[SI-LIST] Re: inner Xtalk of a differential pair
- From: steve weir <weirsp@xxxxxxxxxx>
- To: xlzhou@xxxxxxxxx, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 10 Nov 2003 15:51:08 -0800
Mick, you need to ask the purpose of the practice and then the effects of
the practice. Assuming we are talking about either a microstrip, or
stripline construction, those extra grounds are just signals lines that
"don't switch very often". They are only quiet if stitched to planes at
small, and preferably irregular intervals. They will impact impedance, and
the vias will most likely impact routability.
The arguments for isolation between the members of a diff. pair are usually
for impedance control. The idea is that if the pair is weakly coupled to
the point that the odd mode impedance has an insignificant effect on the
overall impedance, then from an impedance control standpoint the routing
problem is simplified. The cost is space, and that can aggravate the
routing problem that spacing was supposed to help.
So, if you think that allocating space towards reducing odd mode coupling
is the way that you want to go, then calculate how much odd mode coupling
you are willing to accept, and space accordingly. I'll point out here that
if you believe that philosophy, you will similarly need to space adjacent
aggressors to remain below your selected crosstalk limit.
Regards,
Steve.
At 01:57 PM 11/10/2003 -0500, Zhou, Xingling (Mick) wrote:
>Guru,
>
>I have seen some designers try hard and prefer to reduce (single-ended) =
>Xtalk between two trace within a differential pair, sometimes by using =
>GSGSG configuration. Obviously, this is not because of a routing =
>concerns, it is an electrical consideration. Are there any reasons from =
>electrical point of view ? Let's assume symmetric pair. =20
>
>Thanks,
>
>Mick
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