RE: [SI-LIST] : Attenuation on a PCB trace

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From: Degerstrom, Michael J. (degerstrom.michael@mayo.edu)
Date: Tue Apr 24 2001 - 07:12:49 PDT


Ray,

Getting back to Mr. Conn's reply (which I copied below) I think
a lot of us know of the theory that a signal propagating
along a microstrip should be quite different when compared
to a signal that propagates along a stripline. This, as
is mentioned below, is due to the fact that the microstrip
signal must propagate through air and the board laminate.
The theory (also mentioned below) is that the phase response
vs frequency is not nearly as uniform in microstrip as
that in stripline. The result is that the edge rates
in microstrip are not as sharp as that for a stripline.

I would bet that almost all of us simply ignore these
predicted effects for microstrip. Also, I'm not sure
that the HSPICE w-element model could predict these
effects. The older version of the model had frequency
independant L and C - perhaps the updates to the
model are more flexible.

My approach for incorporating very high speed signals
is to route them in microstrip to avoid vias and so that
I can get a wider line than stripline while maintaining
a 50 ohm environment. I would like to hear from others
as to the following:

1) Choice of microstrip or stripline for very high speed
applications.

2) Did microstrip risetime degradation affect your choice
for (1)?

3) Has anyone else tried to characterize the microstrip
risetime degradation effect?

4) What EM tools are suitable for characterizing the
microstrip risetime degradation effect?

5) Can any w-element models model this effect correctly?

Thanks,

Mike Degerstrom

[Mr. Conn's reply now follows]

 Microstrip:
Surface traces will have at least two major elements of signal propagation,
one in the dielectric and the other above the surface. The latter is the
fastest. Summing the two signal elements at the termination causes
considerable risetime degradation for a "digital" signal because of
different
phases (and probably magnitude attenuation) of the same frequency
components.
The phase relationships of the different harmonics are also changed;
therefore, an analytically predicted signal may be considerably off the mark

from the real signal. In short, I would have no confidence in an
analytically
predicted microstrip waveform until proven by correlation with lab
measurements. I have observed about 2:1 risetime degradation in a 9-inch,
50-Ohm microstrip versus the same length of stripline in the same board
during TDR tests; therefore, I would have little confidence in using a
20-inch microstrip interconnection at the bit rate you noted.

 [...]

Michael L. Conn

_______________________________________________________________
Mike Degerstrom Email: degerstrom.michael@mayo.edu
Mayo Clinic; 200 1st Street SW ; Rochester, MN 55905
Phone: (507) 538-5462 FAX: (507) 284-9171
WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html
_______________________________________________________________

> -----Original Message-----
> From: Ray Anderson [mailto:Raymond.Anderson@Sun.COM]
> Sent: Monday, April 23, 2001 4:48 PM
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : Attenuation on a PCB trace
>
>
>
> Dave-
>
> Yes, our group has modeled the rise time degradation in
> simulation using the
> Hspice W-element and have achieved reasonable model to
> hardware correlation.
>
> When using a microstrip model, the fields propagated through
> the dielectric
> and through the air are accounted for in the model, so I
> don't think you
> really want/need to kluge up a model that utilizes a stripline and an
> airline to do the simulation.
>
> -Ray
> Sun Microsystems Inc.
>
> >
> >
> >I interested in whether on not anyone has modeled this
> degradation of rise
> >time (see excerpt below) that Mike has mentioned using a
> simulator. Can it
> >be done by building two transmission lines in the simulator,
> one for air
> >dielectric and one for FR4? If the two parallel
> transmission line method
> >works how would one calculated how much impedence to allocate to each
> >dielectric? Will a field solver tell you this?
> >
> >Dave Lorang
>
>
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