Electromagnetic Interference (EMI)

Transcription

1 Electromagnetic Interference (EMI) IBEX 2011 Speakers: David Gratton- Martek-Palm Beach, FL Johnny Lindstrom- Westport Shipyard, WA

2 Seminar Overview EMI Troubleshooting EMI Prevention with Proper Cabling & Terminations Lightning Protection Corrosion Grounding

3 Electromagnetic Interference Unwanted Periodic Signal/Energy One person s signal is another person s EMI. Interrupts, Obstructs, Degrades, or Limits Equipment Performance

4 Electromagnetic Interference Radiated Emissions RF Energy That Reaches Susceptible Equipment via Broadcast Conducted Emissions RF Energy That Reaches Susceptible Equipment via Common Connections

5 EMI Propagation Radiated Emissions Signals/Energy that Reaches Susceptible Equipment via Broadcast Radiated Power Decreases by Distance Squared Conducted Emissions Signals/Energy that Reaches Susceptible Equipment via Common Connections Combined Modes Signals/Energy that Propagates via Cable Connections that then Become Signal Radiators

6 EMI Sources AC Units Alternators Battery Chargers Blower fans Engines Generators Inverters Propeller Shafts Radars Wiring

7 EMI Recipients Electronic Compasses AV Systems Multifunction Displays Just about any Electronic Device

8 Identifying EMI Sources Trial-and-Error Process of Elimination Turn off All Equipment Except for Affected Device Turn on a Device and Check for Symptoms Repeat Until Interference Symptoms Return Additional Testing May Be Required to Determine If Interference Is Radiated or Conducted Interference May Be Radiated from Cables Connected to Interference Source

9 Avoiding EMI Problems Layout and Space Planning Identify Potential EMI Radiation Sources Identify Potential EMI Conducted Sources Avoid Potential Hot Spots Autopilot RF Transmission Line Rudder Feedback Signal VHF Radio Parallel cable runs are not desirable F

10 Mitigation Is Application Dependent DC Power Supply Leads Internal High Frequency Signal RF Output AC/RF to Antenna (AC Output Signal Wave Form) Signal Output Stage

11 Mitigation for Unintended Signals Objective: Block the Signal Prevent Signal Transmission on Cables Prevent Signal Transmission through Enclosure Shielded Enclosure with Attached Ground Input and Output Cables Active Filters- usually BandPass or Notch Ferrites Best Practice is to attempt to solve problems within the offender rather than the offended.

12 Ferrites as an EMI Suppressor Ferrites Composed of Ferrous Oxide and One or More Powdered Metals Composite Material Resists Imposed EMI Fields by Suppressing Electron Movement Variable Sensitivity to Frequency Lower Frequencies Pass without Significant Loss Above Resonant Frequency Signal Becomes Coupled to Ferrite, Causing a High Impedance Increasing Turns Increases Effectiveness

13 Ferrite Geometry

14 Mitigation for Signal Cables Objective: Preserve Signal on Cable; Prevent Radiation to Other Cables and Devices Balanced Signals Cancel Radiation Shielding Captures Radiation and Diverts Signal to Ground (not applicable to coax). Find it with troubleshooting Type Coverage Conductivity Flexibility Foil 100% poor poor Braid % good good

15 Eliminating Interference: Power cables Shielded Cables Connect One End of Shield to RF Ground Grounding Connect Case to RF Ground Better to Connect Source s Case Less Desirable to Connect Affected Device s Case Filters Install in Power Leads of offender first Ferrites Effective for Conducted and Radiated Noise

16 Other Mitigation Relocate Cable Runs Relocate Equipment Displays Relocate Antennas Consider Antenna Radiation Patterns Consider Cable Lengths Avoid Multiples of ¼ Wave Length with power & data cables This is not an issue with coax cables

17 Signal Loss Calculations Total Loss = Cable Loss + Connector Losses Cable Loss = Loss in db per 100 ft. (Depends on Type) x Cable Length / 100 Connector Loss = Number of Connectors x 0.5 db

18 Connector Selection Connector Type Maximum Frequency Impedance Permitted Uses UHF (PL-259) 300 MHz 50-Ohm VHF, SSB, DGPS, Stereo BNC 4.0 GHz 50-Ohm VHF, SSB, DGPS, Cell, GPS TNC 2.5 GHz 50-Ohm VHF, SSB, DGPS, Cell, GPS, MINI-M N 11.0 GHz 50-Ohm VHF, SSB, DGPS, Cell, GPS F 2.0 GHz 75-Ohm TV, GPS Mini UHF 2.5 GHz 50-Ohm Cell SMA 12.0 GHz 50-Ohm SAT Phone SMB 4.0 GHz 50-Ohm FME 200 MHz 50-Ohm VHF, SSB

19 Cable Bend Radius Cable Type Bend Radius (inches) RG58U 2.0 RG8X 2.4 RG8U 4.5 RG LMR LMR Better to route coax cables using gentle S-curves (green) than tight right-angle turns (yellow) when possible.

