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Top Ten EMC Problems & EMC Troubleshooting Techniquesby Kenneth Wyatt, DVD, Colorado SpringsRev. 1, Feb 26, 2007
EMC Engineering
EMC Engineering Series Page 2
1a. Ground Impedance
The overwhelming majority of high-frequency problems, whetherrelated to emissions, immunity or self-compatibility have highground return impedance as a root cause.
Wires and traces have a high impedance. This is why a groundreturn plane should be used at high frequencies
High-frequency effects start occurring at 1 MHz
The inductance of a wire in free space is about 20 nH/inch
A one-inch wire (or trace) has an impedance ofat 100 MHz
2πfl ≈12Ω
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1b. Slot caught during a design review
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Attempt at shielding module failed to connect halves!
1c. Poor shield integrity
Module halves copper-plated
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2a. Poor Cable Grounding
RF Immunity problems almost always involves a cable
Shield “pigtails” are bad – circumferential (360 degrees) are best
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2b. Poor Cable Grounding (continued)Divert the current back to the shielded enclosure• Proper termination of the cable shield back to the enclosure is the key
Block the current with a high-impedance ferrite choke
PC boardI/O Cable
Enclosure
Icm --->
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2c. Connector penetration through shield
Module was unshielded, main connector ground shells hadno connection with mate, connectors penetrated enclosure
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2d. Connector penetration through shield (cont.)
Module halves copper-plated, finger stock added to connectorsand ribbed main connector ground shell connects with mate
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2e. Good Cable GroundingIdeal I/O connector:
Notice the designincorporates an integratedgrounding connection,which solidly connectsshielded enclosure withconnector ground shellin multiple locations.
The connector groundshell then connects to thecable shield.
Good cable grounding design – cable shield properly connectedto metal enclosure.
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2f. Improved Cable/Connector Grounding
Shim with fingers added to shunt internal noise currents
!!
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3. Emissions From Switching Devices
Switched high-voltages couple to heat sinks and radiate
Use bypassing and ground the heatsink (multiple points)
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4a. LCD Emissions
Display cable radiates and excites the metal LCD housing
Ground the LCD housing to the enclosure in multiple points
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4b. LCD Emissions
Display cable radiates and excites the metal LCD housing
Ground the LCD housing to the enclosure in multiple points
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5a. Stray Internal Coupling Paths
Adding an external ferrite choke on the I/O cable often works best
Interrupt the coupling path with a Faraday shield
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5b. Data lines running underneath xtal oscillator
Data lines couple 100 MHz (and harmonics) through I/O connector to secondPC board; which in that area, is referenced to the power plane (with gaps).
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7a. Crosstalk
Most often due to a self-compatibility problem that occurs whennoisy source currents come into close proximity to sensitiveanalog, or low-level digital, signals
Route power and signals in separate cables
Use shielded cables
For circuit trace couplings, use additional separationor grounded “guard” traces in between the source andreceptor traces
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7b. Crosstalk in Solid Ground PlanesCrosstalk between two conductors depends on their mutual inductanceand capacitance. Usually, the inductive crosstalk is dominant.
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7c. Reducing Crosstalk With A Guard Trace
Ref: Signal Integrity – Simplified, Bogatin
Noise
130 mV
39 mV
22 mV
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7d. Use the “3W” Rule During Layout
Clocks and periodic signals havethe greatest chance of causingcrosstalk in other traces.
Minimize crosstalk effects byusing the “3W” rule; that is,ensure that all critical tracesare “buffered” by at least 3“trace-widths surroundingeach potential source andvictim trace.
Increasing the spacing from 1Wto 3W will decrease far-endcrosstalk by 65%.
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7e. Differential Pairs and the “3W” Rule
I usually consider 3-5W the minimum spacing for any critical traceto board edge spacing.
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8a. Inadequate Signal Returns
A common situation with cables, as well as pin out designs inASICs or other LSI devices is lack of adequate signal returnwires (or pins)
Unless the return path impedance is zero, a fraction of the currentwill likely return on a stray path, creating large loop currents
Route power and signals in separate cables
Use shielded cables
Add more ground return pins to ASIC/FPGA definitions(50/50% is ideal)
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8b. Inadequate Signal Returns
Ensure adequate number andposition of signal return pins.
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9a. Discontinuous Return Paths
Most PC board problems can be traced to discontinuous signalreturn paths. This is becoming more of an issue with increasingclock frequencies used today.
