How many ferrite beads are necessary on a (VGA) cable?



I have a question sparked by my job. A customer asked “Shouldn’t a VGA cable have two ferrite beads?” in regard to a cable he bought online that only had one. I don’t honestly know. I researched ferrites, but my knowledge of electronics is way insufficient to determine an answer on my own. So I’m turning to you folks…
How many ferrites does a cable need? I suppose that depends partly on the type of cable and signal. For VGA, it seems sufficient to me to have only one ferrite, if the connector near the ferrite is the one plugged into the monitor, to reduce interference before the signal gets to its destination… ??



I think, there is no general rule.
In my cables box, there is even a VGA cable without any ferrite at all.
The cable that came with my monitor has one ferrite, the cable of my wife’s monitor has two ferrites.

I think the number of ferrites doesn’t say anything about the cable’s quality.





They’re mostly used to keep the cable from interfering with the outside world and not to keep noise out of the cable, so in theory a cable with no ferrites would be the best shielded/built cable as it meets RFI regulations by virtue of its shielding only.


Hmm… all very interesting. Now to persuade the customer that two is not necessarily better :wink: