debugging emi problems using a real time oscilloscope and near-field probes
DESCRIPTION
Electromagnetic interference is increasingly becoming a problem in complex systems that must interoperate in both high speed digital and RF domains. When failures due to EMI occur it is often difficult to track down the sources using standard test receivers and spectrum analyzers. This session will focus on the ability of modern oscilloscope hardware to view signals in both frequency and time domains to troubleshoot and track down these elusive issues. Deploying techniques such as frequency and time gating as well as time and frequency triggering and mask analysis allow for the capture transient events. These techniques reveal details about the sources of EMI that can quickly lead to mitigation strategies.TRANSCRIPT
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 H-Field
l E-Field
l Measurement considerations for correlating time and frequency
domains
l Frequency analysis capabilities
l Important scope parameters for EMI Debug
l EMI Debug Examples
l Working with FFT’s
l Finding a sources of EMI
l Isolating a source of EMI
l EMI from switch mode power supplies
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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1
2
3
4
5
6
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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 f
ield
H
fie
ld
Distance from DUT
r W
ave
im
pe
da
nce
r = 1.6m for
f > 30 MHz
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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Nov 2010 | Scope Seminar – Signal Fidelity | 8
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H-Field Probe
l Maximum response with probe parallel with current and
closest to the current carrying conductor
06/2009 | Fundamentals of DSOs | 9
Nov 2010 | Scope Seminar – Signal Fidelity | 9
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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 | 10
Nov 2010 | Scope Seminar – Signal Fidelity | 10
Fourier Transform Concept
Any real waveform can be
produced by adding sine
waves
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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 | 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 | 17
Nov 2010 | Scope Seminar – Signal Fidelity | 17
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 W for near-field probes (important for bandwidth)
Vertical sensitivity 1 – 5 mV/div is usually a good setting
Color table &
persistence Easily detect and distinguish CW signals and burst
FFT – Span / RBW Constant Span / resolution bandwidth factor (100 – 1000)
for easy usage when changing frequecy span
Signal zoom & FFT
gating
Easily isolate spurious spectral components in time
domain
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
Objective: Learn how to make frequency domain measurements using an FFT on
an oscilloscope
Example 1: Working with FFT’s
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ı Any real waveform can be
produced by adding sine waves
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Example 2: Finding Sources of EMI Objective: Use a Near Field Probe to locate a narrow band signal and determine
it’s frequency
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Example 2: Finding Sources of EMI Objective: Use a Near Field Probe to locate a narrow band signal and determine
it’s frequency
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Example 2: Finding Sources of EMI Objective: Use a Near Field Probe to locate a narrow band signal and determine
it’s frequency
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Example 2: Finding Sources of EMI Objective: Use a Near Field Probe to locate a narrow band signal and determine
it’s frequency
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Example 2: Finding Sources of EMI Objective: Use a Near Field Probe to locate a narrow band signal and determine
it’s frequency
Example 3: Isolating Sources of EMI Objective: 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.
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Example 3: Isolating Sources of EMI Step 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.
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Example 3: Isolating Sources of EMI ı 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.
ı You should see a strong signature by the pins next to the decoupling cap.
ı 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.
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Example 3: Isolating Sources of EMI ı Step 4: Connect the “blade” E field probe to the scope and probe the traces down near
the 10Mhz crystal.
ı 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
ı 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.
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Example 4: Using a MASK to isolate the event in
frequency/ time
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Place NFP very near or at rest on the SPI through hole connector
Objective: It is important to correlate interfering emissions from signal harmonics
to the time domain. The Gate in the time domain view can reveal much about
where an emission originated.
Example 4: Use a Gate to Correlating frequency/ time
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FFT Gating isolates the spectrum of a time
event
Tg
gTf
1
Example 5: Analyzing power supply emissions
ı Power Supplies are the most common source of EMI and other radiated
emissions.
ı A common DC-DC supply can operate in Buck (Step Down) Boost (step
up) and Inverter modes.
ı Choice of inductor and other elements to match your anticipated load and
current draw can impact EMI emissions
ı Peak output current is an important consideration choice of inductor and
diode for switching converter design.
Buck DC-DC Supply We will
Change
R values
Sources of the EMI Transmission
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Press ‘RUN/CONT’
To stop the
aquisition
Select FFT
and drag a
window
around a
noise burst
and a
“quiet” spot
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.
ı Power Supply design choices have a large impact on EMI emissions, frequency
and time techniques can help unravel the mystery.
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