Figure 1. Magnetic Loop Used to Pickup EMI from an ESD Event
ESD can have significant effects
on electronic system operation. The effects of electric and magnetic field
coupling is explored using E and H field probes. Conclusions are drawn on
resulting system effects and the nature of the fields.
: To gain insight into the nature of the fields radiated
from ESD events, measurements were made using magnetic field and electric
field probes. The source of ESD events used was the generator described on
this website in a Technical Tidbit, June 2001, A Static
Field Powered EMI Source
The source is composed of two lengths of copper tape with a small gap in-between.
Sparks are induced by bringing a static charge close to the copper foil.
In this case, the spark gap is a fraction of a millimeter resulting in fast
di/dt with risetimes of current on the order of a couple hundred picoseconds.
Figure 1 shows the basic test setup. The AM radio in the picture is used
to indicate that the generator is actually generating sparks as evidenced
by "pops" in the radio as the charged styrofoam is moved over the
copper tape. A standard 6 cm 7405 shielded loop
was used to pick up the radiated magnetic fields of the ESD event by responding
(mostly) to magnetic fields (there is some electric field response, see Signal and Noise Measurement Techniques Using
Magnetic Field Probes (~600K
on this site for more information). Voltages picked up by the loop due to
magnetic fields would be comparable to that picked up by loop structures
of similar dimensions, cables for instance, in a system. The loop was connected
using high quality RG-142B/U coaxial cable to a 50 Ohm termination in the
scope. Ferrite cores were used to help isolate the shield of the cable from
becoming part of the antenna structure.
Figure 2 shows the loop output for this test setup. The important
parameter to note is the peak voltage of over 3 volts. Such interference
is a real problem when looking for glitches with oscilloscope probes. Loops
are always formed when making voltage measurements and interference like
this easily gets into the measurement. In addition, it is easy to see how
radiated fields from ESD can effect systems.
Figure 2. Loop Output from Test Setup of Figure 1
Figure 3 shows a similar measurement setup for electric fields. The probe is part of the same 7405 probe set
containing the loop probe in Figure 1. The
probe is essentially an insulated metal ball connected to the center conductor
of the coax. The shield of the cable extends up to the ball but is not connected.
The overall antenna structure has two parts: first, the ball and second,
the shield as far back as the ferrite cores, an asymmetrical dipole of sorts.
The RG142B/U cable connects to a 50 Ohm termination in the scope,
Figure 3. E-field Test Setup - Right Side
Figure 4 shows the measurement result using the ball probe. Note the amplitude
of about 2 volts, a significant amount of EMI. The ringing frequency is likely due
to a combination of the resonant frequency of the copper tape structure at about 500 MHz and the ball and cable shield antenna.
Figure 4. "Ball Probe" Response to an ESD Event
(probe on right side of event)
For the same movement of the charged styrofoam, but with the ball probe near
the left hand side of the copper tape antenna as shown in Figure 5, the opposite
field is picked up by the loop. The result is shown in Figure 6. The amplitude
of the response is about the same, a peak of 2 Volts, but the polarity is
reversed. For a fixed position of the ball, an opposite polarity waveform
can also be made by moving the styrofoam in the opposite direction.
Figure 5. E-field Test Setup - Left Side
Figure 6. "Ball Probe" Response to an ESD Event
(probe on left side of event)
A question: The polarity of the E field probe output inverted when the probe
was moved from the right side of the spark gap in the copper foil to the left
side. What happens to the output of the H field probe if it is moved the
same way? A hint can be found in a pair of Technical Tibits on this site
that cover a completely different topic:
ESD can have strong effects on systems including by radiation. Data presented
shows that even relatively low voltage events from small arcs can produce
significant voltages from radiated EMI in nearby structures. Also notable
is that the opposite polarity of field can be generated from the same discharge
as seen from a different perspective in space.
Additional information on this site regarding ESD effects on systems includes:
November 1999: Transient Suppression Plane
May 2001, Hidden Threats
to Electronic Equipment
- June 2001, A Static
Field Powered EMI Source
January 2002, Cable Effects Part 1: Cable
May 2002, Printed Wiring Board Coupling to
a Nearby Metal Plane, Part 2: ESD Immunity
- February 2003, Crossing Ground Plane Breaks - Part 3, Immunity to Radiated EMI
May 2003, Signal Paths Passing Through Ground and Power Planes, Effects on Immunity
ESD Immunity in System Designs, System Field
Experiences and Effects of PWB Layout (~950K)
(2000 EOS/ESD Symposium paper)
Thanks to Agilent Technologies
the scope for this experiment. The model used for this article was an Agilent Technologies 54845a, an 8 GSa/sec
unit that is now replaced by a much faster scope, the 54853a.