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High Frequency Measurements Web Page
Douglas C. Smith

 Address:  P. O. Box 1457, Los Gatos, CA 95031
 TEL:      800-323-3956/408-356-4186
 FAX:      408-358-3799
 Mobile:   408-858-4528

Technical Tidbit - July 2007
Mobile Phone Induced Circuit Failure

mobile phone and car key

Figure 1.
Mobile Phone in Proximity to a Car Key

Abstract: Cases of circuit failures caused by mobile, or cell, phones are starting to be reported. A brief overview of the problem is given followed by a simple test procedure to gauge the risk to a product.

Discussion: In my consulting work, I see a lot of operational problems in systems, but I have noticed a new kind of problem caused by mobile phones. Recently, a news article appeared in many news outlets about an electronic car key that was destroyed by being in proximity to a mobile phone when the phone rang. One of my clients recently had a similar problem with a small product that resembled an mp3 music player. So what is happening?

Figure 1 shows a car key in proximity to a mobile phone. Although the key above is from an extensively modified 2006 Ford Mustang (a really fun car), the problem in the news story was not with any Ford product, but another company that manufactures automobiles. A factor in common with the small mp3 player like product and many car keys is that the circuit board and/or metal part of the product is about six cm in length, about 1/2 wavelength at the upper mobile phone frequency band in the US, 1850 MHz.

When such a product is very close to a mobile phone operating in its upper frequency band and at its high power level (about 600 mW) significant currents can be induced into the product. This is because it is resonant near the operating frequency of the mobile phone. Enough current can be induced to permanently damage the circuit in some cases. In the car key case, when the key was in a pocket with a mobile phone and the phone became active, the electronic circuit in the key could be damaged. Although the key would still open the door, it would not start the car as a result of the damaged circuitry in the key.

Even if the circuit is not damaged, temporary operational malfunction can occur. I have observed this in a number of automotive and other devices.

In my February 2006 Technical Tidbit, Construction of a Coaxial Antenna for Troubleshooting, a small homemade antenna and method is described for subjecting a small product to an approximation of the very near field of a mobile phone. If your product has an electrical length of about 6 cm (upper mobile phone band) or about 12 cm (lower mobile phone band), I would highly recommend testing it with the simple procedure outlined in that February 2006 Technical Tidbit.

Even for products of other sizes that may be in close proximity to a mobile phone, I would recommend looking for resonances using the method in my June 2006 Technical Tidbit, Measuring Structural Resonances. If a resonance is found in one of the mobile phone bands, then go on to test the product with the method in the February 2006 Technical Tidbit. Even if no resonance is found in the mobile phone frequency bands, testing the device for response to nearby mobile phones may be the safest course of action.

Summary: Mobile, or cell, phones have been causing some interesting problems lately.  Two examples were given and a quick test involving a homemade antenna was discussed. Given the large number of mobile phones around these days, spending a little time on the testing outlined in this article can save a lot of headaches after product introduction.

Additional articles on this website related to this topic is:
  1. September 2004, Mobile Phone Response to EMI from Small Metal ESD
  2. February 2006, Construction of a Coaxial Antenna for Troubleshooting
  3. June 2006, Measuring Structural Resonances
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.

Click here for a description of my latest seminar to be available soon titled:

EMC Lab Techniques for Designers
(How to find EMC problems and have some confidence your system will pass EMC testing while it is still in your lab).

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Copyright © 2007 Douglas C. Smith