Electromagnetic Interference (EMI)

Transcription

Electromagnetic Interference (EMI)
IBEX 2011
Speakers:
David Gratton- Martek-Palm Beach, FL
Johnny Lindstrom- Westport Shipyard, WA
Property of the NMEA. Shall not
be copied or re-distributed.
Seminar Overview
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•
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EMI Troubleshooting
EMI Prevention with Proper Cabling & Terminations
Lightning Protection
Corrosion
Grounding
Electromagnetic Interference
• Unwanted Periodic Signal/Energy
– “One person’s signal is another person’s EMI.”
• Interrupts, Obstructs,
Degrades, or Limits
Equipment Performance
Electromagnetic Interference
• Radiated Emissions – RF Energy That Reaches
Susceptible Equipment via Broadcast
• Conducted Emissions – RF Energy That Reaches
Susceptible Equipment via Common Connections
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
EMI Sources
•
•
•
•
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•
•
•
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AC Units
Alternators
Battery Chargers
Blower fans
Engines
Generators
Inverters
Propeller Shafts
Radars
Wiring
EMI Recipients
•
•
•
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Electronic Compasses
AV Systems
Multifunction Displays
Just about any Electronic Device
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
Avoiding EMI Problems
• Layout and Space Planning
– Identify Potential EMI Radiation Sources
– Identify Potential EMI Conducted Sources
– Avoid Potential Hot Spots
Autopilot
Rudder Feedback
Signal
Parallel cable runs
are not desirable
RF Transmission
Line
VHF
PropertyRadio
of the NMEA. Shall not
F
Mitigation Is Application
Dependent
DC Power
Supply
Leads
RF Output
AC/RF
to Antenna
Output
(AC
Signal
Wave Form)
Internal High
Frequency
Signal
Signal Output
Stage
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”.
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
Ferrite Geometry
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
Foil
Braid
Coverage Conductivity Flexibility
100%
poor
poor
85-95 %
good
good
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
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
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
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
Cable Bend Radius
Cable Type
Bend Radius
(inches)
RG58U
2.0
RG8X
2.4
RG8U
4.5
RG213
5.0
LMR240
0.75
LMR400
5.0
Better to route coax cables using gentle S-curves (green)
than tight right-angle turns (yellow) when possible.
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
Cable Selection
RG58U
RG8X
RG8U
RG213
3/ "
16
1/ "
4
13/ "
32
13/ "
32
1/ "
4
13/ ”
32
Conductor
(AWG)
20
16
13
13
15
9
Impedance
(Ohms)
50
50
52
50
50
50
Nominal
O.D.
LMR240 LMR400
Impedance Match within  2 Ohms
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
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.
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
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
Other Corrosion Sources
• 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
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
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
Grounding System Interconnect
• Objective: Single Point with No Current Flow
• Practical: Few Interconnections with No Common Currents
Vessel Grounding Systems
System
Type
When Required
DC Ground or Negative
Reference
All Vessels with DC Systems
AC Neutral
Reference
AC Grounding (Safety)
Safety
Vessels with AC Shore Power,
Generator, or Inverter Installed
RF Ground
Performance
Vessels with Electronics Equipment
Installed
Single Side Band (SSB)
Ground
Performance
Vessels with SSB Transceiver
Installed
DC Grounding
Reference
Lightning Ground
Safety

Electromagnetic Interference (EMI)

Transcription

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