ast month, we introduced you to radiated emissions pre-compliance testing and what equipment is required. This month, we’ll show you how to actually make the measurement for commercial, industrial, and medical products, including all the system gains and losses. Once we complete the calculation for each dominant harmonic, you can directly compare it to the appropriate test limit. Next month, we’ll cover some details for automotive modules and military pre‑compliance testing.

The generalized test setup for pre‑compliance testing is shown in Figure 1. The unit under test (EUT) is placed on an 80 cm high non‑conductive table, which ideally should be able to rotate. However, a good estimate with fixed tables is to measure at least every face of the EUT in order to maximize the dominant harmonics. The preamplifier may not be required, as most modern analyzers have a 20-dB broadband preamp built in, if needed, to boost signal levels. The attenuator (usually 3 or 6 dB) is fixed to the antenna port to level out the impedance closer to 50Ω.

An additional point is that for most temporary pre-compliance test setups, there will not be a very good reflective floor, so it may not be necessary to adjust the antenna height anyway. You’ll have seen this in the example setups shown in Part 1 of this series, published last month.

The best way to perform this measurement is to use either a spectrum analyzer with a tracking generator (Reference XX) or an XXX network analyzer (VNA). This data will be used in the system gain/loss calculation.

For practical reasons, most pre‑compliance setups will use a 3m test distance. In this case, you can directly compare with the 3m limit chart. This must be done for each harmonic to be measured.

Note that when comparing 3m to 10m test limits, the calculated difference in test limits is actually about 9.5 dB, with the 10m test limit lowered by that much from the 3m limits. For practical reasons, the regulatory authorities simply use a 10 dB difference. The same 10 dB difference is used when comparing Class A or Class B test limits for either 3m and 10m test distances.

To see where the test limit is on the spectrum analyzer, we simply rearrange the equation above. The E-field (in dBµV/m) is taken from the desired test limit. This is the method used to plot test limit lines on your analyzer.

While we can manually calculate each harmonic in question, plugging the measured gains and losses versus frequency into pre-compliance software helps automate this measurement. Most software will also perform the interpolation calculation if a measured harmonic lies between two calibration frequencies. We’ll be discussing and comparing pre‑compliance software next time.

Example Calculation – For example, if we’re measuring a harmonic at 50 MHz at a 3m test distance, then the FCC Class A limit is 50 dBµV/m. For a coax loss of 1 dB, an antenna factor of 9 dB, and no external preamp or attenuator, the signal on the spectrum analyzer of 40 dBµV would be right at the limit.

Calculating Limits for Distances Other Than for 3m or 10m – If we didn’t have enough space and were to set the antenna-to-product distance at 2m rather than 3m, we would merely raise the displayed limit by 20log(3/2), or 3.5 dB. An easy way to remember whether to add or subtract this correction factor is when the antenna gets closer to the product, the harmonics get higher, and so does the regulatory limit (and vice versa).

Once you’ve calculated this limit at the harmonic frequency of concern, you can either move the display line (see below) to that amplitude or use the custom limit feature of your spectrum analyzer. I prefer to use the display line, as it’s faster. Note that if you’re using the built-in preamplifier, it’s not necessary to plug the (typical) 20 dB gain into the equation because the analyzer already compensates for the gain, leaving the signal amplitudes the same and effectively lowering the noise floor by the gain factor.

For pre-compliance testing, antenna height adjustment and EUT rotation are usually limited. Set the test distance between EUT and antenna to 3m. Most companies try to set up the test in an area clear of other equipment or reflecting metal objects. Several examples are shown below.

Set up your spectrum analyzer as follows:

7. Internal attenuation — start with 20 to 30 dB at first and adjust for the minimum attenuation required for the best display without overload.

If you observe a harmonic above the limit, try zeroing in on it using a span of 1 MHz (Figure 3). You can usually ignore the ambients and measure with some accuracy. By accounting for the antenna factor, coax loss, and preamp gain using the above equation, set the Display Line option (if available) to the calculated limit at that frequency. That will show you the margin below or above the limit.

Once you’ve identified the peaks over the limit, if any, the next step is to use the Quasi-Peak detector as required by CISPR 11/32. You’ll need to refer to the user manual as to how to set this up on your analyzer. Some of the affordable analyzers may not have this option built in but may be available as an extra-cost option. Quasi-peak detection uses an averaging method that can reduce the amplitude for non-CW harmonics. However, the measurement takes much longer than an ordinary peak measurement. Typically, you’d make this measurement only for the harmonics over the limit as measured in Peak mode.

Figure 4 shows one good example of a permanent radiated emissions pre-compliance test setup built in a client’s basement. The antenna uses a commercial mast that can raise and lower the antenna, plus turns it from vertical to horizontal polarization. The 12-inch floor tiles were removed to allow 4 x 8-foot sheets of aluminum panels to be set in. They were copper taped together. The turntable was simply a “lazy Susan” placed on top of a roll-around cart. Marks were placed on the floor to indicate a 3m test distance and the product under test was manually rotated and antenna height adjusted for maximum emissions. Despite other racks of equipment in the room, they were able to make reasonable measurements and at least identify “red flags.”

I’ll usually run a baseline plot on the analyzer using “Max Hold” mode for a couple minutes to build up a composite ambient plot. Then, I’ll activate additional traces for the actual measurements. For example, I often have at least two plots or traces on the screen; the ambient baseline and the actual measurement. It greatly helps if you become familiar with the RF spectrum usage in your area.

It’s also a bonus if you already know the top harmonic frequencies already, because it’s easier to identify product emissions from the ambient noise. I often “zero in” by reducing the frequency span on a high harmonic while performing a measurement.

Fortunately, there are three ways around this:

3. Move your testing well away from urban transmitters (easier said than done these days) or test in the early morning hours.

Next month, we’ll cover test setups for automotive modules and MIL‑STD-461, which are similar in that both are tested at a distance of 1m from the test table. We’ll also explain why the short distance!

1. Wyatt, EMC Troubleshooting Trilogy, Volume 2.
2. Wyatt, “Evaluating Reduced-Size EMI Antennas – Part 1,” EDN.