06/2009 | Fundamentals of DSOs | 1 Nov 2010 | Scope Seminar – Signal Fidelity | 1
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Debugging EMI Using a Digital Oscilloscope
06/2009 | Fundamentals of DSOs | 2 Nov 2010 | Scope Seminar – Signal Fidelity | 2
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Debugging EMI Using a Digital Oscilloscope l The problem: isolating sources of EMI after compliance
test failure l Near field probing basics l Measurement considerations for correlating time and
frequency domains l Oscilloscope sensitivity and dynamic range l Frequency analysis capabilities
l Isolating sources of EMI l Probe position and frequency content l Correlating EMI with time domain events using an oscilloscope l Analyzing intermittent EMI
l Measurement example: Isolating intermittent EMI l Measurement example: isolating power supply
switching EMI
06/2009 | Fundamentals of DSOs | 3 Nov 2010 | Scope Seminar – Signal Fidelity | 3
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The Problem: isolating sources of EMI
l EMI compliance is tested in the RF far field l Compliance is based on specific allowable power levels as a
function of frequency using a specific antenna, resolution bandwidth and distance from the DUT
l No localization of specific emitters within the DUT l What happens when compliance fails?
l Need to locate where the offending emitter is within the DUT l Local probing in the near field (close to the DUT) can help physically
locate the problem l Remediate using shielding or by reducing the EM radiation
l How do we find the source? l Frequency domain measurement l Time/frequency domain measurement l Localizing in space
Basic EMI Debug Process
March 2013 EMI Debugging with the RTO 4
Understand your DUT
Clock rates, possible harmonics, frequency of power supplies
Measure DUT in far-field / anechoic chamber
Understand signal behavior of critical frequencies
Identify signal sources with Near-field probes
CW Emission Unknown broadband
noise peak
Noise from power supply
Top Common Causes of EMI Problems (In no particular ranked order)
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Ground Impedance
Poor Cable Shielding
Emissions from Switching Power Supplies
Power Supply Filters
LCD Emissions
Stray Internal Coupling Paths
Component Parasitics
8 Inadequate Signal returns
9 Discontinuous Return Paths
10 ESD in Metallized Enclosures
Ten common EMI Problems by William D. Kimmel and Daryl D. Gerke
06/2009 | Fundamentals of DSOs | 6 Nov 2010 | Scope Seminar – Signal Fidelity | 6
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Near Field Definition
l Low impedance high current fields are predominantly magnetic (e.g. terminated high speed signals)
l High impedance low current fields are predominantly electrical (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
Near-Field "Sniffer" Probes
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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
06/2009 | Fundamentals of DSOs | 9 Nov 2010 | Scope Seminar – Signal Fidelity | 9
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H-Field Probe
l Maximum response with probe parallel with current and closest to the current carrying conductor
Current flow
H field
Vo
06/2009 | Fundamentals of DSOs | 10 Nov 2010 | Scope Seminar – Signal Fidelity | 10
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E-Field Probe
l Maximum response with probe perpendicular with current and closest to the current carrying conductor
Current flow
E field
Vo
06/2009 | Fundamentals of DSOs | 11 Nov 2010 | Scope Seminar – Signal Fidelity | 11
FFT Implementation Digital Down Conversion l Conventional oscilloscopes
l Calculate FFT over part or all of the acquisition
l e.g. 25,000 point FFT for 1 GHz cf and 100 KHz RBW and 100 MHz span
l Improved method:
l Calculate only FFT over span of interest
l fC = center frequency of FFT l e.g. 2500 point FFT for 1 GHz cf
and 100 KHz RBW and 100 MHz span
06/2009 | Fundamentals of DSOs | 12 Nov 2010 | Scope Seminar – Signal Fidelity | 12
Traditional Oscilloscope FFT calculation
FFT calculation with digital downconverter => FFT much faster & more flexible
FFT Implementation Digital Downconversion
t
Time Domain
Record length Windowing FFT
Data aquisition
Zoom (f1…f2)
f
Frequency Domain
Display f2 f1
f
Frequency Domain
t
Time Domain
Record length Windowing FFT
Display Data aquisition
DDC
f4 f3 Span f1…f2 (HW Zoom)
Digital down- conversion fc
SW Zoom (f3…f4)
06/2009 | Fundamentals of DSOs | 13 Nov 2010 | Scope Seminar – Signal Fidelity | 13
FFT Implementation
l Conventional oscilloscopes ⇒ FFT over complete acquisition
l Improved approach ⇒ FFT can be split in several FFTs and also overlapped
FFT 2 second aquisition
FFT 1 first aquisition
FFT 3 third aquisition
FFT 1 FFT 2 FFT 3 FFT 4 first aquisition
FFT 1 FFT 2
FFT 3 FFT 4 50% overlapping
⇒ Faster processing, faster display update rate ⇒ Ideal for finding
sporadic / intermittent signal details
