The answer I've found is - it depends what you're
particular situation is.  There are some specific
uses for guard bands but not necessarilly EMI.
The additional spacing is of course right on as
far a crosstalk is concerned.

Think of crosstalk in the ground plane as a mixing of
return currents in the ground plane.  Try to imagine
with my ASCII diagram below that a trace T1 has a
return current of Ir1 and an ajacent trace T2 has a
return current of Ir2.  How close or far apart from
one another determines the addition of both return
currents in the area marked Ic. As you move out from
under the traces T1 or T2, the return currents fall
off exponentially.  In other words, SEPERATION of traces
should be considered first because it has an exponential
effect of the crosstalk.

If you were to add a guard band in between T1 and T2,
what you are doing is simply seperating the traces
and accomplishing the same thing if you had simply
seperated them without the guard band.


              T1              T2
            =====           =====
           .-----.         .-----.
         .         .     .         .
      .       Ir1    .       Ir2     .
   .             .   Ic  .              .
 __________________________________________
                  GROUND PLANE


Hope that helps ...

Regards,  Doug

Gary Boccoleri wrote:
>
> Good Morning
>
> Can anyone help me with information on guard banding. I'm talking about
> shielding a trace by circling it with a ground trace. This has come up
> as a way to solve all of our EMI problems. I'm of the opinion that the
> additional spacing it requires is what helps to prevent crosstalk and
> the ground traces just take up space.
>
> If anyone has any opinions on this (pro or con) or can point me at some
> resources on the subject I would appreciate it.
>
> Thanks

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