[SI-LIST] Re: Hyperlynx modelling of 'wire over ground'
- From: "Todd Westerhoff" <twesterh@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 21 Feb 2008 14:39:12 -0500
Kevin,
Can you tell me if the following are correct / incorrect?
a) The quality of the original measured signal is poor
b) The quality of the original simulated signal is poor and fairly well matches
the measured signal
c) The quality of the modified (blue wire added) measured signal is still poor
(not much better than
the original)
d) The quality if the modified (wire over ground) simulated signal is
acceptable and does not match
the measured signal
If I got all these right, then I suspect the problem is with the wire over
ground model neglecting
some of the inductance caused by the distance between the blue wire and the
ground reference.
Todd.
Todd Westerhoff
VP, Software Products
SiSoft
6 Clock Tower Place, Suite 250
Maynard, MA 01754
(978) 461-0449 x24
twesterh@xxxxxxxxxx
www.sisoft.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Jennings,
Kevin F
Sent: Wednesday, February 20, 2008 4:00 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Hyperlynx modelling of 'wire over ground'
Curt,
Yes your description of the circuit is accurate and yes there is an actual wire
added on the PCB
which is why I picked the 'wire over ground' element to model this (prior to
actually putting the
wire on the board).
The relatively accurate prediction of signal quality when dealing with PCB
traces but relatively
poor prediction when adding another driver and a single physical wire to the
model was rather
surprising.
My question here was mainly to see whether or not anyone else had tried to
model a rework wire
(and/or multiple drivers) to a PCB and had HyperLynx accurately predict (or
not) the resulting
waveform.
I haven't totally ruled out user error on my part, and still need to go back
and re-validate that my
LineSim representation is an adequate representation of the PCB (apparently if
I had LineSim EXT I
could export the net from BoardSim and import it into LineSim...sigh). From
there maybe try to
trace down what differences between model and reality that could explain the
discrepancy, and if all
that fails, open a case with the good folks at Mentor and see if they can
explain the difference.
I definitely agree about the series termination, that's how I design things but
when tasked with
trying to salvage something one has to do what one has to do.
Thanks for your input (and to all the others who've responded here and
privately).
Kevin Jennings
-----Original Message-----
From: Curt McNamara [mailto:CurtM@xxxxxxxxxxx]
Sent: Wednesday, February 20, 2008 10:10 AM
To: Jennings, Kevin F; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Hyperlynx modelling of 'wire over ground'
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?
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. // principal electrical engineer
Logic Product Development
411 Washington Ave. N. Suite 400
Minneapolis, MN 55401
T // 612.436.5178
F // 612.672.9489
www.logicpd.com
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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=
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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