CISPR 25 Test Setup & Equipment Requirements
Comprehensive guide to the test equipment and setup configurations required for CISPR 25 automotive component emissions testing, including EMI receivers, LISNs, antennas, harness layout, and DUT positioning.
Overview of CISPR 25 Test Setup
A valid CISPR 25 emission measurement depends as much on the test setup and equipment as it does on the device under test (DUT) itself. Incorrect equipment selection, improper harness routing, or inadequate test infrastructure can produce measurements that are either too optimistic (missing real emissions) or too pessimistic (measuring chamber artifacts rather than DUT emissions). This article details the test equipment and setup requirements specified in CISPR 25 for both conducted and radiated emission measurements of automotive components.
All measurements must be performed inside a qualified CISPR 25 anechoic chamber (ALSE) that meets the shielding, absorber, and background noise requirements of the standard.
Required Test Equipment
EMI Receiver / Spectrum Analyzer
The measuring instrument is the core of any CISPR 25 test system. The standard requires a measurement receiver that conforms to CISPR 16-1-1, capable of operating across the full CISPR 25 frequency range (150 kHz to 2500 MHz). Key specifications include:
| Parameter | Requirement |
|---|---|
| Frequency range | 150 kHz to 2500 MHz (minimum) |
| Resolution bandwidths | 9 kHz (below 30 MHz), 120 kHz (30 MHz - 1000 MHz), 1 MHz (above 1000 MHz) |
| Detectors | Peak, quasi-peak, and average |
| Dynamic range | Sufficient to measure from noise floor to maximum expected emission level |
| Input impedance | 50 ohm |
| VSWR | Per CISPR 16-1-1 requirements |
A CISPR 16-1-1 compliant EMI receiver is the reference instrument. A spectrum analyzer may be used for pre-compliance screening, but final compliance measurements should employ a calibrated EMI receiver with the correct detector and bandwidth characteristics.
Line Impedance Stabilization Network (LISN)
The LISN provides a defined, repeatable impedance at the DUT power supply port for conducted emission measurements. It also decouples the DUT from the external power supply to prevent power supply noise from contaminating the measurement. CISPR 25 specifies two LISN types:
5 uH LISN (AN type)
This is the standard LISN for automotive 12V and 24V power supply conducted emission measurements. It presents a 50-ohm impedance at the measurement port and provides 5 uH of inductance in the supply line. The 5 uH LISN is the most commonly used type in CISPR 25 testing.
Key specifications:
- Impedance: 50 ohm +/- 20% from 150 kHz to 108 MHz
- Inductance: 5 uH nominal
- Current rating: Must exceed the maximum DUT operating current
- Measurement port: 50 ohm BNC or N-type connector
- Number of lines: One LISN per power supply line (positive and negative)
25 uH LISN
The 25 uH LISN is used in specific configurations where a higher impedance stabilization network is required. It is less commonly used in CISPR 25 automotive testing but may be specified by certain OEMs or for particular DUT types.
Antennas
CISPR 25 specifies different antenna types for different frequency bands in radiated emission measurements. Each antenna must be calibrated, and its antenna factor must be known across its operating frequency range.
| Frequency Band | Antenna Type | Polarization |
|---|---|---|
| 150 kHz - 30 MHz | Monopole rod antenna (1 m) | Vertical only |
| 30 MHz - 300 MHz | Biconical antenna | Vertical and horizontal |
| 300 MHz - 1000 MHz | Log-periodic dipole array (LPDA) | Vertical and horizontal |
| 1000 MHz - 2500 MHz | Log-periodic or horn antenna | Vertical and horizontal |
Monopole rod antenna (1 m): A 1-meter vertical rod mounted on the ground plane. The rod antenna is used for conducted-current-related radiated emissions in the low-frequency range. It is connected to the EMI receiver through a matching network or directly via a 50-ohm coaxial cable.
Biconical antenna: A broadband antenna used in the VHF range. It provides reasonable gain and broad frequency coverage from approximately 30 MHz to 300 MHz. Both vertical and horizontal polarization measurements are required.
Log-periodic dipole array (LPDA): A directional broadband antenna covering from approximately 200 MHz to several GHz. It is used for the upper frequency bands where higher gain and directional sensitivity are beneficial.
Horn antenna: May be used at frequencies above 1000 MHz where high gain and well-defined radiation patterns are needed. Commonly used in combination with or as an alternative to the LPDA at the highest CISPR 25 frequencies.
