[SI-LIST] Re: Return current at board transitions

One board has 28 mil dielectric which I think of as really thick.
I could try to reduce it but since the board is only 4 layers and 93 mils
thick that could be difficult. The diff pair traces are only 400 mils long
and only 125 mils run too close to another pair so hoping crosstalk will not
be a problem.

Joel


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of steve weir
Sent: Monday, December 29, 2008 4:45 PM
To: Joel Brown
Cc: SI-List
Subject: [SI-LIST] Re: Return current at board transitions

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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