Electronics Guide

Class A and Class B Distinctions

FCC Part 15 divides digital devices into two classes based on their intended use environment. Class A devices are intended for use in commercial, industrial, or business environments, where the electromagnetic environment is assumed to be more controlled and the potential for interference with residential radio and television reception is reduced. Class B devices are intended for use in residential environments and must meet more stringent emission limits to protect nearby radio and television receivers from interference.

The distinction between Class A and Class B is significant for product development. Class B limits are approximately 10 dB more stringent than Class A limits across most of the regulated frequency range. A product classified as Class A must include a warning statement in its documentation indicating that it may cause harmful interference when operated in a residential environment. Products intended for consumer markets or home office use are typically expected to meet Class B requirements.

Class B radiated emission limits at a measurement distance of 3 meters range from 100 microvolts per meter (40 dBuV/m) at 30 MHz to 200 microvolts per meter (46 dBuV/m) at 88-216 MHz and 500 microvolts per meter (54 dBuV/m) above 216 MHz. Class A limits are specified at 10 meters and are correspondingly less stringent, with limits ranging from 90 microvolts per meter at 30-88 MHz to 150 microvolts per meter at 88-216 MHz and 200 microvolts per meter above 216 MHz.

Measurement Procedures

FCC Part 15 measurements are typically performed in accordance with ANSI C63.4, which specifies the measurement procedures and test site requirements for determining compliance. Measurements must be performed at an accredited test facility, which may be an open area test site (OATS), a semi-anechoic chamber, or a fully anechoic chamber. Each facility type has specific validation requirements to ensure measurement accuracy and reproducibility.

During radiated emission testing, the equipment under test (EUT) is configured in a representative operating condition that exercises the digital circuitry and produces maximum emissions. The EUT is placed on a non-conductive table at a specified height, and measurements are taken at multiple azimuthal positions as the EUT is rotated through 360 degrees. The receiving antenna is scanned in height (typically from 1 to 4 meters) to account for ground reflection effects that cause the measured field strength to vary with antenna height.

Both horizontal and vertical antenna polarizations must be measured, as emissions may be polarized in either orientation depending on the radiating structure. The maximum emission at each frequency is recorded regardless of EUT orientation, antenna height, or polarization. Measurements typically cover the frequency range from 30 MHz to 1 GHz for most digital devices, with extended measurements to 6 GHz or higher required for devices operating at clock frequencies above certain thresholds.

Verification and Certification

FCC Part 15 provides multiple authorization procedures depending on the device type. Certification requires testing by an accredited test laboratory and review by a Telecommunication Certification Body (TCB) or the FCC itself. This procedure is mandatory for intentional radiators and certain computing devices. Suppliers Declaration of Conformity (SDoC) allows manufacturers to self-declare compliance based on testing at an accredited laboratory, applicable to most unintentional radiators. The verification procedure has been largely replaced by SDoC for new products.

Documentation requirements include maintaining a test report demonstrating compliance, equipment specifications, and compliance information for consumers. Products must be labeled with the FCC identifier (for certified devices) or compliance statement. The manufacturer must retain documentation for the life of the product plus additional years as specified in the regulations.

Class A and Class B Equipment

Similar to FCC Part 15, CISPR 32 distinguishes between Class A equipment intended for commercial or industrial environments and Class B equipment suitable for domestic use. Class B limits are more restrictive to provide adequate protection for broadcast reception in residential areas. The limits are specified as quasi-peak values measured with a standardized receiver bandwidth and detector characteristics.

CISPR 32 Class B radiated emission limits at 10 meters measurement distance are 30 dBuV/m from 30 to 230 MHz and 37 dBuV/m from 230 to 1000 MHz. Class A limits at 10 meters are 40 dBuV/m from 30 to 230 MHz and 47 dBuV/m from 230 to 1000 MHz. When measurements are performed at 3 meters distance, appropriate conversion factors must be applied, typically adding 10 dB to account for the inverse distance relationship of field strength in the far field region.

Measurement Site Requirements

CISPR 32 specifies measurement procedures and test site requirements that are similar to but not identical with ANSI C63.4 used for FCC testing. Measurements may be performed at an open area test site complying with CISPR 16-1-4 or in a semi-anechoic chamber validated according to the site validation procedures in CISPR 16-1-4. The normalized site attenuation (NSA) methodology ensures that the test facility provides accurate, reproducible measurements.

Antenna factors, cable losses, and receiver characteristics must be calibrated and documented. The measurement receiver must comply with CISPR 16-1-1, which specifies the quasi-peak detector characteristics, measurement bandwidth (120 kHz for frequencies below 1 GHz), and other receiver parameters. These standardized receiver characteristics ensure that measurements at different facilities produce comparable results.

European Union Implementation

In the European Union, compliance with CISPR 32 provides a presumption of conformity with the essential requirements of the Radio Equipment Directive (RED) 2014/53/EU and the Electromagnetic Compatibility Directive 2014/30/EU. Harmonized European standards EN 55032 (emissions) and EN 55035 (immunity) implement the CISPR requirements with any necessary European modifications or additions.

Products sold in the EU must bear the CE marking indicating compliance with applicable directives. The manufacturer must prepare technical documentation including test reports, prepare a Declaration of Conformity, and maintain these records for inspection by market surveillance authorities. While manufacturer self-declaration is permitted for most products, a notified body assessment may be required for certain equipment categories or when harmonized standards are not applied.

Equipment Groups and Classes

CISPR 11 divides equipment into two groups based on whether it intentionally generates RF energy. Group 1 equipment includes devices that do not intentionally generate RF energy for material treatment but may contain incidental RF sources. Group 2 equipment intentionally generates and uses RF energy for material treatment in ISM applications, such as industrial heating, plasma processing, or medical diathermy.

Within each group, equipment is further classified by intended environment. Class A equipment is intended for use in industrial locations and establishments other than domestic premises. Class B equipment is suitable for use in domestic establishments and in establishments directly connected to a low voltage power supply network that supplies residential buildings. Class B limits are more stringent than Class A limits, similar to other CISPR standards.

Group 2 equipment receives additional allowances in the ISM frequency bands (such as 13.56 MHz, 27.12 MHz, 40.68 MHz, 915 MHz, 2450 MHz, and others), where higher emission levels are permitted to accommodate the legitimate use of RF energy for industrial, scientific, and medical purposes. Outside the ISM bands, Group 2 equipment must meet the same limits as Group 1 equipment.

Radiated Emission Limits

CISPR 11 specifies radiated emission limits for frequencies from 30 MHz to 18 GHz, with specific limits depending on equipment group, class, and the frequency range. For Group 1 Class B equipment, the limits at 10 meters distance are 30 dBuV/m from 30 to 230 MHz and 37 dBuV/m from 230 to 1000 MHz, matching the CISPR 32 Class B limits for information technology equipment. Class A limits are 40 dBuV/m and 47 dBuV/m for the same frequency ranges.

Above 1 GHz, both Class A and Class B equipment must meet limits of 70 dBuV/m at 3 meters distance when measured with the peak detector and 1 MHz measurement bandwidth. Average limits are 20 dB lower. Special provisions apply to equipment operating above certain power levels or in specific ISM bands, where alternative limits or measurement procedures may apply.

Medical Equipment Considerations

Medical electrical equipment subject to CISPR 11 must also comply with IEC 60601-1-2, which specifies the EMC requirements for medical electrical equipment and systems. IEC 60601-1-2 references CISPR 11 for emission requirements but adds specific provisions for the medical environment, including consideration of the essential performance of the equipment and the risk management process required by IEC 60601-1.

Medical equipment manufacturers must conduct a risk analysis to determine if the general CISPR 11 limits are sufficient or if more restrictive limits are needed to prevent interference with other medical equipment in the healthcare environment. The professional healthcare environment is generally considered a controlled environment with reduced risk of interference to residential broadcast reception, which may allow the use of Class A limits in many cases.

Test Methods and Setup

CISPR 25 specifies component-level testing methods designed to characterize emissions before installation in a vehicle. Testing is performed in a shielded room or enclosure lined with absorber material to reduce reflections and external interference. The device under test is mounted on a ground plane representing the vehicle body, with the measurement antenna positioned at a standard distance (typically 1 meter) from the DUT.

The standard defines multiple antenna types and positions for comprehensive measurement of radiated emissions. A biconical antenna covers lower frequencies, a log-periodic antenna covers intermediate frequencies, and horn antennas are used for higher frequencies. Both rod (monopole) and loop antennas may be used to characterize electric and magnetic field components separately, as automotive receivers may be sensitive to either field component depending on the antenna type.

