Ferrite Beads

[theplcguy]

You failed to point out that the suppression action of the ferrite bead works both ways. Not only does it prevent the very high frequency signals inside the computer from affecting outside devices by way of the cable it also prevents external high frequency noise from entering by the same path.

[rowlodge]

my xbox 360 has a cordless controller and i think sometines if i have it hooked up to the recharge cord, which has a ferrite bead on it, it will send a clearer signal than if it wasnt attached and just cordless, no scientific evidence of this but id rather have it hooked up than cordless.

[bonsai]

Technically speaking, the ferrite cores act more like an in-efficient inductor than a resistor. By definition, a resistor is frequency agnostic.

Jim is correct about the over-engineering involved. Modern digital differential signaling is so good that they are completely superfluous in all but the the "electrically noisiest" of environments.

Btw, a similar technique is employed for other kinds of transmission lines--including mobile phone antennas. Among other reasons, that's why you see that little pig-tail curly-que helix for mobile phone antennas on cars. The helix acts exactly like a high-frequency inductor to filter incoming noise from the input of the RF amplifier. A similar in-line inductor exists in your hand-held mobile phone, but it's built-in to the circuity on the RF side of the circuit and, at least in modern digital phones, isn't part of the external antenna.

[moss]

Jim is correct about the over-engineering involved. Modern digital differential signaling is so good that they are completely superfluous in all but the the "electrically noisiest" of environments.

Btw, a similar technique is employed for other kinds of transmission lines--including mobile phone antennas. Among other reasons, that's why you see that little pig-tail curly-que helix for mobile phone antennas on cars. The helix acts exactly like a high-frequency inductor to filter incoming noise from the input of the RF amplifier. A similar in-line inductor exists in your hand-held mobile phone, but it's built-in to the circuity on the RF side of the circuit and, at least in modern digital phones, isn't part of the external antenna.

OK, time to add some actual knowledge here. Both the podcast and this thread are full of misconceptions.

1. The main reason for the ferrites on the cables is to block noise to come from the inside of the device to the outside. Otherwise the cable can, and will, become an excellent antenna. In order to be allowed to sell equipment devices must pass some certain EMI tests - this is why the cables that are bundled with gadgets have the ferrites. Cables sold separatly have not to be tested, and thus often do not have any ferrites for cost reasons. But this does not give that the ferrites are not needed! See below.

2. The transfer of data on a cable will only need the ferrites in a very very noisy environment. Which we do not have (yet, see below.)

3. The ferrites are definitly needed if you care about the ambient RF noise level. If this noise level goes up, the use of wireless devices will be hindered, or altogether impossible. One example is that I can not listen to FM radio in my kitchen if I at the same time has to charge my laptop in the same room- the wallwart is leaking so much noise into the radio that it totally drawns the music. So if you care about having good wireless transfer speeds over WiFi, you should care about ferrites and good shielding of elctronics (i.e. no windows on PC cases., no bare high speed digital circuit boards,...)

4. The coils on antennas has nothing to do wth ferrites - their use is totally different! (The only thing connecting them is the theory of induction...) The coils are essentially shortening the physical length of the antenna, and has nothing to do with filtering - that is taken care of by filters in the reciever chain inside the radio.

[md2389]

4. The coils on antennas has nothing to do wth ferrites - their use is totally different! (The only thing connecting them is the theory of induction...) The coils are essentially shortening the physical length of the antenna, and has nothing to do with filtering - that is taken care of by filters in the reciever chain inside the radio.

Absolutely correct. For those that are curious, I suggest you google "SWR" or "Standing Wave Ratio".

[scienceking]

Yeah, I'd be careful about saying such EMI suppression devices are useless. I worked in an EMC lab before, and trust me, we try like heck to not have to use such devices unless we really have to as the extra $0.50 or so of cost it adds to a product can add up to hundreds of thousands of dollars of production cost.

So if its there, its there because it was needed to pass regulations. I do disagree that it will interfere with wireless networking often. Most higher frequency line noise is in the tens and hundreds megahertz range(because this is what non-spread spectrum digital buses operate at), and rarely up in the gigahertz range where it would make a difference. This is however prime real-estate to mess with an FM radio, obviously. It could however interfere with the sensitive electronics in the receiving card itself and lower their baseline sensitivity. But really, to me at least, this would seem like shoddy compatibility work on the wifi card designer's end.

Now a lot of times, its actually there to keep it the 3db or so down noise wise so that its given a safe margin against regulation, but still, you never know, because ferrite cores are so effective, it could be a simple 3db attenuation needed, or a 10db one so it can barely pass! And, you don't know how well your devices take being interfered with. Most of this is set by internal standards, and as much as companies are actually often highly devoted to quality control, when you are up against tough deadlines, motivations to meet internal optional standards are pretty lacking.

It is very very possible that your xbox controller works better when plugged in to a regulated supply with EMI filtering. It makes sense because basically it could filter out self generated noise that would be re-induced into the power bus of your controller, thereby lessening the baseline sensitivity of components in the controller. Its also just possible that the most stable external power supply/voltage/higher voltage(?) used when it is plugged in is helping. But again, I don't know if it does actually do anything in reality.