Why so many cores on the cable?

[Bruce Sparrow]

#25761

I had an interesting call from a customer today. He inherited a project that had to go back through EMI qualification and wondered why there were 5 ferrite cores on his communications cable. He had no information on them. Here’s some possible reasons why:
1.) It is possible that the outage was so bad that multiples of the same core were needed to get the attenuation. Core size, number of turns or number of cores all affect the insertion impedance which attenuates the EMI.
2.) It is possible that the noise was spread over such a wide frequency band that cores of multiple materials were need to get that broad of a band coverage.

A given cable core can only cover a fraction of the frequencies under test for FCC which covers 150kHz to beyond a gigahertz. I advised that he go armed with cores from 31, 44, and 61 materials since he didn’t know what to expect.

I know that when I would go to the test lab I documented every test and the changes occurring leading up to the retest and filed the information for future use.

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