The test harness configuration is critical in CISPR 25 measurements, as cables can be significant radiating structures. The standard specifies harness lengths, routing along the ground plane, and the use of artificial networks (ANs) to provide standardized RF impedances at the power and signal connections. The harness represents typical vehicle wiring but in a controlled, reproducible configuration.

Limit Classes

CISPR 25 defines five limit classes (Class 1 through Class 5), with Class 5 being the most stringent. The appropriate limit class is negotiated between the vehicle manufacturer and component supplier based on the component location in the vehicle, proximity to antennas, and the receiver sensitivity requirements. Components located near antenna modules typically require Class 4 or Class 5 limits, while components in less critical locations may qualify under less stringent classes.

The limits are specified as field strength in dBuV/m across various frequency bands corresponding to broadcast and communications services. Frequency bands of particular interest include AM broadcast (0.15-2 MHz), FM broadcast (76-108 MHz), television (various bands), DAB digital radio (174-240 MHz), GPS (1.2 and 1.575 GHz), cellular communications, and automotive radar (76-81 GHz for newer vehicles). Limits become progressively more stringent from Class 1 to Class 5, with typical differences of 10 to 20 dB between adjacent classes.

Integration with Vehicle-Level Testing

While CISPR 25 addresses component-level testing, the complete vehicle must also be tested according to CISPR 12, which specifies vehicle-level emission measurements. Component-level compliance provides high confidence that the complete vehicle will meet requirements, but vehicle-level testing remains necessary to verify system-level performance including the effects of installation, cable routing, and interaction between components.

Vehicle manufacturers typically flow down CISPR 25 requirements to component suppliers through their EMC specifications, often with additional requirements specific to the vehicle platform or installation location. The automotive supply chain relies heavily on component-level testing to ensure vehicle-level compliance while minimizing costly vehicle-level testing iterations.

Radiated Emission Requirements

MIL-STD-461 specifies multiple radiated emission requirements depending on the platform and installation environment. RE101 covers radiated emissions from magnetic field sources at frequencies from 30 Hz to 100 kHz, applicable to equipment installed on aircraft, ships, and submarines where low-frequency magnetic field emissions could interfere with sensitive navigation or detection systems. RE102 covers radiated emissions of electric field from equipment enclosures and associated cabling from 10 kHz to 18 GHz, representing the primary radiated emission requirement for most military equipment.

The RE102 limits are specified as field strength in dBuV/m at 1 meter distance and vary by platform (ground, ship, aircraft, submarine, space) and frequency. Aircraft platform limits are among the most stringent, reflecting the critical nature of aircraft systems and the proximity of sensitive navigation and communications receivers. The limits typically range from approximately 24 dBuV/m at low frequencies to 54 dBuV/m at higher frequencies, with specific breakpoints and slopes depending on the platform.

RE103 covers radiated emissions from antenna terminals, applicable to equipment with antennas that might radiate spurious emissions outside their intended operating frequency. This requirement limits out-of-band and spurious emissions that could interfere with other onboard or external systems.

Test Methods and Facilities

MIL-STD-461 testing is typically performed in a shielded room rather than an open area test site. The shielded room is lined with RF absorber material on the walls and ceiling to reduce reflections, creating a semi-anechoic environment. The equipment under test is positioned on a ground plane representing the installation platform, with all cables routed in a specified manner to ensure reproducible measurements.

Measurements use broadband antennas positioned at 1 meter from the equipment under test, with the antenna moved along the front, back, top, and sides of the test setup to capture emissions from all directions. Unlike commercial EMC testing that typically seeks the maximum emission, MIL-STD-461 requires emissions to be below limits at all antenna positions. The measurement receiver must meet the requirements of MIL-STD-461 for detector types, bandwidth, and measurement procedures.

Test tailoring is a key aspect of MIL-STD-461 application. The procuring activity specifies which requirements apply to a given equipment item, the applicable limits, and any tailoring based on the specific installation environment. The applicable requirements and limits may be modified from the baseline values in the standard based on the expected electromagnetic environment and the criticality of the equipment function.

Compliance Documentation

MIL-STD-461 compliance requires extensive documentation including a formal test plan, test procedures, and test report. The test plan must detail the equipment configuration, test methods, limit applicability, and any approved tailoring. The test report must include complete data for all required measurements, photographs of the test setup, and analysis of any non-compliances or anomalies.

