[SI-LIST] Re: diff pair questions
- From: "Chris Rokusek" <crokusek@xxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 24 Jul 2001 17:06:04 -0700
Signal,
Try thinking about it as two single ended traces. Then in terms of return
current, a large proportion of the return current for each diff trace
actually returns on the planes rather than the opposite trace. So what you
really have in terms of currents is a separate signal/return loop for each
diff half. These two loops are equal and opposite but spaced very near each
other (in fact on the planes they overlap at all but the highest frequencies
causing cancellation, less noise, good stuff). Another way to visualize
this is that the E/H field lines are "contained" by the planes. Thus
impedance will vary as the plane separation is varied.
Chris Rokusek
Innoveda
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of signal hoss
> Sent: Tuesday, July 24, 2001 1:49 PM
> To: si-list@xxxxxxxxxxxxx
> Cc: simails@xxxxxxxxx
> Subject: [SI-LIST] diff pair questions
>
>
>
> Dear all,
>
> Why does diff pair on pcb always reference to
> gnd/power
> planes, ie., stripline as sandwitch by 2 planes? The
> impedence of diff pair is between positive and
> negative, and should have no relationship to gnd/power
> planes.
>
> Another similar question is I have a 100 ohm diff pair
> stripline lines on pcb and use a diff coaxial cable
> solder to this pcb diff lines to connect to another
> board. Do I need to solder the shield of cable to
> ground ? Please clarify.
>
> Signal Hoss
>
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