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CISPR 16-2-2: Measurement of Disturbance Power
Methods for measuring disturbance power from equipment leads using absorbing clamps
1. Scope and Measurement Concept
CISPR 16-2-2 specifies the methods for measuring the disturbance power generated by equipment on its connecting leads in the frequency range of 30 MHz to 1 GHz. The disturbance power method is an alternative to radiated emission measurements for equipment where the connecting cables are the primary radiation mechanism. The standard provides detailed procedures for positioning the absorbing clamp, scanning for maximum readings, and interpreting the measurement data.
The measurement concept is based on the fact that at frequencies above 30 MHz, the equipment’s connecting cables become electrically long and act as efficient antennas. The disturbance power flowing along the cable is directly related to the radiated emission from the cable. By measuring this power with an absorbing clamp that presents a defined impedance to the cable, the measurement correlates well with the radiated field strength that would be measured at a test site.
The disturbance power method is particularly cost-effective for manufacturers of household appliances, power tools, and lighting equipment. Instead of investing in a full anechoic chamber (costing $100,000–$500,000), a disturbance power measurement setup with an absorbing clamp and an EMI receiver can achieve equivalent results for equipment with dominant cable radiation at a fraction of the cost.
2. Measurement Setup and Procedure
The standard specifies the detailed measurement setup. The EUT is placed on a non-conductive table at 0.8 m height above the reference ground plane. The connecting cable is laid out horizontally in a straight line for a minimum distance of 6 m from the EUT. The absorbing clamp is positioned around the cable and moved along it to find the position of maximum disturbance power indication.
| Frequency Range | Clamp Positioning Increment | Minimum Cable Length | Measurement Distance (EUT to closest clamp position) |
|---|---|---|---|
| 30 – 100 MHz | 5 cm | 6 m | 0.2 m |
| 100 – 300 MHz | 2 cm | 4 m | 0.15 m |
| 300 – 1000 MHz | 1 cm | 2 m | 0.1 m |
The measurement procedure involves: (1) setting the EMI receiver to the measurement frequency with the appropriate bandwidth (120 kHz for 30–1000 MHz), (2) positioning the absorbing clamp at the starting position, (3) measuring the disturbance power with both QP and AV detectors, (4) moving the clamp by the specified increment and repeating the measurement, and (5) continuing until the entire cable length has been scanned or until the maximum position has been passed. The maximum reading across all clamp positions is recorded as the disturbance power at that frequency.
The absorbing clamp measurement is inherently slow because only one frequency can be measured at a time, and the clamp must be physically moved along the cable for each frequency point. A full frequency scan from 30 MHz to 300 MHz with 5 cm increments at 120 kHz bandwidth can take 2–4 hours. Modern FFT-based EMI receivers can reduce this time by simultaneously measuring multiple frequencies, but the clamp positioning remains the dominant time factor.
3. Data Analysis and Interpretation
The disturbance power P is typically expressed in dBpW (decibels relative to 1 picowatt). The measured value at the receiver input (P_meas) is corrected for the absorbing clamp’s transfer impedance and cable losses: P_corrected = P_meas + L_cable — Z_t(dB), where L_cable is the cable loss between the clamp RF output and the receiver input, and Z_t(dB) is the transfer impedance of the absorbing clamp in dBΩ.
The standard provides the correlation between disturbance power and radiated field strength for the typical measurement geometry. For a 3 m measurement distance, a disturbance power of P dBpW corresponds approximately to a radiated field strength of E = P — 20log₁₀(f_MHz) + 44 dBµV/m. This correlation enables comparison with radiated emission limits using the disturbance power method.
For efficient measurement, the standard recommends a two-step approach: (1) perform a fast prescan with peak detection using a current probe at a fixed position (λ/4 from the EUT) to identify critical frequencies, and (2) perform full QP measurements at only those critical frequencies, moving the absorbing clamp to find the exact maximum. This approach reduces total test time by 60–80% while maintaining measurement accuracy.
4. Frequently Asked Questions
Q: What is the advantage of disturbance power measurement over radiated emission measurement?
A: The main advantages are: (1) significantly lower facility cost (no anechoic chamber required), (2) better reproducibility because the measurement is less sensitive to the test environment, and (3) direct identification of cable-related emissions, which simplifies troubleshooting.
A: The main advantages are: (1) significantly lower facility cost (no anechoic chamber required), (2) better reproducibility because the measurement is less sensitive to the test environment, and (3) direct identification of cable-related emissions, which simplifies troubleshooting.
Q: How do I handle equipment with multiple connecting cables?
A: Each cable must be measured separately. While measuring one cable, the other cables must be terminated with their characteristic impedance (50 Ω for coaxial cables, 100–150 Ω for twisted-pair, or the specified impedance in the product standard). The maximum disturbance power among all cable measurements is taken as the equipment’s emission level.
A: Each cable must be measured separately. While measuring one cable, the other cables must be terminated with their characteristic impedance (50 Ω for coaxial cables, 100–150 Ω for twisted-pair, or the specified impedance in the product standard). The maximum disturbance power among all cable measurements is taken as the equipment’s emission level.
Q: What is the minimum cable length required for disturbance power measurement?
A: The minimum length depends on the lowest measurement frequency. For 30 MHz, the cable should be at least λ/2 = 5 m. A 6 m cable is recommended to ensure at least one full standing-wave pattern exists on the cable, allowing the absorbing clamp to find a true maximum position.
A: The minimum length depends on the lowest measurement frequency. For 30 MHz, the cable should be at least λ/2 = 5 m. A 6 m cable is recommended to ensure at least one full standing-wave pattern exists on the cable, allowing the absorbing clamp to find a true maximum position.
Q: Can disturbance power limits be applied to equipment with shielded cables?
A: Yes, but the disturbance power on shielded cables is typically much lower than on unshielded cables because the shield provides common-mode current attenuation. The measurement is performed on the outside of the shield, with the absorbing clamp positioned around the shielded cable.
A: Yes, but the disturbance power on shielded cables is typically much lower than on unshielded cables because the shield provides common-mode current attenuation. The measurement is performed on the outside of the shield, with the absorbing clamp positioned around the shielded cable.
