Mobile device proximity sensor manages RF exposure while maintaining network connectivity – article
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 (Ref. 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 2 W/kg averaged over the 10g of tissue absorbing the most signal.
SAR and RF radiation have made headlines (Ref. 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, California, recently passed a “right to know” law (Ref. 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 pro-actively 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.
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Characteristics of a proximity sensor for SAR applications
There are several factors that suit a proximity sensor to 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.
Robust RF immunity: Sensors are typically operating in a noisy RF environment and susceptible to RF interference. External filters are often needed to filter out RF noise but that requires additional board space. Advanced IC circuit designs integrate the RF filter within the proximity sensor.
Elements of a proximity sensor design
The proximity sensor interface is composed of the sensor, an optional sensor shield, the analogue front-end (AFE) and a digital processor, which outputs a flag that indicates whether there is a proximity event. See Figure 1.
The proximity sensor is typically a simple copper area on the PCB or the FPC that is designed so that its capacitance to ground will vary when a conductive object such as a finger, palm, or face is in the proximity of the sensor. This sensor can detect proximity of objects touching the device (touch sensing) or at a specified distance of a few centimetres from the device (air sensing) depending on resolution.
Figure 1: Proximity sensing interface
An optional shield can be put in place to protect the sensor from potential surrounding noise sources and improve its performance. The shield is composed of another copper area on the PCB or FPC that is below or around the sensor. Another benefit of the shield is that it brings directivity to the sensing allowing the sensor to be programmed to sense objects approaching from top only, for example.
The sensor measures small variations in capacitance between the sensor’s inherent capacitance value (CEnv), and user capacitance (CUser). CEnv is created by the interaction of the sensor’s electrical field with the environment, in particular with ground areas. When the CUser object approaches, the electrical field around the sensor will be modified increasing the total capacitance.
CUser can be estimated by:
Where A is the common area between the user’s finger/palm/face and the sensor; and d is the proximity distance between the user and the system.
ε0 is the free space permittivity and is equal to 8.85 10-12 F/m (constant) and εr is the dielectric relative permittivity.
Typical permittivity of some common materials is given in Table 1.
Table 1: Permittivity of common materials
Based on this formula, the most robust and efficient design can be had by minimising the CEnv value and variations while improving CUser sensitivity.
Smart-device manufacturers are increasingly being proactive in managing SAR absorption. Rather than sacrifice the connectivity that consumers are used to, the use of proximity sensor provides OEMs a smarter solution to manage their device RF performance for staying compliant with the safety limit without comprising the wireless functionality.
References
(1) How Smartphone are Changing Consumers’ Daily Routines Around the Globe; Feb. 24, 2014
(2) Florida Attorney Dies From 3 Different Cell Phone Induced Cancers
(3) Berkeley passes cellphone ‘right to know’ law
The author is with Semtech, www.semtech.con
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