[SI-LIST] Re: Serpentine patterns for length matching

Jim,
My favorite RoT for serpentining is to make sure the spacing from a net
(or pair) to its self is **no less than** the crosstalk spacing to
different net or pair.  Same reason as mentioned by Steve below.


Aubrey Sparkman 
PG Signal Integrity Team 
Dell, Inc. 
Aubrey_Sparkman@xxxxxxxx 
(512) 723-3592 
"There are no constraints on the human mind, no walls around the human
spirit, no barriers to our progress except those we ourselves erect." -
Ronald Reagan

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of steve weir
Sent: Thursday, October 23, 2008 12:30 PM
To: otter30@xxxxxxxxxxxxx
Cc: SI-List
Subject: [SI-LIST] Re: Serpentine patterns for length matching

Jim, are you looking for minimum radius, maximum parallelism type
information or more basic information on how to determine the amount of
serpentine?  Many layout tools calculate the amount of serpentine, but
fail to account for the impacts of coupling.  A first cut RoT ( All RoTs
should be treated with caution. ) is to require serpentine diameter 2X
line to line spacing.  In rough numbers that reduces the cross talk from
serpentine segments to 1/4th normal line to line coupling.  If that
spacing is tight enough that you were to have enough serpentine coupling
to throw your timing, then you would also be a position where your
timing would potentially be thrown by aggressor coupling pull-in or
push-out.

Steve.

otter30@xxxxxxxxxxxxx wrote:
> Hello,
> Does anyone know of general guidelines for the serpentine patterns
used to match net lengths?
>
> We have a board with lots of diff pairs, and our board designer made 
> the serpentines very tight. The separation between the adjacent bends 
> of a pair is the same as the spacing of the traces in the pairs 
> themselves. This seems to imply imedance variations if the trace
widths don't change in the bends.
>
> Any insights would be greatly appreciated, Jim
>
>
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