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Douglas C. Smith

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Technical Tidbit - September 2004
Mobile Phone Response to EMI from Small Metal ESD

test setup

Figure 1.
Test Setup With Antenna to Simulate Mobile Phone and Jingling Change

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 electromagnetic 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.

small coaxial dipole

Figure 2. Small Coaxial Dipole Used for Measurements

E-field setup, right

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.

scope plot of ESD event

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.

expanded view of Figure 4

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.

another event showing measurment EMI

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.
Summary:ESD 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 raises 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 overload.

Additional Material: An in-depth audio format discussion of this article, covering background as well as more technical details, is available at:  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:
Thanks to Agilent Technologies for supplying 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.

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Copyright © 2004 Douglas C. Smith