EMI Troubleshooting App Note 48W 67730 0
EMI Troubleshooting App Note 48W 67730 0
EMI Troubleshooting App Note 48W 67730 0
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
• EMC implies that the equipment being developed is Many product designers may be familiar with how near field
compatible within the expected operating environment. For probes may be used to identify EMI “hot spots” on PC boards
example, a ruggedized satellite communications system and cables but may not know what to do next with this
when mounted in a military vehicle must work as expected, information. We’ll use Tektronix Spectrum View on a 6 Series
even in the vicinity of other high-powered transmitters Mixed Signal Oscilloscope as an example. Here’s a simple
or radars. This implies both emissions and immunity three-step process for EMI troubleshooting.
• EMI (also sometimes referred to as radio frequency or A/D converters, DC-DC converters, and other sources,
interference, or RFI) is more concerned with a product which produce high frequency, fast-edged, digital signals.
interfering with existing radio, television, or other If the product includes a shielded enclosure, probe for
communications systems, such as mobile telephone. leaky seams of other apertures. Record the emission
Outside the U.S. it also includes immunity to external profile of each energy sources.
energy sources, such as electrostatic discharge and Step 2 – Use a current probe to measure high frequency
power line transients. This usually applies to commercial, cable currents. Remember, cables are the most likely
consumer, industrial, medical, and scientific products. structure to radiate RF energy. Move the probe back and
Radiated emissions is usually measured at a 3 m or forth along the cable to maximize the highest harmonic
10 m test distance. This app note will focus on EMI currents. Record the emission profile of each cable.
Troubleshooting.
Step 3 – Use a nearby antenna (typically, a 1 m test
distance) to determine which of the harmonic signals
actually radiate. Record these harmonics and compare
to the near field and current probe cable measurements.
This will help you determine the most likely energy sources
that are coupling to cables or seams and radiating to
the antenna.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
STEP 1 - Near Field Probing - Most near field probe kits Also, note that H-field probes are most sensitive (will couple
come with both E-field and H-field probes. Deciding on the most magnetic flux) when their plane is oriented in parallel
H-field or E-field probes depends on whether you’ll be with the trace or cable. It’s also best to position the probe at
probing currents – that is, high di/dt – (circuit traces, cables, 90 degrees to the plane of the PC board. See Figure 2.
etc.) or voltages – that is, dV/dt – (switching power supplies,
etc.) respectively. Most troubleshooting is done with H-field
probes, because we’re usually interested in tracing high
frequency harmonic currents. The smaller diameter ones
provide higher resolution but may need preamplification to
boost their signals. However, both H- and E-field probes
are useful for locating leaky seams or gaps in shielded
enclosures.
Start with the larger H-field probe and sniff around the
product enclosure, circuit board(s), and attached cables.
The objective is to identify major EMI sources and dominant
narrow band and broadband frequencies. Document the
locations and frequency characteristics observed. As you
zero in on sources, you may wish to switch to the middle-
sized (1 cm) H-field probe (Figure 1), which will offer greater Figure 2. H-field probes offer the best sensitivity when oriented in relation to the
circuit trace or cable, as shown, because they collect the maximum flux lines
resolution (but slightly less sensitivity). You may find most through the loop.
probing is eventually done with this probe.
Remember that not all sources of high frequency energy
located on the board will actually radiate! Radiation requires
some form of coupling to an “antenna-like” structure, such as
an I/O cable, power cable, or seam in the shielded enclosure.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
STEP 2 - Current Probe Measurements - Next, measure STEP 3 - Troubleshooting with a Close-Spaced Antenna
the attached common mode cable currents (including power - Once the product’s harmonic profile is fully characterized,
cables) with a high frequency current probe, such as the it’s time to see which harmonics actually radiate. To do this,
Com-Power CLCE-400, or equivalent (Figure 3). Document we can use an uncalibrated antenna connected to a 4/5/6
the locations of the top several harmonics and compare with Series MSO spaced at least 1m away from the product or
the list determined by near field probing. These will be the system under test to measure the actual emissions (Figure
most likely to radiate and cause test failures because they are 4). Typically, it will be radiation from attached I/O or power
flowing on antenna-like structures (cables). cables, as well as leakage in seams or apertures of the
shielded enclosure. Compare this data to that of the near
Note that it only takes only 5 to 8 μA of high frequency
field and current probes. The actual measured emissions
current to fail the FCC or CISPR Class B test limits. Using the
should indicate the energy source as identified by the
manufacturer’s supplied transfer impedance calibration curve
previous probing.
will help you calculate the current from the analyzer voltage at
the probe port.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
Often, when troubleshooting emissions, you may have already least that, or more, for a safe margin. Therefore, a calibrated
run a formal compliance test and know how far over limit the antenna is not required, as only relative changes are
harmonics are. So, when troubleshooting, the important point important. The antenna also does not necessarily need to be
is that relative measurements are generally important, rather tuned to the frequency of the harmonics. The important thing
than absolute. That is, if we know certain harmonics are 5 to is that harmonic content from the EUT should be easily visible.