Ideally, the signal travels out a trace and returns immediatelyunder that trace
All to often, the return path is broken by a discontinuity, such asa gap or slot in the ground plane or the signal trace passes througha via and changes reference planes
Examine ground (and power) plane layers for gaps and slots
Add extra vias for return currents when switching reference planes
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9b. Generation of differential-mode emission
Montrose, EMC and the Printed Circuit Board, IEEE Press, 1999
YES!
NO!
DM current flows in a loop.The loop radiates energy.
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9c. Discontinuous Return Paths (examples)
!!
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9d. Slots in acquisition board signal return plane
Temporary bridge with copper tape reduced emissions 17 dB!
Noisy IC straddlinggap in ground plane
Large gap in ground plane
Many more gaps
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9e. Discontinuous Return Paths (examples)
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9f. Discontinuous Return Paths (examples)
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9g. Trace passing through two planes with via
Ref: Ott, EMC Aspects of Future High Speed Digital Designs, PCB-01049
Question: where does the return signal current flow?
Source
Termination
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10a. ESD In Metal Enclosures
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10b. Failure Modes Caused by ESD
Typical failure modes include a change of instrument state(front panel control), CPU reset, instrument lockup, loss of data
Fix by shielding, rerouting cables, separation of receptor circuits
EMC Pre-Compliance Testing & Troubleshootingby Kenneth Wyatt, DVD, Colorado SpringsRev 1, Feb 26, 2007
Electromagnetic Compatibility Engineering
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Five Key Threats
These comprise 95% of all EMC problems(EDN Magazine)
1. Radiated Emissions2. ESD (susceptibility)3. RF Fields (susceptibility)4. Power disturbances5. Internal compatibility (crosstalk, etc.)
The solution to most EMC problems is toCONTROL THE PATH OF CURRENT.
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“Source-Path-Receptor” Model
Source Energy Transfer Receptor(emitter) (coupling path) (receiver)
Seam, Slot,Cable TV, Radio
Oscillator
Seam, Slot,Cable
ElectronicCircuitry
RadiatedEmission
RadiatedImmunity
ESD
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EMC Troubleshooting Kit
Pelican 1514 roller caseExtra cables, adapters, etc.Tool kitDMMTin can antennaLine filterHarmonic comb generatorFerritesWrist strap
ESD meterPower adaptersSpectrum AnalyzerSniffer probe set“Bow-tie” antennaESD generatorCurrent probeBroadband preamp
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Major Contents
Current ProbeSpectrum Analyzer
E- and H-Field probes
Wideband Preamp Conductivity ProbeSmall Transmitter
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EMC Troubleshooting Kit
What can you do with it?
The EMC troubleshooting kit may be used for the following:
1. Identifying radiated emission sources on PC boards
2. Discovering leaky seams in enclosures
3. Measuring cable common-mode currents / radiation
4. Making quick “A-B” comparisons for proposed design changes
5. Testing for radiated and ESD immunity
6. Highly portable – may be used as a mini “EMC crash cart”
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Simple Pre-Compliance Tests & Troubleshooting
•Shielding tests•Analyze/measure crystal oscillator harmonics•E- and H-field probing
• PC Boards (E- and H-field probes)• Cables (current probes)• Shields (surface current and 4-point probes)
•Measure bare PC board emissions•Measure conducted emissions in cables
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“Mini” Harmonic Comb Generator Used For Shielding Effectiveness Test
Wyatt/Chaney, RFI Measurements Using a Harmonic Comb Generator, RF Design Magazine, Jan. 1991
Can be placed inside aproduct. Used for shieldingeffectiveness measurements.
Step recovery diode circuit produces5 MHz harmonics from 30 to 1500 MHz
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Comb Generator Taped To Prototype Product
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Then Comb Generator Is Measured Alone
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Shielding Effectiveness Plot Is The Difference!Shielding Effectiveness - MSO6000-Series Prototype
0
5
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0
100
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Frequency (MHz)
SE (d
B)
CablesNo Cables
Ignore this area -running out ofdynamic range.
Need to watch this freq!
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Typical EMI Diagnostic Setup
Wilcox, A Design Process for Achieving Electromagnetic Compatibility, Compliance Engineering
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Examples Of Home Made Loop (H-Field) Probes
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Constructing a simple loop probe
Roleson, Finding EMI Resonances in Structures, EMC Test & Design, Jan/Feb 1992
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“Mini” loop probe
Bend the center conductor of a thin coaxcable into a small loop and solder it to thebraid. Than cover with silicon sealer.
