EMC Pre-Compliance

Technology is rapidly evolving with a vast number of new electronic devices entering the marketplace. Short/Fast time to market and cost-effective product design cycles are the key factors in this highly dynamic environment. Pre-compliance testing is your fundamental tool to meet these requirements. In case electronic products fail the certification step in the certified test house, this delays time to market and the unique opportunity of being the first to launch will be missed. Furthermore, rework and redesign efforts at a very late stage in the product development cycle cause extra costs and reduces revenue. This short introduction into electromagnetic compatibility EMC testing known as EMI or EMC pre-compliance testing demonstrates how to take control of time to market and costs related to EMI certification.

Importance of EMC pre-compliance testing

Any new electronic product – from laptops to tiny sensor nodes – must pass certified electromagnetic interference (EMI) compliance tests in order to prove emission level acceptance and regulatory standard conformance. EMI is an important part of what falls under the term electromagnetic compatibility (EMC).

At pre-compliance test you are mostly concerned about electromagnetic emission coming from your design. In case the electronic product fails the certification step in the test house, you are not allowed to sell the product. This delays your time to market and you might miss the unique opportunity of being the first to launch. Moreover you need to rework your design, which causes further expenses as indicated in the figure below.

Following introduction into electromagnetic compatibility EMC testing known as EMI or EMC pre-compliance testing reveals how you can take control of time to market and costs related to EMI certification.

Basics of EMC pre-compliance testing

EMC is the ability of an equipment or system to work/operate properly in its electromagnetic environment without causing unwanted effects such as electromagnetic interference EMI and intolerable electromagnetic disturbances to anything in that environment. Every electronic device has the potential to emit electromagnetic fields, and compliance testing is the final stage that ensures that an electronic device operates within safe levels.

A certified test house performs a variety of EMI tests to determine if a device has an appropriate level of emission. These tests can be split into two groups – emission and immunity testing. For both groups, radiated and conducted tests are performed.

Radiated tests characterize unintentional electromagnetic radiation released from an electronic device into a non-physical medium which is usually air. Conducted tests characterize the unintentional electromagnetic energy released from an electronic device via a physical medium such as a cable.

Important EMC terms to know

Here is a set of terms which are important to understand EMI pre-compliance and how to perform the tests.

  • Electromagnetic compatibility EMC
    = Umbrella term for the unwanted generation and propagation of electromagnetic energy from a device
  • Electromagnetic interference EMI
    = Disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction
  • Emission
    = During pre-compliance measurements EMI can cause unwanted interference with devices nearby if not properly dealt with during design and test
  • Susceptibility
    = Characteristic of electronic equipment that permits undesirable responses when subjected to electromagnetic energy
  • Radiated Interference
    = Interference resulting from unwanted signals propagating over the air, not through a physical medium such as a cable
  • Conducted Interference
    = Interference resulting from conducted unwanted signals entering a transducer (receiver) by direct coupling (via cables)
    = Comité international spécial des perturbations radioélectriques sets standards to supervise electromagnetic interference in electrical and electronic devices
  • LISN
    = Line Impedance Stabilization Networks isolates the device under test from the AC mains and provides AC power as clean as possible
  • Chamber
    = Shielded room which absorbs sounds or electromagnetic fields and reduce reflections from all internal surfaces
  • Radiation
    = Emissions of energy in the form of electromagnetic waves
  • Immunity
    = Property of an electronic device enabling it to reject a radio disturbance

Pre-compliance vs. compliance testing

Pre-compliance testing is a low-risk and cost-effective method to ensure that your device under test (DUT) will pass final compliance testing. Performing merely a compliance test at the end of a product development cycle is risky due to high costs.
The implications of a failed compliance test at the end of the product development cycles are extensive: First, an expensive redesign is compulsory to meet the regulatory requirements. Second, the time to market is affected negatively. Both it takes time to find the source causing the EMI issue and needs a further trip to the test house. Thorough pre-compliance testing is crucial if you want to save money and avoid nasty surprises during compliance testing.

