EMC PreCompliance App Note

FAQ
Instrument Solution – FAQ
Solution Title
EMC Precompliance: Testing on a budget
Date:05.29.2012
Solution: Almost any electronic design slated for commercial use is subject to
EMC (Electromagnetic Compatibility) testing. Any company intending to sell
these products into a country must ensure that the product is tested versus
specifications set forth by the regulatory body of that country. Here in the US, the
FCC specifies rules on EMC testing. CISPR and IEC are also used throughout the
world.
To be sold legally, a sample of the electronic product must pass a series of
tests performed by accredited compliance companies. This full compliance testing
can be very expensive with many labs charging thousands of dollars for a single
day of testing. Testing a product for full compliance can also require specialized
testing environments and no changes can be made during the testing. Any failures
in compliance testing require that the design heads back to Engineering for
analysis and possible redesign. This can cause delays in product release and an
obvious increase in design costs.
One method to lower the additional costs associated with EMC compliance
is to perform EMC testing throughout the design process well before sending the
product off for full compliance testing. This pre-compliance testing can be very
cost effective and can be tailored to closely match the conditions used for
compliance testing. This will increase your confidence in passing compliance the
first time through, lower your test costs, and speed your time to market.
Measuring Radiated EMI
The most simple form of pre-compliance measurements for radiated emissions can
be performed using a spectrum analyzer, like the Rigol DSA-815 (9kHz to
1.5GHz), and near field electric (E) and magnetic (H) probes.
Figure 1 and 2: Near field E and H probes.
The most simple test is to configure the DSA to use the peak detector and set the
RBW and Span for the area of interest per the regulatory requirements for your
device. Then select the proper E or H probe for your design and scan over the
surface of the design.
Probe orientation (rotation, distance) is also important to consider. The probes act
as an antenna, picking up radiated emissions from seams, openings, traces, and
other elements that could be emitting RF. A through scan of all of the circuit
elements, connectors, knobs, openings in the case, and seams is crucial.
Figure 3: Using a near field H probe
to test a power supply.
For the first pass, configure the spectrum analyzer to use the peak detector. This
will provide you with a “worst case” reading on the radiated RF and it is the
quickest path to determining the problem areas. Larger probes will give you a
faster scanning rate, albeit with less spacial resolution.
Once you have a good idea of your problem areas, you can get more detail by
implementing a few common techniques. If you can, select a spectrum analyzer
that has the standard configuration used in full compliance testing. This includes a
Quasi-Peak detector mode, EMI filter, and Resolution Bandwidth (RBW) settings
that match the full test requirements specified for your product.
This type of setup will increase testing time but should be used on the problem
areas. A full compliance test utilizes these settings.. and so, your pre-compliance
testing with this configuration will provide a greater degree of visibility into the
EMI profile of your design.
On many instruments, you can also store cable and antenna correction factors that
will allow you to see the true signal, without the added errors from the setup.
The next step in radiated testing includes using antennas in place of the near field
probes, a rotating platform for the equipment under test (EUT), and can include a
special room that minimizes environmental factors (semi-anechoic). These setups
are beyond the scope of this document, but there are references at the end that
provide good references for the details of the setup.
Radiated Emission Measurement System
EMI Receiver & Controller
EMI receiver
1m / 3m semi-anechoic chamber
(Shield Room with absorbing materials)
Antenna Tower
1m / 3m
360
。
Metal Plate
Figure 4: Compliance Setup for Radiated Emissions Testing
Measuring Conducted EMI
Conducted EMI testing requires analyzing the RF energy that is coupled from the
instrument or test circuit to the main power line it is connected to.
Like Radiated EMI, Conducted EMI is also measured using a spectrum analyzer,
but it also requires a transient limiter and a Linear Impedance Stabilization
Network (LISN). A LISN isolates the power mains from the equipment under test,
isolates any noise generated by the EUT, and couples the signals generated by the
EUT to the spectrum analyzer.
Figure 3: Standard Conducted Emissions pre-compliance setup.
As with emissions testing, the best start is a scan over the frequency range of
interest using the peak detector on the spectrum analyzer. Then, performing a
quasi-Peak scan using the EMI filter for the problem areas. This will minimize test
time while maintaining a high degree of confidence in your test.
Summary
EMC Compliance testing is mandatory for the majority of electronic products that
are slated for sale throughout the world. For the cost of 1 day of compliance
testing, you can have a pre-compliance setup that you can use to continually
monitor and improve your design. This will help speed product development and
save the company money.
International Regulations:
FCC Sub Part 15
International Electrotechnical Commission (IEC)
Spectrum Analyzers:
http://www.rigolna.com/products/spectrum-analyzers/
Near Field Probes/LISN's/Transient Limiters:
http://www.ets-lindgren.com/7405
http://www.com-power.com/near_field_probes.html
Practical EMC testing series:
www.compliance-club.com/pdf/EMCTestingPart1.pdf
www.compliance-club.com/pdf/EMCTestingPart2.pdf
www.compliance-club.com/pdf/EMCTestingPart3.pdf
www.compliance-club.com/pdf/EMCTestingPart4.pdf
www.compliance-club.com/pdf/EMCTestingPart5.pdf
www.compliance-club.com/pdf/EMCTestingPart6.pdf

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