Ferrite Bead – Does Position Matter on Cables?

A ferrite bead on a conductor creates a very small inductor. The inductance is a function of the size of the bead, the ferrite material used and the number of turns through the bead.

It is extremely unlikely that the ferrite bead will be able to filter switching frequency - they tend to be used to suppress noise well up into the megahertz. I doubt that it will have any effect on ripple (usually tens or hundreds of kilohertz) - it is quieting high-frequency spikes (megahertz and higher) which may be disturbing digital circuits or generating EMI.

Sizing an LC filter for this sort of noise reduction involves measuring the frequency and amplitude of the noise you're trying to suppress. The type of ferrite material as well as the type and value of the capacitor is critical for HF suppression. Once you know the noise you need to quiet, and the characteristics of the LC filter, you can determine how much attenuation you will have .

It can be difficult to design these HF LC filters up-front without knowing what sort of noise you're dealing with. Often, you'll put footprints in the PCB layout for the LC and empirically determine what values you actually need once you get the hardware running.

Here's a fairly generic picture of a ferrite bead: -

• At low frequencies (<10 MHz) its impedance is dominated by its reactance with resistive losses being low. It's basically operating like a piece of ferromagnetic material i.e. it is an inductor
• As frequency rises, inductive reactance drops away due to eddy current losses in the core material. This also gives rise to extra resistive losses.
• At high frequencies "losses" dominate the impedancee curve thus making a series-connected ferrite bead useful as an attenuator of unwelcome high frequency RF interference
• At very high frequencies parasitic capacitance makes the bead less useful

Ferrite beads tend to be useful from 10 MHz onwards. At 8 kHz they are generally useless.