Figure 1. Test Setup With Antenna to Simulate Mobile Phone and Jingling Change
Abstract: ESD events between small pieces of metal, such as
coins, at low voltages produce intense EMI with bandwidths into the
tens of gigahertz. The problem is much worse when an electronic device,
such as a mobile phone, is immersed in the coins as often
happens. Data is presented to show possible voltages induced into
a mobile phone antenna by nearby small metal ESD events. RF front end
damage to the receiver
is a distinct possibility.
Discussion: ESD between small pieces of metal can produce strong
interference and such events are common in the environment. A
particularly nasty case is that of a mobile phone carried in a pocket
or purse along with small metal objects like coins. To investigate this
case, a small coaxial dipole was constructed on the end of a coaxial
cable and placed in a plastic bag with a number of coins as shown in Figure 1.
The antenna was made by extending the center conductor and
folding about the same length of shield back over the cable insulating
sheath, see Figure 2. The exposed center
conductor length was about 2.5 cm giving the antenna a resonant
frequency a little over 2 GHz. Figure 3 shows the antenna enclosed in a
plastic sleeve to prevent discharges directly to the antenna.
Figure 2. Small Coaxial Dipole Used for Measurements
Figure 3. Small Dipole Antenna in Insulating Sleeve
Figure 4 is the scope plot of one of the events that resulted when the
bag containing the
coins and the antenna was shaken. The discharge voltage between the
coins was likely on the order of a few hundred Volts or less. The
displayed peak is over 35 volts!
But that is not the whole story.
Figure 4. ESD Event Picked Up by Small Coaxial Dipole
(V = 5 V/div, H = 5 ns/div)
Figure 5 is an expanded view of Figure 4 (500 ps/div as opposed to 5
ns/div). With sin(x)/x interpolation turned off, the jagged appearance
of the waveform reveals the individual samples connected by straight
lines. The sampling rate used was 8 GSa/sec resulting in 125 ps/sample.
The frequency spectrum of the pulse is well above what this scope
can respond to and display accurately. I would estimate the peak value
of the pulse to be greater than 50 Volts. Such a high value raises
the possibility of damage to the receiver front end.
Figure 5. Expanded View of Figure 4, ESD Event Picked Up by Small Coaxial Dipole
(V = 5 V/div, H = 500 ps/div)
Figure 6 shows another one of the many waveforms than can result when the bag is shaken. In this
case, the waveform is a damped oscillatory wave with an amplitude probably exceeding the displayed 20
Volts peak. Large waveforms such as that shown in Figures 4, 5, and 6
were relatively rare, possibly one in a hundred or a thousand waveforms.
However during bag shaking, hundreds of events occurred over several
seconds so it did not take long to get a large event.
Figure 6. Another ESD Event Showing Unwanted EMI in the Measurement
(V = 5 V/div, H = 2 ns/div)
Figures 4 and 6 exhibit low amplitude noise (about 10% of the peak
value of the main pulse) starting about 10 ns before the main pulse.
This is EMI from the ESD events affecting the measurement. A 10% error
is acceptable for the purposes of this article.
generated EMI can have strong effects on systems including the EMI
generated by low voltage, small metal ESD events. Such ESD events in
close proximity to wireless devices
are a common occurrence in the consumer environment and can pose a
problem. Data presented in this article
the possibility that EMI due to discharges between small metal objects
close to a mobile wireless device, such as a phone, may result in
damage to sensitive receiver front ends, not to mention receiver
An in-depth audio format
discussion of this article, covering background as well as more technical details, is available at: http://emcesd-p.com
Information about the mechanisms, importance, and nature of the
pre-pulse EMI in Figures 4 and 6 as well as how to estimate the
discharge voltage of the ESD events are included in the audio
discussion for this Technical Tidbit.
If you like the information in this article and others on this website, much more information is available in my courses. Click here
to see a listing of upcoming courses on design, measurement, and troubleshooting of chips, circuits, and systems.
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
- August 2004, Wi-Fi (Wireless Lan) Antenna Response to EMI from Small Metal ESD
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.