An Electromagnetic Interference Test Report (EMITR) following a prescribed format is typically required. The government procuring activity reviews and approves the test documentation, and the test laboratory may require accreditation or approval for military testing. Witnessed testing by government representatives is often required for critical systems.

Equipment Categories

DO-160 Section 21 defines multiple equipment categories (A through Z, plus HH through ZZ) based on the installation location in the aircraft and the intended electromagnetic environment. Category B represents equipment installed in areas that are well-separated from sensitive receiver antennas and may use less stringent limits. Category M represents equipment installed in areas that are in close proximity to or may be directly connected to sensitive receivers, requiring the most stringent limits. Other categories address specific installation scenarios with appropriate limits.

The selection of the appropriate category is determined during the aircraft design and certification process, considering the equipment installation location, proximity to antennas, cable routing, and the receiver sensitivity requirements. Aircraft manufacturers specify the required category in their equipment qualification specifications based on the installation analysis.

Frequency Ranges and Limits

DO-160 radiated emission limits cover the frequency range from 100 MHz to 6 GHz (or higher in recent revisions), focusing on the frequency bands used by aircraft radio communications and navigation systems. Key protected services include VHF communications (118-137 MHz), VOR navigation (108-118 MHz), localizer (108-112 MHz), glideslope (329-335 MHz), GPS (1.2 and 1.575 GHz), radio altimeter (4.2-4.4 GHz), and distance measuring equipment (960-1215 MHz).

Limits are specified as field strength in dBuV/m at 1 meter distance, with values depending on the equipment category and frequency band. Category B limits typically range from 45 to 55 dBuV/m across the frequency range, while Category M limits are approximately 20 dB more stringent, ranging from 25 to 35 dBuV/m. The most stringent limits apply in the frequency bands of the most sensitive navigation receivers.

Test Methods and Certification

DO-160 testing is performed in a shielded room lined with RF absorber material, similar to MIL-STD-461 testing. The equipment under test is positioned above a ground plane, with interconnecting cables routed as specified in the test standard. Measurements are made with the antenna positioned at 1 meter from the equipment, measuring emissions from the front, back, top, and sides of the test setup.

The test receiver must meet specifications for bandwidth, detector type, and measurement accuracy. Both peak and quasi-peak measurements may be required depending on the equipment category. Audio frequency modulation testing verifies that emissions in critical receiver bands do not contain modulation components that could interfere with voice communications or navigation displays.

Certification of equipment to DO-160 is typically required for installation on civil aircraft. The certification authority (such as the FAA in the United States or EASA in Europe) reviews the test documentation as part of the equipment approval process. Approved test laboratories must meet specific accreditation requirements for DO-160 testing.

Generic Emission Standards

IEC 61000-6-3 establishes emission requirements for products intended for use in residential, commercial, and light-industrial environments. IEC 61000-6-4 establishes emission requirements for products intended for use in industrial environments. Both standards reference the emission limits and measurement methods of more specific standards such as CISPR 11 or CISPR 32, depending on the product type and operating characteristics.

For radiated emissions, IEC 61000-6-3 typically requires compliance with CISPR 32 Class B limits or equivalent limits from other applicable CISPR standards. IEC 61000-6-4 permits the use of Class A limits appropriate for industrial environments where residential broadcast reception protection is not the primary concern. The standards also require compliance with conducted emission limits and harmonic current limits as appropriate.

Application and Hierarchy

The IEC 61000-6 generic standards apply only when no product-specific or product-family EMC standard exists for the equipment in question. When a product-specific standard exists (such as CISPR 32 for information technology equipment or IEC 60601-1-2 for medical equipment), that standard takes precedence over the generic standards. The generic standards thus serve products that fall outside the scope of existing specific standards or new product categories where specific standards have not yet been developed.

In the European Union, the harmonized versions of the generic standards (EN 61000-6-3 and EN 61000-6-4) provide a presumption of conformity with the essential requirements of the EMC Directive when product-specific standards do not apply. Manufacturers must determine which standards apply to their products based on the product type, intended use environment, and the availability of product-specific standards.

Lighting Equipment

CISPR 15 establishes limits and measurement methods for radio disturbance characteristics of electrical lighting and similar equipment. This standard addresses the unique emission characteristics of lighting products, including the high-frequency switching found in LED drivers and electronic ballasts for fluorescent lamps. The standard covers both conducted and radiated emissions, with specific provisions for different lighting technologies and installation scenarios.

