CORRELATION BETWEEN RADIATED IMMUNITY AND COUPLING ATTENUATION Michiel Pelt

Need for new test method: Introduction Need for new test method: No test methods to measure: symmetrical cables, connecting hardware / cable assemblies balance at higher frequencies combined effect of balance and screening

Introduction Coupling attenuation New and easy test method to evaluate the EMC performance of channels and components (symmetrical + screened) Standard prepared by CENELEC TC 46X / WG3: EN 50289-1-6 Under discussion in ISO / IEC JTC1 SC25 WG 3 and CLC TC 215 WG 1

A near field measurement Introduction A near field measurement In the same way the electromagnetic isolation of a channel can be measured. A signal injected on a pair will make the channel radiate. The radiated power is not captured by an adjacent pair (NEXT, FEXT) but by a current clamp outside the channel.

Simplifications Device under test: unshielded twisted pair cable Introduction Simplifications Device under test: unshielded twisted pair cable coaxial cable dual foil balanced cable Radiated immunity according to IEC 61000-4-3 fixed position antenna and device

Coupling attenuation Coupling attenuation

Definition coupling attenuation ac = 10 LOG10 ( P1 ) max [ P2n; P2f ] ac coupling attenuation P1 input power inner circuit P2n maximum near end peak power P2f maximum far end peak power A very simple mathematical relationship was found by Shannon to calculate the maximum achievable error-free data rate. Shannon’s law tells us that a relationship exists between following parameters: the maximum error-free data rate over the channel the bandwidth of the channel the signal power the noise power Given the bandwidth of the channel and the Signal-to-Noise Ratio (SNR) one can calculate the theoretical maximum data rate at which error-free digits can be transmitted over a bandwidth limited channel in the presence of noise

Limits of coupling attenuation

Unshielded twisted pair cable Coupling attenuation Unshielded twisted pair cable 54 dB First a UTP cable will be considered. Because such a cable has no screen the screening effectiveness equals zero. Its coupling attenuation relies on the balance provided by the cable. The coloured traces present the coupling attenuation of the four pairs. The straight line is the enveloping curve above which all recorded traces remain. For all pairs the coupling attenuation decreases with frequency with the same slope. This means that the coupling attenuation or electromagnetic isolation decreases with frequency. It can be observed that at 100 MHz the value of the coupling attenuation corresponds to 52 dB. Depending on the balance of a UTP cable values between 35 up 55 dB can be expected. Adding a connector may decrease the value of the coupling attenuation by up to 15 dB depending on the quality of the connector.

Coaxial cable 52 dB Coupling attenuation Next, a coaxial cable will be considered. Because such a cable is unbalanced, the balance equals zero. Its coupling attenuation relies solely on the screening effectiveness provided by the outer screen of the cable. The trace represents the measurements of the coaxial cable. The straight line is the enveloping curve above which the recorded trace remains. In contrast to what we found for the UTP cable, the coupling attenuation does not decrease with frequency. It remains fairly constant with frequency up to 1000 MHz. This is perfectly logical because the screening effectiveness remains constant with frequency. We find that at 100 MHz the value of the coupling attenuation corresponds to 52 dB. Depending on the kind of screen of the coaxial cable, values between 30 and 120 dB can be expected.

Dual foil twisted pair cable Coupling attenuation Dual foil twisted pair cable 86 dB Finally a dual foil twisted pair cable will be considered. Because such a cable is balanced and has a screen its coupling attenuation relies on the combined effect of balance and screening effectiveness. The coloured traces show the measurements of the four pairs. The straight line is the enveloping curve above which all recorded traces remain. For all pairs the coupling attenuation decreases with frequency with the same slope. This means that the coupling attenuation or electromagnetic isolation decreases with frequency. However all traces are more than 30 dB higher than those measured for the UTP cable. This because the screen around the signal conductors contributes to the coupling attenuation and the screening effectiveness does not vary with frequency. It can be observed that at 100 MHz the value of the coupling attenuation corresponds to 86 dB. Depending on the balance and the screening of a double foiled twisted pair cable values between 75 up to 95 dB can be expected. Adding a connector may decrease the value of the coupling attenuation by up to 15 dB depending on the quality of the connector.

Radiated immunity Semi-anechoic room (13.2 x 4.7 x 3.0m) Antenna - set-up: 3 m Three amplifiers, two antennas 30 - 200 MHz 200 - 500 MHz 500 - 1000 MHz Incident field: about 3 V/m (not modulated)

Dimensions cabling set-up Radiated immunity Dimensions cabling set-up

Justification for cabling set-up Radiated immunity Justification for cabling set-up Testing active equipment with cabling (proposal to CISPR G) Features cabling set-up: gain / frequency response flat polarization not critical direction not critical (worst case perpendicular to incident field)

Set-up inside semi-anechoic chamber Radiated immunity Set-up inside semi-anechoic chamber

Set-up inside semi-anechoic chamber Radiated immunity Set-up inside semi-anechoic chamber

Uniform area of cabling set-up Radiated immunity Uniform area of cabling set-up

Field uniformity according to Radiated immunity Field uniformity according to IEC 61000-4-3 16 points over 1.5 m x 1.5 m surface 75 % values within 0 - 6 dB of nominal value Field is not constant but uniform ( 83% horizontal; 89 % vertical)

Average field strength Radiated immunity Average field strength

Unshielded twisted pair cable Radiated immunity Unshielded twisted pair cable -35 dBm

Radiated immunity Coaxial cable -35 dBm

Dual foil twisted pair cable Radiated immunity Dual foil twisted pair cable -70 dBm

Conclusion 1: Relative agreement Conclusions Conclusion 1: Relative agreement Same difference between coupling attenuation and induced power for all tested cables For each test method same deviation between curves for balanced and unbalanced cables using

G() = 4 Af()  Gain of set-up G( antenna gain Correlation Gain of set-up G() = 4 Af()  G( antenna gain Af( antenna aperture  wavelength A very simple mathematical relationship was found by Shannon to calculate the maximum achievable error-free data rate. Shannon’s law tells us that a relationship exists between following parameters: the maximum error-free data rate over the channel the bandwidth of the channel the signal power the noise power Given the bandwidth of the channel and the Signal-to-Noise Ratio (SNR) one can calculate the theoretical maximum data rate at which error-free digits can be transmitted over a bandwidth limited channel in the presence of noise

Unshielded twisted pair cable Correlation Unshielded twisted pair cable

Correlation Coaxial cable

Dual foil twisted pair cable Correlation Dual foil twisted pair cable

Conclusion 2: Absolute agreement Conclusions Conclusion 2: Absolute agreement The induced noise power can be calculated using the values for coupling attenuation The calculated induced power fits the measured powers for all cables

Errors during correlation Conclusions Errors during correlation Incident field uniform but not constant Gain set-up varies with frequency Coupling attenuation approached by envelope curve ...