Electronics Guide

Specific Absorption Rate (SAR)

Specific Absorption Rate measures the rate at which RF energy is absorbed by body tissue when a device operates in close proximity to the body. SAR is expressed in watts per kilogram (W/kg) and is the primary metric for devices used within 20 centimeters of the body, such as mobile phones, tablets, smartwatches, and other portable wireless devices.

SAR limits vary by region but are derived from ICNIRP guidelines or IEEE standards. The FCC in the United States sets limits of 1.6 W/kg averaged over 1 gram of tissue for devices used near the head and body. European regulations following ICNIRP guidelines specify 2.0 W/kg averaged over 10 grams of tissue. While the numerical limits differ, both approaches provide equivalent safety margins based on different averaging volumes.

SAR testing requires specialized equipment including anthropomorphic phantoms that simulate human tissue properties, robotic positioning systems, and calibrated electric field probes. Testing follows standardized procedures defined in IEC 62209 for devices used near the head and body. The highest SAR value recorded under specified test conditions must be reported and must remain below applicable limits at the maximum transmit power of the device.

Design techniques to reduce SAR include optimizing antenna placement away from likely body contact points, implementing proximity sensors that reduce transmit power when the device is close to the body, and carefully designing the ground plane and antenna radiation patterns. These considerations must be addressed early in product development since SAR is heavily influenced by device geometry and cannot easily be corrected late in the design cycle.

Maximum Permissible Exposure (MPE)

Maximum Permissible Exposure limits apply to devices that operate at greater distances from the body, typically fixed or mobile transmitters where users are not in direct contact with the radiating antenna. MPE is expressed as power density in milliwatts per square centimeter (mW/cm2) or as electric and magnetic field strengths.

MPE limits are frequency-dependent, with different values for controlled environments (occupational exposure) and uncontrolled environments (general public exposure). The limits account for the body's ability to dissipate heat, which varies with frequency due to different penetration depths and absorption characteristics of tissues.

Compliance with MPE requirements can be demonstrated through measurement or calculation. For fixed installations, site surveys measure field strengths at accessible locations. For portable equipment, MPE evaluation determines the minimum separation distance at which exposure levels fall below limits. This separation distance must be communicated to users through product labeling and documentation.

Products that operate at higher power levels or have antennas positioned close to areas where people may be present require careful MPE analysis. Base stations, access points, and industrial RF equipment often need detailed RF exposure assessments considering all operating modes, antenna configurations, and potential exposure scenarios.

FCC Equipment Authorization

The FCC administers several authorization procedures depending on the device type and potential for interference. Certification is required for most intentional radiators including WiFi devices, Bluetooth equipment, cellular phones, and other wireless transmitters. The process involves testing at an accredited laboratory followed by application submission to an FCC-recognized Telecommunication Certification Body (TCB).

Required testing includes measurement of occupied bandwidth, out-of-band emissions, spurious emissions, and RF exposure. Devices must demonstrate compliance with the specific rule parts that govern their operation, such as Part 15 for unlicensed devices, Part 22 for cellular, Part 24 for personal communications services, or Part 27 for miscellaneous wireless services.

The FCC requires that certified devices bear the FCC ID mark and that certification information be available in the FCC equipment authorization database. Any changes to the device that could affect RF characteristics require evaluation to determine if recertification is necessary through the permissive change process or a new application.

European Radio Equipment Directive

The Radio Equipment Directive (RED) 2014/53/EU establishes requirements for radio equipment placed on the European market. Manufacturers must demonstrate compliance with essential requirements covering safety, electromagnetic compatibility, and efficient use of radio spectrum before affixing the CE mark.

Compliance can be demonstrated through several conformity assessment procedures. For devices operating on harmonized frequencies, manufacturers may use internal production control combined with testing by an accredited laboratory. For devices using non-harmonized frequencies or presenting novel characteristics, involvement of a Notified Body may be required.

RED compliance requires preparation of technical documentation including test reports, risk assessments, and user instructions. The EU Declaration of Conformity must reference applicable harmonized standards or demonstrate equivalent compliance. RED Article 10 requirements address software defined radio aspects, while delegated acts specify additional requirements for categories such as internet-connected devices.

International Certification

Global market access requires certification in each target market. While some mutual recognition agreements exist, most countries maintain independent certification requirements. Key markets include Japan (TELEC/MIC), China (SRRC for radio, CCC for safety), South Korea (KC mark), Australia (RCM mark), and India (WPC).

International certification strategy should consider test report sharing possibilities, regional testing requirements, in-country representative requirements, and labeling obligations. Some certifications can leverage test data from other markets, reducing overall testing burden, while others require testing at locally accredited laboratories or additional market-specific tests.

Licensed and Unlicensed Spectrum

Licensed spectrum is allocated to specific users or services through a licensing process. Cellular networks, broadcast services, and many professional radio systems operate in licensed bands with exclusive or coordinated access rights. Devices operating in licensed bands typically require type approval and may only be used by the license holder or their customers.