Household Appliances

CISPR 14-1 covers electromagnetic emissions from household appliances, electric tools, and similar apparatus. This standard addresses the broad range of consumer products with electric motors, heating elements, electronic controls, and other potentially emissive components. The standard specifies limits and measurement methods appropriate for products intended for residential use, where protection of broadcast reception is a primary concern.

Railway Equipment

EN 50121 series standards address electromagnetic compatibility of railway systems and equipment. These standards cover rolling stock, infrastructure equipment, signaling systems, and telecommunications equipment used in railway applications. Railway environments present unique electromagnetic challenges due to the high-power traction systems, extensive cable runs, and the need to protect safety-critical signaling and communications systems.

Marine Equipment

IEC 60945 and related standards establish EMC requirements for maritime navigation and radiocommunication equipment. These standards address the electromagnetic environment on ships and boats, including the protection of navigation receivers and radiocommunication systems from interference by shipboard electronics. The requirements consider the unique installation environment with extensive metal structures and the critical nature of maritime safety systems.

CISPR and International Adoption

CISPR standards form the foundation for EMC regulations in most countries outside the United States. The European Union adopts CISPR standards as European Norms (EN), often with minor modifications for European regulatory requirements. Australia, Japan, South Korea, and many other countries either adopt CISPR standards directly or develop national standards closely aligned with CISPR requirements. This widespread adoption simplifies international compliance by allowing a single test program to demonstrate compliance in multiple markets.

FCC and CISPR Correlation

While FCC Part 15 and CISPR standards developed independently, the technical requirements are sufficiently similar that equipment designed to one set of standards typically meets or closely approaches the other. The primary differences lie in specific limit values, measurement distances, and procedural details rather than fundamental technical approach. Products designed with adequate margin to Class B limits under either regulatory framework generally achieve compliance under both, though formal testing under each framework is still required to demonstrate compliance.

Accreditation and Laboratory Recognition

International accreditation agreements facilitate mutual recognition of test results between accreditation bodies. Under the International Laboratory Accreditation Cooperation (ILAC) mutual recognition arrangement, test reports from accredited laboratories are accepted by other signatory accreditation bodies. This reduces duplicative testing and facilitates global market access. However, regulatory acceptance of test reports may still require specific accreditation or designation, particularly for certification schemes in major markets.

Design for Compliance

Successful compliance with radiated emission standards begins early in the product development process. Design decisions made at the architecture and schematic stages have profound effects on emission characteristics that are difficult or expensive to correct later. Understanding the applicable standards and their technical basis enables designers to incorporate EMC best practices from the outset, reducing the likelihood of non-compliance and the need for costly redesigns.

Key design considerations include proper PCB layout with attention to loop areas and return current paths, appropriate filtering at power and signal interfaces, shielding and enclosure design, cable management and connector selection, and component selection with EMC performance in mind. Pre-compliance testing during development provides early warning of potential issues while design changes remain feasible.

Test Planning and Margin

Effective test planning considers the applicable standards, measurement uncertainties, production variations, and desired compliance margins. Emissions measured at the regulatory limit provide no margin for measurement uncertainty or production variation, risking failures in verification testing by regulatory authorities or quality variations in production. A design margin of 3 to 6 dB below the regulatory limit provides reasonable assurance of continued compliance considering these factors.

Understanding the statistical nature of EMC testing helps in interpreting results and making appropriate design decisions. Emission levels may vary with test setup configuration, operating conditions, and even ambient temperature. Multiple measurements under varied conditions provide confidence that the equipment meets limits under the full range of expected operating conditions.

Documentation and Traceability

Comprehensive documentation of the compliance testing program is essential for regulatory submissions and ongoing compliance management. Test reports must fully document the equipment under test, test configuration, measurement equipment, test procedures, and results. Any non-compliances, anomalies, or deviations from standard procedures must be documented with appropriate justification.

Change control procedures ensure that modifications to the equipment design do not inadvertently affect EMC compliance. Design changes that could affect emissions, such as modifications to clock frequencies, power supply circuits, PCB layout, enclosure design, or cable configurations, should trigger review and potentially retesting. Maintaining traceability between the tested configuration and production products ensures that compliance demonstrated in testing remains valid for products sold in the market.