EMI Pre-compliance Testeing with a Spectrum Analyzer

EMI Pre-compliance Testeing with a Spectrum Analyzer
Low-cost EMI Pre-compliance
Testing Using a Spectrum Analyzer
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APPLICATION NOTE
Application Note
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Low-cost EMI Pre-compliance Testing Using a Spectrum Analyzer
EMI regulations are in place throughout the world to provide
improved reliability and safety for users of electrical and
electronic equipment. To ensure compliance with these
regulations, many companies employ the services of a
specialized test facility to perform the actual compliance tests
required for EMI certification. The test facility might belong to
an outside company (a “test house”) or to an in-house EMC
department.
Performing basic pre-compliance testing can help minimize
your time and expense at the compliance test house.
Performing pre-compliance testing can help you catch out
of specification conditions before you send your product for
formal testing. If you have already been to a test house and
your product failed the emissions test, testing in your own
lab gives you the time to methodically isolate your problem
areas and apply different corrections.
A great deal of time and effort goes into the design of
today’s products to minimize their EMI signatures. Most
engineers employ good design practices to minimize the
potential for EMI problems. It is common today to perform
pre-compliance measurements during the design and
prototyping stages to identify and address potential EMI
issues before the product is sent out for compliance testing.
These techniques reduce the risk that the product will fail
the final full compliance at the test house.
With the introduction of the Tektronix RSA306 USB based
Real Time Spectrum Analyzer, pre-compliance testing has
never been easier or more cost effective. This application
note provides an overview of EMI compliance testing,
pre-compliance testing, and the measurement regulations.
Test setups using the RSA306 and similar low cost products
are used to perform both radiated and conducted emission
measurements that can help you minimize both your expense
and schedule for getting your products EMI certified.
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Application Note
Compliance Testing
Compliance testing requires methods, equipment and
measurement sites in compliance with international standards.
Compliance tests are commonly done as part of the design
qualification prior to production of a device. Compliance
testing is exhaustive and time consuming, and a failure in EMC
at this stage of product development can cause expensive
re-design and product introduction delays.
The term Radiated Emissions refers to both the intentional
and unintentional release of electromagnetic energy from an
electronic device. To address this concern, a radiated test is
performed to ensure that emissions emanating from the DUT
comply with the applicable limits.
The term Conducted Emission refers to the mechanism that
enables electromagnetic energy to be created in an electronic
device and coupled to its AC power cord. Similar to radiated
emissions, the allowable conducted emissions from electronic
devices are controlled by different regulatory agencies.
Unless you are fortunate enough to have an accredited inhouse full compliance laboratory in your company, the best
choice is to involve a third-party compliance lab at the design
stage and then follow through with testing of your product at
the lab. There are many EMC labs around the world. In the
United States, the FCC maintains a list of laboratories certified
for compliance testing.
The full compliance test in a certificated lab is expensive
with costs ranging from $1,000 to $3,000 per day). Even if
you have your own internal full compliance lab, the time to
perform compliance test is significant. Failure of these tests
can mean some level of costly and time consuming design if
rework is required. It is best to do as much pre-compliance
verification as practical to reduce the risk of a failure during
compliance testing.
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Figure 1. Full compliance test facilities are expensive and are costly to rent. Minimizing
use of these facilities is important from both a cost and a schedule perspective (Image
courtesy of Microwave Vision Group).
Compliance Test Facilities
Some of the cost drivers for compliance testing are the
facilities and equipment needed to perform the testing.
Formal testing requires:
An EMC lab with large anechoic test chamber (Figure 1)
An EMI receiver with Quasi-peak detector and preamplifier,
that can test up to the tenth harmonic or to 40 GHz
Mast and 360° Turn table
EMI software controlling the test equipment like masts, turn
tables, EMI Receiver and report generator
Antennas
Line impedance stabilization network (LISN) and Transient
Limiter (Only for AC Conducted measurements if necessary)
Low-cost EMI Pre-compliance Testing Using a Spectrum Analyzer
Results from a Certified Test Lab
The EMI test house makes their radiated measurements in a
calibrated RF chamber and reports the results as a measure
of field strength. This example report (Figure 2) indicates that
there is a single peak which is above the limit for this specific
standard. Normally in the report you will also receive the
information in tabular format (Figure 3).
