debugging emi problems using a real time oscilloscope and near-field probes

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

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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

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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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H-Field Probe

l Maximum response with probe parallel with current and

closest to the current carrying conductor

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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

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Fourier Transform Concept

Any real waveform can be

produced by adding sine

waves

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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

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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

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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 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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