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CISPR 25: Vehicles, Boats and Internal Combustion Engines – Radio Disturbance Characteristics
Automotive EMC emission limits and test methods for vehicles, boats, and engine-driven equipment from 0.15 MHz to 2.5 GHz
Introduction to CISPR 25
CISPR 25 specifies limits and measurement methods for radio disturbance characteristics of electrical and electronic equipment intended for use in vehicles, boats, and internal combustion engine-driven devices. This standard is fundamental for automotive EMC compliance, covering both conducted and radiated emissions from automotive components and systems in the frequency range 0.15 MHz to 2.5 GHz. CISPR 25 addresses the unique electromagnetic environment of vehicles — where multiple electronic control units (ECUs), sensors, actuators, and infotainment systems must coexist without mutual interference while protecting AM/FM radio, TV, and mobile communication reception within the vehicle.
The vehicle electromagnetic environment is particularly challenging because antennas are often integrated into the vehicle body (roof, rear window, or bumpers) just centimeters away from noise sources like engine ECUs, DC-DC converters, and electric motor drives. CISPR 25 limits are designed to protect receivers with antenna-to-noise-source separations as small as 0.5-2 meters.
Measurement Methods and Test Setup
CISPR 25 specifies two main measurement methodologies: component-level testing (for individual electronic modules) and vehicle-level testing (for the complete vehicle). Component-level testing uses a 5 cm ground plane with the device under test placed 5 cm above it, with specific cable routing and load simulation. Vehicle-level testing places the whole vehicle in an absorber-lined chamber with antennas positioned at standard locations. The standard defines narrowband (continuous carriers, clock harmonics) and broadband (transient, motor noise) disturbance classifications, with different limit lines for each type. AM band (0.15-1.7 MHz) limits are the most stringent, as vehicle AM radio reception is most susceptible to interference.
| Frequency Band | Service Protected | Component Limit (Peak, 1 m distance) | Vehicle Level Limit (Peak, vertical) |
|---|---|---|---|
| 0.15 – 1.7 MHz | AM radio | 56 – 48 dBµV/m | 28 – 20 dBµV/m |
| 1.7 – 30 MHz | Shortwave, amateur radio | 48 – 36 dBµV/m | 20 – 8 dBµV/m |
| 30 – 108 MHz | FM radio, VHF TV | 36 – 44 dBµV/m | 20 – 30 dBµV/m |
| 108 – 470 MHz | VHF/UHF TV, mobile radio | 44 – 52 dBµV/m | 30 – 38 dBµV/m |
| 470 – 2500 MHz | Digital TV, cellular, GNSS | 52 – 62 dBµV/m | 38 – 48 dBµV/m |
AM band (0.15-1.7 MHz) limits in CISPR 25 are the most challenging for automotive designers to meet. Switching power supplies in ECUs produce broadband noise in this range, and the long-wavelength characteristics of AM-band interference make shielding less effective. Special attention to input filtering and PWM frequency selection is required.
Automotive EMC Design for CISPR 25 Compliance
Automotive EMC design for CISPR 25 compliance requires a system-level approach. At the ECU level, key design techniques include: (1) proper PCB stack-up with dedicated ground layers and minimal loop areas for high-current switching paths; (2) spread-spectrum clock generation for microcontrollers and communication busses (CAN, LIN, FlexRay) to reduce peak emission amplitudes by 8-12 dB; (3) local filtering at each power input pin using Pi-filters (ferrite bead + capacitor + ferrite bead) with careful attention to capacitor resonant frequencies; and (4) shielded enclosures for high-frequency modules (radar, telematics, GNSS receivers).
Harness and cabling design is critical because vehicle wiring harnesses act as unintentional antennas. Twisted-pair wiring for all differential signals (CAN, Ethernet, audio), 360-degree shielding for coaxial cables (RF, GPS), and proper return path management using the vehicle chassis as a ground reference are essential practices. Ferrite cores on external harnesses near the ECU connector can provide 10-20 dB of common-mode attenuation. Placement of electronic modules within the vehicle also affects EMC — modules containing sensitive receivers should be located away from known interference sources such as DC-DC converters, inverter drives, and ignition systems.
A well-designed automotive ECU with a four-layer PCB, ferrite-filtered power input, spread-spectrum clocking, and a grounded metal housing typically achieves 6-10 dB margin below CISPR 25 component-level limits across all frequency bands without requiring external ferrite cores on the harness.
Electric and Hybrid Vehicle Considerations
The increasing adoption of electric vehicles (EVs) and hybrid electric vehicles (HEVs) introduces new EMC challenges addressed by CISPR 25. High-voltage traction systems (400-800 V DC) with fast-switching IGBTs or SiC/GaN power devices generate significant broadband noise from DC-DC converters, traction inverters, and onboard chargers. The standard’s frequency range extension to 2.5 GHz covers interference from modern communication services (4G/5G cellular, Wi-Fi, Bluetooth, GNSS) that are now integral to connected vehicles. Special test procedures for EV charging modes — including conducted emissions on the AC charging port and radiated emissions during wireless power transfer (WPT) charging — are incorporated into the latest editions.
EV traction inverters using SiC MOSFETs with switching speeds below 50 ns can generate significant EMI in the 30-300 MHz range. Without proper filtering and shielding, these emissions can exceed CISPR 25 limits by 20-30 dB, causing interference to FM radio and mobile communication receivers in the vehicle.
Frequently Asked Questions
Q: Does CISPR 25 apply to electric scooters and e-bikes?
A: The standard covers vehicles, boats, and engine-driven devices. Small personal mobility devices may be covered by national regulations referencing CISPR 25 or may fall under other EMC standards depending on the jurisdiction.
A: The standard covers vehicles, boats, and engine-driven devices. Small personal mobility devices may be covered by national regulations referencing CISPR 25 or may fall under other EMC standards depending on the jurisdiction.
Q: What is the difference between CISPR 25 narrowband and broadband limits?
A: Narrowband limits apply to continuous, periodic emissions (e.g., clock harmonics). Broadband limits apply to transient or stochastic emissions (e.g., motor commutator noise, relay switching). The detector type (peak vs. quasi-peak vs. average) and measurement bandwidth determine the classification.
A: Narrowband limits apply to continuous, periodic emissions (e.g., clock harmonics). Broadband limits apply to transient or stochastic emissions (e.g., motor commutator noise, relay switching). The detector type (peak vs. quasi-peak vs. average) and measurement bandwidth determine the classification.
Q: How do I test the EMC of a vehicle during wireless charging?
A> CISPR 25 includes provisions for WPT charging testing. The vehicle is positioned over the charging pad in the test chamber, and emissions are measured during the charging process with the vehicle in its normal charging state.
A> CISPR 25 includes provisions for WPT charging testing. The vehicle is positioned over the charging pad in the test chamber, and emissions are measured during the charging process with the vehicle in its normal charging state.
Q: What margin should I target for automotive ECU production?
A> Most automotive OEMs require 6 dB margin below CISPR 25 limits for production ECUs, with some premium manufacturers requiring 10 dB margin to ensure robustness across production tolerances and temperature extremes.
A> Most automotive OEMs require 6 dB margin below CISPR 25 limits for production ECUs, with some premium manufacturers requiring 10 dB margin to ensure robustness across production tolerances and temperature extremes.
