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CISPR 16-2-1: Conducted Disturbance Measurement Methods
Standard methods for measuring conducted EMI at power and signal ports
1. Scope and Measurement Principles
CISPR 16-2-1 specifies the methods for measuring conducted radio disturbance at the power terminals and signal/control ports of equipment under test. It covers the frequency range of 9 kHz to 30 MHz, where the dominant coupling mechanism is via the equipment’s connecting cables acting as transmission lines. The standard applies to all types of equipment that connect to mains power or signal networks and provides the detailed procedures for obtaining reproducible conducted emission measurements.
The fundamental principle of conducted emission measurement is that the disturbance signal generated by the EUT propagates along the connecting cable as a combination of common-mode (CM) and differential-mode (DM) currents. The LISN provides a defined impedance to both modes and separates the total disturbance into measurable components. The measurement instrument (EMI receiver or spectrum analyzer) then measures the voltage developed across the 50 Ω termination resistor of the LISN’s RF output port.
Understanding the ratio of common-mode to differential-mode noise at the measurement port can significantly streamline troubleshooting. Generally, DM noise dominates below 1 MHz and is addressed by X-capacitors and DM chokes, while CM noise dominates above 1 MHz and requires Y-capacitors and CM chokes. A simple current probe measurement on the LISN ground lead can quickly identify which mode dominates.
2. Measurement Procedures and Configurations
CISPR 16-2-1 defines the specific test setup requirements. The EUT is placed on a non-conductive table 0.8 m above a reference ground plane. The ground plane must extend at least 0.5 m beyond the EUT boundaries. The EUT is configured to operate in its worst-case emission mode (typically the highest power or highest processing load condition). For conducted measurements at mains ports, the disturbance voltage is measured between each phase conductor and ground, and between neutral and ground.
| Measurement Port | Coupling Device | Measurement Point | Frequency Range |
|---|---|---|---|
| AC mains (single-phase) | LISN (50 µH/50 Ω) | Phase-to-ground, neutral-to-ground | 150 kHz – 30 MHz |
| AC mains (three-phase) | LISN per phase | Each phase to ground, neutral to ground | 150 kHz – 30 MHz |
| DC power ports | LISN (5 µH/50 Ω) | Positive to ground, negative to ground | 150 kHz – 30 MHz |
| Signal/control ports | Capacitive voltage probe (CVP) or current probe | Each conductor to ground | 150 kHz – 30 MHz |
| Telecom/network ports | Impedance stabilization network (ISN) | Per applicable port specification | 150 kHz – 30 MHz |
The standard specifies that the measurement should be performed with both quasi-peak (QP) and average (AV) detectors. The QP measurement captures the subjective annoyance of the interference, while the AV measurement provides additional information about the nature of the noise. If the QP reading is within 2 dB of the QP limit and the AV reading is below the AV limit for the same frequency, the measurement is considered compliant.
A common source of measurement error in conducted emission testing is the placement of excess cable length. CISPR 16-2-1 specifies that excess mains cable should be arranged in a non-inductive bundle (figure-8 pattern) or cut to the minimum required length. Loosely coiled cables create inductive coupling that can add or subtract from the measured disturbance voltage by 3–6 dB at frequencies above 5 MHz.
3. Special Measurement Conditions
The standard addresses several special measurement conditions. For floor-standing equipment, the EUT is placed directly on the reference ground plane with insulation, and the measurement setup must account for the increased capacitive coupling to ground. For battery-operated equipment, conducted measurements apply only when the equipment is connected to a battery charger or external power supply. For modular systems, each module that can operate independently must be tested both individually and as part of the complete system.
The standard also specifies measurement procedures for equipment with wireless transmission capabilities. During conducted emission measurements, the wireless transmitter must be operating in continuous transmission mode at maximum power to capture the combined emissions from both the power supply and the RF power amplifier stages.
For difficult-to-measure devices such as high-power variable-frequency drives (VFDs) and switching power supplies above 5 kW, the standard allows the use of a current probe as an alternative to the LISN for conducted emission measurements. The current probe is clamped around the power lead, and the measured current is compared to the equivalent current limit derived from the voltage limit and the LISN impedance (I_limit = V_limit / 50 Ω). This technique avoids the LISN saturation issues encountered with high-current equipment.
4. Frequently Asked Questions
Q: Why do conducted emission measurements use both QP and AV detectors?
A: The ratio of QP to AV readings provides information about the nature of the interference. A QP/AV ratio close to 1 indicates continuous narrowband noise (e.g., clock harmonics), while a QP/AV ratio > 6 dB indicates impulsive broadband noise (e.g., motor commutator arcing). This helps identify the noise source for troubleshooting.
A: The ratio of QP to AV readings provides information about the nature of the interference. A QP/AV ratio close to 1 indicates continuous narrowband noise (e.g., clock harmonics), while a QP/AV ratio > 6 dB indicates impulsive broadband noise (e.g., motor commutator arcing). This helps identify the noise source for troubleshooting.
Q: How long should the EUT operate before measurement?
A: The EUT should be operated until it reaches a stable operating temperature (typically 15–30 minutes for small appliances, up to 2 hours for large equipment). Thermal drift can affect switching frequency, component characteristics, and emission levels.
A: The EUT should be operated until it reaches a stable operating temperature (typically 15–30 minutes for small appliances, up to 2 hours for large equipment). Thermal drift can affect switching frequency, component characteristics, and emission levels.
Q: What is the difference between a CISPR 16-2-1 measurement and a pre-compliance scan?
A: Pre-compliance scans typically use peak detection at faster sweep rates and may not use a LISN. Full CISPR 16-2-1 compliance measurements require the specified LISN, proper QP and AV detection with the correct dwell times, and a calibrated measurement setup. Pre-compliance scans may underestimate the actual QP reading by 2–10 dB depending on the signal type.
A: Pre-compliance scans typically use peak detection at faster sweep rates and may not use a LISN. Full CISPR 16-2-1 compliance measurements require the specified LISN, proper QP and AV detection with the correct dwell times, and a calibrated measurement setup. Pre-compliance scans may underestimate the actual QP reading by 2–10 dB depending on the signal type.