10 dB over the limit, the goal would be to reduce these by at
If there are just a few harmonics of concern, often, it’s easier to narrow the frequency span on the spectrum analyzer down
to “zoom in” on a particular harmonic frequency, thus eliminating most of the ambient signals. A common example is
differentiating a 100 MHz clock harmonic from the 99.9 MHz FM broadcast band channel. If the harmonic is narrow band
continuous wave (CW) partially hidden among the modulation frequencies, then reducing the resolution bandwidth (RBW) can
also help separate the harmonic. Just be sure reducing the RBW doesn’t also reduce the harmonic amplitude as well.
Often, it’s good to see the “big picture” of ambient signals within the entire band of interest. To account for ambient signals
using Spectrum View, first turn off the device under test and set up the spectrum limits and Span as desired. For this example,
we’ll use a Center Frequency of 100 MHz and a Span of 200 MHz. A resolution bandwidth (RBW) of 10 kHz for general
purpose troubleshooting can be a good place to start in order to clearly resolve the harmonics. Then, save the spectrum plot
as an ambient baseline by using File > Save As > Ch1 > SV_Normal > then enter a filename > Save (Figure A).
Figure A. With the device under test OFF, save the ambient spectrum using “SV_Normal”.
This will record the various broadcast stations, two-way radios, digital TV, and cellular phone signals. Recall the saved
waveform using Recall > Waveform > select the Filename > Recall (Figure B). Then, turn on the EUT and save a sweep to
record both the ambient and EUT signals. You’ll end up with a screen similar to Figure C.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
You’ll notice a lot of activity around the FM broadcast band (88–108 MHz), the digital TV band (470 to 608 MHz), and cellular
phone (generally 600 to 850 MHz for bands below 1000 MHz). Refer to Reference 6 for more details on cellular frequency
bands within the U.S.
Figure C – An example of an ambient measurement (in gray) and the scan of the device under test (in yellow). Visually noting the differences will reveal the harmonics
from the device under test.
The technique is not foolproof, as there may be additional two-way radio transmissions that are not caught in the ambient
capture, but it will still give you a good idea as to what signals are coming from the device under test. To confirm whether a
harmonic frequency is from the device being measured, you may need to power it off occasionally.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
Time-Correlated Troubleshooting
If you’re unable to stabilize the time domain waveform, and the
signal is “messy”, but arrives in fixed bursts (very common for
digital bus or IoT/wireless circuits), try measuring the period
between bursts and setting the Holdoff time to stabilize the
trigger. If this doesn’t work, you can also Stop the acquisition
to analyze the stored waveform data.
time domain waveform, you may be able to observe time bursts are shown in the time domain screen. Adjust
dependencies on the spectrum plot. See Figures 8 and 9 for the Trigger Level for a stable display or simply stop the
examples of Spectrum Time on the oscilloscope’s display. acquisitions using the Run/Stop front panel button (upper-
right). Then use the Zoom knob (lower-right) turned clockwise
As an example, we’ll measure the DDR RAM bus noise from a to start magnifying the waveform.
fingerprint scanner used as a secure entry access (Figure 7).
There is a flex cable directly adjacent to this memory IC, Notice the zoom window as it progressively gets shorter as
which is coupling to the data bus and radiating a regular you zoom in. By grabbing this zoom window with the mouse
pulse of EMI. Very often a pulse of digital activity can produce or your finger and sliding it back and forth along the time
strong harmonic content. domain waveform, you’ll be able to see the effect on the
harmonics of various portions of the time domain waveform.
From this, you may be able to deduce the origin of these
bursts and apply some mitigations. Figures 8 and 9 show
how the different portions of the time domain waveform affect
the harmonic content and how the Spectrum View window
updates to show the spectrum content that correlates to the
selected Spectrum Time. At the leading edge of the burst,
harmonics increase by 20 dB.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
Figure 8. The zoom window is positioned at a relatively quiet portion of the time domain wave.
