Electronics > RF, Microwave, Ham Radio

Why are USB-C cables with ferrites rare?

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There was a time, if my memory serves me, when adding a ferrite to a power supply cable was a simple mitigation against device EMI. In this USB-C era, despite the cable frequently being used simply as a power supply cable for a radiating device, I don't see products coming with or requiring a ferrite-equipped cable.

Is this:

* A figment of my imagination?
* A technical requirement, as implied here
* Unnecessary because compliant USB-C cables have an impervious shield and compliant USB-C power supplies have significant common mode impedance or something??
* Aesthetically driven?
* A sign engineers have got better at mitigating EMI at the source?
* Because adding a common mode choke to the product itself is cheaper/safer/better/more desirable?

My –°anon scanner came with USB-C cable with ferrite beads. USB-C is mostly used and first appeared in portable devices, so ferrite bead on a cable is a major usability downgrade.

I have some USB cables in my junque box that have ferrites embedded to the cable. These all were included with equipment I bought such as perhaps a camera or some other common consumer device. All the USB cables I buy now don't have the ferrite on them, but I haven't specifically searched for the ones that do. I just tried a search on Amazon for "ferrite USB cable" and yes you can buy them if you choose to. If you don't include "ferrite" in the search you most likely won't be served up that choice, as many consumers will probably do, simply because they don't know what "ferrite" even means. But if you are knowledgeable about RF interference and need a USB cable with a ferrite - you can get it.


--- Quote from: wraper on March 06, 2024, 02:04:48 am ---USB-C is mostly used and first appeared in portable devices, so ferrite bead on a cable is a major usability downgrade.

--- End quote ---

It would still do, to put it on the desktop/hub end, where impedance is likely lower / stray capacitance to ground/surroundings higher.

But ah, that works fine with ye olde USB-A to -B/mini/micro cable, the ordering is well-defined.  With USB-C, it's whatever.

So it kinda makes sense that the common denominator might be "none".

The safer choice would be to put one near both ends (the lighter-loaded end would simply not do much), but that would add more cost and bulk, and they're probably just the plain unadorned cords, cheap, for mobile-to-mobile, etc.

But that's neither here nor there.  I wouldn't say there's much insight here; it's not a high-information question.  The more interesting questions go in different directions.

For example, I wouldn't assume they've solved issues; the safer assumption is a perpetual race to the bottom.  If they can get away with it, they will.  Indeed, chargers tend to have poor CM noise and ground leakage (interference with capacitive touch is a common symptom, as is that rubbery-stiction touch feel when modest AC voltages exist on a surface).  Whether these would be helped with beads on cables, maybe, but mostly it doesn't matter as it's a small device, small system (the charger-cable-cellphone system, give or take someone's hand also holding it), and transiently used -- it might be giving off obnoxious emissions, but good luck locating and prosecuting a source that's only on a couple hours at a time, etc.  And that if licensed spectrum users even notice and report the interference.

Regarding CMCs, the on-board equivalent is an array of data chokes, one side wired in parallel (carrying VCC/GND), the others passing the data signals.  (Data chokes are almost exclusively two-winding transformers, so you need a lot of them wired in parallel to handle all the data signals a USB cable contains.) That way all signals remain coupled to a reference plane, through the chokes, but the reference potential on the cable can be somewhat isolated from the main board ground plane.  Clearly this isn't a step they're going to be embedding in dense electronics like a cellphone, and the better option is to simply ground the connector shield to the board -- extending the reference plane onto the cable itself, maintaining signal quality but also carrying CM noise along it.


I still have one on my USB A to C cable for my keyboard, on the A side. I don't think it has a purpose other than to feel premium.

I remember ferrites mostly on VGA/DVI cables. I suppose these analog carriers could actually be susceptible to common mode noise.

Today I rarely see one, not on USB, HDMI or Displayport. So you're right, something must have changed.
Though sometimes a power brick has one, there I can understand it as afterthough when it failed emc tests.


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