06/2009 | Fundamentals of DSOs | 14 Nov 2010 | Scope Seminar – Signal Fidelity | 14
Tradeoff for Windowing: Missed Signal Events
Original Signal
Signal after Windowing
l All oscilloscope FFT processing uses windowing l Spectral leakage eliminated l However, signal events near window edges are attenuated or lost
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FFT Overlap Processing l Overlap Processing ensures no signal details are missed
Original Signal
…
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
Measurement Consideration: Sensitivity
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Low Noise and High Sensitivity at Full Bandwidth
1mV/div
Locating EMI Faults: First Steps
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General approach Start with the largest loop probe smaller loop probe stub probe
There are many potential sources of EMI on a board. Before you can eliminate an EMI issue you must first identify it.
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
Identifying EMI Through Signal Analysis Understand the DUT Known frequency source (clock and etc.) Possible harmonic frequencies Frequency & power of switching power supply emissions Identify miscellaneous periodic waves
*Take into consideration of technique used such as Spread Spectrum Clocking, frequency hopping and etc. Causes of EMI ı EMI is often caused by the switching of signals, e.g. power supply, clocks,
memory interface, etc. This is referred to as narrowband interference and generally occurs at very specific frequencies related to components on your board.
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Identifying EMI by Frequency Content Understanding the expected signals and their harmonics, analyze possible interference sources in the frequency range of interest
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Signal Harmonics
Correlating Frequency and Time Domains It is important to correlate interfering emission with signal harmonics and also to time domain. Time domain view reveals the period of the interfering source.
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FFT Gating isolates the spectrum of a time event
Capture Intermittent EMI Issues Using a Frequency Mask Stop acquisition on mask violation and observe signals in time domain in order to analyze offending signals.
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Mask Violation acts as a trigger on RF signals
Trigger Acquisition on Time Domain Signal Further to the modern oscilloscopes’ ability to detect weak signals in time domain, it is even more crucial to be able to trigger on weak signals for further analysis
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Trigger on observed time signals at mV/div
EMI Practical Diagnosis: Locating the Source of an Intermittent Spur
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EMI Practical Diagnosis: Locating the Source of an Intermittent Spur
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EMI Practical Diagnosis: Locating the Source of an Intermittent Spur
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EMI Practical Diagnosis: Locating the Source of an Intermittent Spur
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EMI Practical Diagnosis example Locating Broadband Noise Source
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EMI Practical Diagnosis example Locating Broadband Noise Source
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EMI Practical Diagnosis example Locating Broadband Noise Source
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Abnormal Spikes observed
The spike caused the broadband noise
Smaller spike observed too
EMI Practical Diagnosis example Locating Broadband Noise Source
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Abnormal Spikes observed
The spike caused the broadband noise
Smaller spike observed too
SMPS | 34
Synchronous converter using FET switches
l Transistors Qa and Qb act as switches A and B l Must operate in “break before make” mode to prevent high current
through transistors l Voltage at the load determined by the duty cycle
Power Supply Switching
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Time domain
Frequency domain
Analysis of Switching Spectrum
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Measurement Example: Switched Mode Power Supply
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Analysis of the Low Frequency Spur
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Analysis of the Low Frequency Spur
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frequency domain mask captures intermittent spur
Analysis of wideband signal
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Debugging EMI Using a Digital Oscilloscope Summary ı The modern oscilloscope with hardware DDC and overlapping FFT is capable of
far more than a traditional oscilloscope
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 solution quickly.
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