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7/26/2019 Debugging Emi
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06/2009 | Fundamentals of DSOs | 1
Nov 2010 | Scope Seminar Signal Fidelity | 11
Debugging EMI Using a Digital
Oscilloscope
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06/2009 | Fundamentals of DSOs | 2
Nov 2010 | Scope Seminar Signal Fidelity | 22
Debugging EMI Using a Digital Oscilloscopel Lets get familiar with the RTO!
l The problem: isolating sources of EMI after compliance test
failure
l Near field probing basics
l H-Field
l E-Field
l Measurement considerations for correlating t ime and frequency
domains
l Frequency analysis capabilitiesl Important scope parameters for EMI Debug
l Workshop
l Working with FFTs
l Finding a sources of EMI
l Isolating a source of EMIl EMI from switch mode power supplies
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Familiarization
3
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Setting up a multiple channel measurement Press PRESET:
Connect CH1 to RARE_SIG on the demo board,
Connect CH2 to 10_MHZ_CLK Toggle Demo Board DOWN button until 8 is displayed.
Press AUTOSET
Adjust Vertical and Horizontal Position and Scale. (~40ns/div, 1V/Div on each channel).
03.03.2014 Quick Start Guide 4
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Similar Display Using Smart Grid
Press AUTOSET
Minimize both channels (tapping on the channel icon)
Move CH1 onto smart grid Drop CH2 below CH1 on Smartgrid
Change Horizontal scaling to 20ns/div.
03.03.2014 Quick Start Guide 5
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Using Toolbar From same setup
Note the toolbar
Zoom on CH2 to isolate the pulse. Note the Mask Function,
Draw a mask inside the zoomed pulse.
Use the Trash Can to delete mask then zoom window.
Add back the zoom window with undo.
03.03.2014 Quick Start Guide 6
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Display Menu Change demo board to
Press Preset
Press Autoset. Change horizontal scale to 20ns/div
Press the DISPLAY hard Key. Enable infinite persistence.
Raise the intensity of the display to 100%
Shortly a runt pulse should appear.
Note an open voltage level slightly above and below the runt. Jot this down.
03.03.2014 Quick Start Guide 7
Note the dot
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Trigger Menu Keep the same Demo board configuration.
Press PRESET and AUTOSCALE. Change horizontal scale to 20ns/div
Enter TRIGGER system Select trigger type RUNT.
Set the upper and lower limits to the Open space around the runt we saw
Why does it not appear triggered?
03.03.2014 Quick Start Guide 8
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03.03.2014 Quick Start Guide 9
Now lets talk about EMI
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06/2009 | Fundamentals of DSOs | 10Nov 2010 | Scope Seminar Signal Fidelity | 10 10
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 afunction of frequency using a specific antenna, resolution bandwidthand 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 physicallylocate 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
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Basic EMI Debug Process
March 2013 EMI Debugging with the RTO 11
Understand your DUT
Clock rates, possible harmonics,frequency of power supplies
Measure DUT in far-field /
anechoic chamber
Understand signal behaviorof critical frequencies
Identify signal sources withNear-field probes
CW EmissionUnknown broadband
noise peak
Noise from power supply
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Top Common Causes of EMI Problems
(In no particular ranked order)
12
1
2
3
4
5
6
7
Ground Impedance
Poor Cable Shielding
Emissions from SwitchingPower Supplies
Power Supply Filters
LCD Emissions
Stray Internal CouplingPaths
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
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06/2009 | Fundamentals of DSOs | 13Nov 2010 | Scope Seminar Signal Fidelity | 13 13
Near Field Definition
l Sources with Low Voltage, but high current predominantly generate
magnetic fields (e.g. terminated high speed signals)
l Sources with High Voltage, but low current predominantly generate
electrical fields (e.g. unterminated signals)
Near field Transition Far field
Efield
Hfield
Distance from DUT
r
Wave
impedance
r = 1.6m forf > 30 MHz
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Near-Field "Sniffer" Probes
14
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Magnetic and Electrical Near-Field Probes
15
Basically the probes are antennas that pickup the magnetic & electric field variation The output Depends on the position & orientation of the probe
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06/2009 | Fundamentals of DSOs | 16Nov 2010 | Scope Seminar Signal Fidelity | 16 16
H-Field Probe
l
Maximum response with probe parallel with current andclosest to the current carrying conductor
l Traces with relatively high current, terminated wires and
