Chamber vs. Open Area Test Site (OATS)

EMCI performs all of its commercial radiated emissions testing in a NVLAP-accredited, semi-anechoic lined 10-meter chamber. An accredited chamber has six significant advantages over an open area site; these are as follows:

• Reduced test time
• Reduced cost
• Volumetric site attenuation
• Greater repeatability
• Greater resolution
• Greater reliability

Reduced test time.

Open Area Test Sites (OATS) have always had the problem of having to determine whether or not an emission is an ambient or emanating from your product. To help mitigate this, many open area sites were built in remote areas, which meant increased transit time to and from the facility. However, broadband noise emanating from power lines often makes emission measurements below 200 MHz difficult, and sometimes impossible. In addition, the proliferation of cellular telephones, pagers, and now broadband high definition television (HDTV) are taking up larger portions of the RF spectrum, and making emissions measurements at many OATS more time-consuming and less reliable.

Provided that the unit under test can be set up within 30 minutes, a full-compliance radiated emissions scan from 30 MHz to 2 GHz takes only 2 hours in a chamber, as compared with 6-8 hours at a typical OATS.

Reduced cost.

Chambers typically have a higher hourly rate than open area sites. However, the fact that the work can be accomplished in much less time makes the overall cost of doing emissions testing in a chamber significantly less expensive than an OATS. In performing a cost comparison, don't forget to factor in the extra cost of your time.

Volumetric site attenuation.

Open area sites are only required to meet site attenuation at the center of the turntable, while chambers are required to meet volumetric site attenuation. In other words, chambers are required to meet site attenuation not only at the center of the turntable, but also around the perimeter of the chamber's â??quiet zoneâ?�, which is a much more stringent requirement. Open area sites could be 100% in compliance with the requirements, and still have technical problems that would result in the artificial failure of a product. Conversely, since chambers are held to more stringent standards, not only does this reduce the possibility that a product will fail due to a problem with the test site, it also increases the repeatability and reproducibility of the data.

Greater repeatability.

Open area sites are subject not only to increasing ambient noise, but also to changes in the physical environment around the site. High humidity, rain, and snow can dramatically alter test results, often making them appear higher (or lower!) than they actually are. This can result in increased test time, making the manufacturer spend additional time and money reducing an emission which is being enhanced by the environment at the test site. That simply cannot happen at an indoor chamber, whose measurements are not dependent on the environment. If you see an emission, it is coming from your product or the associated support equipment.

Greater Resolution.

First, the basic procedure for radiated electric field emissions testing is as follows: pre-scan, maximization/quasi-peak (QP), cable maximization, final QP. A large part of getting a good set of final data begins with the pre-scan. The following tables quantify the increased resolution seen in a 10-meter chamber over a typical OATS. The pre-scan from 30 MHz to 2 GHz performed in a 10-meter chamber has eight times the resolution of a typical OATS.

Moreover, the pre-scan from 2 to 18 GHz has 12 times the resolution. Not only do you have a better profile of your product (in much less time), better resolution is extremely valuable at frequencies above 500 MHz, where signals become increasingly directional. A highly directional signal could easily be non-compliant if measured, yet it might not even make the final frequency list, given the lack of resolution of the OATS pre-scan data.

Greater Reliability.

Testing at an OATS, the possibility often exists that an emission from your product cannot be measured because it is obscured by the ambient noise. A well-meaning test facility could give you a passing report, however, when self-declaring, it is the manufacturer who has to deal with the implications of selling a non-compliant product. This scenario simply could not happen when using an indoor facility. If you see an emission, you know that it is coming from your product or support equipment.

The following two plots show the ambient environment in EMCI's 10-meter chamber with the door open and with it closed. Even an experienced test engineer would have their work cut out trying to sort through these signals in an attempt to measure the actual emissions coming from the product under test. Even so, there are clearly many places where emissions from a product cannot be measured at an OATS, as they are obscured by ambient noise.

Conclusion.

There are a number of conclusions one may draw from these facts. However, these conclusions may be summed up into one concise statement: emissions testing in a 10-meter chamber, rather than at an OATS, saves both time and money, while resulting in more accurate, reliable and repeatable data.