Figure 9. When Spectrum Time is moved over to the noisy digital burst in the time domain, the EMI increases by 20 dB. With Spectrum Time, it’s possible to analyze
whether this pulse of EMI can couple to cables and radiate.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
Hardware Triggered Troubleshooting - The Tektronix 4/5/6 Magnitude versus Time triggering measures the magnitude
Series MSOs have an RF versus Time hardware trigger option of the frequency content and allows triggering on magnitude
enabled by the RFVT option and found within the Trigger edges, pulse widths and timeout events. This is helpful
badge (RF vs Time trace must be active for trigger option to when the frequency content may correlate to certain
be presented). harmonic conditions, such as harmonics which pulse upward
intermittently.
• Magnitude versus Time - displays the RF Magnitude versus
Time in a separate plot. For example, a 35 MHz harmonic was observed with a
medium-sized (1 cm) H-field probe to pulse upward by 30 dB
• Frequency versus Time - displays a RF Frequency on occasion. This intermittent harmonic could easily result
Deviation versus Time in a separate plot. in compliance failure should it couple out to an I/O cable
and radiate.
• Phase versus Time - displays the RF Phase versus Time in
a separate plot. Let’s assume the probe is connected to Channel 1. To set
up this special triggering mode, double-click the Channel 1
All three of these may be displayed simultaneously and
badge and select Spectrum View. Make sure it is turned ON,
hardware triggering can be selected for edge, pulse width and
and then turn on Normal and Max Hold. Turn on Magnitude in
timeout events on RF Magnitude vs Time and RF Frequency
the RF versus Time Waveforms section (Figure 10).
vs Time plots.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
Now double-click the Spectrum controls box towards the lower-right and set a Center Frequency of 100 MHz and Span of
200 MHz. I also prefer the vertical scale to read “dBuV”. This will display a spectrum from zero Hz to 200 MHz. Set the resolution
bandwidth (RBW) to 10 kHz, leaving all else to the default settings (Figure 11).
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
Finally, double-click the Trigger controls (bottom-right) and set the Trigger Type to Edge (default), the Source to Channel 1 and
hovering the mouse or a single tap with your finger will show the choice “Mag_vs_Time” and select it (Figure 12). Make sure that
“Mag_vs_Time” and Spectrum View are set up, otherwise this option will not be present. Now type in the desired trigger value or
position the “Channel1-Mag” plot so its visible and adjust the yellow arrowhead (within the vertical scale) upwards until the trigger
reference line is above the noise and somewhere within the magnitude pulse.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
If the digital pulses are spaced widely apart, you may need to adjust the Trigger Holdoff. Within the Trigger panel, select “Mode
and Holdoff”, set the Trigger Mode to Normal, the Holdoff to Time, and set the Holdoff Time to something larger than the pulse
width you are isolating. In this case, the pulse width was about 1.3 ms, so the holdoff time was set to 1.5 ms to ensure the trigger
would reset long after the pulse (Figure 13).
Figure 13. Setting the holdoff time, if required to help stabilize the trigger.
Notice the small yellow box along the horizontal axis of the time domain plot. This is the Spectrum Time selector, and its width
is dependent on the RBW. By grabbing the Spectrum Time box and sliding it back and forth along the time domain plot, you will
observe the frequency changes at different times in the spectral display.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
Figure 14. Set the Magnitude Trigger Level (Yellow Arrowhead circled in red) to a position above the residual noise floor and within the Magnitude pulse. The Spectrum
Time box (circled in red, with arrows) can be grabbed with the mouse or finger and slid back and forth to observe the effect on the spectrum display.
For example, with the box slid outside the pulse, the harmonics are at a minimum (Figure 14) and with the box slid into the start
of the pulse, the harmonics reach a maximum (Figure 15). The higher the RBW, the wider the Spectrum Time box, but the less
the resolution, so you may need to experiment to optimize the analysis for your system.
Figure 15. Sliding the Spectrum Time box into the leading portion of the pulse causes the 35 MHz harmonic (and associated spectrum) to increase by up to 30 dB.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
By plugging in an oscilloscope probe to Channel 2 and probing around your board, you should find other digital signals that
correlate to the pulses on Channel 1 and determine whether coupling to cables or enclosure seams is an issue for this 35 MHz
harmonic and what mitigations might be best suited to reduce any couplings.
Next, open the Spectrum View settings and set the Center Frequency to 150 MHz and Span to 200 MHz, with RBW set to
10 kHz. We can observe the oscillation clearly at 155 MHz (Figure 17).
Figure 17. The spurious oscillation can be easily observed in the center of the display.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
Let’s zoom in on the spurious oscillation by opening the Spectrum View settings and selecting a Span of 10 MHz and RBW of
500 Hz, so we can clearly see the frequency response. You may need to move the oscillation back into view by dragging the
Spectrum View to center the oscillation in the display (Figure 18).
