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Electromagnetic compatibility (EMC) is the branch of electronics that concerns the unintentional generation, propagation, and reception of electromagnetic energy.  The formal process of compliance testing ensures that unwanted effects like electromagnetic interference (EMI) or physical damage in operational electronic equipment is not present, making sure that devices will safely work in a reliable manner.  The goal of EMC is to correct operation of different equipment in a common electromagnetic environment. 

 

In the last blog, we talked about the difference between Pre-Compliance and Compliance testing – where Pre-Compliance is an informal, cost-effective, and low risk method to ensure your Compliance testing will pass. 

Today we will talk about the formal Compliance regulatory standards and the general process for final compliance testing.

 

 

EMC Compliance Testing deals with 4 main tests:

  • Emission
  • Susceptibility
  • Immunity
  • Coupling

 

Emission issues involve the deliberate or accidental generation of electromagnetic energy. 

Susceptibility involves the tendency of electrical equipment to malfunction or break down in the presence of unwanted emissions or radio frequency inference (RFI). 

 

Immunity is the opposite of susceptibility; it is the ability of the equipment to function correctly in the presence of RFI.

Coupling is the mechanism by which emitted interference reaches the DUT.  There are several types of coupling:

 

  • Conductive:  When coupling path between source and the receptor is formed by direct electrical contact with a conductive surface (i.e: transmission line, wire, cable, etc.)
  • Inductive:  When a source and receiver are separated by a short distance
  • Capacitive:  When a varying electrical field exists between two adjacent conductors, inducing a change in voltage on the receiving conductor
  • Magnetic:  Type of inductive coupling, when a varying magnetic field exists between two parallel conductors, including a change in voltage along the receiving conductor
  • Radiative:  Occurs when a source and DUT are separated by a large distance.  The source and DUT act as radio antennas, the source radiates an electromagnetic wave

Figure 1:  The use of an anechoic chamber is a crucial component for final compliance testing.

 

The EMC testing process involves open-air test sites that are the reference point in most CISPR standards.  This Is especially useful for emissions testing of large equipment systems.  RF testing of a physical prototype is most often carried out indoors in an EMC test chamber, like an anechoic chamber, which is a room designed to completely absorb reflections of either sound or electromagnetic waves.  This room is isolated from external waves from entering its surroundings.

 

EMC tests are regulated for standard compliance.  These standards help regulate and make uniform product EMC performance.  An example of one of the standards is CISPR, as mentioned in the previous blog.  CISPR’s work involves the equipment and methods for measuring interference, and establishes limits and immunity requirements for electronic devices.  Different countries have different organizations that enforce these requirements.  In America, the Federal Communications Commission (FCC) is the group that enforces these compliance testing and certifications.  This means, in America, the FCC enforces specific CISPR requirements for electronics that are sold, while another country may enforce another group of CISPR requirements.  The FCC requirements only relate to radiated and conducted emissions.  The difference between America and Europe is that there are no immunity limits, this is associated with European EMC certification.  Generally compliance with national or international standards are usually laid down by laws passed by individual nations, so it will vary from place to place.

 

CISPR is divided into various subcommittees depending on the specific type of electronics.  The different subcommittees are:

  • CIS/A - covers radio interference measurements and statistical methods
  • CIS/B - covers interference pertaining to industrial, medical, and scientific RF equipment
  • CIS/D - deals with electromagnetic disturbances that are related to electronic equipment on vehicles, and other devices that are power ed by internal-combustion engines
  • CIS/F - deals with interference relating to household appliances and lighting
  • CIS/H - sets the limits for the protection of radio services
  • CIS/I/ - deals with EMC of information technology, multimedia equipment, and receivers

Figure:  The different CISPR standards provide a guide to which standards your device needs to pass.

Figure 2:  CISPR regulations guide you to what standards your device needs to pass.

 

Compliance testing is a very formal process that is heavily regulated from place to place, so it is best to ensure your devices are likely to pass this compliance testing with the use of pre-compliance testing, which is a low risk, cost effective method to ensure you meet the final compliance requirements, depending on what country your product will be sold.

 

For more information on pre-compliance testing, check out the Making Conducted and Radiated Emissions Measurements application note for more information.  Please like, comment, or share! 

A big question that you might have is, what is the difference between Pre-Compliance and Compliance testing?  What makes Pre-Compliance important?  Pre-Compliance is a low risk, cost effective method to ensure your DUT will pass final Compliance testing.  Waiting until the end of a product development cycle for compliance testing is risky due to its high cost.  The cost includes reserving time in a compliance test lab, and the cost of redesigning your DUT if compliance testing does not pass.  Reserving time in the lab can be difficult, as these labs are in high demand.  It may be a long period of time before the lab is available, which could mean launch delays.  These are all unexpected expenses, in addition to expensive test time.

