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

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From: Lorang, David D (david.d.lorang@intel.com)
Date: Mon Apr 23 2001 - 13:01:57 PDT


 
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

-----Original Message-----
From: MikonCons@aol.com [mailto:MikonCons@aol.com]
Sent: Thursday, April 12, 2001 3:47 PM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Attenuation on a PCB trace

Khalid:

[...]
 
 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
Owner/Principal Consultant
Mikon Consulting
(408)727-5697

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