This document discusses debugging electromagnetic interference (EMI) using digital oscilloscopes, detailing key concepts such as radiated emissions, interference sources, and coupling mechanisms. It outlines practical steps to reduce differential and common mode RF emissions, explores the use of near-field probes for analysis, and emphasizes the importance of both time and frequency domain measurements in EMI debugging. The document concludes that synchronizing time and frequency analysis can help engineers quickly identify and mitigate EMI issues.

1. 06/2009 | Fundamentals of DSOs | 1Nov 2010 | Scope Seminar – Signal Fidelity | 1
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Debugging EMI Using a Digital
Oscilloscope
Dave Rishavy
Product Manager - Oscilloscopes

2. Debugging EMI Using a Digital Oscilloscope
2
l Background – radiated emissions
l Basics of near field probing
l Frequency domain analysis using an oscilloscope
l FFT computation
l Dynamic range and sensitivity
l Time gating
l Frequency domain triggering
l EMI debugging process
l Measurement example

3. Background – radiated emissions

4. Basic Principles: Radiated Emissions
The following conditions must exist
ı An Interference source
 A sufficiently high enough disturbance level
 in a frequency range that is relevant for RF emissions
ı A Coupling mechanism
 transmits disturbance signals from the interference source to the emitting
element
ı An Emitting element (Antenna)
 capable of radiating the energy produced by the source into the far field
4

5. Basic Principles: Radiated Emissions
The following conditions must exist
ı An Interference source
 A sufficiently high enough disturbance level
 in a frequency range that is relevant for RF emissions
ı A Coupling mechanism
 transmits disturbance signals from the interference source to the emitting
element
ı An Emitting element (Antenna)
 capable of radiating the energy produced by the source into the far field
5

6. Interference sources
ı Fast switching signals within
digital circuits
 Single-ended (asymmetrical) data
signals
 Switched mode power supplies -
harmonics
 Differential data signals with
significant common mode
component
6
ı High-order harmonics decrease at 20
to 40 db/decade
ı Structures on the PC board can
begin to resonate at harmonic
frequency

7. Inference Sources:
Differential Mode RF Emissions
ı Emission results when
signal and return are not
routed together
ı Near field probe can detect
this by positioning within
the loop – position of
probe is critical
7

8. General steps to help reduce Differential Mode RF
emissions
ı Reduction of the loop area (i.e. closer routing of the forward and
return conductors)
ı Reduction of the current in the conductor loop (if possible without
impacting the circuit operation.)
ı Reduce the rise/fall times for the transmitted data signals
ı Use filtering to eliminate higher-frequency signal components
(limit the disturbance spectrum.)
8

9. Inference Sources:
Common Mode RF Emissions
ı Common problem in multi-
layer PC boards
ı Caused by parasitic
inductance in return path or
asymmetrical transmission
ı External cable acts as an
antenna
ı Rule of thumb for line length
as an antenna:
 λ/10 not critical
 λ/6 critical
9

10. Common Mode RF Emissions
10
Best Possible Differential mode transmission
Undesired parasitic capacitance in
return path
Unbalanced parasitic terminating
impedances

11. General steps to help reduce common-mode RF
emissions
ı Reduce the RFI current ICM by optimizing the layout, reducing the
ground plane impedances or rearranging components
ı Reduce higher-frequency signal components through filtering or
by reducing the rise and fall times of digital signals
ı Use shielding (lines, enclosures, etc.)
ı Optimize the signal integrity to reduce unwanted overshoots
(ringing)
11

12. Basic Principles: Radiated Emissions
The following conditions must exist
ı An Interference source
 A sufficiently high enough disturbance level
 in a frequency range that is relevant for RF emissions
ı A Coupling mechanism
 transmits disturbance signals from the interference source to the emitting
element
ı An Emitting element (Antenna)
 capable of radiating the energy produced by the source into the far field
12

13. Coupling Mechanisms
ı Three coupling paths:
 Direct RF emissions from the source, e.g. from a trace or an individual
component
 RF emissions via connected power supply, data or signal lines
 Conducted emission via connected power supply, data or signal lines
ı Coupling Mechanisms
 Coupling via a common impedance
 Electric field coupling – parasitic capacitance between source and antenna
 Magnetic field coupling – parasitic inductance between source and antenna
 Electromagnetic coupling – far field coupling (greater than 1 wavelength)
13

