Test & measurement
UNDERSTANDING SAR TESTING:
WHEN IS IT REQUIRED?
plays a crucial role in assessing and ensuring the safety of such electronic devices.
A
SAR measures the rate at which the human body absorbs RF energy from a wireless device. Testing determines the amount of RF energy absorbed by the body, particularly in areas where the absorption is highest, such as the head and torso. The results are expressed in watts per kilogram (W/kg), representing the amount of RF energy absorbed per unit of body mass. Typically the SAR levels are averaged over a 10g mass for European regulatory requirements and a 1g mass for North America.
The primary concern associated with RF radiation exposure is the potential for thermal effects, where the absorbed energy can lead to an increase in body temperature. SAR testing helps establish whether a device emits RF energy at levels that may contribute to thermal effects.
REGULATORY COMPLIANCE SAR testing is a regulatory requirement in many countries to ensure that electronic devices comply with established safety standards. Different regions have specific SAR limits that devices must adhere to be deemed safe for use. For example, the Federal Communications Commission (FCC) and Innovation, Science and Economic Development (ISED) in North America, and the European Union all have stringent SAR regulations.
The simplest way to decide if SAR testing is required is to ask: “Is the device to be used within 20cm of a user?” If the answer to that question is “yes” and the output power of the device is generally above 20mW (for Europe for example) then you would most likely have to carry out some SAR testing.
MOBILE PHONES
SAR testing is mandatory for mobile phones and smartphones. Given the proximity of these devices to the user’s body, especially during calls, SAR testing ensures that the emitted RF energy is within safe limits.
As mobile phones are typically used against the head and the body, they must be considered for all their various user cases, and the regulatory
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s our dependence on wireless devices that emit RF radiation continues to grow, so do the complexities of those devices and their national regulatory requirements. Specific Absorption Rate (SAR) testing
requirement for these conditions may be different. The devices may also operate at different output power levels for those use cases. This needs to be considered both during the testing and in defining a test plan, as testing may need to be repeated for each use case.
Body testing has become every bit as important as head testing, particularly following the introduction of “always on” data services. These technologies can be classed as an un-attended service, meaning that the device can be placed anywhere about the body with the user (in most cases) being unaware of the transmit status of the device. For example, the device could be doing an update, sending a large file, or being used by a third party as a wifi hotspot. Following the introduction of this technology the amount of regulatory testing that is required increased significantly and the allowable separation distance between the device and the body reduced significantly. These changes made the SAR testing of mobile handsets much more demanding.
WIRELESS CONSUMER PRODUCTS Devices like tablets, smartwatches, and other wireless communication devices that are used in close proximity to the body are also subject to SAR testing. This includes devices with wifi, Bluetooth, and cellular connectivity. As some of the devices in this category can be used in the hands or on limbs, a slightly relaxed SAR test limit might be allowed as these extremities do not contain any major organs and are primarily made up of skin, bone, and muscles. Once again, it is important to consider all of the use cases. For the example, a smartwatch could be classed as limb use only. But, if you can speak into the smartwatch then you must consider the proximity of the user’s face to the device when speaking into it. This front of face condition could result in a head SAR use case condition with a separation distance of 10mm, which would need to be tested to the more stringent head and body SAR limits, as well as testing the limb conditions.
WIRELESS INFRASTRUCTURE In addition to individual devices, SAR testing is sometimes applicable to wireless infrastructure components, such as antennas and base stations, to ensure that the overall RF exposure levels are within acceptable limits. As the use of small (pico-cell) office-based base station infrastructure technologies is becoming more widespread, how they are installed in the workplace must be considered. In the case of a base station, it’s often the device's
By Jon Kenny, technical director at TÜV SÜD
August 2024 Instrumentation Monthly
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