EMC Question of the Week: February 24, 2025
Which radiated emissions specification is the most stringent at 100 MHz?
- FCC Part 15 Class B
- CISPR 32 Class B
- MIL-STD-461 RE102
- CISPR 25 (automotive) Class 5
Answer
The best answer is “c” although a good argument can also be made for "d." MIL-STD-461 RE102 specifies a peak electric field measurement of only 24 dB(μV/m) at a 1-meter test distance. CISPR 25 Class 5 is only slightly higher at 25 dB(μV/m) for a quasi-peak measurement. On the other hand, the CISPR 32 Class B quasi-peak limit is 40 dB(μV/m) at a distance of 3 meters. And the FCC Part 15 Class B limit is 43.5 dB(μV/m) at 3 meters.
If we were to assume all measurements were being made in the far-field and adjusted the specifications to the same test distance, the FCC and CISPR 32 quasi-peak limits would be more than 25 dB higher than the MIL-STD-461 and CISPR 25 limits. But the limits don't tell the whole story. FCC and CISPR 32 measurements (which apply to a variety of consumer products) are performed in a relatively well-controlled semi-anechoic environment. For table-top products, there is no local ground structure. Component cables and power cords are positioned to maximize radiated emissions. Strict compliance with commercial requirements is nearly always mandatory.
The MIL-STD-461 (military) and CISPR 25 (automotive) component measurements are made in an absorber-lined shielded room with wire harnesses stretched out horizontally over a metal surface. The test set-up emulates the intended environment of these components, but it does not maximize the far-field emissions in the same way as the commercial test set-ups. Also, compliance with the limits specified in a military or automotive standard is not always mandatory. Exceptions are routinely allowed for products that pose little or no risk of interference in the intended environment.
So, which radiated emissions test is the most difficult to pass? There's no simple answer to that question. But based solely on the specified emissions limit at 100 MHz, MIL-STD-461 RE102 is the most stringent.
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