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Commercial Loop Probes
Commercial probes areavailable from severalsources:Beehive Electronicsprobes pictured…www.beehiveelectronics.com
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Mapping Of PC Board “Hot Spots”½” Magnetic field probe for coursemeasurements
“Micro” magnetic field probe for finemeasurements
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Using A Current Probe To Measure Cable Common-Mode Currents
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Current Probe Transfer ImpedanceRather than carrying out aprecise calculation, it issufficient to measure thevoltage with a known currentand frequency passing throughthe probe.
This measurement is usuallyexpressed as “transferimpedance” (ZT).
Or, expressed in dB:
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Application Of The Current Probe - ExampleFor example, consider the problem of determining the level of probevoltage that will correspond to a CM current on a cable that wouldjust meet the radiated emission regulatory limit.
Clamping the probe around a cable and lumping the net CM currentinto one wire of length L gives the net radiated emission a distance daway. Note we must divide Eq. 8.16 by 2, since that result was fortwo wires carrying a current of IC. Assuming FCC-B limit at 30 MHz(100 uV/m) and 1m cable.
From: Introduction to Electromagnetic Compatibility, Paul
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Ferrite Chokes Block Common-Mode Currents
Often used on keyboard, mouse or video cablesVery useful for troubleshootingManufactures often provide free evaluation kitsBest used on low-Z cables
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Line Cord EmissionsIf the line cord is suspected of emitting HF harmonics, insert an external line filter in seriesIf the emission level (or measured common-mode current) drops, then you need to design in better line filteringAn external filter may be made with an extra power line filter module and spare line cordCut the cord down to minimal size and carefully solder the stripped ends to the filter, observing the correct connections. Wrap with electrical tape.
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Testing A Bare PC Board For Radiated Emissions
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Multiple Crystal Oscillator Analysis
Avoid selecting fundamental oscillator frequencies whose harmonics end upcloser to each other than 120 kHz (CISPR 16 quasi-peak measurement bandwidth)
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Use coolant spray to determine which oscillator is contributing the most to the harmonic of concern
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Example Of A Polar Plot - Wide Lobes
Emissions are coming fromthe sides of the product.
Single- or double-lobed plotis typical behavior at thelower frequencies (<300 MHz).
Probably due to cable radiation.
Polar plot at 216 MHz. Shape indicates a half-wavedipole antenna pattern.
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Typical Polar Plot At Higher Frequencies -Narrow Lobes
Emissions are coming fromall around the product
Typical behavior at thehigher frequencies (>300 MHz)
Probably due to slot radiation.
Polar plot at 848 MHz. Pattern probably indicatesslot radiation and multiple reflections.
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Using a loop probe to measure resonances
Roleson, Finding EMI Resonances in Structures, EMC Test & Design, Jan/Feb 1992
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Resonance measurement
Roleson, Finding EMI Resonances in Structures, EMC Test & Design, Jan/Feb 1992
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External clip lead
Useful for assessing proper signal return (or shield) connection back to enclosureConnect 1m-long clip lead to various shield connection points on all I/O cable portsClamp a current probe around the cable (or measure the amount of radiated emissions)If no current is detected or the clip lead has no effect on radiated emissions, that specific shield connection point is connected back to enclosure shielding OK
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Plated shield conductivity measurement
Use of a “four-point” probe allows you to verify the sheet resistance, and hence the thickness of vapor-deposited (or plated) metallic shieldsA four-terminal (for resistance) DMM is requiredBy knowing the thickness, you can calculate the shielding effectiveness, given the manufacturer’s equation
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Simple ESD Source – Coins In ZipLok Bag (Simulates Pocket Change)
Idea, courtesy of Doug Smith (www.emcesd.com)
Produces 200 ps rise timepulses of several volts
EMC Engineering Series Page 64
Simple ESD Source – Spark Gap Ruler
Idea, courtesy of Doug Smith (www.emcesd.com)
Produces 200 - 500 ps rise timepulses of several volts
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EMC Management Recommendations•Identify a regulatory point of contact within the design team•Develop EMC design goals (6 dB margin for sample of five)•Perform margin testing, rather than testing to limits•EMC engineer is a part of the project management team•Make early design reviews part of the product generation process
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