14. Basic Principles: Radiated Emissions
The following conditions must exist
ı An Interference source
 A sufficiently high enough disturbance level
 in a frequency range that is relevant for RF emissions
ı A Coupling mechanism
 transmits disturbance signals from the interference source to the emitting
element
ı An Emitting element (Antenna)
 capable of radiating the energy produced by the source into the far field
14

15. Emitting Elements (Antennas)
ı Unintentional antennas in electronic equipment
 Connected lines (power supply, data/signal/control lines)
 Printed circuit board tracks and planes
 Internal cables between system components
 Components and heat sinks
 Slots and openings in enclosures
ı Main factor is the length of the antenna with respect to the
wavelength of the interference.
Rule of thumb – antennas with length less than λ/10 are not critical
15

16. Basics of near field probing

17. 17
Near Field Definition
ı Sources with Low Voltage, but high current predominantly generate magnetic
fields (e.g. terminated high speed signals)
ı Sources with High Voltage, but low current predominantly generate electrical fields
(e.g. unterminated signals)
Near field Transition Far field
EfieldHfield
Distance from DUT
r
Waveimpedance
r = 1.6m for
f > 30 MHz

18. 18
EMI Debugging Example
Oscilloscope and accessories
R&S ® RTO
R&S ® RTE
Nearfield probes
R&S ® HZ-15
E- and H-field
30 MHz – 1 GHz
Applicable from 100 kHz
Optional:
R&S ® HZ-16
preamplifier
Current clamp
R&S ® EZ-17

19. Magnetic and Electrical Near-Field Probes
19
ı Basically the probes are antennas that pickup the magnetic & electric field variation
ı The output Depends on the position & orientation of the probe

20. 20
H-Field Probe
ı Maximum response with probe parallel with current and closest to the
current carrying conductor
ı Traces with relatively high current, terminated wires and cables
Current flow
H field
Vo

21. 21
E-Field Probe
ı Maximum response with probe perpendicular with current and closest to
the current carrying conductor
ı Traces with relatively high voltage: unterminated Cables, PCB traces to
high impedance logic (tri-state outputs of logic IC’s)
Current flow
E field
Vo

22. Frequency domain analysis using an
oscilloscope

23. Using an Oscilloscope for EMI Debugging
ıBenefits
 Wide instantaneous frequency coverage
 Overlapping FFT computation with color grading
 Gates FFT analysis for correlated time-frequency analysis
 Frequency masks for triggering on intermittent events
 Deep memory for capture of long signal sequences
ıLimitations
 Dynamic range
 No preselection
 No standard-compliant detectors (i.e CISPR)
23

24. 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 Ω 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 distinguish CW signals and burst
FFT – Span / RBW Easy to use “familiar” interface, Lively Update
Signal zoom & FFT
gating
Easily isolate spurious spectral components in time
domain
24

25. 25
Frequency Domain Analysis
FFT Basics
ı NFFT Number of consecutive samples (acquired in
time domain), power of 2 (e.g. 1024)
ı ∆ fFFT Frequency resolution (RBW)
ı tint integration time
ı fs sample rate
FFT
s
FFT
N
f
t
f ==∆
int
1
Integration time tint
NFFT samples input for FFT
FFT
Total bandwidth fs
NFFT filter output of FFT
FFTf∆ts

26. FFT as Basis for EMI Debugging with Oscilloscopes
Conventional FFT Implementation on a Scope
26
t
Time Domain
Record length T
Windowing FFT
Data acquisition
Zoom
(f1…f2)
f
Frequency Domain
∆t = 1/Fs
f
Display
f2f1
Fmax = Fs/2
S(f) S(f)
x(t)
∆f = 1/T
f1 f2

27. FFT on the Rohde and Schwarz RTE and RTO
Spectrum Analyzer Use Model
ı Use model: Frequency domain
controls time domain
 Time domain parameters automatically changed as
necessary
ı Downconversion FFT (DDC) zooms into
frequency range before FFT
 Largely reduced record length, much faster FFT
Time
Domain
t
Record length T
Data acquisition
Fs=2Β
x(t)
Frequency
Domain
f
Display
f2f1
Β=f2-f1
S(f)
∆f = 1/T
Windowing FFT
27
HW Zoom (DDC)
NCO
Decim-
ation
LP
Zoom happens here –
before the FFT
500 MHz center, 10 MHz span:
Fs = 1 GS/s vs 20 MS/s

