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Re: [Fwd: Re: LAN noise?]



At 04:58 PM 6/1/99 -0400, Glenn Little wrote:
>Data over a CAT 5 cable is high speed.  Ferrites impede changes in
>directio on current flow.  The higher the speed the more the impedement
>of change, up to the point of saturating the ferrite.  If the ferrite is
>to be efficient in eliminating radiated lAN noise, it will also be
>efficient at reducing the amplitude on the LAN.

Sorry, but it doesn't work that way.

The ferrite bead adds mutual inductance between the two sides of the 
twisted pair, which acts to reduce COMMON MODE currents.  The data signal 
in the cable is carried by NORMAL MODE currents.  These currents are not 
reduced by the ferrite bead.

The signal currents in the cable are intended to be balanced.  If they were 
perfectly balanced, the common mode currents would be zero.  The ferrite 
just helps these common mode currents be closer to zero.

The ferrite bead does not impair the data signals at all.

There are many other examples of ferrite beads used to reduce common mode 
currents in cables which carry high speed information.  You may notice that 
almost all computer monitors have a ferrite bead on the video cable these 
days.  This works the same way.  Video signals for modern high resolution 
monitors are very high speed signals.  Here again, the signal is normal 
mode, and the currents that the bead reduces are common mode.  The bead 
doesn't hurt the signal at all.


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