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Chokes vs. small capacitors on signal cables

up vote 14 down vote favorite

I know that many signal cables (usb camera to computer cables, etc.) have bulky ferrite chokes on them to prevent noise.

Why do they all have ferrites instead of ceramic capacitors? Small ceramics also get rid of noise effectively, and would be much smaller (probably cheaper too?) than the ferrites.

2 Answers

up vote 4 down vote accept

A common mode ferrite sleeve that is lossy at radio frequencies and doesn't attenuate the signal is rather more useful on a data cable than shunt capacitors that attenuate the signal and reflect rather than absorb RF.

up vote 3 down vote

Most EMC problems that products have are to do with Radiated EMC. When you have normal tests done like say FCC A and B getting a radiated fail is not uncommon. Radiated EMC is generally due to common mode issues. The standard cable length of most products makes the cabling a good antenna system. The ferrite common mode sleeve[s] that you often see on cabling deals with radiated EMC.


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Chokes vs. small capacitors on signal cables

up vote 14 down vote

I know that many signal cables (usb camera to computer cables, etc.) have bulky ferrite chokes on them to prevent noise.

Why do they all have ferrites instead of ceramic capacitors? Small ceramics also get rid of noise effectively, and would be much smaller (probably cheaper too?) than the ferrites.


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up vote 9 down vote

A common mode ferrite sleeve that is lossy at radio frequencies and doesn't attenuate the signal is rather more useful on a data cable than shunt capacitors that attenuate the signal and reflect rather than absorb RF.

edit

They key is that the ferrite core only acts as an inductor if it passes through the current loop. The signals largely involve currents leaving and returning through the same cable so they are unaffected. EMI largely involves currents leaving and returning through different cables, so it's suppressed. - Peter Green Apr 18 at 1:09

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