The report in Figure 3 shows the test frequency, measured
amplitude, calibrated correction factors, and adjusted field
strength. The adjusted field strength is compared to the
specification to determine the margin, or excess.
Figure 2. Compliance testing shows a failure around 90 MHz.
Figure 3. This data shows the failure from Figure 2 at approximately 89 MHz.
www.tektronix.com/emi
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Application Note
Country
Approval Regulatory Bodies
United States
Federal Communications Commission (FCC)
Canada
Industry Canada (IC)
Japan
Ministry of Internal Affairs and Communications (MIC)
China
Ministry of Industry and Information Technology (MIIT)
Table 1. Examples of regulatory bodies.
Figure 4. Affordable pre-compliance testing can easily be set up to uncover
potential problems so that you can minimize test time in more expensive compliance
test facilities.
Pre-Compliance Testing
In the EMI world, different equipment and techniques are
used at different stages of design and qualification. At the
early stages of development, design-for-EMC techniques
are combined with diagnostics to produce low compliance
signatures and low susceptibility to both external and internal
interference. Pre-compliance testing may be used to catch
compliance problems early and greatly improve the probability
of successful first pass of full EMC compliance testing without
additional re-design. If early compliance testing has identified
problem areas, pre-compliance testing offers a fast, low cost
method for evaluating modifications to your design.
Pre-compliance testing is not required to conform to
international standards; the goal is to uncover potential
problems and reduce risk of failure at the expensive
compliance test stage. The equipment used can be
noncompliant and have lower accuracy and dynamic range
than compliant receivers if sufficient margin is applied to the
test results. Pre-compliance testing requires:
Spectrum analyzer with peak detector (quasi-peak optional)
Preamplifier (optional)
Antenna with non-metallic stand for radiated emissions
Line impedance stabilization network (LISN) for conducted
Power limiter for conducted
Near field probes for diagnostics (optional)
Pre-compliance testing may be done in a certified lab using
fast measurement techniques intended to give a ‘quick look’
at problem areas, or done at a temporary site by engineering
personnel.
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When selecting a test site it is best to pick a location that will
minimize external signal sources. Rural areas, conference
rooms or basements are good because they minimize signals
that might mask the DUT emission levels you are trying to
measure (Figure 4).
General-purpose spectrum analyzers, such as the Tektronix
RSA306, that contain general purpose filters and detectors are
often employed in pre-compliance. They are fast measurement
tools that often are already used in the design process, so no
additional capital expense is required.
Spectrum Analyzer Settings for
EMI Measurements
Spectrum analyzers used for EMI measurement have a
defined receiver bandwidth, method of signal detection,
and method of averaging results to achieve signal levels.
In the case of many commercial EMI measurements, these
measurement elements are defined by the Comité International
Spécial des Perturbations Radioélectriques (CISPR), a
technical organization within the International Electrotechnical
Commission (IEC), an international standards body. Other
standards and certification bodies, such as TELEC in Japan,
also have requirements for measurement methods and
certification techniques. In the US, the Department of Defense
has developed the MIL-STD 461E with special requirements
for military equipment.
Some other requirements are not specified by the standards,
and are only subject to local geographic regulations. Operation
in countries within defined regulatory domains may be subject
to additional regulations. Implementers need to refer to the
country regulatory sources for further information. Table 1 lists
some of the current regulatory bodies and the countries in
which they have jurisdiction.
Low-cost EMI Pre-compliance Testing Using a Spectrum Analyzer
Frequency Range
Bandwidth (6 dB)
Reference BW
9 kHz to 150 kHz
(Band A)
100 Hz to 300 Hz
200 Hz
0.15 MHz to 30 MHz
(Band B)
8 kHz to 10 kHz
9 kHz
30 MHz to 1000 MHz
(Bands C and D)
100 kHz to 500 kHz
120 kHz
1 GHz to 18 GHz
(Band E)
300 kHz to 2 MHz
1 MHz
Table 2. Measurement Bandwidth versus Frequency specified by CISPR 16-1-1.
Frequency Range
Bandwidth (6 dB)
10 Hz-20 kHz
10, 100, and 1000 Hz
10-150 kHz
1 and 10 kHz
150 kHz-30 MHz
1 and 10 kHz
30 MHz-1 GHz
10 and 100 kHz
1-40 GHz
0.1, 1.0 and 10 MHz
Table 3. Bandwidths versus frequency specified for peak, average and RMS detectors
by ANSI C63.2.
