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

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From: MikonCons@aol.com
Date: Thu Apr 12 2001 - 15:47:17 PDT


Khalid:

Given that you have the board to test, you could use a spectrum analyzer with
a tracking generator to obtain frequency domain performance of your 50 Ohm
lines. Make sure you have a match to the Zo of the line at the launch
(generator) and termination (measurement) points to the cables used (usually
50 Ohms). You can then obtain a display of the attentuation as a function of
frequency.

From your statement of "...frequency of 2.5 Gbps..." I interpret you to be a
"digital type" where 2.5 Gbps really means a fundamental frequency of 1.25
GHz, plus all its harmonics. For a well-grounded signal, you can get by with
only considering the odd harmonics.

Stripline:
For a 20-inch, high-Tg FR4 stripline trace, you can expect to only have to
consider the fundamental, third, and fifth harmonics for the bit rate you
specified.

Another approach is to use a field solver that allows you to specify the
analysis frequency and includes the attenuation (usually expressed in
dB/meter) at the analysis frequency as part of its output data. Such a
program of course requires you to specify the loss tangent (typicall 0.02 for
high-Tg FR4). Run analyses at the fundamental and the above noted harmonic
frequencies. Use the resulting attenuation factors to compute the amplitudes
of each harmonic (for the desired length of line) and combine them in a math
program (e.g., MathCAD or Mathematica) for the expected output/termination
waveform. You could also sum the three signals in a simulation program to
obtain a display of the predicted waveform. This technique assumes there is
no appreciable chage in Er over the range of the harmonics used in the test
and calculations, otherwise the higher harmonics would slightly shift in
phase and distort the real signal. (Note that this effect might be a good
indicator of an Er dependence on frequency.)

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.

Other:
Result to basic equations if you know the material and/or line
characteristics. Review the E-mail from Raymond Waugh today on the SI List
titled "LOSS IN HIGH VSWR TX LINE (STUB Q)."

Comments from others who have done this type of thing are welcome.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting
(408)727-5697

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