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Let’s Talk About Unconventional EMC Chambers

We previously covered the typical anechoic and semi-anechoic electromagnetic compatibility (EMC) test chambers used for testing mainstream commercial products like information technology equipment, industrial control equipment, multi-media equipment. In this article we will address less mainstream, non-typical EMC test chambers that are often used in automotive, military, and other pre-compliance types of test environments.

Advantages of Testing in Shielded Enclosures

You may recall the advantage of performing EMC testing in an all-metal shielded box is having a virtually RF noise free ambient surrounding to work in. For emissions work, this means not having to “tip-toe” around real ambient noise that might be present from radio stations and other legal emitters located nearby. For immunity work, this means containing RF generated by signal generators and RF amplifiers and not having to worry about violating RF spectrum rules. Because of these benefits, the testing performed in shielded enclosures requires less of our time. We’re not wasting it struggling with problems arising from imperfect test chambers.

Disadvantages of Testing in Shielded Enclosures

A chamber lacking RF absorber treatment will experience resonances and reflections at certain frequencies of applied RF from the test antenna and/or equipment under test (EUT). This makes taking accurate measurements in these types of chambers very challenging because of the constructive and destructive interference caused by standing waves that bounce off the sides of the chamber. Some signals act as though they’re amplified and others like they’re attenuated. It’s impossible to know which frequencies interact constructively or destructively and it changes from chamber to chamber depending on its size. Also, in a non-absorber lined chamber, forget about trying to obtain a uniform RF field as required by IEC 61000-4-3 due to these unwanted reflections.

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VSWR and its Effects on Power Amplifiers

Voltage Standing Wave Ratio results from an impedance mismatch between a source (an amplifier) and a load (test application). This mismatch can influence the performance of the source.

Reverberation Chambers

The first type of chamber I think of when anyone mentions unconventional EMC chambers is the reverberation chamber. Unlike the plain old shielded enclosure minus any absorber treatment, this type of chamber utilizes resonances and reflection modes to its advantage. It uses tuner or paddle/stirrer mechanisms to maximize RF signals injected into the chamber. The primary advantage of this type of chamber is cost. There is no expensive ferrite absorber material required and because of the high Q-factor smaller wattage (less-expensive) RF amplifiers can be utilized while still generating high E-field strengths. Because of the randomly varying wave polarization there is no need to re-orientate the EUT to expose all its sides to the field as is required by more traditional immunity test methods as described in standard such as IEC 61000-4-3 and others. These types of chambers can also accommodate large complex arrangements of test systems and devices. The construction can easily handle high field strengths required by today’s military and aerospace test standards. The disadvantage of working in a reverberation chamber is not being able to effectively perform testing at lower frequencies due to required larger size of the chamber. This type of chamber is used for both emissions and immunity testing.

TEM and Other Unconventional Test Chambers

The other specialized test chambers that may be encountered in your career are the Transverse Electromagnetic (TEM) cell or its closely related, higher frequency capable cousin called the Gigahertz Transverse Electromagnetic (GTEM). The TEM cell uses an expanded coaxial design and has an inner conductor called the septum. The GTEM has a larger and unique tapered transmission line design terminated with absorber material to reduce reflections at the back wall. As its name implies, the GTEM can operate at higher frequencies than the TEM. Both TEM and GTEM are used for emissions and immunity testing. Advantages include reduced floor space requirements and lower wattage power amplifier output capability than what is required of a traditional measurement setup. Emissions testing can be completed quickly as there is no need to maximize emissions by scanning the antenna from one to four meters in height. This fast scan capability can be a useful characteristic when trying to troubleshoot an emissions problem. Disadvantages of the TEM and GTEM are that only small EUTs with a limited amount of cabling can be tested efficiently. So, if you have a large EUT or one with tons of cabling then testing in a TEM or GTEM is probably not for you.

Closing

Several unconventional types of EMC chambers were described in this brief article. Now you know what they are and their advantages and disadvantages. This information and knowledge about your specific EUT and testing needs can help you determine if one of these unconventional EMC test chambers is right for you.

References and Further Reading

  1. In Compliance Magazine. (2018, December 7). What Every Electronics Engineer Needs to Know About EMC Chambers.
  2. Montrose, M. I. and Nakauchi, E. M. Testing for EMC Compliance, Approaches and Techniques, IEEE Press/Wiley-Interscience, 2004.
  3. ETS-Lindgren. “Reverberation Chambers – Design and Construction Considerations for Aerospace and Military Test Requirements.” 2018.

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