Frequency Range
Bandwidth (6 dB)
30 Hz – 1 kHz
10 Hz
1 kHz-10 kHz
100 Hz
10 kHz-150 kHz
1 kHz
150 kHz-30 MHz
10 kHz
30 MH-1 GHz
100 kHz
Above 1 GHz
1 MHz
Table 4. Bandwidths versus Frequency specified by Mil-STD-461E.
Resolution Bandwidth (RBW)
The bandwidth of the measurement is defined by a receiver
bandwidth shape or a resolution bandwidth (RBW) filter in
the case of a spectrum analyzer. The bandwidths used are
representative of the perceived threats within the spectrum,
and the bandwidths vary with the receive frequency.
The level measured by a receiver or spectrum analyzer of
any non-continuous signal will depend upon the measurement
bandwidth used. To achieve consistent results, regulatory
Figure 5. Most EMI standards specify 6 dB bandwidth. Using 3 dB bandwidth will give
very different results.
agencies have defined the bandwidth and shape of the filters
used in compliance measurements. Filter bandwidths specified
by CISPR for peak, RMS, and average detectors are shown
in Table 2. The American National Standards Institute (ANSI)
and MIL-STD-461E bandwidths are shown in Tables 3 and 4,
respectively.
Figure 5 shows the differences in filter shape for 3-dB vs
6-dB filters. Both filter shapes are Gaussian, but the width
is different. The measurement filter bandwidth is specified at
some amount of power down from the peak. So a 100 kHz
3-dB filter is described by the yellow trace in Figure 5, where
the 100 kHz width occurs 3 dB down from the peak. The
100 kHz 6-dB filter has the same width, but is specified 6 dB
down from the peak.
RBW filters are generally specified at -3 dB for most
spectrum analyzer measurements outside of EMI testing.
However, 6-dB filters are used for most EMI measurements,
and CISPR16-1-1, ANSI, and MIL-STD 461E all specify
6-dB filters.
This is important because measurements will differ with filter
shape. While the peaks of the signals should be the same
level for a given 3-dB or 6-dB filter, the measured noise would
be lower for the same RBW setting between a 3-dB and a
6-dB filter.
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Application Note
Detection Methods
A detector calculates a single point that represents the
signal over a defined sampling interval. Detection methods
can calculate the positive or negative peak, the RMS or
average value of voltage, or the Quasi-Peak (QP) value. The
compliance labs use quasi-peak (QP) detectors for the full
compliance test, but the pre-compliance measurements can
be made with simple peak detectors for a more conservative
test margin. The EMI department or the external labs typically
begin their testing by performing a scan using simple peak
detectors to find problem areas that exceed or are close to the
specified limits. For signals that approach or exceed the limits,
they perform QP measurements. The QP detector is a special
detection method defined by EMI measurement standards.
The QP detector serves to detect the weighted peak value
(quasi-peak) of the envelope of a signal. It weights signals
depending upon their duration and repetition rate. Signals that
occur more frequently or last longer will result in a higher QP
measurement than infrequent, short impulses.
An example of peak and QP detection is seen in Figure 6.
Here, a signal with an 8 μs pulse width and 10 ms repetition
rate is seen in both peak and QP detection. The resultant
QP value is 10.1 dB lower than the peak value.
A good rule to remember is QP will always be less than or
equal to peak detect, never larger. The RSA306 offers peak
detection to do your EMI troubleshooting and diagnostics.
You don’t need to be accurate to perform an EMI department
or lab scan, since it is all relative. If your lab report shows
the DUT was 3 dB over the limit and your peak detected
measurement is 6 dB over, then you need to implement fixes
that reduce the signal by 3 dB or more.
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Figure 6. Quasi-peak responses will always be less than or equal to peak
detection, never larger. So you can use peak detection to do your EMI troubleshooting
and diagnostics.
