Updated! Debugging EMI Problems Using a Digital Oscilloscope
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Transcript of Updated! Debugging EMI Problems Using a Digital Oscilloscope
06/2009 | Fundamentals of DSOs | 1 Nov 2010 | Scope Seminar – Signal Fidelity | 1
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Debugging EMI Using a Digital Oscilloscope
Dave Rishavy Product Manager - Oscilloscopes
Debugging EMI Using a Digital Oscilloscope
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l Background – radiated emissions l Basics of near field probing l Frequency domain analysis using an oscilloscope
l FFT computation l Dynamic range and sensitivity l Time gating l Frequency domain triggering
l EMI debugging process l Measurement example
Basic Principles: Radiated Emissions The following conditions must exist ı An Interference source A sufficiently high enough disturbance level in a frequency range that is relevant for RF emissions ı A Coupling mechanism transmits disturbance signals from the interference source to the emitting
element ı An Emitting element (Antenna) capable of radiating the energy produced by the source into the far field
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Basic Principles: Radiated Emissions The following conditions must exist ı An Interference source A sufficiently high enough disturbance level in a frequency range that is relevant for RF emissions ı A Coupling mechanism transmits disturbance signals from the interference source to the emitting
element ı An Emitting element (Antenna) capable of radiating the energy produced by the source into the far field
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Interference sources ı Fast switching signals within
digital circuits Single-ended (asymmetrical) data
signals Switched mode power supplies -
harmonics Differential data signals with
significant common mode component
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ı High-order harmonics decrease at 20 to 40 db/decade
ı Structures on the PC board can begin to resonate at harmonic frequency
Inference Sources: Differential Mode RF Emissions
ı Emission results when signal and return are not routed together ı Near field probe can detect
this by positioning within the loop – position of probe is critical
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General steps to help reduce Differential Mode RF emissions ı Reduction of the loop area (i.e. closer routing of the forward and
return conductors) ı Reduction of the current in the conductor loop (if possible without
impacting the circuit operation.) ı Reduce the rise/fall times for the transmitted data signals ı Use filtering to eliminate higher-frequency signal components
(limit the disturbance spectrum.)
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Inference Sources: Common Mode RF Emissions ı Common problem in multi-
layer PC boards ı Caused by parasitic
inductance in return path or asymmetrical transmission ı External cable acts as an
antenna ı Rule of thumb for line length
as an antenna: λ/10 not critical λ/6 critical
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Common Mode RF Emissions
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Best Possible Differential mode transmission
Undesired parasitic capacitance in return path
Unbalanced parasitic terminating impedances
General steps to help reduce common-mode RF emissions ı Reduce the RFI current ICM by optimizing the layout, reducing the
ground plane impedances or rearranging components ı Reduce higher-frequency signal components through filtering or
by reducing the rise and fall times of digital signals ı Use shielding (lines, enclosures, etc.) ı Optimize the signal integrity to reduce unwanted overshoots
(ringing)
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Basic Principles: Radiated Emissions The following conditions must exist ı An Interference source A sufficiently high enough disturbance level in a frequency range that is relevant for RF emissions ı A Coupling mechanism transmits disturbance signals from the interference source to the emitting
element ı An Emitting element (Antenna) capable of radiating the energy produced by the source into the far field
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Coupling Mechanisms
ı Three coupling paths: Direct RF emissions from the source, e.g. from a trace or an individual
component RF emissions via connected power supply, data or signal lines Conducted emission via connected power supply, data or signal lines
ı Coupling Mechanisms Coupling via a common impedance Electric field coupling – parasitic capacitance between source and antenna Magnetic field coupling – parasitic inductance between source and antenna Electromagnetic coupling – far field coupling (greater than 1 wavelength)
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Basic Principles: Radiated Emissions The following conditions must exist ı An Interference source A sufficiently high enough disturbance level in a frequency range that is relevant for RF emissions ı A Coupling mechanism transmits disturbance signals from the interference source to the emitting
element ı An Emitting element (Antenna) capable of radiating the energy produced by the source into the far field
