EMC Bench Notes: Pre-Compliance Testing for Radiated Emissions, Part 2 – Making the Measurement

Introduction

Last 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.

Notice that with a reflective floor, there will be a direct and reflected wave. In the official commercial compliance test, the antenna is raised or lowered between 1 and 4 m in order to find the point where the direct and reflected waves add up to a maximum. In reality, most pre-compliance setups simply adjust the antenna height to some nominal height, say 1.2 or 1.5m, depending on the tripod design.

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.

Finding Test Limits

The test limits will be located in the appropriate EMC standard used for your product type. Most commercial or medical products will use one of the IEC or European EN standards, which will refer to CISPR 11 or CISPR 32. For automotive modules, you’d refer to CISPR 25 (usually Class 5), and for military or aerospace ground equipment, MIL-STD-461. Limits for commercial aircraft are found in DO-160, which often refers to MIL-STD-461.

Pre-Test Calibrations

Before you’re ready to make an actual measurement, you’ll need to have characterized the gain versus frequency for any broadband RF preamplifiers (if used), the fixed attenuator (if used), plus the total loss in coax cables.

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

Antenna Factor

One important loss is the antenna factor (AF). Antenna factor is merely the transfer function of incident E-field impinging at the antenna versus the voltage measured at the antenna port. The unit will be 1/m. Calibrated antennas will come with a calibration chart of frequency versus AF. This will usually be in steps of 5 MHz or greater. If the harmonic lies between two calibration factors, you’ll need to interpolate an AF.

Making the Measurement

You can calculate the E-field (dBμV/m) by recording the dBμV reading of the spectrum analyzer and factoring in the coax loss, external preamp gain (if used), any external attenuator (if used), and antenna factor (from the antenna calibration provided by the manufacturer). See Figure 2 for the gain and loss diagram.

This calculation can then be compared directly with the 3m or 10m radiated emissions test limits using the formula:

E-field (dBμV/m) = SpecAnalyzer (dBμV) – PreampGain (dB) + CoaxLoss (dB) + AttenuatorLoss (dB) + AntFactor (dB/m)

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.

SpecAn (dBµV/) = E-field (dBµV/m) + PreampGain (dB) – CoaxLoss (dB) – AttenLoss (dB) – AntFactor (dB)

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 month.

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.

Performing the Commercial Test

Typically, this test will be in accordance with CISPR 11 or 32. Ideally, the antenna height should be adjusted between 1 and 4m so the vector sum of the direct and reflected emissions are maximized and the EUT rotated to at least all faces at each harmonic frequency.

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:

1. Frequency 30 MHz to 1,000 MHz (or higher, as specified by the standard)
2. Resolution bandwidth = 120 kHz, per the standard, or 100 kHz is close enough
3. Preamp = Off (turn on if you need to boost the signal more)
4. Set the vertical units to dBμV
5. Adjust the Reference Level so the highest harmonics are displayed and the vertical scale is reading in even 10 dB increments
6. Use peak detection initially and quasi-peak detection on any over-limit peaks later
7. Internal attenuation — start with 20 to 30 dB at first and adjust for the minimum attenuation required for the best display without overload.

I prefer setting the vertical units from the default dBm to dBμV so the displayed numbers are positive. Then, adjust the Reference Level for even increments along the vertical axis. This is also the same unit used in the test limits of most EMI standards. I also like to set the horizontal scale from linear to log (if possible) so frequencies are easier to read out. Now, start measuring!

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.

Alternatively, you can set the display line to the harmonic amplitude as a reference while troubleshooting, comparing progress against the display line. This is a fast and efficient way to troubleshoot and keep track of progress (or lack of)!

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.”

Dealing with Ambient Transmissions

One problem you’ll run into immediately when testing radiated emissions outside of a shielded room, is the number of ambient signals from sources like FM and TV broadcast transmitters, cellular telephone, and two-way radio. This is especially an issue when using external antennas.

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:

1. In most cases, you’ll observe a range of product emissions in a harmonic relationship. Very often, these harmonics are created from the same source and if one, or more, are masked by ambient signals, then working on the others that are more visible will generally bring the whole batch down, as well.
2. In some cases, there will be a critical harmonic masked by an ambient transmitter. A common example is a 100 MHz harmonic hidden underneath a strong FM broadcast station at the 99.9 MHz channel. In this case, I’ll try reducing the resolution bandwidth from 100 or 120 kHz down to as little as 1 kHz, or less. This often “filters out” the modulation from the FM station, allowing you to observe the hidden harmonic. This also presumes the harmonic is an unmodulated continuous wave (CW) signal. Just be sure reducing the RBW doesn’t also reduce the harmonic amplitude. If your harmonic is modulated, this may not work, so you could try selecting a higher related harmonic, as in (1) above.
3. Move your testing well away from urban transmitters (easier said than done these days) or test in the early morning hours.

Remember that strong nearby transmitters can affect the amplitude accuracy of the measured signals, as well as create mixing products that appear to be harmonics, but are really combinations of the transmitter frequency and mixer circuit in the analyzer. You may need to use an external bandpass filter at the desired harmonic frequency to reduce the effect of the external transmitter. An example would be an FM broadcast band “stop band” filter.

Summary

Very few companies today can afford permanent EMC test facilities, and I hope this has given you some ideas for performing pre-compliance testing at your own facility. While ambients are particularly difficult to deal with and can hide important harmonics, it is still possible to use these temporary sites for troubleshooting and a certain level of pre-compliance testing. This could save you thousands of dollars in wasted testing at your local compliance test facility.

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!

References

1. Wyatt, EMC Troubleshooting Trilogy, Volume 2. https://www.amazon.com/stores/Kenneth-Wyatt/author/B00SNQ1LJ2
2. Wyatt, “Evaluating Reduced-Size EMI Antennas – Part 1,” EDN. https://www.edn.com/evaluating-reduced-size-emi-antennas-part-1/