Determining The Size Of Cabinet Apertures For Effectively Mitigating Radiated Emissions. By David Norte Thursday, April 7 th, 2005

Transcription

1 The EMC, Signal And Power Integrity Institute Presents Determining The Size Of Cabinet Apertures For Effectively Mitigating Radiated Emissions By David Norte Thursday, April 7 th,

2 Motivation For This Research Cabinet apertures are required for cooling the internal electronics of any electrical system. As the size of the apertures increases, the thermal performance of the system also improves, however, at the expense of shielding effectiveness against radiated electromagnetic interference (EMI). In this case, it is of interest to understand the link between the size of cabinet apertures and their shielding effectiveness. This research addresses this issue with respect to studying the radiated emissions from an enclosed box with a single aperture. The size of this aperture is varied and the resultant radiated emissions leakage is measured with the use of field probes. A point source at a given frequency and with a 1-volt amplitude provides the excitation within the box at its central location. Since it is expected that the highest electrical frequencies will produce the most EMI, this research is targeted at a 200ps risetime frequency of 2.5GHz. This risetime component is routinely encountered in most very high-speed electrical systems. In addition to studying a single aperture, an array of identical apertures is also investigated in order to further our understanding of how arrays of apertures affect radiated EMI. 2

3 Analysis Setup The red borders indicate the metallic perimeters of the enclosure. At the center of the enclosure resides the point source that provides the excitation. The opening at the right side of this enclosure is the aperture. Immediately to the left and to the right of this aperture are two field probes that are used to measure the radiated EMI. Point source at 2.5GHz, 1volt amplitude Aperture with two field probes Aperture size 1.25cm 3

4 Baseline Point Source Radiation Without The Enclosure Note the uniform radiation. 4

5 Time Domain Radiated Fields Detected By The Two Probes Note the slight phase difference between the two probe signals and the sinewave shapes. The peak-to-peak radiated electric field is about 3.96(10-2 ) volts/meter at the Xo probe. 5

6 Consider The Radiated Emissions Without An Aperture 6

7 Example Radiated Emissions, At A Given Point In Time, With No Aperture 7

8 Time Domain Radiated Emissions At The Two Field Probes Note the deviation from the sinewave nature of the detected radiation. The peak-to-peak radiated electric field is about 1.6(10-2) volts/meter at the Xo probe. 8

9 Consider The Radiated Emissions From A 3.75cm Aperture 3.75cm 9

10 Example Radiated Emissions, At A Given Point In Time, With A 3.75cm Aperture Field Leakage 10

11 Time Domain Radiated Emissions At The Two Field Probes The peak-to-peak radiated electric field is about 4.0(10-2) volts/meter at the Xo probe and is 3.5dB (33%) lower at the X1 probe. 11

12 Consider The Radiated Emissions From A 2.5cm Aperture 2.5cm 12

13 Example Example Radiated Radiated Emissions Emissions, WithAtA A2.5cm GivenAperture Point InAt Time, A Given WithPoint A 2.5cm In Time Aperture Field Leakage 13

14 Time Domain Radiated Emissions At The Two Field Probes The peak-to-peak radiated electric field is about 3.04(10-2) volts/meter at the Xo probe and is still about 3.5dB (33%) lower at the X1 probe. 14

15 Consider The Radiated Emissions From A 1.93cm Aperture 1.93cm 15

16 Example Radiated Emissions, At A Given Point In Time, With A 1.93cm Aperture 16

17 Time Domain Radiated Emissions At The Two Field Probes The peak-to-peak radiated electric field is about 2.8(10-2) volts/meter at the Xo probe and is about 4.1dB (37.6%) lower at the X1 probe. 17

18 Consider The Radiated Emissions From A 1.25cm Aperture 1.25cm 18

19 Example Radiated Emissions, At A Given Point In Time, With A 1.25cm Aperture 19

20 Time Domain Radiated Emissions At The Two Field Probes The peak-to-peak radiated electric field is about 2.8(10-2) volts/meter at the Xo probe and is now 7.5dB (57.8%) lower at the X1 probe. 20

21 Consider The Radiated Emissions From A 1.0cm Aperture 1.0cm 21

22 Example Example Radiated Radiated Emissions Emissions, WithAtAA1.0cm GivenAperture Point InAt Time, A Given WithPoint A 1.0cm In Time Aperture 22

23 Time Domain Radiated Emissions At The Two Field Probes The peak-to-peak radiated electric field is about 2.8(10-2) volts/meter at the Xo probe and is now 10.5dB (70.1%) lower at the X1 probe. 23

24 In all previous cases with a single aperture, the shape of the original sinewave excitation at the point source was not maintained at either probes. 24

25 db Reduction In EMI vs. Aperture Size It appears that apertures with dimensions on the order of 1cm provide significant mitigation against radiated EMI. Also, note the nonlinear dependence of the shielding effectives on the aperture size. 25

26 % Reduction In EMI vs. Aperture Size 26

27 Consider An Array Of Seven 1-cm Apertures 27

28 Consider The Radiated Emissions From An Array Of Seven 1.0cm Apertures 1.0cm 28

29 Example Radiated Emissions, At A Given Point In Time, With An Array Of Seven 1.0cm Apertures Nearly Uniform Phase Front 29

30 Example Radiated Emissions With An Array Of Seven 1.0cm Apertures At A Different Point In Time 30

31 Time Domain Radiated Emissions At The Xo And X1 Field Probes The peak-to-peak radiated electric field is about 2.2(10-2) volts/meter at the Xo probe and is now 8.0dB (60%) lower at the X1 probe. 31

32 Time Domain Radiated Emissions At All Probes Inside Of The Enclosure Peak-to-Peak EMI = 2.25e-2 volts/meter 32

33 Time Domain Radiated Emissions At All Probes Outside Of The Enclosure Peak-to-Peak EMI = 8.84e-3 volts/meter (8.1dB [60%] decrease relative to the collection of inner probes) 33

34 The array of seven 1-cm apertures produced significant mitigation against radiated EMI on the order of 8.1dB (60%). It is noted that a loss of about 2.4dB (24%) in shielding effectiveness was obtained relative the case of a single 1-cm aperture. 34

35 Consider the case of a fewer number (4) of larger apertures (1.75cm) such that the total linear dimension is maintained at 7.0cm. 35

36 Consider The Radiated Emissions From An Array Of Four 1.75cm Apertures 36

37 Example Radiated Emissions, With An Array Of Four 1.75cm Apertures, At A Given Point In Time 37

38 A Temporal Display Of The Radiated Emissions With Four 1.75cm Apertures T1 T2 T3 T4 T5 T6 38

39 Time Domain Radiated Emissions At All Probes Inside Of The Enclosure Peak-to-Peak EMI = 3.05e-2 volts/meter 39

40 Time Domain Radiated Emissions At All Probes Outside Of The Enclosure An additional 6.2dB (51%) of peak-to-peak radiation occurs relative to the case of seven 1-cm apertures. Peak-to-Peak EMI = 1.81e-2 volts/meter (4.5dB [40%] decrease relative to the collection of inner probes) 40

41 For mitigating EMI, it is better to choose a larger number of smaller apertures than a smaller number of larger apertures. 41