cables
Current flow
H field
Vo
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06/2009 | Fundamentals of DSOs | 17Nov 2010 | Scope Seminar Signal Fidelity | 17 17
E-Field Probe
l
Maximum response with probe perpendicular with currentand closest to the current carrying conductor
l Traces with relatively high voltage: unterminated Cables,
PCB traces to high impedance logic (tri-state outputs of
logic ICs)
Current flow
E field
Vo
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06/2009 | Fundamentals of DSOs | 18Nov 2010 | Scope Seminar Signal Fidelity | 18 18
Debugging EMI Using a Digital Oscilloscope
l The problem: isolating sources of EMI after compliance test
failure
l Near field probing basicsl Measurement considerations for correlating time and frequency
domains
l Frequency analysis capabilities
l Oscilloscope sensitivity and dynamic range
l Isolating sources of EMIl 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: Locating Broadband Noise Source
l Measurement example: isolating power supply switching EMI
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19
Fourier Transform Concept
Any real waveform can beproduced by adding sine waves
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20
Frequency Domain AnalysisFFT Basics
l NFFT Number of consecutive samples (acquired in
time domain), power of 2 (e.g. 1024)l fFFT Frequency resolution (RBW)
l t int integration time
l fs sample rate
FFT
sFFT N
ft
f ==int
1
Integration time tint
NFFTsamples input for FFT
FFT
Total bandwidth fs
NFFTfilter output of FFT
FFTfts
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21
Measurement Consideration:FFT Implementation
l Conventional oscilloscopesl Calculate FFT over entire acquisition
l Improved method: Digital Down
Conversion
l Calculate only FFT over spanof interest
l fC= center frequency of FFT
=> FFT much faster & more flexible
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22
Measurement Consideration: Time Gating
Signal characteristics change over the acquisition intervalGating allows selection of specific time intervals for analysis
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23
Measurement Consideration: Time GatingTg
gTf
1=
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Ability to detect weak SignalsEMI 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
24
Low Noise and High Sensitivityat Full Bandwidth
1mV/div
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25
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/N
FFTl 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
FFTN
1log10 10
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26
Signal to Noise
>80 dB
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06/2009 | Fundamentals of DSOs | 27Nov 2010 | Scope Seminar Signal Fidelity | 27
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 for0 1 GHz frequency range
Coupling 50 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 distinquish CW signals and burst
FFT Span / RBW Easy to use familiar interface, Lively Update
Signal zoom & FFTgating
Easily isolate spurious spectral components in timedomain
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Objective: Learn how to make frequency domain measurements using an FFT on
an oscilloscope
Lab 1: Working with FFTs
28
Any real waveform can be
produced by adding sine waves
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Lab 1: Working with FFTs
29
SETUP
CH1= SMA-BNC cable
SMA-BNC= Demo board RF out
Demo board setting =1
Press PRESET.
Ch1=50 Ohm
AUTOSCALE
With the FFT window up, try different Resolution BW
settings. What is the trade off?
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Locating EMI Faults: First Steps
30
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.
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Locating EMI Faults: First Steps
31
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.
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Observe the Spectrum While Scanning With a Near-
Field Probe
32
I) General Approach
Wide Span scan fundamental of interfering signals are usually lower than 1GHz,
a span of
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Observe the Spectrum While Scanning With a Near-
Field Probe
33
I) General Approach
Wide Span scan fundamental of interfering signals are usually lower than 1GHz,
a span of
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Observe the Spectrum While Scanning With a Near-
Field Probe
34
I) General Approach
Wide Span scan fundamental of interfering signals are usually lower than 1GHz,
a span of
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Observe the Spectrum While Scanning With a Near-
Field Probe
35
I) General Approach
Wide Span scan fundamental of interfering signals are usually lower than 1GHz,
a span of
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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
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.