Unlicensed spectrum, also called license-exempt or ISM bands, allows operation without individual user licensing subject to technical rules that limit power, bandwidth, and duty cycle. Popular unlicensed bands include 2.4 GHz, 5 GHz, and 6 GHz for WiFi, as well as various sub-GHz bands for IoT applications. While no user license is required, devices must still be certified to demonstrate compliance with technical requirements.

Spectrum allocations differ between regions, and a frequency permitted in one country may be unavailable or subject to different rules elsewhere. Multi-region products must be designed to operate within the intersection of allowed parameters or implement region-specific configurations that adapt to local requirements.

Band Edge and Channelization Requirements

Regulators specify exact frequency ranges for radio services and require devices to contain their emissions within allocated bands. Band edge requirements limit out-of-band emissions that could interfere with adjacent services. Devices must demonstrate that emissions fall rapidly outside the authorized band, typically meeting specified attenuation levels at defined frequency offsets.

Many bands specify channelization plans that define center frequencies and channel bandwidths. Devices must operate on authorized channels and may need to support specific channel numbering schemes. Regulatory databases maintained by organizations such as ETSI provide detailed channelization information for European markets.

Emission Limits

Intentional radiators must meet limits on both in-band and out-of-band emissions. In-band limits typically specify maximum effective isotropic radiated power (EIRP) or conducted power combined with antenna gain restrictions. These limits balance the need for adequate coverage against interference potential to other users of the same band.

Out-of-band and spurious emission limits protect services operating on nearby frequencies. Harmonic emissions, intermodulation products, and broadband noise must remain below specified levels. Testing verifies compliance across a wide frequency range, often from below the operating frequency to several harmonics above.

Coexistence Testing

Devices sharing spectrum must coexist without causing unacceptable degradation to other systems. Coexistence testing evaluates how devices perform in the presence of other transmitters and how their transmissions affect other receivers. Standards such as IEEE 802.19 address coexistence between wireless standards in TV white space bands.

Bluetooth, WiFi, and cellular technologies include coexistence mechanisms defined in their respective standards. Devices combining multiple radios must demonstrate that their radios can operate simultaneously without mutual interference. This often requires careful frequency planning, time-domain coordination, or adaptive mechanisms that detect and avoid interference.

Testing laboratories perform coexistence evaluations using standardized test setups and procedures. Results demonstrate compliance with coexistence requirements and provide data for optimizing coexistence performance through firmware and configuration adjustments.

DFS Requirements and Testing

DFS-capable devices must implement radar detection algorithms that identify specified radar waveforms within required detection thresholds and time windows. Upon detecting radar, devices must cease transmission on the affected channel within a specified time and not return to that channel for a defined non-occupancy period.

Before transmitting on a DFS channel, devices must perform a Channel Availability Check (CAC) by monitoring the channel for a minimum period to verify no radar is present. The CAC period varies by regulatory domain, typically 60 seconds for most channels with longer periods for certain weather radar frequencies.

DFS testing requires sophisticated radar simulators that generate the specified waveforms at precise power levels. Testing verifies detection probability, response time, and proper channel management behavior. Both master devices (such as access points) and client devices have DFS requirements, though client requirements are generally less stringent since they operate under master device control.

Regional DFS Variations

DFS requirements vary significantly between regulatory domains. The FCC, ETSI, and other regulators specify different radar waveforms, detection thresholds, and timing parameters. Multi-region products must implement DFS algorithms that satisfy the requirements of all target markets or support region-specific configurations.

Some regulatory domains are introducing updated DFS requirements that mandate detection of additional radar waveforms or modify timing parameters. Products must track regulatory changes and update DFS implementations through firmware to maintain compliance in all markets.

TPC Implementation

TPC-capable devices must support a defined power reduction range, typically 3 dB or 6 dB below maximum power. The device must implement mechanisms to select appropriate power levels based on link conditions and reduce power when possible while maintaining communication quality.

For WiFi devices, TPC is often implemented in conjunction with link adaptation mechanisms that adjust modulation and coding based on signal conditions. Standards such as IEEE 802.11h define TPC procedures for 5 GHz operation. Implementation must ensure that power adjustments occur appropriately and that the device can respond to TPC requests from infrastructure equipment.

Power Limits by Device Type

Regulations often specify different power limits for different device categories. Fixed point-to-point links may be permitted higher EIRP than mobile devices. Indoor-only devices may have different limits than devices approved for outdoor operation. Client devices operating under infrastructure control may have different limits than master devices.

Understanding device classification is essential for determining applicable power limits. A device certified as portable may be subject to SAR requirements and lower power limits, while the same device classified as mobile might have different constraints. Manufacturers must carefully consider intended use cases when selecting device classifications during the certification process.