28. Measurement Consideration
Gated FFT in the RTE and RTO
Practical Time-Frequency Analysis
50% overlap (default setting)Gated FFT:
|---------------------------------- One complete Time-Domain capture ----------------------------|
The Key to unraveling the time domain
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29. FFT Gating
29

30. Measurement Consideration: Sensitivity
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
Low Noise and High Sensitivity
at Full Bandwidth
1mV/div
30

31. 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/
NFFT
l 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
1
log10 10
31

32. Measurement Consideration: Signal to Noise
>80 dB
32

33. ı Mask Tool
ı 6 dB EMI filter?
 Not critical for precompliance, will change results only slightly.
Measurement Consideration: Limit Lines
33
Upper for limit line usage
Mask definition
in units of FFT
Upper region
mask acting
as limit line
Stop-on mask violation setting is very useful!

34. Measurement Consideration:
Frequency Mask Triggering
34

35. EMI debugging process

36. The Problem: isolating sources of EMI
ı EMI compliance is tested in the RF far field
 Compliance is based on specific allowable power levels as a function of
frequency using a specific antenna, resolution bandwidth and distance from
the DUT
 No localization of specific emitters within the DUT
ı What happens when compliance fails?
 Need to locate where the offending emitter is within the DUT
 Local probing in the near field (close to the DUT) can help physically locate
the problem
 Remediate using shielding or by reducing the EM radiation
ı How do we find the source?
 Frequency domain measurement
 Time/frequency domain measurement
 Localizing in space
36

37. E) Nearfield probe
to localize the interferer
source
D) Interferer current
measurement to find
out the coupling type
B) „Know your DUT“:
List of potential interferer sources
37
A) Far-field measurement C) Reference measurement without DUT
Source Frequency
Clock frequency e.g. 25 MHz + Multiples
Ethernet PHY e.g. 125 MHz + Multiples
Voltage converter / power
adapter
broadband
…
EMI compliant testing / Test lab EMI Debugging / R&D
F) Analysis of counter-measures
EMI Debugging Procedure
Analysis steps

38. Observe the Spectrum While Scanning With a Near-
Field Probe
38
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

39. Observe the Spectrum While Scanning With a Near-
Field Probe
39
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

40. Observe the Spectrum While Scanning With a Near-
Field Probe
40
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

41. Measurement Example

42. Measurement Example – IP Phone
42

43. Example: IP-phone
Situation
ı IP-phone components
 Complex processor unit
 DDR2 memory
 Ethernet Layer 2 Switch
 2 x Gigabit Ethernet PHYs
 Several DC/DC Converters
 SPI-Interface to keyboard module
 Analog circuits (loudspeaker, Microphone)
ı Failed in EMI compliant test
43
Frequency
(MHz)
Level
(dBµV/m)
Limit
(dBµV/m)
Margin
(dB)
Height
(cm)
Azimuth
(deg)
Polarization
248.68 41.20 47.50 6.30 0.0 157.00 HOR
250.00 44.50 47.50 3.00 0.0 293.00 HOR
375.00 52.30 47.50 -4.80 0.0 359.00 HOR

44. Far Field Test Result
44

45. RFI Current Measurement
375 MHz Spur
Peak detect separates
intermittent interference
45

46. ı Additionally detected emissions on following frequencies:
 CW: 375 MHz
 Broadband: 250 MHz
46
200 MHz 425 MHz
250 MHz
375 MHz
Example: IP-phone
Current-probing on interface lines

47. Example: IP-phone
Nearfield-probing for source localization
Lokalisierung
47
DC/DC-converter no. 2
No significant emission
Nearfield spectrum in the area of
the processor module;
among others 375 MHz interferer

48. Correlating Time and Frequency Domains
48

49. Example: IP-phone
Results
ı Interferer signal detected on interface lines
 The interferer is probably transferred via common-mode coupling
ı Interferer sources localized
 DC/DC converter no. 1
 Processor module respectively LAN PHY interfaces
ı Analysis of layout and implementation of counter measures.
49

50. Debugging EMI Using a Digital Oscilloscope
Summary
ı If we can measure something in the far field, it must have an electric and
magnetic near field source.
ı The conditions required for a radiated emission allow us insight to track down a
source and mitigate potential interferers.
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 source and solution quickly.
ı Please see this shortcut to our application note for additional information:
http://goo.gl/rvpfCK
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