36
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Identifying EMI by Frequency ContentUnderstanding the expected signals and their harmonics, analyze possible
interference sources in the frequency range of interest
37
Signal Harmonics
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38
Lab 2: Finding Sources of EMI
Objective: Use a Near Field Probe to locate narrow band signals and determine
the frequency
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Lab 2: Finding Sources of EMI
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SETTING(same as scope exit configuration from last lab)
CH1= Change to Loop Near Field Probe (NFP)
Config (if needed) Demo Board = #1 (seven segment display should read=
1
)
Press PRESET..
Ch1=50 Ohm
Vertical Scale=1mV/div
Perform FFT on this signal (settings, 825MHz CF, 50MHz Span,
100KHz RBW).
Adjust the Color table=false colors
Locate the 825MHz CW with the NFP by scanning
the probe above the surface of the PCB.
Note that we can see the same signal as before, but now we are not directlyconnected to it. This emission seems to be coming from the digitalattenuation IC path.
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Lab 2: Finding Sources of EMI (Cont)
40
SETTING(Change to scan entire board)
CH1= Change to Loop Near Field Probe (NFP)
Config (same except FFT settings) Demo Board = #1 (seven segment display should read=
1
)
Press PRESET..
Ch1=50 Ohm
Vertical Scale=1mV/div
Perform FFT on this signal (settings, 500MHz CF, 1GHz Span, 2MHz
RBW).
Adjust the Color table=false colors
Note the 825MHz CW with the NFP by scanning the probe above the surface of the
PCB. Also note the power supply and other harmonic emissions. Note that thereis something down around 10MHz we want to look at further (there is a small, but
larger spike down there. We can admit that we are focused on the process not that
this might be an actual problem.
L b 3 I l i S f EMI
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Lab 3: Isolating Sources of EMI
SETUP
Ch1=Large Loop NFP
Preset
CH1=50 Ohm
Vertical Scale=1mV/div
FFT=10MHz CF, 20MHz Span, 20KHz RBW
41
This lab utilizes larger to smaller probes to
demonstrate the isolation of an EMI issue through
progressive probing. It requires 4 steps and the
change of 4 NFPs.
L b 3 I l ti S f EMI
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Lab 3: Isolating Sources of EMIObjective: Learn how to use different size probes and both E-Field and H-Field
probes to localize a 10MHz emission
Step 1: move around the board and show that there is a stronger 10MHz
emission around the main IC in the middle of the board.
42
L b 3 I l ti S f EMI
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Lab 3: Isolating Sources of EMIStep 2: Switch to smaller loop probe and have the user take the probe around all
4 sides of the IC.Note that the smaller probe can isolate which side of the IC has
the most emission in the H field.
43
L b 3 I l ti S f EMI
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Lab 3: Isolating Sources of EMI Step 3: Switch to the smallest Magnetic probe (stub probe). This probe can be placed on
each pin of the IC to look for the one with the largest emission.
You should see a strong signature by the pins next to the decoupling cap.
This area which includes the local oscillator as well as the pins of the IC by the decoupling
path are the source of the emissions.
44
L b 3 I l ti S f EMI
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Lab 3: Isolating Sources of EMI Step 4: Connect the blade E field probe to the scope and probe the traces down near
the 10Mhz crystal.
You should see an emission is coming from the clock trace evident by the E field probes
ability to detect the signal when the probe is placed right on top of the clock trace
You can also probe the trace leading to the 10MHz Clock pin. There is a strong Electrical
field on this due to the trace length.