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Emitting Elements (Antennas)
ı Unintentional antennas in electronic equipment Connected lines (power supply, data/signal/control lines) Printed circuit board tracks and planes Internal cables between system components Components and heat sinks Slots and openings in enclosures
ıMain factor is the length of the antenna with respect to the
wavelength of the interference. Rule of thumb – antennas with length less than λ/10 are not critical
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Near Field Definition
ı Sources with Low Voltage, but high current predominantly generate magnetic fields (e.g. terminated high speed signals)
ı Sources with High Voltage, but low current predominantly generate electrical fields (e.g. unterminated signals)
Near field Transition Far field
E fie
ld
H fi
eld
Distance from DUT r
Wav
e im
peda
nce
r = 1.6m for f > 30 MHz
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EMI Debugging Example Oscilloscope and accessories
R&S ® RTO R&S ® RTE
Nearfield probes R&S ® HZ-15
E- and H-field
30 MHz – 1 GHz Applicable from 100 kHz
Optional: R&S ® HZ-16
preamplifier
Current clamp R&S ® EZ-17
Magnetic and Electrical Near-Field Probes
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ı Basically the probes are antennas that pickup the magnetic & electric field variation ı The output Depends on the position & orientation of the probe
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H-Field Probe
ı Maximum response with probe parallel with current and closest to the current carrying conductor
ı Traces with relatively high current, terminated wires and cables
Current flow
H field
Vo
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E-Field Probe
ı Maximum response with probe perpendicular with current and closest to the current carrying conductor
ı Traces with relatively high voltage: unterminated Cables, PCB traces to high impedance logic (tri-state outputs of logic IC’s)
Current flow
E field
Vo
Using an Oscilloscope for EMI Debugging
ıBenefits Wide instantaneous frequency coverage Overlapping FFT computation with color grading Gates FFT analysis for correlated time-frequency analysis Frequency masks for triggering on intermittent events Deep memory for capture of long signal sequences
ıLimitations Dynamic range No preselection No standard-compliant detectors (i.e CISPR)
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Important Scope-Parameters for EMI Debugging
Parameter Description
Record length Ensure that you capture enough
Sample rate >2x max frequency, start with 2.5 GS/s for 0 – 1 GHz frequency range
Coupling 50 Ω for near-field probes (important for bandwidth)
Vertical sensitivity 1 – 5 mV/div is usually a good setting across full BW
Color table & persistence Easily detect and distinguish CW signals and burst
FFT – Span / RBW Easy to use “familiar” interface, Lively Update
Signal zoom & FFT gating
Easily isolate spurious spectral components in time domain
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Frequency Domain Analysis FFT Basics
ı NFFT Number of consecutive samples (acquired in time domain), power of 2 (e.g. 1024)
ı ∆ fFFT Frequency resolution (RBW) ı tint integration time ı fs sample rate FFT
sFFT N
ftf ==∆int
1
Integration time tint NFFT samples input for FFT
FFT
Total bandwidth fs NFFT filter output of FFT
FFTf∆ts
FFT as Basis for EMI Debugging with Oscilloscopes Conventional FFT Implementation on a Scope
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t
Time Domain
Record length T
Windowing FFT
Data acquisition
Zoom (f1…f2) f
Frequency Domain
∆t = 1/Fs
f
Display f2 f1
Fmax = Fs/2
S(f) S(f) x(t)
∆f = 1/T
f1 f2
FFT on the Rohde and Schwarz RTE and RTO Spectrum Analyzer Use Model
ı Use model: Frequency domain controls time domain Time domain parameters automatically changed as
necessary
ı Downconversion FFT (DDC) zooms into frequency range before FFT Largely reduced record length, much faster FFT
Time Domain
t
Record length T
Data acquisition
Fs=2Β
x(t)
Frequency Domain
f
Display f2 f1
Β=f2-f1
S(f)
∆f = 1/T
Windowing FFT
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HW Zoom (DDC)
NCO
Decim- ation
LP
Zoom happens here – before the FFT
500 MHz center, 10 MHz span: Fs = 1 GS/s vs 20 MS/s
Measurement Consideration Gated FFT in the RTE and RTO Practical Time-Frequency Analysis
50% overlap (default setting) Gated FFT:
|---------------------------------- One complete Time-Domain capture ----------------------------|
The Key to unraveling the time domain
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Measurement Consideration: Sensitivity Ability to detect weak Signals EMI tends to be weak and near field probes have low gain, the oscilloscope needs to be able to detect small signals over its full bandwidth
Low Noise and High Sensitivity at Full Bandwidth
1mV/div
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Signal to Noise and ENOB Higher ENOB => lower quantization error and higher SNR => Better
accuracy
l Thermal noise is proportion to BW. l An FFT bin is captures a narrow BW proportional to 1/
NFFT
l Noise is reduced in each bin by a factor of l The limit approaches sum of all non-random errors.