Figure 18. The spurious oscillation is zoomed in and centered in the display. We’re triggering on the time domain waveform and can see it expanding and contracting as
the oscillation varies in frequency.
Now go back to the Channel 1 >> Spectrum View panel and select Frequency in the RF versus Time section. You’ll need to adjust
the Vertical Scale of the waveform and slow down the Horizontal Scale time base to 40 us/div so its easily viewable. This will
display the Frequency Versus Time plot of the oscillation (Figure 19).
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
Figure 19. Selecting the Frequency box in the RF versus Time section of the Spectrum View panel.
Now, open the Trigger panel and select Channel 1 >> Freq_vs_Time (Figure 20). This will allow triggering on the frequency
deviation versus time waveform at the bottom of the display. Adjust the trigger level (yellow arrowhead) to stabilize the
RF frequency vs time waveform on your display. You can also optionally stop the acquisitions using the Run/Stop button
(upper-right).
Figure 20. Open the Trigger panel and select “Freq_vs_Time” trigger.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
Note that the lower peak at 155.34 MHz is higher than the right-hand peak at 157.55 MHz (Figure 21). Examining the frequency
versus time plot at the bottom of the screen shows a peak deviation of about +1.5 MHz and a lower deviation of –1 MHz. This
difference approximately corresponds to the difference between the two end peak frequencies as shown by the markers of
157.55 – 155.34 = 2.21 MHz. Observe that the frequency deviation plot (red boxes) lingers at the lower –1 MHz more so than at
the +1.5 MHz, which is why the lower frequency peak is higher than the other. All standard time and frequency measurements
can be added to the RF vs Time plot and displayed in the measurement results badge on the right of the screen.
Figure 21. Frequency deviation analysis of the spurious oscillation indicates a deviation of about 2.2 MHz and that the oscillation lingers at the lower frequency
(155.5 MHz) longer than the higher frequency (157.2 MHz).
In the end, the spurious oscillation was easily controlled by adding output capacitance at the op-amp to control the open-loop
gain. This example does show the power of the frequency deviation versus time trigger in helping analyze the characteristics of
an unusual harmonic signal that varies in frequency. This would also be a powerful technique for characterizing spread spectrum
clock signals.
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Step by Step EMI Troubleshooting with 4, 5 and 6 Series MSO Oscilloscopes APPLICATION NOTE
Summary References
By combining the Tektronix 4, 5 and 6 Series MSOs with André and Wyatt, EMI Troubleshooting Cookbook for Product
integrated Spectrum View multi-domain analysis and the Designers, SciTech, 2014.
time versus frequency triggering, you’ll be able to capture
Ott, Electromagnetic Compatibility Engineering, Wiley, 2009
elusive EMC issues faster and easier. By developing your
own EMI troubleshooting test lab for radiated and conducted Tektronix 4 Series MSO oscilloscopes, https://www.tek.com/
emissions, you’ll save time and money by moving the oscilloscope/4-series-mso-mixed-signal-oscilloscope
troubleshooting process in-house. This will save you time
and cost when compared to performing troubleshooting at Tektronix 5 Series MSO oscilloscopes, https://www.tek.com/
commercial test labs. oscilloscope/5-series-mso-mixed-signal-oscilloscope
As technology continues to advance, we EMC engineers and Tektronix 6 Series MSO oscilloscopes, https://www.tek.com/
product designers need to upgrade our usual analysis test oscilloscope/6-series-mso-mixed-signal-oscilloscope
tools to stay one step ahead and be able to better capture and
Tektronix EMI home page, https://www.tek.com/application/
display the more unusual emissions expected. Oscilloscopes,
electromagnetic-interference-emi-and-electromagnetic-
such as the 4/5/6 Series MSOs, with time-corelated or
compatibility-emc
hardware (amplitude or frequency) triggering have already
proven to be invaluable for EMI debug and troubleshooting. 2017 EMI Pre-Compliance Test Guide (Interference
Advanced spectral analysis will be especially important Technology), http://learn.interferencetechnology.com/2017-
as mobile devices continue to shrink and more products emc-pre-compliance-test-guide/
incorporate wireless and other advanced digital modes.
Cellular frequencies in the U.S. (Wikipedia), https://
en.wikipedia.org/wiki/Cellular_frequencies_in_the_US
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Rev. 02.2018
Copyright © Tektronix. All rights reserved. Tektronix products are covered by U.S. and foreign patents, issued and pending. Information in this publication supersedes that
in all previously published material. Specification and price change privileges reserved. TEKTRONIX and TEK are registered trademarks of Tektronix, Inc. All other trade
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08/20 SBG 48W-67730-0