Understanding the Unintended Antenna Behavior of a Product

Transcription

1 Understanding the Unintended Antenna Behavior of a Product Colin E. Brench Southwest Research Institute Electromagnetic Compatibility Research and Testing colin.brench@swri.org

2 Radiating System Source of RF energy Radiator Coupling Updated April 2008 Southwest Research institute EMC Research and Testing 2

3 Source Properties Current loop Potential difference Impedance Updated April 2008 Southwest Research institute EMC Research and Testing 3

4 Basic Antenna Structures Slot antennas Seams Unused connectors Monopole and dipole antennas Interface cables Other conductors Loop antennas Cables Other conductors Updated April 2008 Southwest Research institute EMC Research and Testing 4

5 Typical Unwanted Slot Antennas Gaps in an EMI shield Splits or void areas in a plane (power or return) Updated April 2008 Southwest Research institute EMC Research and Testing 5

6 Slot Antenna - Diverted Current Sheet Updated April 2008 Southwest Research institute EMC Research and Testing 6

7 Typical Unwanted Loop Antennas Etch route to decoupling capacitor Terminations with shared return Within a large VLSI device Poorly implemented return path Updated April 2008 Southwest Research institute EMC Research and Testing 7

8 Loop Antenna - defined current path Updated April 2008 Southwest Research institute EMC Research and Testing 8

9 Heat pipes or sinks Power wiring Interface wiring Typical Unwanted Dipole Antennas Updated April 2008 Southwest Research institute EMC Research and Testing 9

10 Dipole Antenna - RF potential exists between two conductors Heatsink VLSI Device Printed circuit card Updated April 2008 Southwest Research institute EMC Research and Testing 10

11 PDA Antenna Example Ear phone / mic PDA Power cord Updated April 2008 Southwest Research institute EMC Research and Testing 11

12 Coupling Mechanisms Close and small Directly coupled Inductance Capacitance Small and remote Uncoupled Point source Current loop Current element Large and very close Tightly coupled Distributed inductance Distributed capacitance Complex EM coupling Resistive Common paths Intentional and unintentional chassis to logic return connections Updated April 2008 Southwest Research institute EMC Research and Testing 12

13 Source Examples Current loops Multi-point chassis to logic Cable shield currents Voltage potentials Between VLSI device and heat-sink Between mother and daughter boards Common impedance Updated April 2008 Southwest Research institute EMC Research and Testing 13

14 Radiator Properties Terminal Impedance Radiation resistance represents energy radiated Terminal reactance the energy stored in the non radiating fields Updated April 2008 Southwest Research institute EMC Research and Testing 14

15 External Conductor Example Rack mount sub systems from a variety of vendors were mounted in a rack All sub systems were compliant alone Total system emissions were marginal at high frequencies Total system emission profile was changed when the doors were closed Updated April 2008 Southwest Research institute EMC Research and Testing 15

16 Level (dbµv/m) 60 Product Emissions with Doors Open G Frequency (MHz) Updated April 2008 Southwest Research institute EMC Research and Testing 16

17 Level (dbµv/m) 60 Product Emissions with Doors Closed G Frequency (MHz) Updated April 2008 Southwest Research institute EMC Research and Testing 17

18 Level (dbµv/m) 60 Product Emissions with Doors Closed Increased Emissions Decreased Emissions G Frequency (MHz) Updated April 2008 Southwest Research institute EMC Research and Testing 18

19 Computational Model Shield with apertures Observation points Arbitary Conductor Source Arbitary Conductor Updated April 2008 Southwest Research institute EMC Research and Testing 19

20 Shield Performance With no Extra Conductors Field Strength (dbuv/m) No Shield Shield Only Frequency (MHz) Updated April 2008 Southwest Research institute EMC Research and Testing 20

21 Shield Performance with Internal Conductor Field Strength (dbu/v/m) Frequency (MHz) Updated April 2008 Southwest Research institute EMC Research and Testing 21

22 Shield Performance with an External Conductor No Shield Shield + External Conductor 80 Field Strength (dbuv/m) Frequency (MHz) Updated April 2008 Southwest Research institute EMC Research and Testing 22

23 Shield Performance with Both Conductors 110 No Shield Shield + Both Conductors Field Strength (dbuv/m) Frequency (MHz) Updated April 2008 Southwest Research institute EMC Research and Testing 23

24 Adding Details Refined source model includes direct coupling between source and shield or external conductor Imperfections induce some cross polarization More complex external structures Updated April 2008 Southwest Research institute EMC Research and Testing 24

25 Multi-Wire FDTD Model Updated April 2008 Southwest Research institute EMC Research and Testing 25

26 Animated Field Plots Time domain view of electric fields propagating through an aperture Electric fields propagating in the presence of external conductors Slots in infinite plane Slots in a real enclosure Grounded external wire Isolated external wire Updated April 2008 Southwest Research institute EMC Research and Testing 26

27 Field Strength with External Wires Updated April 2008 Southwest Research institute EMC Research and Testing 27

28 Product Realities Antennas happen! Result from unintentional discontinuities in a current path and RF potentials between conductors Cannot be completely avoided Design requires a balance of minimizing: RF energy source Coupling Antenna size and geometry Updated April 2008 Southwest Research institute EMC Research and Testing 28

29 Measurement Antenna Example 30MHz half wave dipole Terminal impedance Ideal As used on site 4m horrizontally polarized 1m vertically polarized With feed cable present Field distribution Updated April 2008 Southwest Research institute EMC Research and Testing 29

30 30 MHz Dipole Impedance for Different Environments Condition Resistive Value Reactive Value Mismatch Loss (ohm) (ohm) (db) Free j Space Horizontal j Vertical j April 2008 Southwest Research institute EMC Research and Testing 30

31 Dipole With Feed Cable Even with a perfect balun current is still coupled on to the copolarized Feeder. This unbalances the antenna and acts a secondary radiator affecting gain and the antenna impedance i When the spacing varies so does the antenna behavior Updated April 2008 Southwest Research institute EMC Research and Testing 31

32 30 MHz Vertical Dipole Impedance for different feed locations Feed Location Resistance Reactance Mismatch Loss (m) (ohm) (ohm) (db) Antenna Alone j j j j j j j j j j j j Updated April 2008 Southwest Research institute EMC Research and Testing 32

33 30MHz Dipole Impedance Feed Cable Position Updated April 2008 Southwest Research institute EMC Research and Testing 33

34 Mismatch Loss Feed Cable Position Updated April 2008 Southwest Research institute EMC Research and Testing 34

35 Field Variation 30MHz Vertically Polarized Dipole 6 5 Height (m) Field Strength (db) Updated April 2008 Southwest Research institute EMC Research and Testing 35

36 Product Antenna Summary It is important to recognize and separate the antenna effects from the coupling effects It is important to identify the true source Confusion between the source the coupling mechanisms and the radiator can cause an engineer to chase phantoms during EMI failure analysis Updated April 2008 Southwest Research institute EMC Research and Testing 36