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EMC Fundamentals Presented By: Mike Violette Washington Laboratories, Ltd. September 14, 2007

EMC Fundamentals Presented By: Mike Violette Washington Laboratories, Ltd. September 14, 2007. http://www.wll.com. Introduction Elements of an EMI Situation. Source " Culprit " Coupling method " Path " Sensitive device " Victim ". VICTIM. SOURCE. PATH. http://www.wll.com.

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EMC Fundamentals Presented By: Mike Violette Washington Laboratories, Ltd. September 14, 2007

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  1. EMC FundamentalsPresented By:Mike VioletteWashington Laboratories, Ltd.September 14, 2007 http://www.wll.com

  2. IntroductionElements of an EMI Situation • Source "Culprit" • Coupling method "Path" • Sensitive device "Victim" VICTIM SOURCE PATH http://www.wll.com

  3. Let’s see how this all got startedDead Smart Guys • First Transmitters: Spark Devices • Heinrich Hertz (1857-1894) clarified and expanded on • James Clerk Maxwell’s Electromagnetic Theory • Marconi: first use & patent Maxwell Hertz Marconi http://www.wll.com

  4. How Does EMI Affect Electronics? • Radiated and conducted interference • Conducted Interference Enters and Exits Equipment through Wiring and Cabling • Radiated Interference Enters and Exits Equipment through Wiring and Enclosure Penetration Radiated Susceptibility Radiated Emissions Conducted Susceptibility Conducted Emissions http://www.wll.com

  5. Interference to TV Reception No Interference Two Interfering Signals Injected into TV http://www.wll.com

  6. Common “Coupling Modes” Common and Differential Mode • Crosstalk (cabling and conductors) • Field to cable (“Antenna”) • Conducted (direct) • Field to enclosure http://www.wll.com

  7. SOURCE VICTIM Crosstalk(cable-to-cable coupling) http://www.wll.com

  8. Induced Current Radiated Coupling: Field to Cable Electromagnetic Wave Loop Area Coupling proportional to: E/H Field, Loop Area, Frequency http://www.wll.com

  9. VDM VCM INoise COMMON and DIFFERENTIAL MODE • COMMON-MODE: “Line to Ground” • DIFFERENTIAL MODE: “Line-to-Line” (Normal Mode) http://www.wll.com

  10. Loop Area Induced Current Radiated Coupling: Field to Cable Radio Electromagnetic Wave Patient Monitor VCM http://www.wll.com

  11. NOISE Frequency (MHz) Frequency (Hz) Real Response Ideal Response Instrumentation Interference EKG Signal Interference Current, If http://www.wll.com

  12. AMPLITUDE MODULATION AMPLITUDE MODULATION AMPLITUDE MODULATION Interference Current, If Effect of Modulation http://www.wll.com

  13. How Does EMI Affect Electronics? • Electrostatic Discharge & Transient Pulses • ESD can induce “glitches” in circuits, leading to false triggering, errors in address & data lines and latch-up of devices • Upset • Damage • Degradation leading to future failure(s) Gee, the humidity is low in here. What’s this for? http://www.wll.com

  14. C EKG Signal Interference Current C EKG Signal Interference Current Filtering Please, I’m very ticklish http://www.wll.com

  15. Direct Indirect Surge Coupling • Lightning and pulse sources cause high-energy transients into power and data cables http://www.wll.com

  16. T A F(t) Log F f = 1/T 2f 3f Spectrum of a Trapezoidal Wave (Characteristic of Digital Devices) T f =1/ptr A t F(t) tr Log F f = 1/pt Digital Equipment SourcesFourier Analysis Spectrum of a Square Wave http://www.wll.com

