Hi again,

ANY opening in the ground/powerplane will have 2 (at least ) side
effects.

1) It will -as discussed increase the impedance of any trace passing
   over an opening -circular, slot, rectangular... the shape and size
   of the opening controls how much the impedance will change.
   Of course, a large hole has larger effect than a small
   In this respect, a square hole will not differ much from a
   circular hole.
   - A large number of small holes will also have effect -such as
   having many viaholes going through the plane.
   -Any change in impedance will give reflections of the signal
   but of course, it's not always a signal is so sensitive to this
   type of 'distorsion'

2) If a trace passes over a hole in the ground plane -or power plane!!!
   the signal return path that (approximately goes) in the plane
   directly under the trace (on medium to hi frequency designs),
   will have to go round the hole and thereby creating a so called
   loop area. This will give differential mode EMI directly
   proportional to the size of the loop area and signal current and
   proportional to the square of the frequency.
   It will also open for the possibility of common mode EMI
   which is much worse but is more difficult to describe here.
   You could say that the loop makes current flow in directions that
   you don't want it to. The amount of emission will then depend on how
   good an antenna the rest of the circuitry will act as.

Hope this helps

Per Viklund


Patrick Jabbaz wrote:
>
> I was also wondering what effect do holes have in the ground plane,
> I know that slots are a big problem, but circular holes (such as pad or vias
> clearances) I'm not so sure.
> Could someone clear this up for me.
> Thanks
>
> Patrick Jabbaz
> Halliburton Energy Services
> Circuit Board Designer
> (281) 596-6697
> [log in to unmask]
> -----Original Message-----
> From: Per Viklund <[log in to unmask]>
> To: [log in to unmask] <[log in to unmask]>
> Date: Tuesday, January 13, 1998 9:09 AM
> Subject: Re: [DC] Trace Impedance over a gridded ground plane ...
>
> >Hi,
> >
> >There is no doubt that the impedance do increase if the ground plane
> >(or the power plane -or both depending on which is the reference plane)
> >is broken up in some way. Just as Dough McKean so rightly points out,
> >the impedance is the square root of per unit length inductance divided
> >by per unit length capacitance.
> >Again, as Dough McKean points out, the capacitance between the plane and
> >the conductor will be lower than if the plane was solid.
> >-This is also the case of a heavily perforated solid plane.
 snip...

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