[SI-LIST] Re: Is a copper plane not tied to any net a reference plane?
- From: Ege Engin <engin@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 06 May 2004 09:20:50 +0200
I think my original post didn't make it to the list. So here it is
again:
Eric,
I will assume a simpler case in that the medium is homogeneous (for
example all planes are embedded in FR4). The two planes and the signal
line can be considered as a 3-conductor transmission line. Consequently,
the two TEM modes of concern here are the stripline and parallel-plate
modes. For signal transmission, stimulation of the parallel-plate mode
is not desired, since it results in voltage fluctuations between the
planes.
Both modes are excited in this 3-conductor transmission structure
assuming that it is terminated in a way that the return current is
forced to flow on the GND at the driver and receiver. The parallel-plate
mode is excited with a mode current equal to I*H1/(H1+H2), where I is
the current on the signal line, H1=100mil, and H2=5mil. For wide planes,
the parallel-plate mode can be considered as a radial wave (Larry gives
a qualitative description of how this wave propagates). This current is
normally not compensated by the signal return current; therefore, there
is a current on both planes.
Ege
eric steimle schrieb:
>
> I?m trying to settle an argument without saying which
> side I?m on, and I was hoping someone could give a
> quick and easy example to prove this.
>
> You have a four layer board that looked like this
>
> SIG1
> GND
> VCC
> SIG2
>
> And the maximum rise time on the board was about 2ns
> (say a 100MHz clock), then if I ran a trace from one
> IC to another on SIG1 the return current would flow
> along the GND plane (assuming no splits in the plane
> etc.)
>
> What if I covered SIG1 with a plane of copper say
> hovering 5 mil above SIG1, and then made the distance
> between SIG1 and GND 100mil so..
>
> Plane sheet of Copper
> 5 mil of air
> SIG1
> 100 mil FR4
> GND
> VCC
> SIG2
>
> Now that plane sheet is not GND and it has no
> association to any net it is just a continuous sheet
> of copper about the size of the board. One of us
> argues that the return current will continue to flow
> along the GND plane as it did before. The other
> argues that the return current will instead flow
> mostly along that sheet of copper unitl it jumps back
> to the GND plane as it gets closer to the IC. Any
> help in this would be much appreciated.
>
> Thanks
>
>
>
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