Application Note

6 Fluke Corporation A first look at DSOs
Troubleshooting
Whichever troubleshooting
scenario below is appropriate
at the time, remember that it’s
important to inspect waveforms
for fast-moving transients or
glitches, even if a spot check of
the waveform reveals no anoma-
lies. These events can be difficult
to spot, but the high sampling
rate of today’s DSOs, together
with effective triggering, makes
it possible.
DUT with KGU. This approach
assumes that you have access to
a KGU and a reference library.
1. Make sure the DUT and KGU
are set up in identical operat-
ing modes.
2. Starting at a high-level point
in the system or block dia-
gram, use the DSO to look
for the presence or absence
of fundamental signals. For
example, look for a line-
voltage supply, as well as
subsequent dc supply voltages
to the various subsystems.
This requires probing the
major input and output signals
at major nodes in the system.
3. Compare signals at key nodes
while changing the operat-
ing mode to see if a failure
occurs. With signals from both
devices available, you have
two options:
• Display the live waveform
from the KGU on Channel 1
of the DSO, and display the
live waveform from the DUT
on Channel 2.
• Capture a trace from the
KGU and overlay it with a
trace from the DUT. Perform
a waveform-compare or
pass/fail test.
• The ScopeMeter 190 Series
II has 4 input channels
which means that 4 signals
from 4 different sources can
be compared.
4. Continue with this process
until you note a variance
between the DUT and KGU
waveforms.
DUT with Circuit Diagrams.
This approach assumes that no
KGU and no waveform reference
library for the DUT are available,
but that circuit diagrams of the
DUT can be located.
1. Review the circuit diagrams to
understand the basic opera-
tion of the DUT.
• Analog circuits such as
oscillators, amplifiers and
signal conditioners (attenu-
ators, filters and dividers)
should exhibit uniform
waveform patterns.
• Digital circuits such as
gates, switches and pro-
cessors should display
waveforms with predictable
amplitudes, pulse periods
and even pulse patterns.
2. Starting at a high-level point
in the system or block dia-
gram, use the DSO to look
for the presence or absence
of fundamental signals. For
example, look for a line-
voltage supply, as well as
subsequent dc supply voltages
to the various subsystems.
This requires probing the
major input and output signals
at major nodes in the system.
3. Use the storage capability
of the DSO to capture and
compare waveforms while
changing the operating mode
of the DUT.
• Visualize a theoreti-
cal “good” waveform and
compare it to the waveform
displayed on the DSO. Try
to identify any obvious
anomalies.
4. Use the horizontal or vertical
cursors to quickly evaluate if
the time or amplitude of the
trace falls within the time or
amplitude ranges suggested
by the circuit design.
Complex DUT, No Circuit
Diagrams. This approach
assumes that the DUT is a fairly
complex system, that no KGU is
available, and that only limited
DUT documentation is available.
1. Study the circuit cards, look-
ing for common components
and circuits, and identify
high-level test points in the
system and check for the
presence or absence of funda-
mental signals. As before, start
at one point and work your
way backwards probing the
major input and output signals
at major nodes in the system.
2. Compare waveforms at key
nodes while changing the
operating modes to see if a
failure occurs. Store these
waveforms.
3. If an examination of the DUT
and analysis of waveforms at
key circuit nodes reveal no
obvious faults, use the DSO’s
storage capability to solicit the
aid of peers.
• Identify “suspect” wave-
forms from the DUT.
• Use the DSO’s Extract or
Output mode to save these
waveform files in a bitmap
(.bmp) format.
• Email the files to a peer or
factory expert anywhere in
the world for aid in trouble-
shooting the circuit.
4. Using outside experts, go
through each key node and
one by one eliminate the obvi-
ous good nodes, eventually
narrowing your focus to obvi-
ous faulty or suspect nodes.
Fluke Corporation
PO Box 9090, Everett, WA 98206 U.S.A.
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
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©2005-2011 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 3/2011 2432735B D-EN-N
Pub_ID: 10882-eng, rev 01
Modification of this document is not
permitted without written permission
from Fluke Corporation.