How emission frequency is defined for EMI testing? I think most of electronics devices, For FCC and CE testing, they conducted 30Mhz to 6GHz as the emission frequency.. Is less than and higher than this frequency is not harmful??
For radiated emissions testing, frequencies down to ~150kHz may be included in a sweep test. Low frequency clocks can contribute to EMI at higher frequencies in this bandwidth however. The bandwidth to sweep will depend on which specific standard you are testing for compliance with
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I have some follow-up to Lane's answer, which goes into greater detail.
The ITU recommendations generally refer to CISPR standards such as CISPR 32:2015 (which recently subsumed CISPR 22:2008) which have historically only checked emissions frequencies up to 6GHz (though there are other standards such as FCC section 15, which includes an evaluation up to 40GHz depending on the class of equipment). For more information on these emissions tests, please see https://www.ti.com/lit/wp/slyy142/slyy142.pdf.
Historically, very few pieces of standard consumer equipment operated above 6GHz. Theoretically higher frequency emissions could introduce noise-folding issues, but any noise-folding effects could always be seen by lower-frequency tests anyway. Sensitive measurement equipment typically includes antialiasing filters to cut off out-of-band interference, and equipment that is susceptible to >6GHz interference signals was up until recently rare enough to not warrant significant regulatory oversight, or else deployed in a manner that prevents general interference - think satellite links in empty fields, radio telescopes in empty deserts, or radar systems pointed at the atmosphere, strategically placed to avoid interference from commercial communications.
However, with the advent of improved satellite telemedia systems, 5G networks allocating higher frequency spectrum than traditional 4G systems, etc, telecommunications companies have been pointing out for a few years that the existing emissions standards do not cover a sensible range of frequencies for future systems. This is an active and evolving area of work for standards bodies, with the expectation that emissions requirements up to 40GHz will be adopted from FCC part 15 and some requirement from 40-100GHz must be defined as well. A good example of modern considerations for standards updates can be found in the ETSI 5G workshop presentations, particularly from Huawei and Nokia in this link.
So in summary, higher emissions frequencies could be considered less harmful in the past for most consumer devices, but definitely will be more harmful in the future. We live in an interesting slice of history where the problem has been identified, but no new standard has been codified by CISPR as of yet.
As for below 30MHz, there's a few factors that contribute to the lesser emphasis on frequencies this low:
There are exceptions, like automotive systems that must interoperate with long-wave radio, older analog cable television systems, and a handful of other systems with long enough wire lengths to behave as sub-30MHz antennas or specific needs for sub-30MHz wireless communications. For these limited cases, more stringent requirements are applied.
So in summary, a combination of physics limitations, low available bandwidth, and existing tests more likely to catch problems with sub-30MHz conducted emissions, all combine to limit the need for sub-30MHz radiated emissions testing to a handful of specific, well-understood circumstances.
In reply to Derek Payne:
Many thanks for clear explanation.
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