20 Coax Cables Attenuation Transmission Losses (db per ft.) Connector Losses (db) Impedance Mismatch (Avoidable) 50 Ω - 75 Ω Also affected by installation Cable Length Minimum Length Necessary (Most systems have no tuned length) Equipment Connections Match Application and Cable Type Extensions beware of the allowed loss

21 Cable Selection RG58U RG8X RG8U RG213 LMR240 LMR400 Nominal O.D. Conductor (AWG) Impedance (Ohms) 3 / 16 " 1 / 4 " 13 / 32 " 13 / 32 " 1 / 4 " 13 / Impedance Match within 2 Ohms

22 Shielding Connections of Power Shields Connected to RF Ground System Effectiveness Dependent on Low Impedance to Ground Best Ground Path for High Frequency EMI signals is via Copper Foil #8 AWG Stranded Copper Wire Acceptable

23 Lightning Strikes- GOOD LUCK! Refer to ABYC TE-4 Lightning Protection for more information Direct Strike Direct Hit on a Part of the Vessel, Such as an Antenna or Mast. Conductive Strike Strike on a Utility Line and Conducted Aboard Through the AC Power Cord Inductive Strike Nearby Strike Causing a Large Magnetic Field, Which in Turn Induces a Voltage in the Vessel s Wiring There is no sure way to prevent damage from any type of strike.

24 Lightning Protection: Two schools of thought Refer to ABYC TE-4 Lightning Protection for more information Conductive and Inductive Commercially Available Surge Protection Products Direct Strikes Air Terminals Connected to Properly Sized Conductors, with Relatively Straight Paths to Ground Towers Grounded Port and Starboard

25 Corrosion Refer to ABYC E-11 AC & DC Electrical Systems on Boats for more information Electrolysis Chemical and/or Electrochemical Change Due to Electric Current Galvanic Corrosion Corrosion from Electric Current Flow between Connected but Dissimilar Metals in the Same Electrolyte

26 Other Corrosion Sources Refer to ABYC E-11 AC & DC Electrical Systems on Boats for more information Stray Current Corrosion Electric Current Flow Caused by an Outside Source. (In a Marina). Velocity Corrosion Electric Current Flow Caused by Strong Water Currents Selective Corrosion Electric Current Flow Caused by Areas with Different Composition within the Same Alloy Oxygen Starvation Electric Current Flow Caused by Trapped Electrolytes Making Adjacent Surfaces More Positive to the Remaining Metal Surface

27 To Bond or Not to Bond Age Old Debate! Refer to ABYC E-2 Cathodic Protection for more information Decision Is Vessel Dependent Bonding Is Fairly Common in the Industry Aluminum Is Less Noble Than Almost Everything Else Always Isolate From Bonding System And/or Bond by Itself to a Sacrificial Anode Selected Especially for Aluminum

28 DC Common Grounding System Refer to ABYC E-11 AC & DC Electrical Systems on Boats for more information Shorts Stray Potentials to Ground May Provide Cathodic Protection Usually Main Connection between Vessel and Earth Ground

29 Grounding System Interconnect Refer to ABYC E-11 AC & DC Electrical Systems on Boats for more information Objective: Single Point with No Current Flow Practical: Few Interconnections with No Common Currents

30 Vessel Grounding Systems Refer to ABYC E-11 AC & DC Electrical Systems on Boats for more information System Type When Required DC Ground or Negative Reference All Vessels with DC Systems AC Neutral Reference Vessels with AC Shore Power, AC Grounding (Safety) Safety Generator, or Inverter Installed RF Ground Performance Vessels with Electronics Equipment Installed Single Side Band (SSB) Ground DC Grounding Performance Reference Vessels with SSB Transceiver Installed Lightning Ground Safety