Anti RFI ferrite rings - do they work?

Of course they work. No sane manufactory would intentionally design cables with giant blobs near the ends.

There are multiple flavors of material, composition, size, and shape. The ferrite clamp on cylinder that I think you're referring to is usually necesary to prevent the laptop and the switching power supply, from radiating EMI and RFI. The FCC gets rather irate when that happens. Everything will work without the ferrite cylinder, but will not pass Part 15.

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I can see a simple insulated pair of cables fed through and looping around a few times within a ferrite ring working, but does a cylinder over shielding work. ?

I guess it would be a bit dumb of the manufacturers to imagine that they do, and then waste their money fitting them on the *offchance* that they do. I imagine that the testing bodies would have rapidly told them if they didn't accomplish what they were put there for ...

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At one time I did a lot of materials testing to BS standards and over the years we came across a few significant errors that got into the published standards. I doubt things are much different in other sets of standards. conjecture Someone notes that these cylinders suppress harmonics of greater than 100 Mhz compared to nothing added on very lossy , little metal coverage, wire; so stipulates they must be added to all such wires

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The ferrite cylinders provide a high impedance for common-mode emissions coming from the source onto the cable so that the cable does not act as an antenna/wire. It should have no effect on signals traveling on the cable that are "balanced". The cylinder is just an elongated toroid, and the additional material raises the common-mode impedance more at the lower frequencies. A series of smaller toroids would have the same effect at lower frequencies, and would probably be even more effective at higher frequencies. There are a LARGE number of ferrite compositions, designed to have peak impedance at various frequencies, and to have a "high Q" or a "low Q", depending on what frequenciesa are needed to be suppressed. And as another poster said, tha manufacturers would not go to the trouble and expense if they did not have to. Back in the 1970's, when I first got into EMC, there were complaints about interference to airplanes in the Baltimore Washington area, and it was traced to computers. That led to the first FCC rules on interference. Eventually we issued ANSI C63.4 in conjunction/cooperation with the FCC, setting up rules on how to make measurements to ensure standards were used to measure equipment to the FCC emission limits.

I worked in this area for 40+ years before retiring from Bell Labs 10 years ago. I am still on ANSI C63 and various IEEE EMC Society committees. In that 50-year time, emissions have increased from the 1

- 100 MHz area up to the 10 MHz - 10 GHz area. WIth the increase in frequency, it has become harder to shield noise as the smallest openings that were not a problem at 10 MHz become slot antennas at 10 GHz. Ferrites are now being incorporated into the connectors themselves as putting them on to cables allows for noise to be radiated from the short stretch of cable between the connector and the point of application of the ferrite. Good design will help reduce the problem, but there aren't enough well-trained EMC designers/engineers out in the field. The IEEE EMC Society is constantly working to get more engineers interested in the field. It is hard to get people interested in something they cannot feel, see, hear, smell or touch. But it's a fun way to be an engineer!

I worked in this area for 40+ years before retiring from Bell Labs 10 years ago. I am still on ANSI C63 and various IEEE EMC Society committees. In that 50-year time, emissions have increased from the 1

- 100 MHz area up to the 10 MHz - 10 GHz area. WIth the increase in frequency, it has become harder to shield noise as the smallest openings that were not a problem at 10 MHz become slot antennas at 10 GHz. Ferrites are now being incorporated into the connectors themselves as putting them on to cables allows for noise to be radiated from the short stretch of cable between the connector and the point of application of the ferrite. Good design will help reduce the problem, but there aren't enough well-trained EMC designers/engineers out in the field. The IEEE EMC Society is constantly working to get more engineers interested in the field. It is hard to get people interested in something they cannot feel, see, hear, smell or touch. But it's a fun way to be an engineer!

But is that still the case where the wire is coaxial with a thick layer of metal, shielding the core wire? This type is high current carrying coaxial wire , not signal level co-axial cable which can have relatively open mesh shielding.

It's the equivalent of a one-turn winding on a ring. Yes, it will work, at least to some extent The amount of inductive reactance and series resistance it adds to common-mode current flow on the outside of the cable will depend on the size, the ferrite mix, and the frequency.

Yes. You can (and do) get current flowing on the *outside* of the shield - an unbalanced flow. Common-mode chokes such as these ferrites are intended to block this type of current flow.

For the thing to works, a certain amount of current flow needs to exist On a high Z line they do nothing. You need bypass caps for that, or combinations of inductors and caps.

greg

Yes, the ferrite is effective for any common mode signals that occur when the outbound signal on the center conductor is not EXACTLY equal to the return on the outer conductor/shield. Any net difference between the two signal levels is common mode and that is what the ferrite will reduce by increasing the common mode impedance.

Mostly, these are inductors to suppress common-mode RF currents, but they are also made lossy (like a resistor coupled to a secondary winding), so they dampen ringing.

Even if the cable is already shielded, the shield is a potential antenna. Damping the antenna current is sometimes necessary.

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I think they work great. I made a hat out of them, and almost all of the voices have stopped.