Test Setup Configuration
Ground Plane Setup
The ground plane forms the foundation of the CISPR 25 test bench. Requirements include:
- Material: Copper, aluminum, or steel with minimum thickness of 0.5 mm.
- Size: Extends at least 200 mm beyond the DUT and harness edges on all sides.
- Surface: Clean, free of oxidation, and electrically continuous.
- Bonding: Connected to the chamber shielding wall with low-impedance bonds.
- Placement: Positioned on the chamber floor, serving as the reference surface for all measurements.
DUT Positioning
The DUT is placed on the ground plane and elevated 50 mm above the surface using a non-conductive support (such as a low-permittivity foam block or plastic stand). The DUT orientation and position relative to the measurement antenna must be documented and maintained consistently across all measurements.
For DUTs with a metallic housing that is grounded in the vehicle, the housing is bonded directly to the ground plane. For DUTs with plastic housings or non-grounded configurations, the DUT is isolated from the ground plane by the non-conductive support.
Wiring Harness Layout
The wiring harness configuration is one of the most critical aspects of the CISPR 25 test setup, as it directly affects both conducted and radiated emission levels. Requirements include:
- Harness length: 1500 mm is the standard length unless otherwise specified by the OEM. If the actual vehicle harness is shorter, the actual length is used.
- Harness routing: The harness runs in a straight line from the DUT across the ground plane, parallel to the front edge of the ground plane (facing the antenna for radiated measurements).
- Harness height: 50 mm above the ground plane, supported by non-conductive spacers at regular intervals (typically every 100-150 mm).
- Harness bundling: All wires in the harness are bundled together (using non-conductive cable ties or loom) to replicate the vehicle installation.
- Harness termination: Wires not connected to active loads or the LISN are terminated with appropriate load impedances as defined in the test plan.
LISN Placement
The LISN is placed on the ground plane at the end of the wiring harness, opposite the DUT. The LISN ground terminal is bonded directly to the ground plane with a short, low-impedance connection. The power supply cable from the LISN to the external DC supply exits the chamber through a filtered feedthrough.
Antenna Positioning for Radiated Measurements
- Distance: 1 meter from the nearest point of the wiring harness to the antenna reference point.
- Height: Antenna center aligned with the harness height (approximately 50 mm above ground plane for the monopole; at harness height for biconical and LPDA).
- Scanning: The antenna is moved along the length of the harness to find the position of maximum emission. The antenna position that yields the highest reading is documented.
Power Supply
A stabilized DC power supply provides the nominal battery voltage:
| Vehicle System | Nominal Test Voltage |
|---|---|
| 12V system | 13.5 V DC |
| 24V system | 27.0 V DC |
The power supply must have low RF noise to avoid contaminating the measurement. It is placed outside the shielded chamber, with power routed through the LISN and filtered feedthrough panel.
Load Simulators
Load simulators replicate the electrical loads that the DUT controls in the vehicle. These may include resistive loads (for lamp drivers), inductive loads (for motor controllers), or electronic loads with specific impedance characteristics. The load simulator design must minimize its own electromagnetic emissions to avoid contaminating the DUT measurement.
Common Setup Errors to Avoid
- Harness routed too close to absorber walls: Can couple energy into the absorber and produce artificially low readings.
- Ground plane bonds missing or corroded: Poor grounding creates common-mode noise paths that affect measurement validity.
- LISN not bonded to ground plane: Introduces impedance in the measurement reference, degrading conducted emission accuracy.
- Antenna distance incorrect: Even small deviations from the 1 m measurement distance produce significant errors due to the near-field measurement geometry.
- External equipment noise leaking into chamber: Power supplies, load simulators, and control PCs must be properly filtered or placed outside the chamber.
How TESTUPS Can Help
TESTUPS supplies complete CISPR 25 test systems, including EMI receivers, calibrated LISNs, antenna sets, ground planes, harness fixtures, and load simulators. We also provide turnkey CISPR 25 anechoic chamber installations with integrated test systems ready for accreditation. Our application engineers can review your test setup, optimize harness configurations, and provide training on CISPR 25 measurement techniques. For laboratories seeking accreditation support, TESTUPS offers gap analysis and technical consultation services.
Contact TESTUPS to discuss your CISPR 25 test equipment needs or request a system quotation.
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