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There are ferrite chokes, rings to suppress CM EMI and they are also used for USB cables as shown in below example:

enter image description here

But isn't USB differential signaling type of transmission? That means to be the common mode signals at USB receiver end are already subtracted resulting zero. Just like in a differential ended amplifier with a balanced signal transmission.

If my knowledge above is correct I don't quite understand the need for the choke for USB. The only thing comes to my mind is to mitigate the CM signals if the interference is huge. Where am I wrong? I mean why is USB being a differential type of receiver not enough to subtract the CM interferences and needs chokes?

JYelton
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cm64
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  • The choke is to supress unwanted common-mode signals. Generally it is there to help you pass radiated emissions standards testing. The signals being suppressed may not even originate from USB at all. It may just be some other clock or high-speed signal. – user57037 Dec 10 '18 at 07:29

2 Answers2

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The chokes used on most cables, including USB cables are placed there to keep the cable from being used as an antenna for RF signals that come out the equipment that the cable is attached to.

There are other types of chokes used such as pictured below that are molded inside plastic on the cable.

enter image description here

(Picture Source: https://www.amazon.com/Tripp-Lite-Hi-Speed-Ferrite-U023-003/dp/B008VOPCGY)

Michael Karas
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  • A choke molded into the cable is highly effective. But often all that is needed to pass radiated emissions is to use a small PCB mount common mode choke for the D+/D- signal lines, and one each single-ended ferrite bead choke for the power and GND. Then it is not necessary to ship a special cable with the product. – user57037 Dec 10 '18 at 07:29
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There‘s always a bit of mismatch in a differential transmission. (Mismatch in source impedance, Receiver impedance, ...) So a common mode choke can still improve common mode rejection in USB transmission.

Stefan Wyss
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