Application Note

4 Fluke Corporation A first look at DSOs
Understanding and
reading waveforms
The majority of electronic wave-
forms encountered are periodic
and repetitive, and they conform
to a known shape. Here are the
factors to consider in analyzing
waveforms:
Shape. Repetitive waveforms
should be symmetrical. That is,
if you were to print the traces
and cut them in two like-sized
pieces, the two sides should be
identical. A point of difference
could indicate a problem.
Rising and falling edges.
Particularly with square waves
and pulses, the rising or fall-
ing edges of the waveform can
greatly affect the timing in digi-
tal circuits. It may be necessary
to decrease the time per division
to see the edge with greater
resolution.
Amplitude. Verify that the
level is within the operating
specifications of the circuit.
Also check for consistency, from
one period to the next. Monitor
the waveform for an extended
period of time, watching for any
changes in amplitude.
Amplitude offsets. DC-couple
the input and determine where
the ground reference marker
is. Evaluate any dc offset and
observe if this offset remains
stable or fluctuates.
Periodic waveshape. Oscil-
lators and other circuits will
produce waveforms with con-
stant repeating periods. Evaluate
each period in time using cursors
to spot inconsistencies.
1
2
Figure 8. If the two components of the
waveform are not symmetrical, there may be
a problem with the signal.
t t
Figure 9. Use cursors and the graticule
marks to evaluate the rise and fall times
of the leading and trailing edges of a
waveform.
Figure 10. Use horizontal cursors to identify
amplitude fluctuations.
F i g u r e 11. Evaluate waveform DC offsets.
t t
Figure 12. Evaluate period-to-period time
changes.
Waveform anomalies
Here are typical anomalies that
may appear on a waveform,
along with the typical sources of
such anomalies.
Transients or glitches. When
waveforms are derived from
active devices such as transistors
or switches, transients or other
anomalies can result from timing
errors, propagation delays, bad
contacts or other phenomena.
Noise. Noise can be caused
by faulty power supply circuits,
circuit overdrive, crosstalk, or
interference from adjacent cables.
Or, noise can be induced exter-
nally from sources such as dc-dc
converters, lighting systems and
high-energy electrical circuits.
Figure 13. A transient is occurring on the
rising edge of a pulse.
Figure 14. A ground reference-point mea-
surement showing induced random noise.