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