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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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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.

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  • \$\begingroup\$ I thought ferrites were like small inductors at <10Mhz. Wouldn't an inductance dissipate the signal as well? \$\endgroup\$ – dpdt Apr 17 '16 at 20:42
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    \$\begingroup\$ 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. \$\endgroup\$ – Peter Green Apr 18 '16 at 1:09
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    \$\begingroup\$ The key is that the ferrite sleeve is in common_mode. Signal currents that go out on the signal wire return on the ground wire, so net zero current, so they don't see the ferrite. Which is why signals propagating down the cable do see the ferrite, and get dissipated in the loss. \$\endgroup\$ – Neil_UK Apr 18 '16 at 7:57
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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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Ferrites on a cable is essentially a sign of a failure to do proper EMI design in the actual PCB. The fact that you see them so often should tell you something..

Without going to gruesome details of EMI design issues, it can be said that a ferrite on a cable is a) ineffective and b) expensive band-aid to a design which didn't pass the CISPR test requirements for consumer electronics. Ferrites do not work very well on cables because the cable already typically has high impedance and you're essentially doing energy division..

Capacitors are perfectly good solution for EMI, when they're on PCB itself. Typically you would use three terminal flow-through caps to eliminate differential noise. Capacitors may also eliminate common mode noise when connected from both signals to reference plane (usually GND) but this tends to kill your differential signal as well. Or single-ended signal. In short, capacitors are bad for your signal.

Trumping all that is a common mode choke. These are available on bifilar configurations that work even on high-speed data lines such as gigabit ethernet. Even better is having proper shielding and grounding strategy which will short the common mode noise back to the source instead of passing it to the cable.

Incidentally ferrites can work very well if you put them INSIDE the case and use a cap to form a RC (well, LC) circuit.

Clip-on Ferrites have their place but they should be a last resort/stopgap solution or used for difficult cases such as high-speed V-by-one or LVDS signal cables.

Here's a primer from Murata on the subject: http://www.murata.com/~/media/webrenewal/products/emc/emifil/knowhow/26to30.ashx

edit Corrected some nonsense about capacitors across differential pairs.

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