[SI-LIST] Hyperlynx modelling of 'wire over ground'
- From: "Jennings, Kevin F" <Kevin.Jennings@xxxxxxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 19 Feb 2008 07:59:28 -0600
I'm consulting on a signal integrity issue on a board and trying to use Hyp=
erLynx to model proposed changes to the net. The BoardSim model of the PCB=
net appears to be a fairly accurate model of reality. When I construct a =
LineSim model of the net it also seems to be a fairly accurate model of rea=
lity. However, when I then add a 'wire over ground' to the Linesim model t=
he HyperLynx predicted waveforms seem to be nowhere near what is actually s=
een on the reworked PCBA.
The net in question is ~20 inches long, ~40MHz clock driven by a single dri=
ver into ~50 ohm PCB with 7 loads distributed along the line; the single dr=
iver does not have nearly enough oomph to produce a clean edge on the incid=
ent wave. What I was proposing as a rework was a wire from a second driver=
into an appropriate spot in the net. The Linesim model shows a relatively=
clean edge now but adding the wire on the PCBA does not. Instead there is=
a ~1V 5 ns wide dip in the signal after the first incident wave switch tha=
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=
from ground) suggests that the proposed rework is relatively insensitive t=
o both which then suggests that the major contributor to the change in the =
signal (from the model's perspective) is the change in the net topology and=
available drive that occurs by adding the second driver and wire, which is=
as I expected.
The fact that reality diverges so far from the model basically renders the =
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=
een model and reality when wiring in a second driver given that the model w=
as fairly accurate with the unmodified PCBA?
Thanks for your input.
Kevin Jennings
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