EMC Fundamentals Presented By: Mike Violette Washington Laboratories (301) 417 -0220 web: www. wll. com Washington Laboratories, Ltd. September 14, 2007 7560 Lindbergh Dr. Gaithersburg, MD 20879
Introduction Elements of an EMI Situation • Source "Culprit" • Coupling method "Path" • Sensitive device "Victim" VICTIM SOURCE PATH
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 Maxwell Hertz • Marconi: first use & patent Marconi
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 Conducted Susceptibility Radiated Emissions Conducted Emissions
Interference to TV Reception No Interference Two Interfering Signals Injected into TV
Common “Coupling Modes” Common and Differential Mode • Crosstalk (cabling and conductors) • Field to cable (“Antenna”) • Conducted (direct) • Field to enclosure
Crosstalk (cable-to-cable coupling) SOURCE VICTIM
Radiated Coupling: Field to Cable Electromagnetic Wave Loop Area Induced Current Coupling proportional to: E/H Field, Loop Area, Frequency
COMMON and DIFFERENTIAL MODE COMMON-MODE: “Line to Ground” • DIFFERENTIAL MODE: “Line-to-Line” (Normal Mode) • VDM VC M INoise
Radiated Coupling: Field to Cable Radio Electromagnetic Wave Patient Monitor VC M Loop Area Induced Current
Instrumentation Interference EKG Signal Interference Current, If NOISE Frequency (Hz) Ideal Response Frequency (MHz) Real Response
Effect of Modulation Interference Current, If
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?
Filtering Please, I’m very ticklish C EKG Signal Interference Current C
Surge Coupling • Lightning and pulse sources cause high-energy transients into power and data cables Direct Indirect
Digital Equipment Sources Fourier Analysis Spectrum of a Square Wave T A F(t) f= 1/T Log F 2 f 3 f Spectrum of a Trapezoidal Wave (Characteristic of Digital Devices) T f =1/ptr A t F(t) tr f= 1/pt Log F
Equipment Emissions Limits
The decibel (d. B) • • • The d. B is used in Regulatory Limits (FCC, CISPR, etc. ) The d. B 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) deci. Bel provides a more realistic scale: d. B = 10 LOG 10(P 2/P 1) Voltage & Current are expressed as follows: d. B (V or I) = 20 LOG 10(V 2/V 1) “ 20 LOG” derives from the conversion from Power to Voltage (ohm’s Law: P = E 2/R) Named after me!
d. B • Can have several reference units: • • Watt: d. B above one Watt (d. BW) Milliwatt: d. B above one milliwatt (d. Bm) Volt: d. BV Microvolt: d. Bu. V Microamp: d. Bu. A picotesla: d. Bp. T Electric Field: d. Bu. V/m Radio Receiver Sensitivity ~ 10 d. Bu. V • E-Field Limit for FCC: ~40 -60 d. Bu. V/m • Distance to moon: 107 d. Bmile (20 LOG 2. 5 E+5 miles) • National debt: 128 d. B$ (10 LOG 6 E+12) •
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
Pulsed Sources Fourier Analysis Fourie Do you like my new shirt? r-> A f =1/ptr t F(t) tr f= 1/pt Log F Spectrum of a Pulse
Urban Ambient Profile Cell phone Switching noise FM Radio
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-tocable 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 l= c/f = 3 X 108/frequency l = 300/f. MHz • For example, wavelength at FM Radio Band (100 MHz) is 1
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
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!
Cables - Loops • • Emissions are a function of 1) Current; 2) Loop Geometry; 3) Return Path of the 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: I V~ Area E (& H)
Filters - Overview • • • Passband High pass Low pass Single component, L, Pi, T Common mode; differential mode Placement Components Lead length Leakage Limitations
Low Pass Filter C EKG Signal Noise Current EKG Signal C Noise Current Rejection Noise EKG Signal Attenuation of Noise Frequency (Hz)
Filters - Types
Filters - Components • Discrete Component Filters • Component selection • Lead length considerations Power Filter Modules • Filtered Connectors • • Construction • Selective loading • Termination (bonding and grounding)
Circuit Design – Real Performance
Filters Power Line Filter Signal Line Filter (Screw-in Type) Typical Schematic Signal Line Filter
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 • Low impedance to ground termination • Minimize lead length
Filter Performance Poor Installation = Poor Performance Filter OUT Filter IN Filter
Filter Placement
Shield Concepts Electric Field Coupling + E-Field V+ - Field Terminations on Inside Metal Sphere “Faraday Cage” - + V+ V=0 “Ground” 0 V Potential
Shield Concepts Magnetic Field Coupling I V Magnetic Field Shielding Low residual field Ferrous Shield Common at powerline and low frequencies; High-current conditions m >>1 I V
Effects of Openings Cable Leakage V+ Metal Sphere “Faraday Cage” + + V=? V=0
Radio Frequency Effects Shielded Enclosure VRF ~ RF Source
RF Leakage Metal Box VRF ~ L RF Source L ~ l/2 Perfect Transmission
Shielding The Business Card Test Good to about 1 GHz
Shielding - Overview • Shields - conductive barriers • Reflection • Absorption • Materials • Electric field - conductivity • Magnetic field - permeability • Discontinuities • • • Windows Vents Seams Panel components Cable connections
Shielding Effectiveness Incident Field E 1 Resultant Field E 2 SHIELD Reflected ER SE = E 2/E 1 (d. B)
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
Shielding - Material All are good electric field shields Need high u for Mag Field Shield
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
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)
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
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)
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
Galvanic Scale
Component Selection Spectrum of a Square Wave T A F(t) f= 1/T Log F 2 f 3 f Spectrum of a Trapezoidal Wave (Characteristic of Digital Devices) T f =1/ptr A t F(t) tr f= 1/pt Log F
Circuit Design – Component Selection MAX 485 • • • MAX 487 Circuits available in an EMI version Specify logic of necessary speed - not faster than required EMI performance varies between manufacturers
Switching Power Supplies • Two Sources: • • Harmonics of switching power supply Broadband emissions due to ringing waveforms & f f
Underdamped (Ringing) Waveform • Typical in switching circuits 10 s k. Hz Broadband (radiated & conducted) d. V/d. T = 100 s. MV/s 100 s Volts 100 MHz+ f
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
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. •
Isolated Power/Grounding • Example Trace Layout (Bad Idea!) Exception: Analog circuit isolation
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
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
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
CHECKLIST
WLL Contact Information www. wll. com; info@wll. com Phone: 301 216 -1500 Fax: 301 417 -9069 INFO@WLL. COM
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