[SI-LIST] Re: Hyperlynx modelling of 'wire over ground'
- From: "Curt McNamara" <CurtM@xxxxxxxxxxx>
- To: "Jennings, Kevin F" <Kevin.Jennings@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 20 Feb 2008 09:09:42 -0600
There are several possibilities. From your description it appears you =
have a net with driver and multiple receivers in LineSim. You then add =
another transmission line with another driver and tie it into the first =
transmission line. Is that correct? Do you have both drivers active at =
the same time?
In the real board, is this second transmission line an actual wire on a =
PCB?=20
If these are all correct, here are some possibilities:
The actual wire does not have the impedance you model.
The actual wire has significant parasitics which you haven't modeled.
In general we avoid multiple drivers on a single line. There are several =
issues: output circuitry may not parallel nicely; propagation delay to =
inputs may be different; pulses originating at different line lengths =
arrive at receivers at different times, leading to reflections like you =
observe.
For the case you described we would commonly use series termination at =
the driver. Small resistors can often be placed between package leads =
and solder pads to prove this out.
Curt
Curt McNamara, P.E.=A0// principal electrical engineer=20
Logic Product Development
411 Washington Ave. N. Suite 400
Minneapolis, MN 55401
T // 612.436.5178
F // 612.672.9489
www.logicpd.com=20
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On Behalf Of Jennings, Kevin F
Sent: Tuesday, February 19, 2008 7:59 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Hyperlynx modelling of 'wire over ground'
I'm consulting on a signal integrity issue on a board and trying to use =
Hyp=3D
erLynx to model proposed changes to the net. The BoardSim model of the =
PCB=3D
net appears to be a fairly accurate model of reality. When I construct =
a =3D
LineSim model of the net it also seems to be a fairly accurate model of =
rea=3D
lity. However, when I then add a 'wire over ground' to the Linesim =
model t=3D
he HyperLynx predicted waveforms seem to be nowhere near what is =
actually s=3D
een on the reworked PCBA.
The net in question is ~20 inches long, ~40MHz clock driven by a single =
dri=3D
ver into ~50 ohm PCB with 7 loads distributed along the line; the single =
dr=3D
iver does not have nearly enough oomph to produce a clean edge on the =
incid=3D
ent wave. What I was proposing as a rework was a wire from a second =
driver=3D
into an appropriate spot in the net. The Linesim model shows a =
relatively=3D
clean edge now but adding the wire on the PCBA does not. Instead there =
is=3D
a ~1V 5 ns wide dip in the signal after the first incident wave switch =
tha=3D
t in some of the loads is dipping back near the Vih spec limit.
Varying the Linesim 'wire over ground' parameters (wire radius and =
distance=3D
from ground) suggests that the proposed rework is relatively =
insensitive t=3D
o both which then suggests that the major contributor to the change in =
the =3D
signal (from the model's perspective) is the change in the net topology =
and=3D
available drive that occurs by adding the second driver and wire, which =
is=3D
as I expected.
The fact that reality diverges so far from the model basically renders =
the =3D
model useless for testing changes from a model perspective.
Does Linesim maybe not do well when modeling multiple drivers on a net?
Does Linesim maybe not model 'wire over ground' very well?
Anything else that I'm missing that might explain the wide discrepancy =
betw=3D
een model and reality when wiring in a second driver given that the =
model w=3D
as fairly accurate with the unmodified PCBA?
Thanks for your input.
Kevin Jennings
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