  17. Equipment Emissions Limits http://www.wll.com

  18. The decibel (dB) Named after me! • The dB is used in Regulatory Limits (FCC, CISPR, etc.) • The dB is a convenient way to express very big and very small numbers • The “Bel” was named after Alexander Graham Bell Bel = LOG10(P2/P1) • deciBel provides a more realistic scale: dB = 10LOG10(P2/P1) • Voltage & Current are expressed as follows: dB (V or I) = 20LOG10(V2/V1) “20LOG” derives from the conversion from Power to Voltage (ohm’s Law: P = E2/R) http://www.wll.com

  19. dB • Can have several reference units: • Watt: dB above one Watt (dBW) • Milliwatt: dB above one milliwatt (dBm) • Volt: dBV • Microvolt: dBuV • Microamp: dBuA • picotesla: dBpT • Electric Field: dBuV/m • Radio Receiver Sensitivity ~ 10 dBuV • E-Field Limit for FCC: ~40-60 dBuV/m • Distance to moon: 107dBmile (20LOG2.5E+5miles) • National debt: 128dB$ (10LOG6E+12) http://www.wll.com

  20. Broadband Sources • Man-made noise dominates • Intended transmissions, switching transients, motors, arcing • Intermittent operation of CW causes transient effects • Digital Switching • Inductive kick • Switch bounce • Digital Signaling • Broad spectrum based on pulse width & transition time • HDTV • CDMA • UWB Technologies http://www.wll.com

  21. Pulsed SourcesFourier Analysis Fourier-> Do you like my new shirt? A f =1/ptr t F(t) Log F f = 1/pt tr Spectrum of a Pulse http://www.wll.com

  22. Urban Ambient Profile Cell phone FM Radio Switching noise http://www.wll.com

  23. Cables - Overview • Major coupling factor in radiating emissions from an equipment and coupling of emissions from other sources into an equipment • Acts as radiating “antenna”, receiving “antenna”, and cable-to-cable coupling mechanism • External cables are not typically part of the equipment design but the installation requirements must be considered during the design • Problem is a function of cable length, impedance, geometry, frequency of the signal and harmonics, current in the line, distance from cable to observation point • Frequency Effects: Tied into Cable Wavelength • For example, wavelength at FM Radio Band (100 MHz) is 1 meter (Human Body Resonance) • = c/f = 3X108/frequency • = 300/fMHz http://www.wll.com

  24. Cables - Length/Impedance • Efficiency as an antenna - function of length compared to wavelength • At typical data transfer rates - length is short • At harmonics or spurs the length may become long • Impedance mismatch creates a high SWR http://www.wll.com

  25. How very important • Frequencies of testing from 26 MHz to 1 GHz • Corresponding cable lengths: • L ~ 11 meters @ 26 MHz to 30 cm @ 1 GHz • “Short” cables can be large contributors to Interference Problems • Power cables • Grounding wires • Patient cables • Data cables • Control harnesses • Structures! http://www.wll.com

  26. E (& H) I V ~ Area Cables - Loops • Emissions are a function of 1) Current; 2) Loop Geometry; 3) Return Path of the Current • Current flow creates a magnetic field H=I/2R for a single wire model • Single wire case is not realistic • Loop geometry formed by the current carrying conductor and the return line contribute to the field strength • Electric field strength: http://www.wll.com

  27. Filters - Overview • Passband • High pass • Low pass • Single component, L, Pi, T • Common mode; differential mode • Placement • Components • Lead length • Leakage Limitations http://www.wll.com

  28. C EKG Signal C Noise Current Rejection Noise EKG Signal Attenuation of Noise EKG Signal Frequency (Hz) Noise Current Low Pass Filter http://www.wll.com

  29. Filters - Types http://www.wll.com

  30. Filters - Components • Discrete Component Filters • Component selection • Lead length considerations • Power Filter Modules • Filtered Connectors • Construction • Selective loading • Termination (bonding and grounding) http://www.wll.com