The biggest difference is that compliance testing must follow exactly procedure written down in the applicable standard. This is associated with large costs because special measurement equipment (EMI receiver), an anechoic chamber (which is a large room) and special antenna equipment is needed. In contrast, a pre-compliance test can be done with a spectrum analyzer in your development environment.
In addition, a compliance test delivers an EMI failure report. However, it cannot identify the cause of the failure due to the nature of how the test is carried out. A pre-compliance test can pinpoint for example the source of radiation on your electronic design in the form of an EMI risk.

Obviously, not many companies want to invest in their own EMC test facility due to high costs associated. But: Pre-compliance testing is a widely used und highly recommended approach to control design project costs and ensure time to market.

How to test emission

With emission tests the amount of electromagnetic energy emitted from your device is measured. Emission tests are crucial because your device should not affect nearby devices by accident. Pre-compliance emission tests are mandatory when performing the final certification test.

  • Radiated emission tests quantify the electromagnetic energy strength propagating over-the-air from your device. This test ensures that your device is within the allowable field strength that is prescribed by the applicable standard. Many devices, however, fail this type of measurement.

Needed equipment for a radiated emission test

  • Calibrated EMI antenna, mounted on a tripod, positioned either 1 or 3 meters away from the device under test (DUT)
  • Spectrum analyzer with pre-amplifier
  • EMC pre-compliance measurement software

It is recommended to perform this kind of measurement far away from other equipment to avoid any interference. A large parking lot or an empty conference room are suitable locations. Any transmissions from broadcast radio, TV, or cell phones should not interact with the device under test or the antenna during testing.

  • Conducted emission tests focus on unwanted signals generated by the DUT. Conducted EMI emissions are electromagnetic disturbances created by the device under test that propagate out of the DUT along power cords, signal and data cables. Such kind of unwanted signals travel along the coupling paths directly into other electronic devices.

Needed equipment for a conducted emission test

  • Spectrum analyzer with EMC pre-compliance measurement software
  • Line impedance stabilization network (LISN)
  • Limiter to protect the input of the spectrum analyzer

How to test immunity

To determine the susceptibility of a device under test to the electromagnetic energy emitted from other devices nearby, immunity testing is recommended. However, immunity testing is not required by the CISPR standard and therefore often not carried out. Nevertheless, it should not be neglected because your device has not to be affected by other devices in its environment.

Needed equipment for an immunity test

  • Signal generator with enough frequency coverage
  • Set of near-field probes

How to troubleshoot while testing

When an emission test indicates that one or several signals surpass the prescribed limit lines then you have a good indication of an EMI design issue. Before final EMC testing at a certified test-house it makes sense to track down the source of emission and fix it. The simplest way to figure this out is to use a spectrum analyzer and a set of near-field probes. Knowing which portion of the devices shows the unwanted emission (i.e. which component, trace, etc.), the details of the design and the signal frequencies (i.e. clock frequency, data transmission frequency, etc.) used within the design allows to pinpoint the cause of the unwanted emission. By applying one of the various approaches to reduce EMI issues, the problem can be fixed.

Speed up time to market and lower costs

Failing EMC certification at the end of the product development cycle is not only expensive but impacts the time to market negatively as well. Obvious costs come from failing an EMC certification at the end of the product development cycle or additional time required to track down the issue on the design. Opportunity costs for fixing the issue and fitting additional circuitry into the device are even worse. Finally, the delay in time to market misses the window of opportunity and planned revenue cannot be realised. In our fast-evolving business environment being the first on the market is crucial for revenue planning and brand recognition. It is therefore recommended to perform EMC pre-compliance tests from an early stage in the product development cycle.
EMC pre-compliance testing is not for free. But costs associated to failing the EMC certification test can easily exceed the expenses for an EMC pre-compliance test system.

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