Video Filters
Video filters are specified in some EMI measurements and
were the original method used in spectrum analyzers to
reduce the effects of noise variations in measurements. The
name video filter derives from the original implementation,
when low-pass filters were placed between the detected
output and the Y-axis analog drive input of the CRT on the
spectrum analyzer. RTSAs and some modern spectrum
analyzers use digital techniques to achieve this smoothing
of the noise on the signal. In most EMI measurement cases,
video filters are specified to be either off, or the video filter is
specified to be at least 3 times greater than the specified RBW
of the measurement.
Low-cost EMI Pre-compliance Testing Using a Spectrum Analyzer
Intentional Radiator Measurements
As the wireless revolution enters its next phase of deployment,
there is a shift to add wireless capabilities on a wide variety
of non-traditional products like thermostats, coffee makers,
and even toothbrushes. One challenging part of this revolution
is that it demands that product manufacturers learn how to
properly add this new wireless capability to their products.
From an EMI perspective, it will require additional intentional
radiator testing. An intentional radiator is a device that
broadcasts radio energy (not infrared or ultrasonic energy)
to perform its function. Intentional radiation is produced by
devices like:
Cell phones
Citizen’s band (CB) radios
Figure 7. Shows our pre-compliance setup located in a basement of one of our
buildings.
Walkie-talkies
Wireless connections
Bluetooth devices
Short range broadcast equipment
Wireless key-access systems
Active near field communications (NFC) and
radio frequency identification (RFID) systems
Clearly radio waves are needed for the energy transfer.
These devices intentionally use the radio spectrum and
therefore always require FCC or other equivalent equipment
authorization. Devices that are intentional radiators are also
subject to unintentional testing requirements. Emissions at
frequencies other than those the device is designed to use
can occur because of internal circuitry.
When selecting a spectrum analyzer for this type of testing,
it is important to select an instrument that can capture at least
the third harmonic (if not more) of the radiated signals being
generated within the device. The test setup for an intentional
radiator is the same as the radiated emissions setup shown
before. However, in this case, the frequencies of interest are
limited to the radiated frequencies and frequency masks
defined by the specifications, such as WiFi, Bluetooth, etc.
Tektronix has an application note that focuses on the details
of this measurement using a WiFi example.1
Case Study: Radiated Emission
Measurements
In our pre-compliance testing we used a distance of both
one meter and a few centimeters. Reducing the distance
between the DUT and the test antenna increases the
ratio of the DUT signal strength to RF background noise.
Unfortunately, near field results do not translate directly into the
far field tests used in EMI compliance testing, so one has to
be careful about drawing conclusions. (See the sidebar, “Near
Field vs. Far Field Measurements”.) Adding a pre-amplifier
is another good way to boost the relative DUT signal levels.
Figure 8 (on page 8) shows a block diagram of our setup.
1 “Regulatory Pre-compliance Testing for Wireless LAN Transmitter”,
http://www.tek.com/dl/55W-30065-2%2520WLAN%2520Pre-compliance
%2520App%2520Note_1615_0.pdf
www.tektronix.com/emi
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Application Note
Type N Connector
RF
DUT
0 to 1 meter
Non-metallic
DUT platform
Figure 8. Block diagram of pre-compliance radiated emissions test setup.
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Optional
Pre-Amp
USB 3.0
Low-cost EMI Pre-compliance Testing Using a Spectrum Analyzer
Figure 9. Test antennas. An Electro-Metrics EM-6912A Biconical Antenna, and a PCB log periodic antenna ordered from www.wa5vjb.com
Selecting Antennas
For our measurements we used three very low cost PC
board log periodic antennas2 and a biconical antenna. These
antennas were mounted on a tripod for easy placement.
The Antenna Factors (AF) and cable loss can be input into the
RSA306 for field strength correction (Figure 10). A biconical
antenna was used for the 20 to 200 MHz frequencies. The
longer 20 to 200 MHz wavelengths require a larger antenna,
and the background noise may also be an issue as it includes
many radio broadcast frequencies.
Characterizing Your Environment and
Test Results
After inputting the antenna correction factors and cable loss
to the RSA306, turn on the peak detectors and set the limit
lines. The limit lines were adjusted to adapt to our testing
environment.
Prior to turning on your DUT it is important to evaluate and
characterize your test environment. Is there enough signal
room between the limit line and your ambient noise floor?
Are there known signals that can be reduced? Do you need
to move your test set up to a quieter environment?
