Application Note

3 Fluke Corporation A first look at DSOs
Trigger position. Sets the trig-
ger position to the 50 % point of
the vertical amplitude. Depend-
ing on the signal characteristics,
this action may or may not result
in a stable display.
At this point you should see
a trace that 1) lies within the
vertical range of the display,
2) shows at least three periods
of a waveform, and 3) is stable
enough to allow you to recognize
the overall characteristics of the
waveform. Next, start fine-tuning
the settings.
2) Adjust vertical and
horizontal settings
Start by adjusting the horizontal
timing, increasing the time per
division so that you see a wide
time span of the unknown wave-
form. From that view, reverse the
adjustment as required to narrow
the view to just what you want
to display.
Now adjust the vertical sensi-
tivity, expanding the waveform
vertically but ensuring that the
high and low points of the wave-
form do not exceed the vertical
span.
3) Adjust trigger settings
If needed, adjust the trigger set-
tings to stabilize the waveform
display. Or, you may want to
adjust the trigger delay to see
pre- or post-trigger details on the
waveform. Always start with the
trigger-level setting, adjusting
it so that it falls on a repetitive,
unique point on the rising or fall-
ing edge of a waveform.
As an example, with the oscil-
loscope trigger set to the rising
edge and the level set to the 50
% point, the following figures
illustrate the cause of an unsta-
ble waveform display.
After two successive updates
based on triggers 1 and 2, the
resultant trace will appear
unstable.
However, simply by manually
adjusting the trigger point to a
repetitive, unique point on the
edge, you can solve this problem
and produce a stable waveform
display.
When working with complex
signals like a series of pulses,
pulse width triggering may be
required. With this technique,
both the trigger-level setting
and the next falling edge of the
signal must occur within a speci-
fied time span. Once these two
conditions are met, the oscillo-
scope triggers.
Another technique is single-
shot triggering, by which the
oscilloscope will display a trace
only when the input signal meets
the set trigger conditions. Once
the trigger conditions are met,
the oscilloscope acquires and
updates the display, and then
freezes the display to hold the
trace.
Trig
Figure 5a. Trigger point set to 50 % point
on trace.
Trig
Figure 5b. Trigger point is set to the 50 %
point but due to the aberration on the lead-
ing edge in the second period, an additional
trigger results in an unstable display.
1
st
Trig
2
nd
Trig
Figure 6a. At the first update, the scope
triggers on the first edge. On the second
update, the scope may trigger on the
second trigger point indicated.
Trig
Figure 6b. Unstable waveform display
caused by incorrect trigger level setting.
Trig
Figure 6c. Trigger level adjusted to a unique
repetitive position, outside the aberration on
the second period.
Trigger
Level (V)
Pulse Width (t)
Figure 7. Pulse width triggering will allow
you to set up the oscilloscope to trigger on a
specific pulse defined by level and time.