(Measurement induced errors are still present)
FFTf∆
∗
FFTN1log10 10
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ı Mask Tool ı 6 dB EMI filter? Not critical for precompliance, will change results only slightly.
Measurement Consideration: Limit Lines
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Upper for limit line usage Mask definition in units of FFT
Upper region mask acting as limit line
Stop-on mask violation setting is very useful!
The Problem: isolating sources of EMI
ı EMI compliance is tested in the RF far field Compliance is based on specific allowable power levels as a function of
frequency using a specific antenna, resolution bandwidth and distance from the DUT
No localization of specific emitters within the DUT ı What happens when compliance fails? Need to locate where the offending emitter is within the DUT Local probing in the near field (close to the DUT) can help physically locate
the problem Remediate using shielding or by reducing the EM radiation
ı How do we find the source? Frequency domain measurement Time/frequency domain measurement Localizing in space
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E) Nearfield probe to localize the interferer source
D) Interferer current measurement to find out the coupling type
B) „Know your DUT“: List of potential interferer sources
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A) Far-field measurement C) Reference measurement without DUT
Source Frequency
Clock frequency e.g. 25 MHz + Multiples
Ethernet PHY e.g. 125 MHz + Multiples
Voltage converter / power adapter
broadband
…
EMI compliant testing / Test lab EMI Debugging / R&D
F) Analysis of counter-measures
EMI Debugging Procedure Analysis steps
Observe the Spectrum While Scanning With a Near-Field Probe
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I) General Approach ı Wide Span scan – fundamental of interfering signals are usually lower than 1GHz,
a span of <1GHz is sufficient as a start
Observe the Spectrum While Scanning With a Near-Field Probe
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I) General Approach ı Wide Span scan – fundamental of interfering signals are usually lower than 1GHz,
a span of <1GHz is sufficient as a start ı Identify abnormal spurious or behavior and its location while moving the probe
around
Observe the Spectrum While Scanning With a Near-Field Probe
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I) General Approach ı Wide Span scan – fundamental of interfering signals are usually lower than 1GHz,
a span of <1GHz is sufficient as a start ı Identify abnormal spike or behavior and its location while moving the probe around ı Narrow down to smaller span and RBW, change to smaller probe for better analysis
Example: IP-phone Situation ı IP-phone components Complex processor unit DDR2 memory Ethernet Layer 2 Switch 2 x Gigabit Ethernet PHYs
Several DC/DC Converters SPI-Interface to keyboard module Analog circuits (loudspeaker, Microphone)
ı Failed in EMI compliant test
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Frequency (MHz)
Level (dBµV/m)
Limit (dBµV/m)
Margin (dB)
Height (cm)
Azimuth (deg)
Polarization
248.68 41.20 47.50 6.30 0.0 157.00 HOR 250.00 44.50 47.50 3.00 0.0 293.00 HOR
375.00 52.30 47.50 -4.80 0.0 359.00 HOR
ı Additionally detected emissions on following frequencies: CW: 375 MHz Broadband: 250 MHz
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200 MHz 425 MHz
250 MHz 375 MHz
Example: IP-phone Current-probing on interface lines
Example: IP-phone Nearfield-probing for source localization Lokalisierung
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DC/DC-converter no. 2 No significant emission
Nearfield spectrum in the area of the processor module; among others 375 MHz interferer
Example: IP-phone Results ı Interferer signal detected on interface lines The interferer is probably transferred via common-mode coupling
ı Interferer sources localized DC/DC converter no. 1 Processor module respectively LAN PHY interfaces
ı Analysis of layout and implementation of counter measures.
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Debugging EMI Using a Digital Oscilloscope Summary ı If we can measure something in the far field, it must have an electric and
magnetic near field source.
ı The conditions required for a radiated emission allow us insight to track down a source and mitigate potential interferers.
ı EMI Debugging with an Oscilloscope enables correlation of interfering signals with time domain while maintaining very fast and lively update rate.
ı The combination of synchronized time and frequency domain analysis with advanced triggers allows engineers to gain insight on EMI problems to isolate and converge the source and solution quickly.
ı Please see this shortcut to our application note for additional information: http://goo.gl/rvpfCK
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