Anechoic Chambers and Smart Antennas In Cell Phone EMC/EMI Testing

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When dealing with EMC/EMI testing and mobile technology the use of Anechoic Chambers and Smart Antennas will provide customers with better service, improved quality, and reduced health risks. EMC/EMI testing is vital to ensuring that mobile phones and Cellular antennas function correctly in the increasingly narrow bandwidths that modern systems often are functioning on. An anechoic chamber designed to stop reflection of radio length waves can be a critical component to successful EMC/EMI testing on the most sensitive of equipment where standard environmental conditions can contaminate the readings. These rooms are insulated from noises originating outside of them, in addition to absorbing waves created inside them to keep them from bouncing about. The effect is to mimic quiet open-space of near infinite size that allows for very sensitive EMC/EMI testing to be conducted.

These chamber can be small compartments or as large as an aircraft hanger, and the size chosen is directly related to the objects being tested and the frequency range of the signals. It should be noted that in some cases scale models are sufficient for short wavelength testing. In the context of mobile technology and cell phones these rooms are very useful for testing many parts of the overall system from bandwidth limits to electromagnetic interference. One can find both full anechoic and semi-anechoic chambers. The difference is that a full one absorbs energy in all directions while a semi-anechoic chamber has solid floors for supporting heavy items. The RF version of these chambers use radiation absorbent material or RAM instead of material designed to dampen sound waves. Equipment used to take measurements of antenna radiation patterns, EMC, and radar cross section readings are typically housed inside such chambers. Like other forms of testing that uses anechoic chambers, those for radio waves can be either done on full scale objects or on scale models with correctly scaled radio frequencies. RF versions of these chambers using pyramidal RAM can function partly as acoustic dampeners also because the material does attenuate sound waves and shield from exterior noise.

Because measurements in EMC/EMI testing rely on negligible spurious signals in order to eliminate errors and ambiguities, RAM used inside RF anechoic chambers is very important. Ideally this material should reduce the RF radiation as much as possible. It has been found that pyramid shaped arrays if RAM material covering all internal surfaces. The majority of the time this material is a rubberized foam substance infused with specific levels of carbon and iron with the size of the pyramid chosen to match the lowest frequency expected. This can range from two feet to 3 or 4 inches depending on radio wavelength. Another form, useful for ranges between 30 and 1000 MHz, are ferrite tiles. While easier to fit and more durable, it does have a limited range. A recent innovation is the hybrid ferrite material formed into a pyramid. This has the advantage of wider range of absorption with a smaller pyramid size.

Such chambers can be very useful for testing the newest types of smart antennas. These can be found under such names as Adaptive Array Antennas, Multiple Antennas, and MIMO. These arrays possess smart signal processing algorithms that allow it to identify spatial signal signatures (or direction of arrival) and then calculate beamforming vectors which locate the mobile/target antenna which is common is W-CDMA and UMTS cellular formats. It should be noted that the DOA estimates are computationally intensive no matter what algorithm is used. Some common ones include MUSIC, ESPRIT, and Martix Pencil. Once the DOA is determined then Beamforming takes place. This is the formation of a radiation pattern in a directional format to hit the target/mobiles desired while nulling those that are not. A FIR tapped delay line filter is often used for this, because it can be altered on the fly internally by the antenna and it reduces MMSE quite well. For this type of filter Stepest Descent and LMS algorithms are commonly used.

In testing these types of antenna the RF anechoic chamber is vital to ensuring proper function of the computations and physical beaming of the signal. Smaller antenna can be tested inside screen rooms designed with a Faraday cage concept. This eliminates outside interference that might impact the function of the antenna or create false readings in the test measurements. It also prevents leakage from tested systems getting outside the chamber and causing harm to technicians who might be running the tests. These chambers, while expensive to construct, remain vital tools in EMC/EMI testing.

The use of anechoic chamber is a must for testing and getting the full benefit from mobile technology. There are many other terms used in this procedure which include EMC/EMI testing, bandwidth testing as well as antenna pattern measurement. Having a sound knowledge of all these terms will enable you to use your cellular service in a much better way.

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