth; it does so in a highly regular and
measurable manner.
Generally the biggest knob on a scope is the one labeled something
like “TimeDivision.” Try cranking it through all of its positions. Go all the
way counterclockwise: the beam will be a single dot, either stopped or
moving very slowly to the right.
As with the amplitude control, this switch is calibrated. The slowest
sweep rates (all the way counterclockwise) might be as much as 5 seconds
per division. Slowly rotate the knob and watch as the dot picks up speed.
5 sec/div, 2 sec/div, 1, .5, .2, .l-pretty soon the dot will be moving so fast
it will start to look like a line. Rotate it all the way. Now, the dot is mov-
ing at perhaps 50 nanoseconds per division. That’s fast!
The horizontal system is frequently called the “time base,” because it
provides all basic timing functions to the scope.
A cardiac monitor is nothing more than a specialized oscilloscope. A
very slowly moving beam shows the patient’s heart rate. The signal beats
only 70 timedsec, so a slow rate is best to represent the input.
Suppose the signal moves not at 70 beatdsec, but at 7 million (say,
for a hummingbird on speed). At the slow sweep rate of the cardiac mon-
itor the beam will move up and down so fast compared to the left-to-right
sweep that a band of light will appear. You’ll see no recognizable signal.
Crank up the sweep rate. The band will eventually resolve itself into the
familiar cardiological shape. At first, the signal will be all squished to-
gether. Perhaps three beats will be in each division. Rotate the knob again.
Now, only one beat is in a division. With each rotation the horizontal
image expands. With each rotation you can still measure the beat fre-
quency by counting divisions and applying the Timemivision parameter
listed on the control.
The Horizontal control, then, lets you pick a sweep rate that generates
a recognizable picture of the signal you are measuring.
There’s always one little detail to complicate matters. So far we’ve
ignored the issue of synchronizing the sweep to the signal.
In the case of the cardiac input, suppose on one sweep the beam starts
off on the left side of the screen when the signal is halfway up the slope,
and the next sweep starts when the input is at 0 volts. The position of the
display will shift left or right on every sweep, creating an image impossi-
ble to focus on.