[time-nuts] Noise and non-linear behaviour of ferrite

[Charles Steinmetz]

Kit wrote:

> 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.

Best regards,

Charles