45
L b 4 U i G t t C l ti f / ti
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Lab 4: Using a Gate to Correlating frequency/ time
46
Objective: Use a MASK to stop and help correlate an emission in thefrequency domain to a pulse train on the SPI bus in the time domain
Demo board=#4 (seven segment display =4) Ch1= Small H 2.5-2 NFP Ch2= Passive probe with retractable hook and attach probe to SPI DATA
through hole connection on edge of board
Preset. Ch1=50 Ohm Ch1=Vertical Scale 1mV/div Ch2=500mV/div Setup FFT to find the pulse causing an emission at around 35MHz FFT= 100MHz CF, 200MHz Span, 2MHz RBW Adjust the horizontal to have a few bursts of traffic. ~5-10us/div
See next slide for next steps
L b 4 U i MASK t C l ti f / ti
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Lab 4: Using a MASK to Correlating frequency/ time
47
Place NFP very near or at rest on the SPI through hole connector
Note: WrongProbe Shown here
L b4 U i M k t C l t f / ti
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Lab 4: Using a Mask to Correlate frequency/ time
48
You should see something similar to the screen below withoutthe NFP near
the emitter. Note that CH2 is minimized
With the NFP in place near the emitter a rise in harmonic distortion can beseen spanning between 30 MHz to 160 MHz
L b4 U i M k t C l t f / ti
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Lab 4: Using a Mask to Correlate frequency/ time
49
Move probe away so the transmission is not being captured
Set a MASK to stop on this emission and move the probe back into position
Lab4 UsingaMask toCorrelate freq enc / time
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Lab 4: Using a Mask to Correlate frequency/ time
50
Add back in the analog wave form of CH2 and show the correlation of thebursts of SPI data to the noise captured by the NFP
You canzoom if
neededhere
Lab4:UsingaMask toCorrelate frequency/ time
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Lab 4: Using a Mask to Correlate frequency/ time
51
Instructor ONLY: Perform a math function on CH2 to show the derivative (dx/dt) of CH2. This derivative is
the energy of the edge. Some of this energy is what is picked up by the NFP as it
emits from the signal connection. The math and CH2 waveform should look similar.
(Note that you need to scale the math function WAY down to have it appear on screen).
Lab5:Analyzingpowersupplyemissions
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SMPS | 52
Lab 5: Analyzing power supply emissions
l Power Suppl ies are the most common source of EMI and
other radiated emissions.
l A common DC-DC supply can operate in Buck (Step Down)
Boost (step up) and Inverter modes.
l Choice of inductor and other elements to match your
anticipated load and current draw can impact EMI emissions
l Peak output current is an important consideration choice of
inductor and diode for switching converter design.
BuckDCDCSupply
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SMPS | 53
Buck DC-DC SupplyWe willChange
R values
Lab 5: Isolating Sources of EMI
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Lab 5: Isolating Sources of EMI
SETUP
Ch1=Large Loop NFP
Preset
CH1=50 Ohm
Vertical Scale=10mV/div
FFT Settings (500MHZ CF, 1GHz Span, RBW:5MHz)
Display set to False Colors
Demo Board set to:2
54
BuckConverterEMIemissions
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Buck Converter EMI emissions
55
EMI Profile No Output Load
BuckConverterEMIemissions
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Buck Converter EMI emissions
56
EMI Profile Matched Load
Changeto
3
BuckConverterEMIemissions
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Buck Converter EMI emissions
57
EMI Profile Oversized Load
Changeto
4
Sourcesof theEMITransmission
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Sources of the EMI Transmission
58
PressRUN/CONT
To stop theaquisition
Select FFTand drag awindowaround anoise burst
and aquiet spot
InductorVoltageView OversizedLoad
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Inductor Voltage View Oversized Load
59
Remove FFTsPress RON/CONT
Add a voltage probe
to CH2 to view theVoltage at theinductor
Cursors can help verify time alignment
InductorVoltage MatchedLoad
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Inductor Voltage Matched Load
60
Changeto
2
Debugging EMI Using a Digital Oscilloscope
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Debugging EMI Using a Digital Oscilloscope
Summary The modern oscilloscope with hardware DDC and overlapping FFT is capable of
far more than a traditional oscilloscope
EMI Debugging with an Oscilloscope enables correlation of interfering signals
with time domain while maintaining very fast and lively update rate.
The combination of synchronized time and frequency domain analysis withadvanced triggers allows engineers to gain insight on EMI problems to isolate
and converge the solution quickly.
Power Supply design choices have a large impact on EMI emissions, frequency
and time techniques can help unravel the mystery.