Pre-Compliance Testing for Radiated Emissions

expert insights

EMC Bench Notes

Part 1: Equipment Needs

By Kenneth Wyatt

n past articles, we’ve discussed troubleshooting techniques for dealing with radiated emissions. Let’s turn our attention towards performing our own radiated emissions pre-compliance testing in-house.

The purpose of pre-compliance testing is an attempt to duplicate the test setup as used by your third-party test lab (Figure 1). Because these test chambers are fully shielded and lined with expensive ferrite and carbon-loaded RF absorber material to reduce reflections, they can cost several million dollars to construct.

Figure 1: A typical commercial 10m semi-anechoic chamber

Most companies will not want to invest this amount, so rely on third-party test labs. In order to get a more accurate measurement of radiated emissions without the cost, we’ll show you how to set up your own pre-compliance test in‑house. I’ve used these methods successfully for many of my clients.

Ideally, you should procure a copy of the appropriate EMC test standard used, depending on the product type. For example, for military testing, you’ll need a copy of MIL‑STD-461. For commercial, industrial, or medical products, you’d use one of the IEC standards, such as IEC/EN 61326, IEC/EN 60601, or the generic IEC/EN 61000-6-3, which will refer back to CISPR 11 or CISPR 32. For automotive modules, you’ll need a copy of CISPR 25. These will describe the equipment and setups and test limits required.

Equipment Required

Let’s start off with the basic equipment you’ll need. This will include a good spectrum analyzer or EMI receiver, a calibrated EMI antenna, a tripod and test bench or table for the equipment under test (EUT) and a large enough space in which to test.

I’ve listed many choices of analyzers and antennas in volume 1 and standards and test setups in volume 2 of my EMC Troubleshooting Trilogy (Reference 1). Figure 2 shows a popular example of an affordable bench top spectrum analyzer.

Figures 3, 4 and 5 show example antennas.

Figure 2: An example of an affordable spectrum analyzer usable for pre-compliance testing of radiated emissions.

Figure 3: A typical full-sized calibrated EMI antenna. These can be 1m or more in size.

Figure 4: A physically small calibrated antenna that could work well at a 1m or 3m test distance. Because of the small dimensions, it would not be suitable at 10m.

Figure 5: For a reduced-size calibrated antenna, I like an active antenna with built-in broadband RF preamplifier. This helps reduce the noise floor to compensate for the very short antenna elements.

You’ll want to specify an analyzer with the required test frequency range as specified in the appropriate standards your product requires. Most tests will go up to at least 6 GHz. Military tests go as high as 18 GHz, but depending on the product type, they can go higher. If you plan to use the analyzer for conducted emissions, the lower frequency will need to be 9 kHz.

You’ll find the larger EMI antennas to be more sensitive in the frequencies below 300 MHz. This is helpful, because we’ll often see emissions in the 50 to 300 MHz frequency band.

Some of the physically smaller antennas (Figure 4), may work at a 1 or 3m test distance, but probably not at 10m due to lack of sensitivity.

If you’ve no room to store one of the larger full-sized EMI antennas, then you might consider a broadband active antenna in Figure 5. A built-in RF preamplifier brings the sensitivity up to about the same as a full-sized EMI antenna. I’m currently using this antenna because it packs up nicely in a small transit case. I’ve compared the performance of these three antennas in Reference 2.

Often, you’ll also require a 20-dB broadband RF preamplifier in order to boost the signals to a useable level. Some analyzers may have this capability built-in, though.

One last item you’ll need is a sturdy antenna tripod. Several companies make these and I describe several in volume 1 (Reference 1). For the larger antennas, see examples of heavy-duty tripods in Figure 6. Lighter antennas can use a lighter tripod. Check volume 1 of my trilogy for many more choices.

Figure 6: Examples of heavy-duty tripods. This style is best for full-sized EMI antennas.

Example Test Setups

Where to test? Because the measurement assumes a reflective surface, most of the time, we’ll just use the earth or floor and assume we’ll get some fraction of the reflecting wave. Commercial labs will raise and lower the antenna at the dominant harmonic frequencies in order to maximize both the direct and reflected wave. Some companies use their parking lot away from other vehicles or metal reflective objects. I’ve also used office cubicles or conference rooms. Figure 7 shows the basic test setup.

Figure 7: Block diagram of a typical radiated emissions test setup. The 3m or 10m test distance is measured from the front of the product under test and a reference point on a broadband log-periodic (usually about the mid‑point along the boom) or center support of a broadband dipole.

Military and automotive module testing uses a fixed table with antennas spaced at a 1m test distance. Commercial, industrial, and medical products are tested on a rotating table at either a 3m or 10m test distance.

I’ve found conference rooms work pretty well. They are out of the weather, resources are usually close at hand, and it’s easier to clear out an area to test. Figure 8 shows one of my early setups while testing an industrial alarm system at the client’s facility. Notice the client constructed a wooden turntable, which allowed rotation of the product under test to find the maximum emission.

Figure 8: An example 3m test setup in a conference room.

Figure 9 is a similar 3m setup in an office cubicle. The back end of the antenna had to stick out partway into the hallway, though, attracting some attention. I used the opportunity to discuss the basics of radiated emission pre-compliance testing with a few interested employees.

Figure 9: Another example of a 3m test setup in an office cubicle.

Summary

If you already have the equipment for benchtop troubleshooting, then all you’ll need to add would be a calibrated antenna, tripod, connecting coax cables and possibly a low-noise broadband RF preamplifier. An optional, but highly recommended addition would include a 6-dB attenuator placed at the antenna port, which will help stabilize a 50Ω load impedance across the test frequency band.

Next time, we’ll discuss the details of making the measurement and add some additional examples of successful test locations and setups.

References

Wyatt, EMC Troubleshooting Trilogy.
Wyatt, “Evaluating Reduced-Size EMI Antennas – Part 1,” EDN.

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