     

What is EMC Compliance Testing?

Electromagnetic Compatibility (EMC) Testing, is the interaction of electrical equipment with its electromagnetic environment, and other equipment.  All electronic devices have the potential to emit electromagnetic fields, and compliance testing is the final stage of testing  that ensures the electronic devices operate safely.

An example of poor compliance is when your TV picture quality is wrecked by wavy interference lines each time you turn on the kitchen blender.  This is something that shouldn’t happen – you should be able to have your TV operate normally, regardless of a kitchen blender running.  The blender produces electromagnetic waves that interfere with the TV signal.  Electronics need proper shielding to avoid interference with other devices.  This example is quite harmless, but if you think about unintentional electromagnetic interference on a larger scale, it could be a safety hazard, for example, it could corrupt data. 

Figure 1:  Final compliance testing is performed in an anechoic chamber

Figure 1:  An anechoic chamber is required for compliance testing

 

Common Measurements for Compliance

Developing your own EMC test lab will help you check your designs for compliance while they are in development and undergoing revisions.  Verifying your designs on your own is called pre-compliance testing.  Pre-compliance testing closely simulates the way compliance test s are run – putting your designs to test against actual test limits.  Once you are confident in your design, you can take it to a third party lab for final compliance testing.  This will make your testing more efficient and cost effective than relying on limited external laboratory tests.

The top most common compliance test failures are:

  1. Radiated Emissions
  2. Radiated Immunity
  3. Conducted Emissions
  4. Electrostatic Discharge (ESD)

Let’s go over what each of these are:

 

Radiated emissions testing

Radiated emission testing measures the radiated E-fields emanating from the DUT, and is usually the most common test failure. All devices will have some amount of emissions, but as long as they meet the requirements of your standards body you will be compliant.  A radiated emissions test involves measuring a DUT’s radiated emissions using a signal analyzer and an antenna.

 

Radiated immunity testing

The next common point of failure is radiated immunity.  Radiated immunity is a measure of how much external electric fields from external sources the CUT can tolerate before its performance starts to degrade.  This test set up requires 3 signal generators to cover the entire frequency range, RF broadband power amplifiers, and 2 – 3 antennas.

Figure 3:  Shows the relationship between radiated emissions vs. radiated immunity 

Figure 3:  Demonstration of the difference of Emissions and Immunity / Susceptibility

 

Conducted emissions testing

Conducted emissions testing focuses on the unwanted signals the DUT generates on the AC mains.  Both radiated and conducted testing are very important as you will not pass compliance testing if either of these fails.  For conducted emissions testing, your will need a spectrum analyzer equipped with EMC pre-compliance measurement software, line impedance stabilization network (LISN), and a limiter.  For more info refer back to the blog, "Complete your EMC conducted emissions testing in just 7 steps".

Figure 2: Radiated emissions set-up

Figure 2:  Sample set-up of a radiated emissions measurement

 

Electrostatic Discharge testing

ESD shield testing checks how immune the DUT is to static discharges, usually from operators touching key pads or touchscreens. The set up for ESD testing requires a ground plane that the DUT is connected to and several sheets of metal of various thickness to observe how the DUT interacts with the various planes, thickness and materials.    

 

Proper pre-compliance testing is crucial if you want to avoid surprises during compliance testing.  By checking your designs for electromagnetic compatibility during your design and verification work, you can ensure that you will pass compliance testing on the first try.  If you fail compliance testing, there’s a best-case scenario.  A simple design tweak fixes the issue and your only added cost is more time with the compliance lab.  As you probably know, the best-case scenario rarely happens.  Many times a failed compliance test means you have to do a significant design rework, which can mean a delayed product ship date.

 

For more information on pre-compliance testing, check out the Making Conducted and Radiated Emissions Measurements application note for more information.  Please like, comment, or share!  Stay tuned for the next one!

There are two kinds of EMC Pre-Compliance tests you can perform – radiated and conducted.  Today, we will review conducted emissions testing – what it is and why it’s important.  For a similar discussion on radiated testing, check out EMC Basics:  What is Radiated Emissions & Immunity Testing?.

 

Conducted emissions tests focus on the unwanted signals that are on the AC mains generated by the device under test (DUT).  Conducted RF emissions are electromagnetic disturbances (noise voltages and currents) that are caused by electrical activity in a DUT and is conducted out of the DUT along its interconnecting cables – for instance, power, signal, or data cables.  Conducted disturbances, in particular, a conductor, can couple directly into another electronic device or component within the same device.  This will provide unwanted signals that could lead to issues, like inaccurate performance.  This type of testing is one of the first group of tests performed in the process for EMC Pre-Compliance, followed by Radiated Emissions testing, Radiated Immunity, and conducted immunity testing.  The general procedure is to connect the appropriate equipment, load the limit, and load the correction factors.

