Test & measurement


compliance distance which is important. If the minimum distance a user can get to a device and still meet the regulatory requirements is known, then the placement of that device in the workplace can be easily assessed. For example, if the base station is compliant at 0mm separation distance then it can effectively be placed anywhere. Alternatively, if the separation distance is relatively large then this could be mitigated by specifying a minimum height at which the base station could be mounted on an office wall. Certain medical devices that use wireless technology, such as wearable health monitors or implantable devices, may also require SAR testing to national regulatory requirements. As can be seen from the various descriptions above, it’s vitally important to understand and define how a product will be used to understand when and how SAR tests will be applied. Another key requirement is to understand and apply all “reasonably foreseeable use” conditions. Examples of this would be for something like a mobile phone where a manufacturer has stated


Instrumentation Monthly August 2024


that it’s used in a body condition at a separation distance of 15mm, when in reality most users are placing the device in their pocket. In this case it is the pocket use case at 0mm to 5mm that must be assessed. Similarly, if a device is classified as only for use in the hands when it can be placed in a coat or hoodie type garment pocket while it is transmitting, then body conditions would also need to be applied.


The other conditions that are important to consider is whether the device under test is for public use or for occupational use. If a device is for occupational use, the SAR limits are less stringent. The reason behind this is that professional users can be given training on the operation of their equipment and on how to limit their exposure to the RF radiation from those devices. This would result in an awareness of the equipment with regards to what it is doing and how to use it. If a device is classed as occupational use, it may have been designed to the limits associated with occupational use and manufacturers must be certain that the device cannot be easily obtained


by the general public. If this is not the case, then the device must be tested to the general public limits. Such an issue would be particularly difficult to deal with as SAR testing is a physical test that must be carried out on hardware that is representative of that being placed on the market. Any issues that need to be fixed at the end of a product development cycle would be far more costly to rectify than if they were found earlier in the development cycle.


In most cases a fix for SAR compliance issues is to turn down the output power of a product but this will negatively affect device performance. When a device is being developed, it is therefore important to fully understand SAR testing requirements from the start. This will ensure that a practical and realistic SAR test programme is thought out and carried out as soon as possible in a product’s development cycle, to minimise the likelihood of design reworks which can be costly and cause time-to-market delays.


TÜV SÜD www.tuvsud.com/uk 19


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