Practical approach
to EMI diagnostics
21 JUNE 2016
Pierre Dupont
Field AE
EMC? What does that even look like?
• Electromagnetic Compatibility: an electronic or electrical product
shall work as intended in its environment. The electronic or
electrical product shall not generate electromagnetic disturbances,
which may influence other products, and shall tolerate
interference from other devices.
• Sounds simple, but EMC must be considered and tested during
design, and products must pass many international standards
◦ Failure to comply means you can’t sell your product, or face penalties
◦ Failing EMC testing after the product is designed is a major problem,
and customers spend money to make sure it doesn’t happen
• Increasingly, the problem is interference between devices in your
own product, not interference from the outside
◦ This means your product doesn’t work, and it’s your fault
2
EMI Standards
Standards Bodies define receivers in terms of
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Frequency range
Resolution bandwidth
Detectors and averaging
Accuracy, sensitivity and dynamic range
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Types of EMI tests
Radiated emissions > 30 MHz
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Performed in RF isolated chamber
Far field measurements (3m or 10m)
EUT is placed on a turn-table, idle state
Fully calibrated setup
o Chamber
o Receiver
o Antenna’s
Conducted emissions < 30 MHz
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For devices which connect to power grid
Characterize energy conducted to AC grid
Line Impedance Stabilization Network
o Connected Rx between AC and EUT
EUT is in operational state
4
EMI Testing Work Flow
SCHEDULE TIME AT TESTING LAB
Catch problems early
Save time
Help design
Design ~90%
Completed
Pre-compliance
Test with
spectrum analyzer
In House
Fail
$$$
Time consuming
Report failures only
Expensive
Compliance Test
Test House
Pass
Fail
EMI Troubleshooting In House
EMI Pre-Compliance testing will save time/money by identifying problem areas
before they become expensive re-design issues
5
Measurement settings: bandwidth
effects
Analyzer with selectable -3 dB (RBW)
and -6 dB filter definitions, 1 dB/division
Random noise measured with 100 kHz
filters.
-3dB, 100 kHz response in yellow,
-6dB, 100 kHz response in blue
10*Log10(BW1/BW2), or 10*Log(71/100)=-1.5dB difference from using wrong BW
EMI filters (CISPR, MIL) are specified at the -6 dB bandwidth
6
Quasi peak detection
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Detectors were designed to place emphasis on frequently occurring signals that would annoy a listener or viewer of broadcast communications
Now that communications are bursted and digital, these detectors no longer measure the effect of EMI on communications very well, but regulations are very slow to change
•
Originally, the QP detector really was a RC circuit and a voltmeter‐ now it’s implemented digitally
Sin
S1
R
1
C
S2
R
2
The Quasi-Peak Detector and Associated Voltmeter
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Calculated response of the QP detector
and meter to pulse stimulus
Quasi peak detection
• Average or QP+ Meter is always ≤ Peak measurement
• Measured CW power are equal for Average, QP and Peak detectors
Peak Response
8 us PW, 10 ms rep. rate signal
QP Response
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Measurement settings: video filter
• Video filtering was the original trace smoothing technique to reduce
variations in signals
• Specified off for all but TELEC
• Widely used for many other SA applications
◦ Required by many legacy measurements, and preferred by many SA users
◦ Sometimes yields faster smoothing compared to waveform averaging
Standards
VBW Requirements
Analyzer VBW setting
CISPR
VBW not used
Maximum value or
disabled
TELEC
VBW = RBW or VBW ≥ 3*RBW
VBW=RBW or disabled
MIL
Greatest value or not used
Maximum value or
disabled
Video bandwidth requirements specified for EMI measurements 9
Report generation
10
EMI Challenges
• EMI debugging is painful work. Engineer need to find the root
cause of noise source to fix it.
◦ Debugging process by changing components,
◦ Cutting PCB lines & reconnect them until they find the noise source.
◦ Difficult to pin point the analog and/or digital signals causing the EMI.
• Difficult to capture infrequent EMI bursts.
◦ Need real-time SA with long instrument acquisition time.
◦ Most spectrum analyzers are not real time.
◦ Most real time SAs are expensive and not portable.
• When a device is transmitting RF power, the RF may adversely
affect circuit. This can dramatically change EMI signature.
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Is an In-house EMI pre-certification
test affordable?
Parts
Cost
RSA306
$3,500
Laptop PC
$800
SignalVu‐PC with EMC pre‐compliance masks
Included with the RSA306
Antenna & accessories
$3400 Total Cost
$7,700
$7,700 investment for both radiated and conducted test setups is lower than $10,000 cost of a single turn at certification house
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EMI debug & investigation
Near field probing
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Help isolate source of emission
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E-field
• High voltage, low current source
• Stub
• Max sensitivity perpendicular to source
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H-field
• Low voltage, high current source
• Loop
• Max sensitivity parallel to source
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Measures relative changes
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Repeatability is important
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EMI debug & investigation
Using RSA306
• Transient EMI missed in peak scan with swept analyzer (yellow trace),
found after 1 minute of Max-hold (blue trace)
while DUT was cycled through disk-cache operation.
• Infrequent transient discovered with DPX after 5 seconds.
The red areas are frequently-occurring signals,
and the blue and green portions are transients.
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EMI debug & investigation
Using RSA306
The transient
that occurs at
1 second
repetition rate
is captured
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EMI debug & investigation
Identifying Coincident Signals & Events
Spectral
peak occurs
during burst
Spectral
peak lower
off of burst
• Coincidence is KEY to fixing transient EMI issues
• Locate source/cause of the emission
• Simultaneous capture on ALL inputs
• Common trigger across all channels
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EMI debug & investigation
World’s first multi domain scope
Mixed Signal Oscilloscope
Controls
Time
Domain
Display
Spectrum
Analyzer
Controls
Frequency
Domain
Display
RF Input
Analog Inputs
Digital Inputs
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Thank you
Pierre Dupont
Pierre.dupont@tektronix.com