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CISPR 16-1-3: Specification for Radio Disturbance and Immunity Measuring Apparatus — Disturbance Power Measurement
Absorbing clamp methods for measuring disturbance power from equipment leads
1. Scope and Principle of Disturbance Power Measurement
CISPR 16-1-3 specifies the requirements for measuring the disturbance power generated by equipment, particularly from the connecting leads (mains cables, signal cables, and control cables) in the frequency range of 30 MHz to 1 GHz. The disturbance power method was developed as an alternative to radiated emission measurements for equipment where the primary radiation mechanism is the connected cables acting as unintentional antennas.
The principle is based on the fact that at frequencies above 30 MHz, the electrical length of typical equipment connecting cables exceeds λ/10, making them effective radiators. By measuring the net power flowing along the cable (the disturbance power), the standard provides a measurement that correlates well with the radiated emission from the cable. The absorbing clamp converts cable-borne common-mode currents into a power measurement by absorbing the RF energy traveling along the cable.
Disturbance power measurement is particularly valuable for equipment with long connecting cables such as household appliances, power tools, and lighting equipment. The method eliminates the need for a full anechoic chamber or open-area test site for frequencies up to 300 MHz, significantly reducing compliance testing costs for smaller manufacturers.
2. Absorbing Clamp Specifications
The absorbing clamp consists of a current transformer, a series of ferrite rings, and decoupling ferrite elements that together provide a defined transfer impedance and absorption characteristic. CISPR 16-1-3 specifies that the absorbing clamp must have a directivity of at least 10 dB across the frequency range, ensuring that power flowing toward the EUT (reflected from the mains) is attenuated relative to the power flowing from the EUT.
| Frequency Range | Transfer Impedance (Zt) — Typical | Minimum Directivity | Insertion Loss |
|---|---|---|---|
| 30 – 100 MHz | 5 – 15 Ω | 10 dB | < 3 dB |
| 100 – 300 MHz | 10 – 20 Ω | 10 dB | < 2 dB |
| 300 – 1000 MHz | 15 – 25 Ω | 8 dB | < 3 dB |
The absorbing clamp must be positioned at specific distances from the equipment under test. The measurement is performed by sliding the clamp along the cable to find the position of maximum indicated disturbance power (the “antenna current maximum” position), which occurs at distances corresponding to multiples of λ/4 from the EUT for the cable acting as a standing-wave antenna.
The positioning of the absorbing clamp is critical and is a common source of measurement uncertainty. The clamp must be moved in small increments (typically 5 cm at 30 MHz, decreasing to 2 cm at 300 MHz) along the cable to find the true maximum. With improper positioning, disturbance power readings can be underestimated by 6–10 dB. Automated clamp positioning systems are strongly recommended for reproducible results.
3. Measurement Procedure and Calibration
The disturbance power measurement procedure involves placing the EUT on a non-conductive table at a height of 0.8 m above the reference ground plane. The connecting cable is routed horizontally for a minimum distance of 6 m, with the absorbing clamp positioned along the cable. The measurement is performed with both quasi-peak and average detection, and the maximum reading over all clamp positions is recorded.
Calibration of the absorbing clamp requires a vector network analyzer (VNA) and a calibration fixture that provides a known common-mode current. The transfer impedance is derived from S-parameter measurements. The standard specifies that the calibration uncertainty should be less than 2 dB (k=2). Regular verification using a reference current source is recommended between full calibrations.
For pre-compliance testing, a simplified approach using a current probe and a spectrum analyzer with a 6 dB attenuator can provide approximate disturbance power measurements. Position the current probe at λ/4 from the EUT (where λ is the wavelength at the measurement frequency) and read the common-mode current. The disturbance power can be estimated as P = I² × R_clamp, where R_clamp is the nominal resistive component of the absorbing clamp impedance (typically 100–200 Ω).
4. Frequently Asked Questions
Q: Why measure disturbance power instead of radiated emissions directly?
A: The disturbance power method is simpler, more reproducible, and requires less expensive facilities than radiated emission measurements. It is particularly suitable for equipment where the connecting leads are the primary radiation mechanism — which is the case for most household appliances and power tools below 300 MHz.
A: The disturbance power method is simpler, more reproducible, and requires less expensive facilities than radiated emission measurements. It is particularly suitable for equipment where the connecting leads are the primary radiation mechanism — which is the case for most household appliances and power tools below 300 MHz.
Q: Can disturbance power measurements be correlated to radiated field strength?
A: Yes, there is a well-established correlation for the typical geometry: a disturbance power of P dBpW at the absorbing clamp corresponds approximately to a radiated field strength of E = P + 10log₁₀(120π) — 20log₁₀(λ) — 10log₁₀(R) dBµV/m at distance R. However, this correlation depends on the cable configuration and is only approximate for engineering evaluation.
A: Yes, there is a well-established correlation for the typical geometry: a disturbance power of P dBpW at the absorbing clamp corresponds approximately to a radiated field strength of E = P + 10log₁₀(120π) — 20log₁₀(λ) — 10log₁₀(R) dBµV/m at distance R. However, this correlation depends on the cable configuration and is only approximate for engineering evaluation.
Q: Is the absorbing clamp method applicable to all product categories?
A: It is most applicable to products with a single main connecting cable (mains-powered appliances). For equipment with multiple cables, the measurement must be performed on each cable individually while terminating the others with defined loads, which complicates the procedure.
A: It is most applicable to products with a single main connecting cable (mains-powered appliances). For equipment with multiple cables, the measurement must be performed on each cable individually while terminating the others with defined loads, which complicates the procedure.
Q: What are the limitations of disturbance power measurement?
A: The method is limited to frequencies where the cable is an efficient radiator (generally above 30 MHz). Below 30 MHz, cable dimensions are electrically short, and the disturbance voltage method (LISN) is more appropriate. Above 1 GHz, waveguide modes in cables become significant, complicating the power measurement.
A: The method is limited to frequencies where the cable is an efficient radiator (generally above 30 MHz). Below 30 MHz, cable dimensions are electrically short, and the disturbance voltage method (LISN) is more appropriate. Above 1 GHz, waveguide modes in cables become significant, complicating the power measurement.