Before we go through the steps to complete the conducted emissions testing process, let’s gather the equipment required.  These are common items that a test bench should have – these include:

 

  • Spectrum Analyzer equipped with EMC pre-compliance measurement software
  • Line impedance stabilization network (LISN) - The LISN is important because it isolates power mains from the DUT, which must have as clean of a signal as possible
  • Limiter
  • DUT

 

Now let’s go through the conducted emissions testing process in seven steps:

 

1.  Set up your test

Connect the signal analyzer to the limiter, LISN, and DUT.  Make sure the cord between the DUT and LISN is as short as possible to avoid the power cord from becoming an antenna.  Measure the signals on the power line with the DUT off.  If you see a signal approaching the established limit lines, you’ll want to set up some additional shielding so that these signals do not interfere with your possible conducted emissions from your DUT.  Shielding isolates components from each other to avoid coupling and interference that unwanted.

 

2.  Select your frequency range

Be sure you are measuring within 150 kHz and 30 MHz, which is the correct bandwidth for this measurement.   This is the corresponding frequency span that meets the CISPR requirement, which is a standard that is used for compliance testing. We will talk more about CISPR in another blog.

 

3.  Load the limit lines and correction factors

 

The two limit lines used for conducted emissions are EN5502 Class A quasi-peak and EN55022 Class A EMI average.  To compensate for measurement errors, add a margin to each limit line.

 

Figure 1:  Scan table where you can select the frequency span needed for the corresponding measurement.

 

Figure 2:  Conducted emissions display with limit lines and margin set

 

4.  Correct  for the LISN and the transient limiter

 

The transient limiter is used to protect the input mixer, basically acting as a filter or attenuator and is used with the LISN.  The correction factors for the LISN and the transient limiter are stored within the signal analyzer and can be easily recalled.  Correction factors adjust the reference plane for the DUT compensate for any loss through cables, space, etc.  Now you are able to view ambient emissions.  During this step, the DUT must be turned off.  If your emissions are above the limit, the cord between the LISN and DUT may need to be shortened.

Most radiated and conducted limits in EMC testing are based on quasi-peak detection mode.  Quasi-peak detectors weigh signals according to their repetition rate, which is done by having a charge rate faster than the discharge rate.  As the repetition rate increases, the quasi-peak detector does not have enough time to discharge completely, resulting in a higher voltage output.

 

The quasi-peak and average of the signals need to be measured and compared to their respective limits.  There are three detectors – Detector 1 will be set to peak, Detector 2 to Quasi-peak, and Detector 3 to EMI average.

 

Figure 3:  Loading correction factor files

 

5.  Locate signals above the limit lines

 

Switch on the DUT to find signals above the limit lines.  This is a good time to check to make sure the input of the signal analyzer is not overloaded by stepping the input attenuator up in value and seeing if they display levels do not change.

 

Figure 4:  Scan and search for signals above the limit lines

 

6.  Measure the Quasi-peak and average of the signals 

 

Most radiated and conducted limits in EMC testing are based on quasi-peak detection mode, which is available in the EMC X application.  Quasi-peak detectors weigh signals according to their repetition rate, which is done by having a charge rate faster than the discharge rate.  As the repetition rate increases, the quasi-peak detector does not have enough time to discharge completely, resulting in a higher voltage output. 

 

The quasi-peak and average of the signals need to be measured and compared to their respective limits.  There are three detectors – Detector 1 will be set to peak, Detector 2 to Quasi-peak, and Detector 3 to EMI average. 

 

Almost there!  We’ve got one more step to go!

 

7.  Review the measurement results 

 

The quasi-peak detector delta to Limit Line 1 & average detector delta to Limit Line 2 should all have negative values.  If there are some measurements that are positive, then there is a problem with conducted emissions from the DUT.  Before redesigning / troubleshooting the DUT with these results, check to ensure there is proper grounding if there are conducted emissions problems.

 

Figure 5: Quasi-peak and average delta to limit - the measurement results

 

Check these tips out for any troubleshooting issues: 

  • If the signals you are looking at are in the lower frequency range of the conducted band (2MHz or lower), you can reduce the stop frequency to get a closer look
  • You can add more data points by changing the scan table
    • The default scan table is two data points per bandwidth, or 4.5 kHz per point

 

To get more data points, change the points per bandwidth to 2.25 or 1.125 to give four or eight points per bandwidth. 

 

For more details on conducted emissions testings, check out the Making Conducted and Radiated Emissions Measurements application note for more information.  Please like, comment, or share!  Stay tuned for the next one!