[SI-LIST] Re: Adding trace length for timing adjustment
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <doug@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 16 May 2003 18:54:21 -0700
First, a question of mine (for anybody): is there some theory that =
declares that the effect I describe below (coupling across serpentine =
legs causes part of the wave to bypass the serpentine) is zero in =
stripline?
Now that I've asked that, here's some of what I've found:
________________________________
No intense research here, but I did TDR the Front-Side Bus of a product =
board that had a variety of serpentine types (a few loooong legs, many =
short legs, some in-between), and here's what I found.
Conclusions:
1. Serpentining was >80% effective on this Front Side Bus (FSB). I.E., =
we achieved at least 80% of the expected delay from serpentines.
2. Vp variation due to serpentining induced about 30pS of skew on this =
3", 50ohm bus.
3. A few long legs of serpentining were better than several short ones.
4. There won't be any "Rules of Thumb" which will easily describe the =
issue (how effective are serpentines?). When we need to worry about pS, =
simulations have to be performed on expected worst-case nets.
Other Notes
1. Only layer 1 was represented here. The effects have been less on =
stripline.
2. The dielectric is 4mils thick; serpentine legs are separated by =
20mils center to center (same as trace-to-trace separation); trace width =
=3D 7mils.=09
Some general conclusions of mine:
Increasing trace length with serpentines (AKA "meanders") does not give =
an increase in flight-time directly proportional to the increase in =
trace length. Coupling across the serpentine legs causes part of the =
wave to bypass the serpentine (I would refer to it as a "barreling =
through the switchbacks" phenomena), reducing the flight-time. The =
speed-up effects are reproducible in simulations and seem to be only =
weakly tied to rise-time.=20
The effect can be lessened by separating serpentine legs, or routing in =
stripline. A "flat spiral" (AKA "bifilar spiral") is an option mentioned =
in some papers, but my experience with it has shown it to be terrible, =
S.I.-wise.
Take pains to avoid serpentines - they're not free.=20
If serpentining is necessary, keep the adjacent traces far apart - =
perhaps an S/H (trace separation to dielectric thickness) ratio of about =
5 to 1.=20
Routing in stripline reduces the effect.
Jeff Loyer
-----Original Message-----
From: Doug Brooks [mailto:doug@xxxxxxxxxx]
Sent: Thursday, May 15, 2003 10:09 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Adding trace length for timing adjustment
I know there are several different views on this subject.
Suppose I need to add some additional length (time) to a trace and am=20
considering three different strategies:
1. a randomly meandering length
2. a "trombone-like" length down and back
3. more, shorter snake-like loops
What do people see as the different trade-offs for these three different =
strategies under differing conditions?
Doug Brooks
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