  31. Circuit Design – Real Performance http://www.wll.com

  32. Power Line Filter Typical Schematic Signal Line Filter Signal Line Filter (Screw-in Type) Filters http://www.wll.com

  33. Filter - Placement • Isolate Input & Output • Establish boundaries with filters between • Input or Output interfaces and active circuitry • Digital and Analog • Compartments and Modules • Prevent bypass coupling • Control line exposure on line side of filter • Use dog-house compartment • Shielded cables to control exposed cable runs • Terminate - Terminate - Terminate • Low impedance to ground termination • Minimize lead length http://www.wll.com

  34. Filter Filter IN Filter OUT Filter PerformancePoor Installation =Poor Performance http://www.wll.com

  35. Filter Placement http://www.wll.com

  36. Electric Field Coupling + - E-Field V+ Field Terminations on Inside Metal Sphere “Faraday Cage” - + V+ V=0 “Ground” 0V Potential Shield Concepts http://www.wll.com

  37. I V Low residual field Ferrous Shield m >>1 I V Shield Concepts Magnetic Field Coupling Magnetic Field Shielding Common at powerline and low frequencies; High-current conditions http://www.wll.com

  38. V=? + Effects of Openings Cable Leakage + - V+ Metal Sphere “Faraday Cage” V=0 http://www.wll.com

  39. ~ Radio Frequency Effects Shielded Enclosure VRF RF Source http://www.wll.com

  40. RF Leakage Metal Box L VRF ~ RF Source L ~ l/2 Perfect Transmission http://www.wll.com

  41. Shielding The Business Card Test Good to about 1 GHz http://www.wll.com

  42. Shielding - Overview • Shields - conductive barriers • Reflection • Absorption • Materials • Electric field - conductivity • Magnetic field - permeability • Discontinuities • Windows • Vents • Seams • Panel components • Cable connections http://www.wll.com

  43. Shielding Effectiveness Incident Field E1 Resultant Field E2 SHIELD Reflected ER SE = E2/E1 (dB) http://www.wll.com

  44. Shielding -Reflection/Absorption Plane wave occurs when E to H wave impedance ratio = 1 k = 3.4 for t in inches and k = 134 for t in meters http://www.wll.com

  45. Shielding - Material All are good electric field shields Need high u for Mag Field Shield http://www.wll.com

  46. Shielding - Seams/Gaskets • Required openings offer no shielding in many applications • Apertures associated with covers tend to be long or require many contact points (close screw spacing) • Large opening treatment • Screens, ventilation covers, optic window treatments • WBCO formed to effectively close opening • Seam opening treatments • Overlapping flanges • Closely spaces screws or weld • Gasket to provide opening contact • Gasketed SE http://www.wll.com

  47. Shielding - Penetration • Conductors penetrating an opening negates the shielding provided by absorption and reflection • Cables penetrations require continuation of the shield or • Conductors require filtering at the boundary • Cable shields require termination • Metal control shafts serve as a conductor • Use non-metallic • Terminate shaft (full circle) http://www.wll.com

  48. Grounding - Overview • Purpose • Safety protection from power faults • Lightning protection • Dissipation of electrostatic charge • Reference point for signals • Reference point is prime importance for EMC • Potential problems • Common return path coupling • High common impedance • High frequency performance http://www.wll.com

  49. Grounding - Impedance • Establish a low impedance return • Ground planes • Ground straps for high frequency performance • Establish single point or multipoint ground • Single point for low frequency or short distance • Distance(meters) < 15/f(MHz) • Multipoint for high frequency or long distance • Distance(meters) > 15/f(MHz) http://www.wll.com

  50. Bonding • Bonds should have two basic characteristics • Low impedance < 2.5 milliohms • Mechanical & electro-chemical stability • Low impedance • Avoid contamination • Provide for flush junction to maximize surface contact • Use gaskets or fingerstock for seam bonds • Provide a connecting mechanism • Mechanical and electro-chemical stability • Torque to seat for the mechanical connection • Lock washers to retain bond • Allow for galvanic activity for dissimilar metals http://www.wll.com

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