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Ferrite beads prevent electromagnetic interference from affecting a cable. Without fundamentally understanding how EMI works, this is obviously a beneficial effect for the stability and integrity of electronic devices.

However, commercial cables have very inconsistent configurations regarding ferrite beads:

Many Serial/VGA and DVI cables have ferrite beads attached to both ends. However, very few HDMI and DisplayPort cables have any ferrite beads, although they can transmit more signals of different types (video/audio/ehternet).

On USB cables, it's rare to find a Micro-USB cable of any length with a ferrite bead, while sometimes even short Standard- and Mini-B cables are rather common to have one or two attached to them, even though all connectors have exact same data and power specifications.

I have also seen a lot of older PC cases which have a ferrite ring around the front-panel cables (just LEDs and power switches), while I've never seen newer cases with any such rings or on any outlet power cables, however some laptop power supplies have one on the device end.

Finally, I have never seen an ethernet cable with a ferrite bead, although they can transmit both data and power over long distances and are very delicate in regard to signal quality (categories, twisted pairs, shielding and so on).

So, this makes me wonder:

  1. What kinds of singals ferrite beads have an effect on: Just analog, data cables, power cables, all or none ("snake oil")?
  2. How great the effect is on the cable (this will likely depend on the environment)?
  3. Whether cable length is important (since no 50m ethernet cable has them, but 10cm USB cables do)?
  4. Whether cable gauge is of any importance?
  5. If there can be any adverse effects of using a cable with a ferrite bead (as it increases the impedance)?
  6. Are two beads always "better" than one?

Background of the question:

We have a bag full of aftermarket ferrite beads at work and I'm considering if it's a good idea to use them on cables that run close to big lighting and sound equipment. Although there were never any apparent signal issues, stability is of the highest priority and adding the beads would cost nothing.

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  • \$\begingroup\$ Any cable transporting data faster than about 20 Mbps, probably won’t have ferrite beads because they’ll start to wreck the data as you go higher and higher in frequency. \$\endgroup\$
    – Andy aka
    Feb 1, 2021 at 20:39
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    \$\begingroup\$ Hi Andy, thank you. Is this really generally the case? Because USB 2.0 is a lot faster than 20 Mbps and I've seen a lot of USB cables sporting them. Are those really exclusively meant for USB 1.1 applications? I've also seen some on USB 3.0 cables, albeit much less, but I have one attached to my monitor's powered hub right now and I think it even came with it. \$\endgroup\$
    – Prototype700
    Feb 1, 2021 at 20:46
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    \$\begingroup\$ You're overthinking what these ferrites do. Their purpose is nearly always to prevent the cable from being a radio transmitter. Here's the extremely precise and scientific method of how we pick a ferrite in the engineering world. 1) You send your product for an FCC test 2) FCC guy says "You fail" 3) Apply different ferrites until you get a passing test. VERY VERY RARELY would a consumer notice the difference if the ferrite were removed completely. \$\endgroup\$
    – Kyle B
    Feb 1, 2021 at 21:33
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    \$\begingroup\$ Okay, so which one is it? Is there any benefit to having them on a cable at all or not? And is there a drawback with data cables (or any other)? @Justme: I believe you, the fake-cylinders don't make understanding their usage much easier, though. Thank you for the explanation on ethernet. \$\endgroup\$
    – Prototype700
    Feb 1, 2021 at 22:45
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    \$\begingroup\$ "Consumer" meaning anybody who isn't an electronics engineer with an oscilloscope and a spectrum analyzer handy. Basically, lacking one of these ferrites, the neighbors TV or radio might go "buzzzzzzz", maybe their WiFi will act up, or perhaps cell phone calls will get dropped. Or nothing at all would happen. There are very few scenarios where a ferrite would be protecting your circuit from external noise, these really are almost exclusively used to make sure your device doesn't interfere with your neighbors wireless stuff. \$\endgroup\$
    – Kyle B
    Feb 2, 2021 at 0:31

1 Answer 1

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The purpose of a ferrite bead around a pair of cables is to balance the impedance of unbalanced signals like VGA signals and microphones and more over the spectrum that may cause interference. By raising the CM impedance the differential impedance remains constant while the percentage of imbalance is reduced.

While not used on most HDMI cables they are implemented on board for HDMI on MOBO’S with tiny SMT CM chokes for the VHF/UHF to GHz range.

Similarly, Ethernet signals use a hybrid transformer to separate full duplex signals which may a CM choke in a Magjack.

Select the optimum impedance matching components in accordance with the frequencies at which noise is a problem, the space, and cost.

CM chokes often serve a bidirectional purpose for both ingress and egress to reduce crosstalk as well as reduce unintended radiation.

The ferrite often is limited in useful range to 2 decades of spectrum, so the type of ferrite ,the size and the number of conductor turns all have an impact on the results. Ferrite permeability has a wide range so careful selection is required.

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  • \$\begingroup\$ Hi Tony, thanks for the answer. So, what I read from your explanations is that it is almost impossible to even select a matching ferrite bead for a cable from a device with unknown specifications and lacking measuring equipment? Could you possibly go a bit more into depth on my sub-questions 5 and 6? Concerning that if I had an unbalanced cable possibly picking up any interference, could it be detrimental to just slap a handful of ferrite beads of different sizes on? (Except for the cost-aspect) and do you know of any problems with data connections as Andy mentioned in the first comment above? \$\endgroup\$
    – Prototype700
    Feb 3, 2021 at 23:45
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    \$\begingroup\$ Ferrites don't work when the differential impedance is already high but true 1:1 Xfmr with high inductance do work at low f and high Z as Baluns. SMD CM chokes work well at UHF and help to reduce the SMPS noise interference that couples ground paths from systems with their own noise generation. I discovered this on an ASUS MOBO to a 54" TV on HDMI with some pixelation noise in high contrast zones that went away when I put my finger across the tiny CM choke when a long cable was used. so yes more CM chokes and small RF ground shunt caps can help a bit. I have yet to see any ill effects. \$\endgroup\$
    – Tony Stewart EE75
    Feb 4, 2021 at 0:13

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