CISPR 25 — Automotive Radio Disturbance Testing Standard
Complete guide to CISPR 25, the international standard for measuring radio disturbance from vehicle components and modules, covering test methods, frequency ranges, limit classes, and setup requirements.
What Is CISPR 25?
CISPR 25 is the international standard published by the International Special Committee on Radio Interference (CISPR), a technical committee of the International Electrotechnical Commission (IEC). Its full title is “Vehicles, boats and internal combustion engines — Radio disturbance characteristics — Limits and methods of measurement for the protection of on-board receivers.” The standard defines measurement methods and emission limits for electromagnetic disturbances produced by electronic components and modules installed in vehicles.
Unlike vehicle-level EMC regulations such as ECE R10, CISPR 25 focuses specifically on component-level emissions testing. Its primary goal is to protect radio reception systems onboard the vehicle, including AM/FM broadcast radio, DAB, GPS, Bluetooth, Wi-Fi, cellular, and keyless entry receivers. Automotive OEMs routinely require their suppliers to demonstrate CISPR 25 compliance as a condition of component approval.
Frequency Range and Measurement Categories
CISPR 25 covers a broad frequency range from 150 kHz to 2500 MHz, divided into measurement bands that correspond to different on-board receiver types. The standard defines both conducted emission and radiated emission measurement methods.
Conducted Emissions
Conducted emissions are measured on the power supply lines and signal lines of the device under test (DUT) using a Line Impedance Stabilization Network (LISN). Two types of LISN are specified:
| LISN Type | Impedance | Application |
|---|---|---|
| 5 uH LISN | 50 ohm / 5 uH | 12V and 24V power supply lines |
| 25 uH LISN | 50 ohm / 25 uH | Alternative for specific configurations |
Conducted emission measurements are performed in the frequency range of 150 kHz to 108 MHz using peak and average detectors. The measurement uses a spectrum analyzer or EMI receiver connected to the LISN measurement port.
Radiated Emissions
Radiated emissions are measured using antennas positioned at a defined distance from the DUT and its wiring harness. The measurement method is known as the Absorber Lined Shielded Enclosure (ALSE) method, performed inside a CISPR 25 anechoic chamber.
Radiated emission measurements cover the full frequency range from 150 kHz to 2500 MHz, using different antenna types for different frequency bands:
| Frequency Band | Antenna Type | Measurement Distance |
|---|---|---|
| 150 kHz – 30 MHz | Monopole rod antenna (1 m) | 1 m from harness |
| 30 MHz – 300 MHz | Biconical antenna | 1 m from harness |
| 300 MHz – 1000 MHz | Log-periodic antenna | 1 m from harness |
| 1000 MHz – 2500 MHz | Log-periodic / horn antenna | 1 m from harness |
All radiated emission measurements in CISPR 25 are performed at a 1-meter measurement distance, which is closer than many other EMC standards. This short distance provides higher sensitivity and better correlation with the actual electromagnetic environment inside a vehicle.
Emission Limit Classes
CISPR 25 defines five limit classes, numbered Class 1 through Class 5, with Class 1 being the most relaxed and Class 5 being the most stringent. The standard itself does not mandate which class a component must meet. Instead, the vehicle OEM specifies the required limit class based on the component’s installation location, proximity to antennas, and the sensitivity of nearby receivers.
| Class | Stringency | Typical Application |
|---|---|---|
| Class 1 | Least stringent | Components far from antennas |
| Class 2 | Low | Non-critical locations |
| Class 3 | Moderate | General automotive components |
| Class 4 | High | Near antenna or sensitive receivers |
| Class 5 | Most stringent | Directly adjacent to antenna systems |
Most OEMs require Class 3 or higher for components installed in the engine compartment or passenger cabin. Components installed near the vehicle’s antenna module or infotainment system are typically held to Class 4 or Class 5 limits.
Test Setup Requirements
A proper CISPR 25 test setup is critical for obtaining valid and reproducible measurement results. Key setup elements include:
Ground Plane
The DUT and its wiring harness are placed on a metallic ground plane that serves as the reference conductor. The ground plane must extend at least 200 mm beyond the DUT and harness on all sides. It must be made of a conductive material such as copper, aluminum, or steel with a minimum thickness of 0.5 mm.
Wiring Harness Configuration
The test harness must replicate the actual vehicle installation as closely as possible. The harness length is typically 1500 mm (or as specified by the OEM), routed 50 mm above the ground plane using non-conductive supports. The harness routing and bundling directly affect emission measurement results.
DUT Positioning
The DUT is placed on the ground plane at a specified height (typically 50 mm above the ground plane on a non-conductive support). The orientation of the DUT and the routing of its harness relative to the measurement antenna are defined in the test plan.
Power Supply and Load Simulation
A stabilized DC power supply provides the nominal vehicle battery voltage (typically 13.5 V for 12V systems or 27 V for 24V systems). Load simulators replicate the electrical loads that the DUT drives in the vehicle. The LISN is inserted between the power supply and the DUT to provide a defined impedance and measurement port for conducted emissions.
Detector Types and Bandwidths
CISPR 25 specifies different detector types and resolution bandwidths depending on the frequency band:
| Frequency Range | Bandwidth | Detectors |
|---|---|---|
| 150 kHz – 30 MHz | 9 kHz | Peak, Average |
| 30 MHz – 1000 MHz | 120 kHz | Peak, Average |
| 1000 MHz – 2500 MHz | 1 MHz | Peak, Average |
Both peak and average detector results are compared against their respective limit lines. A component must meet both the peak and average limits to be considered compliant.
Relationship to ECE R10
While CISPR 25 is a voluntary standard (its application is determined by OEM requirements), the test methods and measurement techniques it defines are closely aligned with the component-level emission tests in ECE R10. Many of the ESA emission test procedures in ECE R10 Annex reference CISPR 25 methodology. Achieving CISPR 25 compliance at the appropriate class level is often a practical prerequisite for ECE R10 type approval.