[SI-LIST] Re: Return current at board transitions
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Joel Brown <joel@xxxxxxxxxx>
- Date: Mon, 29 Dec 2008 16:44:53 -0800
Mode conversion happens. But not nearly as much as you would think.
Unless the board with the large dielectric spacing has really thick
dielectric, most of the energy is still confined between each trace and
the nearest plane. That's a good thing too, otherwise your pair to pair
crosstalk would be terrible.
Steve.
Joel Brown wrote:
> Two boards are mated together with impedance controlled high speed
> connectors.
> One board has diff pairs with a large dielectric spacing so a large
> percentage of the return currents flow in the opposite trace of the diff
> pair.
>
> The other board has a small dielectric height so a large percentage of the
> return currents flow in the adjacent ground planes.
>
> What happens at the transition? How does the return current change from
> flowing in the opposite signal trace to a ground plane?
>
> Is this a discontinuity even though the impedances are the same? Does it not
> matter since the differential currents cancel?
>
>
>
> Thanks - Joel
>
>
>
>
>
>
>
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