debugging emi using a digital oscilloscope - west … use for emi problems.pdftest failure l near...

06/2009 | Fundamentals of DSOs | 1 Nov 2010 | Scope Seminar – Signal Fidelity | 1

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Debugging EMI Using a Digital Oscilloscope


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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

Presenter

Presentation Notes

Lower noise results in


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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

Presenter

Presentation Notes


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


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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

Presenter

Presentation Notes


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


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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

Presenter

Presentation Notes



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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

Presenter

Presentation Notes



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

Presenter

Presentation Notes

Another limitation of the FFT in spectrum analysis applications is the inefficiency of the computation for analysis of narrow spans at high frequencies. A conventional FFT is computed on a single record of data samples acquired by an A/D converter and the spectrum is computed from DC to ½ the sample rate. The desired resolution bandwidth is set by the number of time samples used in the computation and the span is set by zooming in on the desired section of the spectrum. A better approach is to select the desired center frequency using a down converter to shift the center frequency of the span of interest down to DC and then sampling this bandwidth limited spectrum at a lower rate consistent with the span of interest. The lower sampling rate allows a much narrower resolution bandwidth using a smaller number of time domain samples. The shorter time record improves the computational throughput dramatically.


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 downconversion fc

SW Zoom (f3…f4)

Presenter

Presentation Notes

This slide shows a more detailed comparison of the two methods of computation. The conventional FFT is computed on the complete data record and then zooming in on the desired span. The R&S RTO uses a digital down converter to shift the center frequency and span of interest to DC and reduce the sample rate at its output. The window function and FFT is computed on the shorter time record at a much faster rate. Zooming is also possible after the FFT.


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

Presenter

Presentation Notes

The fast computation of the FFT that results from the use of the down converter can be exploited in overlap processing. Conventional oscilloscopes compute FFT‘s on time records that are separated in time by an amount equal to the processing time for each FFT. Overlap processing allows the computation of multiple FFT‘s on overlapping data records in such a way that data making up one FFT consists of a percentage of the data from the previous FFT with the remaining part being new data. The percentage of overlap can be set by the user. In this slide, a 50% overlap is shown. Overlap processing is valuable because most signals are dynamic and change over time and, as you recall, the window function attenuates the signal at the edges of the inteval. Intermittent signals that appear near the edges of the interval will not be seen due to the weighting of the window but with a 50% overlap, the signal attenuated in one FFT will be seen in the next one.


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

Presenter

Presentation Notes

A non-integral number of cycles cause discontinuities in the waveform produce high frequency transients which add false frequency information => Leakage Multiply the waveform record by applying a window function that is zero at the both ends, reducing the discontinuities, resulting a more accurate FFT measurement.


FFT Overlap Processing l Overlap Processing ensures no signal details are missed

Original Signal

…

Presenter

Presentation Notes

A non-integral number of cycles cause discontinuities in the waveform produce high frequency transients which add false frequency information => Leakage Multiply the waveform record by applying a window function that is zero at the both ends, reducing the discontinuities, resulting a more accurate FFT measurement.

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


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a span of <1GHz is sufficient as a start ı Identify abnormal spurious or behavior and its location while moving the probe

around


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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 example Locating Broadband Noise Source

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Abnormal Spikes observed

The spike caused the broadband noise

Smaller spike observed too


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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

Presenter

Presentation Notes

A simplified implementation of a synchronous buck converter is show here. The switches are replaced with FET’s . The transistors must operate in a break before make mode so that both are not conducting at the same time otherwise, large amounts of power will be dissipated. The control signal C is applied to Qa while its inverse is applied to Qb to ensure this operation. Unlike ideal switches, however, FET’s do not switch instantly and have a small amount of loss in the on state.

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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debugging emi using a digital oscilloscope - west … use for emi problems.pdftest failure l near...

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