debugging emi

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.



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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.



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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.


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?



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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)

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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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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

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Magnetic and Electrical Near-Field Probes

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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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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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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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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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Fourier Transform Concept

Any real waveform can beproduced by adding sine waves


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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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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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Measurement Consideration: Time Gating

Signal characteristics change over the acquisition intervalGating allows selection of specific time intervals for analysis


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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

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Low Noise and High Sensitivityat 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/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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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

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Any real waveform can be

produced by adding sine waves


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Lab 1: Working with FFTs

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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

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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.


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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.


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Observe the Spectrum While Scanning With a Near-

Field Probe

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I) General Approach

Wide Span scan fundamental of interfering signals are usually lower than 1GHz,

a span of


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Field Probe

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I) General Approach


a span of


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Field Probe

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I) General Approach


a span of


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Field Probe

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I) General Approach


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.

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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

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Signal Harmonics


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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)

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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


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


FFT=10MHz CF, 20MHz Span, 20KHz RBW

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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.

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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.

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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.

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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.

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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

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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

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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

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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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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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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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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



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SETUP

Ch1=Large Loop NFP

Preset

CH1=50 Ohm


FFT Settings (500MHZ CF, 1GHz Span, RBW:5MHz)

Display set to False Colors

Demo Board set to:2

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BuckConverterEMIemissions


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Buck Converter EMI emissions

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EMI Profile No Output Load



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EMI Profile Matched Load

Changeto

3



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EMI Profile Oversized Load

Changeto

4

Sourcesof theEMITransmission


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Sources of the EMI Transmission

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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

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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

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Changeto

2



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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.

debugging emi

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