Using proximity sensing to meet mobile device FCC SAR regulations

Riaan du Toit, Azoteq

April 17, 2012

Riaan du Toit, AzoteqApril 17, 2012

In this Product How-To article, Riaan du Toit of Azoteq, describes how to use the company’s patented proximity sensing technology to measure and adapt capacitive sensors in wireless mobile devices that meet the FCC specific absorption rate (SAR) regulations.

The use of wireless mobile devices has increased rapidly in the last few years, with high demands on wireless connection performance. The regulations for human exposure to electromagnetic radiation have become a limiting factor in the performance of wireless communication.

FCC SAR regulations dictate reduced output power levels in the presence of a human body. The effect of reduced power levels may lead to a connection interrupt, therefore the accurate detection of a human body is critical.

The limitations are explored through the use of specific examples where sensors are triggered falsely by non-human objects, hence limiting the output power of the device when that is not required. Capacitive sensing techniques are proposed to distinguish between human and non-human sensor activation.

Further design criteria for the sensor location, sensor performance and software detection algorithms are discussed. The examples include most common triggers, such as mobile device covers, glass and metal. The ability of the sensor to adapt to the environment is a key aspect in accurate sensing when in proximity to human and non-human objects.

FCC Specific absorption rate regulations
The regulations imposed by the FCC (Federal Communications Commission) define a specific absorption rate (SAR), which is a safe measure of the rate at which RF signals are absorbed by the human body. Designers of wireless mobile devices tend to prefer higher output power levels for optimal performance of their product.

By adhering to the FCC SAR regulations, output power is reduced in cases where it is required, although also due to a variety of false triggers. Dropped connections and degraded upload performance are more likely to occur as a result of lowered wireless signal output power. The rejection of false triggers is a key aspect of optimizing the performance of a wireless connection.

Solutions are required to distinguish between relevant activations and false activations. Offered in this article are techniques dependent on human behavior and human capability. Proposed is the use of multiple capacitive sensors with advanced adaptive characteristics, strategically placed in order to effectively solve the problem at hand.

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