Application Note

5 Fluke Corporation A first look at DSOs
Ringing. Ringing can be seen
mostly in digital circuits and in
radar and pulse-width-modula-
tion applications. Ringing shows
up at the transition from a rising
or falling edge to a flat dc level.
Check for excessive ringing,
adjusting the time base to give a
clear depiction of the transition-
ing wave or pulse.
Momentary fluctuation.
Momentary changes in the
measured signal generally result
from an external influence such
as a sag or surge in the mains
voltage, activation of a high-
power device that is connected
to the same electrical grid, or a
loose connection. Use the DSO’s
slowest time-base setting or
the paperless recording or “roll”
mode. Start at the input and
watch the acquired waveform
over long time spans to track
down the source of the problem.
Drift. Drift—or minute changes
in a signal’s voltage over time—
can be tedious to diagnose. Often
the change is so slow that it is
difficult to detect. Temperature
changes and aging can affect
passive electronic components
such as resistors, capacitors and
crystal oscillators. One problem-
atical fault to diagnose is drift in
a reference dc voltage supply or
oscillator circuit. Often the only
solution is to monitor the mea-
sured value (V dc, Hz, etc.) over
an extended time.
Figure 15. Excessive ringing occurring on
the top of the square wave.
Figure 16a. A momentary change of
approximately 1.5 cycles in the amplitude of
the sinewave.
110 Vac
Figure 16b. Using an oscilloscope with a
paperless recorder mode allows plotting of
the amplitude (voltage level) over time.
1. 01 kHz
Figure 17. Performing a frequency
measurement on a crystal oscillator that
has been trend-plotted over an extended
period (days or even weeks) can highlight
the affect of drift caused by temperature
changes and aging.
Diagnosing problems
Although successful trouble-
shooting is both an art and a
science, adopting a troubleshoot-
ing methodology and relying on
the functionality of an advanced
DSO can greatly simplify the
process.
Good troubleshooting practices
will save time and frustra-
tion. The time-tested approach
known as KGU, Known Good Unit
comparison, accomplishes both
goals. KGU builds on a simple
principle: an electronic system
that is working properly exhibits
predictable waveforms at critical
nodes within its circuitry, and
these waveforms can be cap-
tured and stored. This reference
library can be stored right on the
DSO as an online resource, or
can be printed out to serve as a
hard-copy reference document. If
the system or an identical system
later exhibits a fault or failure,
waveforms can be captured from
the faulty system—called the
Device Under Test (DUT)—and
compared with their counterparts
in the KGU. Consequently, the
DUT can either be repaired or
replaced.
To build a reference library,
start by identifying appropriate
test points, or nodes, on the DUT.
Now, run the KGU through its
paces, capturing the waveform
from each node. Annotate each
waveform as required.
Get into the habit of always
documenting key waveforms
and measurements. Having a
reference to compare to will
prove invaluable during future
troubleshooting.