[time-nuts] Noise and non-linear behaviour of ferrite
csteinmetz at yandex.com
Sun Jul 20 10:40:33 EDT 2014
> From my (past) CATV experience, ferrite-based devices don't introduce
>"noise" as such but they can certainly pick up noise (or transmit) unless
>they are toroidal or otherwise well screened.
Ferrites do create noise. The permeability of ferrites is a complex
quantity, so it has a real part. This real part, like any
resistance, generates thermal noise. It is sometimes referred to as
"secondary permeability" (as opposed to the "initial permeability"),
and typically rises rapidly above some frequency (the initial
permeability often falls off starting at about this frequency, as
well). That transition is where the ferrite turns from a good
transformer core to a good RF suppression core.
A separate phenomenon, magnetic hysteresis, also causes losses in
ferrites that produce noise when the magnetic field changes.
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