Module Certification Requirements

To qualify for modular approval, modules must meet specific criteria defined by regulators. FCC requirements include having their own shielding, modulation capability, and power supply regulation. Modules must have buffered data inputs, maintain compliance with all applicable rules when installed in any host, and be labeled with their certification identifier.

Limited modular approval may be granted for modules that do not meet all standard criteria but can demonstrate limited host dependencies. Such modules may require additional evaluation when integrated into specific hosts or may have restrictions on the types of hosts into which they can be installed.

Host Integration Considerations

Host products using certified modules must maintain the module's certified configuration. Antenna specifications, including type, gain, and cable loss, must remain within certified parameters. Host manufacturers must follow module integration guidelines and may need to perform limited testing to verify that integration does not affect module compliance.

Labeling requirements for host products depend on module visibility. If the module's certification label is visible after installation, no additional host labeling may be required. If the label is not visible, the host must display the module's certification identifier or its own identifier if certified as a composite device.

Class II permissive changes allow host manufacturers to add or change antennas within specified parameters without module recertification. Understanding permissive change rules enables flexibility in product design while maintaining compliance.

SDR Security Requirements

Regulators require that SDR devices implement security features to prevent unauthorized modification of RF parameters. The FCC requires manufacturers to describe security measures that prevent third-party software from modifying RF parameters beyond certified limits. Security descriptions must be provided confidentially as part of the certification application.

The EU Radio Equipment Directive Article 3(3)(i) requires that radio equipment support features ensuring software can only be loaded when compliance has been demonstrated. Delegated Regulation (EU) 2022/30 implements these requirements for categories including cellular equipment, WiFi devices, and IoT products.

Security measures may include cryptographic verification of software updates, secure boot mechanisms, hardware-enforced parameter limits, or access controls that prevent unauthorized configuration changes. The specific measures required depend on the device type, software architecture, and regulatory interpretation.

Reconfigurable Radio Systems

ETSI has developed standards for Reconfigurable Radio Systems (RRS) that enable authorized reconfiguration while maintaining compliance. These standards define architecture, interfaces, and security requirements for software-reconfigurable devices.

Reconfigurable radio equipment may support multiple radio access technologies, frequency bands, or power configurations that can be activated through software. Compliance requires that all possible configurations meet applicable requirements and that the device cannot be configured to operate outside approved parameters.

Pre-Compliance and Design Verification

Pre-compliance testing during development identifies potential issues before formal certification testing. Evaluating prototype devices against expected requirements allows design modifications while changes are still practical. Many accredited laboratories offer pre-compliance services, and some design teams invest in in-house test equipment for early screening.

Design verification testing covers all operating modes, channels, and power levels to ensure the device meets requirements across its full operating envelope. This comprehensive evaluation often reveals edge cases or specific configurations that require optimization before certification.

Certification Testing

Formal certification testing must be performed at laboratories accredited for the specific test methods and regulatory requirements. In the United States, laboratories must be accredited to ISO/IEC 17025 and recognized by the FCC for the applicable rule parts. European testing requires accreditation by a national accreditation body and, for certain product categories, designation as a Notified Body.

Testing covers RF parameters (frequency, power, bandwidth, emissions), SAR or MPE evaluation, and any technology-specific requirements such as DFS. Test reports must follow prescribed formats and include all information required by the certification authority.

Documentation and Application

Certification applications require comprehensive documentation including test reports, device descriptions, block diagrams, schematics (often submitted under confidentiality), user manuals, and labeling artwork. Applications must be complete and accurate to avoid delays or rejection.

Response to certification body questions and resolution of any issues identified during application review require technical expertise and may involve additional testing or documentation. Maintaining good relationships with certification bodies and understanding their requirements facilitates efficient processing.

Change Management

Any change to a certified product that could affect RF performance requires evaluation for compliance impact. Component substitutions, firmware updates, antenna modifications, and mechanical changes may require retesting or recertification. Manufacturers must maintain change control procedures that identify potentially impactful changes and route them for compliance evaluation.

Permissive change rules define when modifications can be made without full recertification. Understanding these rules enables efficient product updates while maintaining compliance. Changes exceeding permissive change allowances require new testing and updated certification.

Post-Market Obligations

Certified devices must continue to meet requirements throughout their market life. Regulators may conduct market surveillance, request device samples for testing, or investigate interference complaints. Manufacturers must be prepared to demonstrate ongoing compliance and respond to regulatory inquiries.

Product labeling and user documentation must accurately reflect certified parameters and provide required user information. Updates to regulatory requirements may necessitate label or documentation changes even for products already on the market.

Regulatory Monitoring

Wireless regulations evolve continuously as spectrum allocations change, new technologies emerge, and regulators respond to interference issues or policy objectives. Manufacturers must monitor regulatory developments to ensure continued compliance and identify opportunities enabled by regulatory changes.

Industry associations, regulatory newsletters, and specialized consultants provide regulatory intelligence services. Active participation in standards development and regulatory proceedings enables manufacturers to influence future requirements and gain early insight into upcoming changes.