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Washington Laboratories (301) 417-0220 web: www.wll.com7560 Lindbergh Dr. Gaithersburg, MD 20879 EMC Fundamentals Presented By: Mike Violette Washington.

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Presentation on theme: "Washington Laboratories (301) 417-0220 web: www.wll.com7560 Lindbergh Dr. Gaithersburg, MD 20879 EMC Fundamentals Presented By: Mike Violette Washington."— Presentation transcript:

1 Washington Laboratories (301) 417-0220 web: www.wll.com7560 Lindbergh Dr. Gaithersburg, MD 20879 EMC Fundamentals Presented By: Mike Violette Washington Laboratories, Ltd. September 14, 2007

2 Introduction Elements of an EMI Situation Source "Culprit" Coupling method "Path" Sensitive device "Victim" SOURCE PATH VICTIM

3 Let’s see how this all got started Dead Smart Guys First Transmitters: Spark Devices Heinrich Hertz (1857-1894) clarified and expanded on James Clerk Maxwell’s Electromagnetic Theory Marconi: first use & patent HertzMaxwell Marconi

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 SusceptibilityRadiated Emissions Conducted Susceptibility Conducted Emissions

5 Interference to TV Reception Two Interfering Signals Injected into TV No Interference

6 Common “Coupling Modes” Common and Differential Mode Crosstalk (cabling and conductors) Field to cable (“Antenna”) Conducted (direct) Field to enclosure

7 Crosstalk (cable-to-cable coupling) SOURCE VICTIM

8 Radiated Coupling: Field to Cable Loop Area Induced Current Electromagnetic Wave Coupling proportional to: E/H Field, Loop Area, Frequency


10 Radiated Coupling: Field to Cable Patient Monitor Loop Area Induced Current Electromagnetic WaveRadio VCMVCM

11 Instrumentation Interference Interference Current, If Ideal Response Frequency (Hz) EKG Signal Real Response Frequency (MHz) NOISE

12 Effect of Modulation Interference Current, If

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?

14 Filtering Interference Current EKG Signal C C Interference Current EKG Signal Please, I’m very ticklish

15 Surge Coupling Lightning and pulse sources cause high-energy transients into power and data cables Indirect Direct

16 Digital Equipment Sources Fourier Analysis F(t) Log F f = 1/T 2f3f T A Spectrum of a Square Wave T A Log F F(t) f = 1/  f =1/  r rr  Spectrum of a Trapezoidal Wave (Characteristic of Digital Devices)

17 Equipment Emissions Limits

18 The decibel (dB) 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 = LOG 10 (P 2 /P 1 ) deciBel provides a more realistic scale: dB = 10LOG 10 (P 2 /P 1 ) Voltage & Current are expressed as follows: dB (V or I) = 20LOG 10 (V 2 /V 1 ) “20LOG” derives from the conversion from Power to Voltage (ohm’s Law: P = E 2 /R) Named after me!

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)

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

21 Pulsed Sources Fourier Analysis A F(t) Spectrum of a Pulse  Log F f = 1/  f =1/  r rr Fourier-> Do you like my new shirt?

22 Urban Ambient Profile Switching noise Cell phone FM Radio

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 = 3X10 8 /frequency = 300/f MHz

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

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!

26 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: V ~ I Area E (& H)

27 Filters - Overview Passband High pass Low pass Single component, L, Pi, T Common mode; differential mode Placement Components Lead length Leakage Limitations

28 Low Pass Filter Noise Current EKG Signal C C Noise Current EKG Signal Frequency (Hz) Rejection EKG Signal Noise Attenuation of Noise

29 Filters - Types

30 Filters - Components Discrete Component Filters Component selection Lead length considerations Power Filter Modules Filtered Connectors Construction Selective loading Termination (bonding and grounding)

31 Circuit Design – Real Performance

32 Filters Power Line Filter Typical Schematic Signal Line Filter (Screw-in Type) Signal Line Filter

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

34 Filter Performance Poor Installation = Poor Performance Filter Filter IN Filter OUT

35 Filter Placement

36 Shield Concepts + - Field Terminations on Inside Metal Sphere “Faraday Cage” “Ground” 0V Potential V+ V=0 + - Electric Field Coupling E-Field V+

37 Shield Concepts Magnetic Field Shielding Common at powerline and low frequencies; High-current conditions I V  Ferrous Shield Low residual field Magnetic Field Coupling V I

38 Effects of Openings + - Metal Sphere “Faraday Cage” V=0 V+ V=? Cable Leakage +

39 Radio Frequency Effects V RF ~ Shielded Enclosure RF Source

40 RF Leakage V RF ~ Metal Box RF Source L L ~ /2 Perfect Transmission

41 Shielding The Business Card Test Good to about 1 GHz

42 Shielding - Overview Shields - conductive barriers Reflection Absorption Materials Electric field - conductivity Magnetic field - permeability Discontinuities Windows Vents Seams Panel components Cable connections

43 Shielding Effectiveness SHIELD Incident Field E 1 Resultant Field E 2 SE = E 2 /E 1 (dB) Reflected E R

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

45 Shielding - Material All are good electric field shields Need high u for Mag Field Shield

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

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)

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

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)

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

51 Galvanic Scale

52 Component Selection T A Log F F(t) f = 1/T 2f3f T A Log F F(t) f = 1/  f =1/  r rr  Spectrum of a Square Wave Spectrum of a Trapezoidal Wave (Characteristic of Digital Devices)

53 Circuit Design – Component Selection Circuits available in an EMI version Specify logic of necessary speed - not faster than required EMI performance varies between manufacturers MAX485 MAX487

54 Switching Power Supplies Two Sources: Harmonics of switching power supply Broadband emissions due to ringing waveforms & f f

55 Underdamped (Ringing) Waveform Typical in switching circuits f 100 MHz+ 100s Volts 10s kHz dV/dT = 100sMV/s Broadband (radiated & conducted)

56 Circuit Design - Summary Consider EMI at the beginning Understand requirements Select components Design in protection Circuit Design - Layout Design in ground planes, guards, segregation EMI gains from layout has virtually zero recurring cost Grounds and Returns Develop a ground scheme Consider digital, analog, return, and shield terminations Design in hooks Provide space for potential fix actions that may be required

57 Decoupling & Power Distribution Connect all ground pins of high frequency circuits together in the same ground structure. Do not separate, isolate, break or otherwise “cut” the ground plane. Do not separate, isolate, break or otherwise “cut” the power plane. Do not insert impedances into Vcc/power traces.

58 Isolated Power/Grounding Example Trace Layout (Bad Idea!) Exception: Analog circuit isolation

59 Top 10 Common Mistakes 1.Improperly shielded cables: The principal problem is the cable-to-backshell termination 2.Unfiltered cable penetrations 3.High Frequency sources with poor termination: High frequency sources: signals and power supplies 4.Case seams and apertures: bad/no gasket, or improper mating surfaces 5.Poor bonding between metal parts of unit

60 Top 10 Common Mistakes 5.Long ground leads on shields and bonding conductors 6.No high frequency filtering on analog inputs: Radiated and conducted immunity 7.Not accounting for the high frequency effects of ESD 8.Inadequate filters on I/O cables for emissions 9.Inadequately-installed power line filters

61 The Ten Steps to Avoiding EMI Problems 1. Signal Termination 2. Layout 3. Decoupling & Power Distribution 4. Grounding 5. Bonding 6. Filtering 7. Cabling 8. Shielding 9. Surge Suppression 10. CHECKLIST


63 WLL Contact Information; Phone: 301 216-1500 Fax: 301 417-9069 INFO@WLL.COM

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