Controlling the Unwanted

If there�s one electronic gadget that most of us own or at least have some familiarity with, it�s the cell phone. Almost like the message proclaimed in an ad by a major credit card company, �You wouldn�t leave home without it.�

Today�s cell phones are complex electronic systems expected to work flawlessly in just about any environment. Most users don�t have a clue about some of the challenges designers have to overcome to deliver a versatile and reliable product. Because so many diverse technologies�analog, digital, power, and RF�are crammed in such a small space, eliminating or, at least, minimizing circuit interference is at the top of the list of product requirements. Here�s how one EMI problem was solved in a flip-type phone.

This example was related by Mike Evans, director of applications at California Micro Devices. The EMI problem was caused by the wiring that carries the digital signals between the baseband microprocessor on the bottom flip of the phone to the LCD module and the camera on the top flip. This wiring is a few centimeters long and comprises between 20 and 60 data lines, making an excellent antenna.

Unwanted carrier frequency signals picked up by the data lines on the base flip and EMI generated from the data signals during transition time were present on the wiring. With both sources of EMI, the phone most likely would fail EMI certification. Also, there is a potential ESD problem at the hinge between the flips so the data lines must be able to withstand ESD events as well.

The solution was to place EMI filters with ESD protection on the PCB nearest the hinge. Because RC-type filters can eliminate the noise and are available in chip-scale packaging, the requirements for small size and low cost were met.

Each filter provides 15 kV of ESD protection for each of the eight data lines, all in a 4 mm � 1.4 mm package. Because of the low parasitic inductance and low equivalent series resistance of the capacitors in each filter, attenuation of 35 dB to 40 dB can be achieved from 800 MHz to 6 GHz. Most flip phones now incorporate this type of filter.

Another example, as reported by Rick Johnson at R & F Products, a Laird Technologies Co., shows how microwave absorbers were used to suppress EMI in a high-speed switching network. High-frequency energy was leaking out where fiber-optic transceivers were installed in the front of the switching unit.

The front panel consisted of 32 transceiver plug-in modules with two LEDs next to each module. Initial shielding already had been put in place but could not contain the >6-GHz leakage from around the transceivers. The energy apparently was manifesting itself as surface currents along the front of the equipment.

By using a die-cut magnetic absorber bonded with pressure-sensitive adhesive to the front of the unit, the leakage was effectively eliminated. The absorber was 0 .060� thick, loaded with magnetic filler, and optimized for the frequency range of 6 GHz to 18 GHz. The strip of elastomeric magnetic absorber was die cut to fit over all of the modules and LEDs.

Trends
For EMI suppression components, small size coupled with improved performance are driving the development of new products. Manufacturers are continually pressured to cram more functionality into smaller packages.

As noted by Dave Armitage, components engineering manager at Schaffner EMC, ��there is a clear move from the single EMC component toward a total solution. Wherever possible, customers like to have one solution covering harmonics, power quality, and EMC from one supplier. Of course, it goes without saying that lower cost of goods tops the list of requirements for customers.�

EMC Products
To help you find the right EMC solution for your application, a collection of products is presented, from absorbers to transient suppressors and active EMI filters to subminiature chokes. Be sure to access the rsleads link following each product for more information from the manufacturer.