Ensuring SAR Compliance With Smart Proximity Sensors In 5G Devices

By David Wong, senior director of consumer sensing products, Semtech

Just five years ago, 5G networks enabled consumer devices to push beyond the limits of 4G. While that achievement was significant, the impact on the market had not yet been realized.

Today 5G is everywhere. Globally, 5G is expanding rapidly and estimations indicate that 1.5 billion people will have access to 5G networks by 2024. To support this growth, 5G deployments from both regional and nationwide carriers are continuously going live.

In the U.S., 5G smartphone revenue is expected to reach $337 billion by 2025 . More than two-thirds of the smartphone market is 5G-enabled phones (Apple devices accounting for 85% and Android devices more than 50%) and growing. This presents an opportunity for RF designers to address the forecasted surge of 5G devices, and deliver high performance connectivity and industry-compliant products. These devices need to support many different wireless protocols such as 5G and Wi-Fi 6 that can add complexity during product development.

Designing smartphones, tablets and laptops to support the fastest 5G network can be challenging. Typically, to support high-band applications, RF engineers must add more RF antennas into the design to accommodate the additional frequencies. While uncommon, adding the antennas can cause potential design flaws. For example, the increase in RF connectivity may require the form factor to be larger than originally anticipated.

In designing these products, engineers need to design proximity sensors that accurately address the industry’s Specific Absorption Rate (SAR) requirements. SAR is the measure of the amount of RF energy absorbed by the body when using a wireless device measured in watts per kilogram (W/kg). SAR sensors allows original equipment manufacturers (OEMs) to build products that reduce RF power only when necessary, thus ensuring good link quality and range.

In the U.S., wireless devices must pass SAR testing required by Federal Communications Commission (FCC) before they can be shipped to retailers. In the U.S. and Canada, SAR compliance is 1.6 W/kg (over 1g of tissue) with a separation distance of 25mm. For Europe, the SAR limit is 2 W/kg (over 10g of tissue) with a separation distance of 5mm.

Smart proximity sensors improve RF performance and throughput in smartphones, tablets and laptops, while maintaining global compliance in these devices. Smart sensors monitor human presence and enable active RF power management when a user is in close range. These sensors are highly accurate and can distinguish an inanimate object from a human, ensuring that the device is operating at peak performance and in compliance at all times.

How Human Proximity Sensors Work

To understand the intelligence of a smart sensor and how it can detect then reduce RF power when a human is in close proximity, let’s take a look under the hood of a four channel SAR controller (SX9330). This device accepts four sensor inputs and can accurately discriminate between an inanimate object and a human body. When the chip senses a human body nearby, it sends an alert via an I2C serial bus to its host processor to indicate detection.

The sensor input can be relatively simple and is often not much more than a copper plate on a PCB that is surrounded by a ground shield for noise immunity. Both the plate and the ground shield are connected to separate inputs to the SX9330 device.

The smart sensor detects the presence of a conductive object such as a finger or a palm by monitoring the capacitance on its sensor inputs. When there is no conductive object close by, the sensor only sees an inherent capacitance value created by the interaction of its electric field and the environment. An approaching conductive object modifies the electric field around the sensor, and the total capacitance the sensor sees will rise.

The main challenge with a proximity sensor is that is has to be accurate enough to differentiate an approaching human from environmental noise. Additionally, temperature and other environmental factors can make capacitance vary slowly as well. Consequently, the chip includes an auto offset compensation mechanism which dynamically monitors and removes environmental effects to yield only the capacitance change caused by a nearby human.

Key Benefits Of Proximity Sensors

• Best-in-Class Performance: Advanced AFE (Analog Front End) design with exceptional performance enables higher detection distance or smaller sensor area. It also enables high signal-to-noise ratio analog and digital processing chain with strong immunity.
• Built-In Intelligence: Innovative, smart persons-sensing feature differentiates between human and inanimate objects to enhance device response.
• Mitigation of False Positives: Enables low temperature variation over a longer duration. This limits the chances of false human detections with environmental temperature changes.
• Ease of Use: Ultra-small footprint with robust package provides versatility for a wide range of applications and product implementations. It also offers low power consumption, which is ideal for battery-operated devices (such as connected devices).

The imminent rise of 5G devices is the biggest driver for new complaint designs powered by SAR sensors. Across the globe, consumers will be purchasing new 5G-enabled devices and expect that each of these products deliver the most optimal performance, extended battery life and a robust wireless connection. Implementing smart proximity sensors into connected devices is vital to the future of widespread 5G access, implementation, and adoption.