1. CORRELATION BETWEEN
RADIATED IMMUNITY
AND
COUPLING ATTENUATION
Michiel Pelt

2. 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

3. 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
Under discussion in ISO / IEC JTC1 SC25 WG 3 and CLC TC 215 WG 1

4. 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.

5. 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
fixed position antenna and device

6. Coupling attenuation
Coupling attenuation

7. Definition coupling attenuation
ac = 10 LOG10 ( P )
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

8. Limits of coupling attenuation

9. 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.

10. 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.

11. 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.

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

13. Dimensions cabling set-up
Radiated immunity
Dimensions cabling set-up

14. 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)

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

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

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

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

19. Average field strength
Radiated immunity
Average field strength

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

21. Radiated immunity
Coaxial cable
-35 dBm

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

23. 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

24. 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

25. Unshielded twisted pair cable
Correlation
Unshielded twisted pair cable

26. Correlation
Coaxial cable

27. Dual foil twisted pair cable
Correlation
Dual foil twisted pair cable

28. 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

29. 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
...