Application Note

2 Fluke Corporation A first look at DSOs
Triggering
Trigger controls allow you to
stabilize and display a repetitive
waveform.
Edge triggering is the most
common form of triggering. In
this mode, the trigger level and
slope controls provide the basic
trigger point definition. The slope
control determines whether the
trigger point is on the rising or
the falling edge of a signal, and
the level control determines
where on the edge the trigger
point occurs.
For even greater control and
visibility into signal phenom-
ena, you can use the ability of
some DSOs to capture events
leading up to the trigger point
(“pre-trigger”) or after the trig-
ger point (“post-trigger”) on the
input waveform. As an example,
by using pre-triggering or post-
triggering, you may catch a spike
that occurs in between two
occurrences of a signal.
Pulse width triggering trig-
gers on specific pulses within a
series or it can identify one-time
or sporadic problems in a pulsed
signal. In this mode, you can
monitor a signal indefinitely and
trigger on the first occurrence of
a pulse whose duration, or pulse
width, is either outside of or
within set limits. The goal is to
isolate and display a pulse that
meets the predetermined time
criteria.
Single-shot triggering is
useful for capturing a one-time
event—such as electrical arcing
or a relay closure. A DSO with a
single-shot mode waits until it
receives a trigger and then sets
itself in a hold mode to store the
signal at the moment the one-
time event occurs.
Video triggering is a power-
ful feature of advanced DSOs.
Video signals can be extremely
complex, providing no unique
edge that is repetitive and that
can be isolated to stabilize the
signal. With an extensive range
of signaling protocols in use in
today’s electronic video equip-
ment and systems, an effective
DSO is one that recognizes the
predominant video protocols and
provides appropriate triggering
functionality.
Setup
The task of capturing and ana-
lyzing an unknown waveform on
an oscilloscope can be routine, or
it can seem like taking a shot in
the dark. However, in most cases,
taking a methodical approach to
setting the oscilloscope will cap-
ture a stable waveform or help
you determine how the scope
controls need to be set so that
you can capture the waveform.
1) Start with Auto
Connect the ground reference
lead and then connect the probe
tip to the circuit test point. Most
oscilloscopes have the ability to
either perform a one-time auto
setup or continuously analyze the
unknown input signal. Press the
AUTO button or verify that the
scope is already in Auto mode.
Pressing the AUTO button will
typically set up the oscilloscope
to automatically adjust three key
parameters:
Vertical sensitivity. Adjusts
the vertical sensitivity so that the
vertical amplitude spans approxi-
mately three to six divisions.
Horizontal timing. Adjusts
the horizontal time per division
so that there are three to four
periods of the waveform across
the width of the display.
Figure 2. Sampling and interpolation, sampling is depicted by the dots while interpolation
is shown as the black line.
t
Figure 3. Unknown trace adjusted for 3-6
vertical divisions.
Figure 4. Unknown trace adjusted for 3-4
periods horizontally.