Tag Archives: EMC

Anechoic Chambers for RF and Electromagnetic Testing

As the meaning of anechoic is ‘without echo’, an anechoic chamber represents a room that has minimal wave reflections from the floor, ceiling, and walls. Anechoic chambers are, therefore, suitable for testing Radio Frequency or RF, electromagnetic interference or EMI, and electromagnetic compatibility or EMC. Special materials on the floor, ceiling, and walls of the chamber help to absorb electromagnetic waves.

Another type of anechoic chamber is suitable for audio waves. The design of such chambers is meant for testing audio recording. The floor, ceiling, and walls have special material and their design helps to absorb sound waves.

A wide range of application areas requires accurate measurements of the electromagnetic spectra. For instance, the testing of an antenna requires measuring the electromagnetic energy levels that it is sending or receiving in all directions. Engineers call this the radiation pattern of the antenna, and the pattern can be in three dimensions, or in the principal plane.

When testing an antenna in an anechoic chamber, engineers use a reference antenna for transmitting a known level of power. They rotate the antenna under test to a known angle and allow the measurement system to record the power it receives. By rotating the position of the antenna under test to a different angle, they can take another measurement of the power it is now receiving. By combining all the measurements, they can form a polar plot representing the radiation pattern in that elevation or azimuthal plane.   

Conducting this exercise in the open area test site offers several disadvantages.

The test environment may have extraneous electromagnetic waves that the antenna can pick up along with the test signal. This will introduce errors in the measurement. A variety of sources can supply these extraneous waves, including air traffic, cell phones, FM radio transmitters, and more.

Moreover, weather conditions like rain and wind may also easily affect outdoor measurements of electromagnetic radiation.

Additionally, there can be reflections from nearby structures and the floor. The antenna under test will likely pick up these unwanted reflections as well.

Testing inside an anechoic chamber helps engineers avoid the above disadvantages. Typically, anechoic chambers use metal walls as a shield for preventing external radio signals from impinging on equipment inside the chamber. Special RF absorbing materials on the interior walls, floor, and ceiling of the chamber help in absorbing unwanted reflections of radio waves.

In fact, a shielded and non-reflecting anechoic chamber represents an infinitely large room, where the reflections do not reach the device under test, thereby enabling repeatable and accurate measurements.

Available anechoic chambers range in size from a typical room to a small tabletop enclosure. In fact, some anechoic chambers are so big engineers can easily walk inside, while some are as large as an aircraft hangar.

Pyramidal foams with a loading of conductive carbon often cover the internal surfaces of anechoic chambers. The tapered structure of the pyramidal shapes ensures minimal wave reflections for radio waves hitting them, while the presence of conductive carbon helps to absorb the waves. The RF absorbing material converts the absorbed incident electromagnetic energy to heat.

Do You Need EMC Testing?

Any electronic product faces a necessary hurdle before it goes to the market. It must clear the Electromagnetic Compatibility (EMC) test. This being a critical test in the design journey of an electronic product and passing this crucial test proves the design is right.

However, most designers relegate this important emissions testing to a late part of the design lifecycle of the product. This unnecessarily increases the risk of cost overruns and project delays shortly before the planned launch. Therefore, it is necessary to test for emissions at various stages of the product design plan.

When testing for EMC, you are actually minimizing the possibility of the radiated or conducted emissions from your device interfering with other electronic products nearby. Simultaneously, EMC testing ensures that the product under design is impervious to electromagnetic emissions coming from other sources in the vicinity.

Electromagnetic emissions are the energy the product emits in the radio frequency (RF) range. The device may emit these energies in either conducted or radiated form.

Below about 30 MHz, conductors and cables are not very efficient as antennas. At these frequencies, they are rather good at conducting the RF energy through shared loads and power sources. The conducted emissions, when passing through them may start interfering with other electronic equipment.

As the frequency goes up, beyond 30 MHz, conducted emissions are no longer an issue. At these high frequencies, cables and conductors start behaving more as antennas radiating the energy, thereby causing interference with other equipment.

Engineers use different test procedures and equipment for measuring conducted and radiated emissions. Although they use almost similar filter components for mitigating their effects, the electrical values involved are different.

Standards for measurement and testing electromagnetic emissions for both the conducted and radiated type differ in the US and Europe. While the US uses FCC Part 15, Europe uses CISPR 22/EN 55022. However, both approaches are very similar, and if the equipment meets the requirements of one of the standards, you can rest assured that it will meet the needs of the other standard as well.

Both the US and European standards set separate specifications for conducted and radiated emissions. The two types of emissions have their own limits applicable to the final system and its power supply.

Manufacturers making internal mountable power supplies often test them to meet regulations as standalone products. However, this is not enough if your design is using one of these power supplies with a load. In such a case, it is necessary that the complete system meet the EMC regulations. As a metal box encases the power supply, meeting the EMC challenges requires using external components.

Additionally, as most power supplies use switching topologies, they produce high levels of radiated and conducted emissions. Although the manufacturers may have already mitigated these emissions during the design phase, adding load to the power supply may produce further emissions. Therefore, it is necessary to test the combined system to ensure it meets the requirements of the EMC standards. Usually, a certified lab using calibrated test kits does the final testing. However, certain in-house testing is also possible, not requiring much equipment.