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Minimising SAR with proximity sensor

28 Dec 2015  | David Wong

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The Federal Communications Commission (FCC) last set U.S. mobile phone RF exposure limits in 1996, recommending a maximum of 1.6 watts per kilogram specific absorption rate (SAR). Back then, the Motorola StarTAC was the industry's best selling phone, less than 20% of the country had mobile devices and when they did they wore them away from their bodies in belt holsters.

A lot has changed now that we are in the smartphone era, starting with much higher RF power levels to support higher data rates and a more extensive range of smart mobile devices including smartphones, phablets and tablets. Usage of these devices is increasing dramatically. In a 2014 survey of consumers, research firm Nielsen discovered that American consumers spent 34 hours on average per month using the mobile apps on their phones – which is more time than they spent online via PC(1).

This increased RF power and increased exposure has caused the industry to anticipate ways to better manage SAR reduction. Proximity sensors are one tool that have long been used in tablets to detect a user's body before the device comes closer than the minimum distance (dmin) for safe use as defined by the FCC. Once the device reaches dmin, the proximity sensor can then trigger a reduction in RF power to limit the user's RF exposure

SAR background
SAR is the measure of the amount of RF power that is radiated into the human body when in a close proximity to a mobile device. It is defined as the power absorbed per mass of tissue and is measured in units of watts per kilogram (W/kg).

In the U.S., the FCC sets SAR standards and these limits are followed in many other countries around the world. Standards for European countries are determined by CENELEC, and are currently set at 2W/kg averaged over the 10 g of tissue absorbing the most signal.

SAR and RF radiation have made headlines (2) recently with several high profile brain cancer deaths, even though there is not a scientific link between the two. Also, the city of Berkeley, Calif., recently passed a "right to know" law (3) that all cell phones sold in the city must be labelled with the SAR level and a warning. These headlines have raised some customer concern, which has led to mobile device manufacturers looking at new ways to proactivity manage their SAR levels.

Proximity sensor reduces SAR
One solution for managing SAR is to build a capacitive proximity sensor into a mobile device that can determine when the device is close to being in contact with parts of the body and can optimise the RF power to a level that reduces overall SAR, yet still lets the device stay connected to the network.

Proximity sensors are an established SAR management solution in tablets, which have increased RF power for wireless radios in order to deliver a better connectivity and data throughput.

Characteristics of a proximity sensor for SAR applications
There are several factors that make a proximity sensor ideal for SAR applications. As with any smart device component, lower power and smaller footprint are essential. Other functional capabilities that are an important part of the solution:

High-Resolution Sensing: The resolution of the proximity sensor expands the flexibility of the solution and enables very small sensor design to support the required sensing distance for SAR management in these space-constrained devices.

Permittivity Filter: It's essential for the proximity sensor to have the ability to detect and discriminate between low permittivity surfaces (a table top, for instance) and high permittivity (human body) surfaces. This allows the device to remain in full RF transmit power mode while on a tabletop, minimising any unnecessary RF back off and to be in reduced power mode only when near the human body.

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