High Frequency Measurements
Douglas C. Smith
Address: P. O. Box 1457, Los Gatos,
Technical Tidbit - April 2004
Paper Clips, They're Not Just for Emissions Any More
Abstract: The following
article describes the adaptation of a commercial ESD gun to apply local H-field
pulses for troubleshooting sensitive circuits. The key components are a section of brass tubing, some brass
strap and a paper clip!
Discussion: In the
development/pre-test laboratory a standard commercial ESD gun is rather
analogous to a shotgun. When you
pull the trigger the product either passes or fails. The two most valuable pieces of knowledge to be developed in
the mitigation of a failure are:
In trying to
find the actual victim circuit, we would like to have something a bit more like
a rifle than a tactical nuclear weapon.
With a couple of cheap accessory devices, the same ESD generator can be
made into a tool for rather high-resolution circuit inspection. The following pictures and waveforms
used a Schaffner 435 ESD gun, but the methods can be adapted to almost any ESD
generator that has both air and contact discharge modes.
- What is the
current path of the discharge?
- Which is
the most sensitive victim circuit?
In Figure 1, a
simple grounded brass tube extending beyond the tip of the air discharge point
is used to produce locally high E-field pulses without discharging directly to
sensitive board level components (which is a really good idea if you don’t want
to destroy a whole lot of test boards during your investigation). This was the first adaptation of the
ESD gun for board level investigations, but its limitations soon became
obvious, as noted below. The brass
strip from the ESD gun grounding point is held in contact with the brass tube
by a rubber O-ring. This allows
for quick attachment when changing from normal discharge use to local
Figure 2 shows
the second and generally more useful adaptation. It is simply a paper clip soldered to the same size
grounding tube and connects to the point of the contact discharge tip of the
ESD gun. The adapter has been
dipped in an insulating plastic coating to reduce the risk of shorting board components
during the investigation.
Two examples of
the induced H-field interference are shown below. In Figures 3 and 4, the H-field pulse is induced into the
ground lead of a 10:1 high impedance oscilloscope probe. Ringing
in the scope probe lead makes for a rather poorly behaved waveform, but it
is indicative of the kind of voltages that can be developed into a high impedance
system. For a “sanity check”, the ground lead
was detached from the probe at the lead to probe body end, leaving the tip
connected to the lead. The induced
voltage to the probe with the loop opened was quite small (<100 mV),
indicating that the H-field component was doing the real work.
In Figures 5 and
6, the field is coupled into a Fischer Communications Corporation F-301 near
field probe into a 50 ohm terminated input. As would be expected, the waveform shows considerably better
damping, but still enough voltage that it could drive against an active logic
output hard enough to change the logic level seen by an input.
In both cases,
the waveform was recorded by a Tektronix TDS 684B oscilloscope with a 1 GHz
analog bandwidth and a 5 GS/sec rate with the ESD gun was set for a 6 kV
Figure 4 (V=10 V/div, H=5 ns/div)
cases it can be seen that the coupled pulses are both quite energetic and
fast. By starting at high
voltage and a rapid repeat rate (5-10 pulses per second), you can find the
general area of the sensitivity quickly.
Then begin stepping down the voltage and change to single discharge to
more finely localize the sensitive circuit. This is where the H-field version has significant advantage
over the E-field version. At low voltages
it is much more difficult to create a consistent air discharge induced field
into the circuit.
very simple accessory for a standard ESD gun can provide a useful
troubleshooting tool that can be used with cables, back planes, board traces
and individual components to identify ESD sensitive victim circuits.
Figure 6 (V=1 V/div, H=5 ns/div)
There are several articles on this website covering
useful ways to use a paper clip. Brent's article ties in well with these articles. The articles are linked below.
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Questions or suggestions? Contact me at firstname.lastname@example.org
Copyright © 2004 Douglas C. Smith