Figure 10. Low cost PC board log periodic antennas were used to cover the frequency
range of 400 MHz to 11 GHz and AF corrections were added in the RSA306.
2 The concept of using low-cost PC board log periodic antennas and the creative mounting design were taken from Ken Wyatt’s “The EMC Blog”, http://www.edn.com/
electronics-blogs/the-emc-blog/4403451/PC-board-log-periodic-antennas
www.tektronix.com/emi
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Application Note
Figure 11a. Environmental background results. Broadcast signals are clearly visible in the VHF band.
Figure 11b. Test results of DUT. No out of limit conditions were attributable to the DUT.
Once you are satisfied with your background noise, turn on
the power to your DUT. The differences between the two
measurements are the emissions from the DUT (Figure 11b).
For our testing, we used a Tektronix WiFi demo board that had
already been through EMI compliance testing, so there were
no failures to detect. The good news is that if you have setup
your testing correctly and nothing comes close to the limit line,
it may mean that you are ready for compliance testing.
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If problems are uncovered at this stage, further diagnosis and
design modifications may be required. The features available
on the RSA306 allow for both pre-compliance measurements
and diagnostics. Problem signals may be identified by
engineers familiar with the DUT design. Near field probing tools
may also be useful and are discussed in a later section.
Low-cost EMI Pre-compliance Testing Using a Spectrum Analyzer
Near Field vs. Far Field Measurements
Near Field
Ele
ctr
E
H
Wave Impedance
Zw =
ic
Transition
Region
Fie
c
eti
Far Field
ld
ld
Fie
gn
Ma
Distance
λ
2π
Figure 12. In the near field wave impedance depends on the nature of the source and the distance from it. In the far field the
impedance is constant.
In a full compliance lab, EMI receivers and well-calibrated
antennas are used to test the electronic devices over
a distance of 3 or 10 meters. In other words, the
measurements might be done in the far field. These test
chambers are designed to eliminate or greatly reduce
all the unwanted RF signals so that only the DUT’s EMI
signals are measured.
While every effort needs to be taken to ensure that the
RF background noise is minimized for your pre-compliance
testing, the background noise may still be significant.
Reducing the distance between the test antenna and the
DUT boosts the signal level of the DUT relative to the RF
background.
Figure 12 shows the behavior of wave impedance in the
near and far fields, and the transition zone between them.
We can see that in the near field region fields can range
from predominantly magnetic to predominantly electric
wave impedance. Near field measurements are used for
troubleshooting, since they allow one to pinpoint sources
of energy and they may be performed without the need
for a special test site.
However compliance testing is performed in the far
field and predicting far field energy levels from near field
measurements can be complicated because the strength
of the far field signal is dependent not only on the strength
of the source, but also the radiating mechanism as well as
any shielding or filtering that may be in place. As a rule of
thumb we must remember that we if are able to observe
a signal in the far field then we should be able to see the
same signal in the near field. However it is possible to
observe a signal in the near field and not see the same in
the far field.
www.tektronix.com/emi
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Application Note
For conducted measurement instead of antennas you use
3
a LISN (line impedance stabilization network). A LISN is a
low-pass filter which is placed between an ac or dc power
source and the DUT to create a known impedance and to
provide an RF noise measurement port. It also isolates the
unwanted RF signals from the power source. Again, adding
a pre-amplifier is a good way to boost the relative DUT signal
levels. Figure 14 (on page 13) shows a block diagram
of our setup.
Figure 13. Basic pre-compliance conducted emissions test setup.
Case Study: Conducted Emission
Measurements
Figure 13 shows our pre-compliance setup for conducted
emissions testing. The device under test is a universal AC/DC
power adapter for a laptop computer.
LISN (Line Impedance Stabilization Network)
Caution! It is critical that the spectrum analyzer input
is disconnected from the LISN prior to unplugging
the source power to the LISN! The discharge levels from
the LISN can damage the front end of the spectrum
analyzer.
3 We used the Solar Electronics, Type 8028-50-TA-24 BNC LISN.
http://www.solar-emc.com/LISN.html
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Note that the interference being conducted on a 60 or 50 Hz
power supply can also be an issue for some. While most of
the conducted EMI tests specify a measured frequency range
of 9 kHz to 1 GHz, it can be useful to measure the signals at
lower frequencies when the need arises. For low frequency
measurements, RSA5100 Series real time spectrum analyzers
are a good choice since they can cover frequencies down to
the sub-hertz frequency ranges.
