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CISPR 16-1-2: Specification for Radio Disturbance and Immunity Measuring Apparatus — Coupling Devices for Conducted and Radiated Disturbance Measurement
LISNs, current probes, CDNs, and coupling networks for EMC testing
1. Scope and Overview of Coupling Devices
CISPR 16-1-2 specifies the characteristics and calibration requirements for coupling devices used in conducted and radiated disturbance measurements. These devices provide a defined impedance for the measurement of disturbance signals while isolating the equipment under test (EUT) from the power mains or communication networks. The standard covers line impedance stabilization networks (LISN), current probes, coupling/decoupling networks (CDN) for conducted RF immunity testing, capacitive voltage probes, and absorbing clamps.
The primary function of these coupling devices is to establish reproducible measurement conditions. Without standardized coupling devices, the impedance presented to the EUT’s power port would vary with the mains wiring impedance, making measurements inconsistent between laboratories. The LISN provides a stable impedance of 50 µH || 50 Ω (or 5 µH || 50 Ω for higher current ratings) across the 150 kHz to 30 MHz frequency range.
The 50 µH || 50 Ω LISN impedance was chosen based on extensive studies of typical mains wiring impedance in European and North American installations. For DC-powered equipment, a 5 µH LISN is typically used, reflecting the lower inductance of DC supply wiring. Always verify that the LISN type matches your EUT’s power configuration and current rating.
2. LISN Characteristics and Specifications
The LISN is the most widely used coupling device in EMC testing. It serves three functions: providing a stable RF impedance to the EUT power port, isolating the EUT from the mains power source at RF frequencies, and presenting the disturbance signal to the measuring receiver via a 50 Ω output port.
| Parameter | Specification for 50 µH/50 Ω LISN | Specification for 5 µH/50 Ω LISN |
|---|---|---|
| Frequency range | 150 kHz – 30 MHz | 150 kHz – 30 MHz |
| Impedance (EUT port) | 50 µH || 50 Ω ± 20% | 5 µH || 50 Ω ± 20% |
| RF output impedance | 50 Ω | 50 Ω |
| Isolation (mains to RF port) | > 20 dB | > 20 dB |
| Max continuous current | 16 A (typical) | 100 A (typical) |
| Max voltage rating | 250 V AC / 400 V DC | 250 V AC / 400 V DC |
The standard also specifies the LISN’s phase characteristics and the requirement that the phase shift between the EUT port and the RF output port does not exceed ±20° across the frequency range. This phase requirement is critical for accurate measurements of reactive disturbance sources.
A frequently overlooked issue is LISN saturation when testing high-power equipment with high inrush currents or significant harmonic content. The LISN inductors can saturate under high DC current bias (from half-wave rectification) or during transient inrush events, causing the impedance to deviate from the specified characteristic. For equipment with high peak currents, a 5 µH LISN should be used instead of the 50 µH type.
3. Other Coupling Devices
Current probes are used for non-invasive measurement of conducted common-mode currents on cables and wire bundles. CISPR 16-1-2 specifies the transfer impedance (Zt) characteristics of current probes, typically 1–10 Ω in the 150 kHz to 1000 MHz range. The probe’s bore diameter must accommodate the cable bundle under test without compression.
Coupling/decoupling networks (CDNs) are used for conducted RF immunity testing per IEC 61000-4-6. They inject RF disturbance signals onto power, signal, or communication lines while maintaining the functional signal integrity. The CDN must present a defined common-mode impedance of 150 Ω at the EUT port across the 150 kHz to 80 MHz (extended up to 230 MHz) frequency range.
Absorbing clamps are used for measuring disturbance power on leads in the 30 MHz to 1 GHz range. The clamp consists of a combination of current transformers and ferrite absorbers that measure the power flowing along the lead. CISPR 16-1-2 specifies the insertion loss and directivity characteristics of approved absorbing clamp designs.
For pre-compliance testing, a DIY LISN can be constructed using a 250 µH inductor, a 0.1 µF capacitor, and a 50 Ω termination resistor, but the impedance tolerance (±20%) and phase characteristics (±20°) must be verified with a vector network analyzer. Commercial LISNs include calibration data traceable to national standards, which is essential for formal compliance testing.
4. Frequently Asked Questions
Q: Can I use a single LISN for both single-phase and three-phase equipment?
A: Yes, but each phase of a three-phase supply requires its own LISN, and the LISN must be rated for the line-to-line voltage of the three-phase system. For 400 V three-phase systems, LISNs rated for 400 V AC (line-to-neutral) must be used.
A: Yes, but each phase of a three-phase supply requires its own LISN, and the LISN must be rated for the line-to-line voltage of the three-phase system. For 400 V three-phase systems, LISNs rated for 400 V AC (line-to-neutral) must be used.
Q: What maintenance does a LISN require?
A: Regular calibration verification (annually minimum), inspection of the high-current relay contacts for pitting, and verification of the 50 Ω termination resistor accuracy. The LISN’s internal capacitors should be checked for degradation every 2–3 years.
A: Regular calibration verification (annually minimum), inspection of the high-current relay contacts for pitting, and verification of the 50 Ω termination resistor accuracy. The LISN’s internal capacitors should be checked for degradation every 2–3 years.
Q: Do I need different LISNs for different frequency bands?
A: The standard LISN covers 150 kHz to 30 MHz. For measurements below 150 kHz (9–150 kHz), a separate LISN with 200 Hz bandwidth filters and appropriately scaled impedance components is required as specified in CISPR 16-1-2 Annex A.
A: The standard LISN covers 150 kHz to 30 MHz. For measurements below 150 kHz (9–150 kHz), a separate LISN with 200 Hz bandwidth filters and appropriately scaled impedance components is required as specified in CISPR 16-1-2 Annex A.
Q: What is the difference between a V-network and a LISN?
A: In CISPR terminology, a V-network is the specific coupling circuit that presents a defined impedance (usually 50 µH || 50 Ω) at the EUT power port. The term LISN (Line Impedance Stabilization Network) is the practical implementation of a V-network in a laboratory instrument. The terms are often used interchangeably.
A: In CISPR terminology, a V-network is the specific coupling circuit that presents a defined impedance (usually 50 µH || 50 Ω) at the EUT power port. The term LISN (Line Impedance Stabilization Network) is the practical implementation of a V-network in a laboratory instrument. The terms are often used interchangeably.
