A Guide to Radiated Emissions Testing

Electromagnetic interference (EMI) can occur when controlled power and data signals are passed across the isolation barrier. Isolation design tries to prevent and mitigate such events, but it can be very complex—and so can the electromagnetic compatibility (EMC) testing process!

EMC labs test a product’s radiated emissions, radiated immunity, conducted immunity, and conducted emissions. You can get an overview of these concepts in my “Visit to the EMC Lab” miniseries. In this post, we’ll focus on radiated emissions testing specifically for industrial applications.

“Radiated emissions” refers to the unintentional propagation of electromagnetic energy from an electronic device. Devices with high levels of radiated emissions can interfere with the operation of nearby electronic systems and networks or even disrupt their own functionality. To ensure devices work in harmony, various organizations and agencies regulate the allowable radiated emissions from electronic systems.

Industrial EMC Standards: CISPR11 and CISPR32

For industrial applications, I have typically tested to CISPR11 and CISPR32. These tests are conducted across a broad frequency range of 30mHz to 6GHz, so they typically involve two different test set-ups. Since CISPR32 is the more stringent standard, this post will focus on testing in the 30MHz -1GHz range.

Figure 1 shows the radiated emissions limits for CISPR32. Whether you’re testing to Class A or Class B limits depends on the end environment and applies to both peak and quasi peak measurements.

Figure 1 - CISPR32 30MHz - 1GHz Limits

10-Meter Radiated Emissions Test Setup

Radiated emissions are measured using an antenna and a spectrum analyzer or EMI receiver that sweeps across a predefined frequency band. For the 30MHz to 1GHz range, testing is conducted in a 10-meter semi-anechoic chamber (Figure 2), which allows the equipment under test (EUT) to be placed 10m from the antenna calibration point. The antenna is adjustable from 1m to 4m.

Figure 2 - 10m Semi-Anechoic Chamber

The EUT is powered on and placed on a nonconductive turntable 0.8 m above the horizontal ground reference plane, as specified in the CISPR32 standard. It is important that no other conductive surfaces are within proximity of the EUT, as this will affect results.

All auxiliary equipment should be placed outside the chamber. There may be a well under the turntable that allows you place equipment in close proximity to the EUT.

The device will be tested with the antenna in both horizontal and vertical polarizations.

Radiated Emissions Testing: Peak Measurements

We will now step through the process of capturing the radiated emissions data according to CISPR32. We will use measurements for ADI's next generation isoPower - ADuM6421A - as an example

1. EUT placed 10m from the calibration point with its front facing the antenna.
2. Antenna set to 1m.
3. EMI receiver sweeps across the frequency range (30MHz to 1GHz, in this example) and generates a peak plot (Figure 3).

Figure 3 - 10m Peak Plot - ADuM6421A

You may notice in Figure 3 that the peak plot appears to fail Class B. However, that doesn’t mean the device has failed EMC testing entirely! How can it still get a passing grade? Let’s talk about peak vs. quasi-peak measurements in more detail.

Radiated Emissions Testing: Quasi-Peak Measurements

Each identified peak undergoes its own series of quasi-peak measurements. For these tests, the spectrum analyzer is centered on the peak and the turntable is rotated 360° to locate the highest emission. Then, keeping the EUT in this orientation, the antenna height is raised from 1m to 4m.

The worst-case quasi-peak measurement is recorded and compared to the required limit lines. This is what determines if you pass or fail the specification requirements. In this case the ADuM6421A passes Class B (<1GHZ) by 3.6dB.

Table 1 - Quasi-Peak Measurements for Figure 3 - ADuM6421A

EMC Testing Equipment: Peak Vs. Quasi-Peak Detector

A peak detector captures and holds the highest voltage level of the input AC signal. The input signal passes through a diode, which rectifies it, and then through a capacitor, which charges to the peak voltage of the rectified signal and holds this peak value. When the output voltage is acquired, the analyzer moves to the next frequency and the detector is reset by discharging through a resistor.

The quasi-peak detector has a similar structure with one key design difference: there is one additional discharge mechanism with a slower time constant than the charging time constant. Quasi-peak detectors weigh signals according both to their amplitude and how often they occur, whereas the peak detector depends on amplitude only. This is the reason you will often see quasi peak measurements lower than peak measurements or peak plots.

Figure 4 - Simplified peak and quasi-peak EMI receivers, featuring a diode and capacitor in series.

Impulse Rate

To understand the difference between these devices, let’s consider two input signals: one with a slow impulse rate and a second with a fast impulse rate.

The output of the peak detector is the peak of the input signal. Therefore, the output is the same for both the high and low impulse rates. For the quasi-peak detector, however, a faster impulse rate results in a higher voltage output, while a slower impulse rate gives a lower output voltage—one that might, perhaps, fall below the radiated emissions limit for its device class.

Figure 5 – Illustration of effects of impulse rate on quasi-peak output.

Follow the blue lines in Figure 5. The first illustration shows that, as the repetition rate of the signal increases, the quasi-peak detector does not have time to discharge as much, resulting in a higher voltage output (shown in green). The second illustration shows that, if the repetition rate is lower, then the quasi-peak detector has more time to discharge, hence giving a lower (weighted) output voltage

Conclusion

The rules and methods of emissions measurements have evolved with changing EMC standards over time. Radiated emissions are one of the most sensitive measurements made, requiring accurate calibration, antenna and product positioning, and shielding from the normal electromagnetic environment we live in. Not to mention the wall absorption inside the anechoic chamber so that the emissions only come from a combination of the product directly and the ground (floor) reflection wave. We will go into more detail on this in a future blog.

Banner image attribution: Mihaela Wojcik via Wikimedia Commons. Image used under the Creative Commons Attribution 4.0 International license.