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Radiated Emissions Measurements: OATS and ALSE Methods

This article is devoted to the radiated emissions measurements. These measurements can be performed either at the Open Area Test Site (OATS) or in an Absorber-Lined Shielded Enclosure (ALSE) [1].

Radiated Emissions Measurements – OATS (FCC and CISPR 22)

FCC and CISPR 22 radiated emissions measurements are performed in the frequency range starting at 30 MHz and extending into the gigahertz territory.  CISPR 22 allowable limits for both Class A and Class B devices are shown in Figure 1.

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Figure 1: CISPR 22 radiated emissions limits

 

FCC allowable limits for Class A and Class B devices are shown in Figure 2 and Figure 3, respectively.

Figure 2: FCC Class A radiated emissions limits

 

Figure 3: FCC Class B radiated emissions limits

 

Both the FCC and CISPR 22 measurements can be performed at an OATS or in an ALSE. Figure 4 shows an example of an OATS, while Figure 5 shows an example of an ALSE.

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Figure 4: Open-Area Test Site (OATS)

 

Figure 5: Absorber-Lined Shielded Enclosure (ALSE)

 

In this part we focus on the OATS measurements, while in the second part we concentrate on the ALSE setup.

The ideal OATS is a flat piece of land, free of overhead wires and nearby reflective structures, away from any and all external signals, with a perfectly reflective ground plane. Weather protection is usually needed, but the structure should not contain any metallic material (beams, nails, door hinges, etc.).

Since the OATS should be away from all reflective structures, this requires the control room to be remotely located or located underneath the ground plane. The test site should be sufficiently large to permit antenna placing at the specified distance. Ground plane should extend at least 1 m beyond the periphery of EUT and the largest measuring antenna, and cover the entire area between the EUT and the antenna.

The boundary of the area is defined by an ellipse, as shown in Figure 6.

Figure 6: OATS ground-plane boundary

 

Since the test area is open it is absolutely critical that prior to measuring the radiated emissions for a device under test (DUT), the ambient measurements are taken to reveal any potential external sources of emissions. Such a measurement is shown in Figure 7.

Figure 7: OATS ambient measurement

 

The ambient measurement is then followed by the DUT emissions measurement, like the one shown in Figure 8.

Figure 8: OATS DUT radiated emissions measurement

 

Radiated Emissions Measurements – ALSE (CISPR 25)

CISPR 25 radiated emissions measurements are performed in the frequency range 150 kHz – 2.5 GHz.  Peak limits for broadcast bands are shown in Figure 9, while the peak limits for mobile services are shown in Figure 10.

Figure 9: Peak limits for broadcast bands

 

Figure 10: Peak limits for mobile services

 

The measurement setup inside the ALSE for the radiated emissions measurements using a monopole antenna is shown in Figures 11 and 12. The monopole antenna measurement results are shown in Figure 13. The measurement setup inside the ALSE for the radiated emissions measurements using a biconnical antenna is shown in Figures 14 and 15.

Figure 11: Measurement setup with a monopole antenna (top view)

 

Figure 12: Measurement setup with a monopole antenna (side view)

 

Figure 13: Monopole antenna measurements results

 

Figure 14: Measurement setup using a biconical antenna (top view)

 

Figure 15: Measurement setup using a biconnical antenna (side view)

 

The biconnical antenna measurement results are shown in Figure 16.

Figure 16: Biconnical antenna measurements results

 

Finally, the measurement setup inside the ALSE for the radiated emissions measurements using a log-periodic antenna is shown in Figures 17 and 18.

Figure 17: Measurement setup using a log-periodic antenna (top view)

 

Figure 18: Measurement setup using a log-periodic antenna (side view)

 

The log-periodic antenna measurement results are shown in Figure 19.

Figure 19: Log-periodic antenna measurements results

 

References

  1. Bogdan Adamczyk, Foundations of Electromagnetic Compatibility with Practical Applications, Wiley, 2017.

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