For best conducted EMI measurements, it is advisable to use
2 LISN’s: one for a defined impedance to the DUT, and one to
go to the spectrum analyzer or receiver. However, one LISN is
better than none, but 2 is best.
Low-cost EMI Pre-compliance Testing Using a Spectrum Analyzer
Type N Connector
USB 3.0
RF
Optional
Power Filter
LISN
Optional
Pre-Amp
DUT
Power
Prefer Metallic Ground Plane
Figure 14. Block diagram of pre-compliance conducted emissions testing.
www.tektronix.com/emi
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Application Note
Figure 15. Conducted emissions test showing an over-limit condition at the lower end of the spectrum.
Power Filter
For conducted measurements the background noise comes
from the power source. While the LISN will provide some
isolation, many times you will need additional power filtering.
For our measurements, the noise from our building power
dominated our results. By adding a power filter4 we were able
to reduce the incoming noise to a sufficient level for making
our conducted measurements.
Characterizing Your Environment and
Test Results
First we input the LISN correction factors to the RSA306,
turn on the peak detectors and set the limit lines. Again,
prior to turning on your DUT it is important to evaluate and
characterize your test environment. Is there enough room
between the limit line and your noise floor? Do you need to
add a power filter?
4
We used a Filter Concepts SX30, http://www.filterconcepts.com/ac_filters.html
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Once you are satisfied with your background noise, turn on
the power to your DUT and attach the LISN output to the
spectrum analyzer, in that order. The differences between the
two measurements are from the DUT (Figure 15).
For the conducted measurements our DUT was a very low
cost laptop power supply purchased over the internet. We
used a spare laptop as a load for the power supply. In this
case we were able to see a failure. Figure 15 shows the DUT
conducted emission going above the limit at approximately
172 Hz. The features available on the RSA306 allow for
pre-compliance measurements and diagnostics. Problem
signals may be identified by engineers familiar with the DUT
design. Again near field probing tools may be useful. If you
have setup your tests correctly and nothing comes close
to the limit line, it may mean that you are ready for your
compliance testing.
Low-cost EMI Pre-compliance Testing Using a Spectrum Analyzer
Near-field Probes for EMI are electromagnetic pickups used
to capture either the electric (E) or magnetic (H) field at the
area of interest and are used with the spectrum analyzer.
Manufacturers provide kits of probes that offer the best
compromise between size, sensitivity and frequency range,
and you may need all the sizes in your toolkit to solve your
problem. Selection between an H-field or E-field probe may be
driven by location of a signal in your design, or by the nature
of its source (voltage or current). For example, the presence of
a metal shield may suppress the E-field, making it necessary
to use an H-field probe for the application. Near-field probes
must be used to either pick up the signal near the device
under test. For further information Tektronix has an application
note that focuses on troubleshooting EMI problems using near
field probes.5
Figure 16. A near field probe can be used to discover the location of unintended
RF emissions.
Near Field Tools for Debugging
In essence, the far field test can accurately tell whether the
product passes or fails as a whole but cannot pinpoint the
source of a problem. Using only the far-field test, one cannot
isolate problems down to specific components or locations,
like too much RF energy “leaking“ from an opening in a
metal enclosure or help identify a cable radiating too much
RF energy. The near-field test is a good way to locate such
emission sources and is typically performed using a spectrum
analyzer and near-field probe.
Conclusion
Failing an EMI compliance test is expensive and can put a
product development schedule at risk. However, setting up
your own pre-compliance testing can help you isolate any
problem areas and fix them before you go to the complaint
test house. The Tektronix RSA306 offers a new low cost
pre-compliance capability that will help you minimize both your
expense and schedule in getting your products EMI certified.
5 “Practical EMI Troubleshooting”, http://www.tek.com/document/application-overview/
troubleshooting-emi-problems
www.tektronix.com/emi
17
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Rev. 01/16
TEK.COM
For Further Information
Tektronix maintains a comprehensive, constantly expanding collection of application notes, technical briefs and other resources to help engineers working on the cutting edge
of technology. Please visit tek.com
Copyright © 2016, 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 names
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