**From:** *[email protected]*

**Date:** Thu Apr 12 2001 - 15:47:17 PDT

**Next message:**Paul Levin: "[SI-LIST] : Attenuation and Delay on a PCB Trance"**Previous message:**Mike Saunders: "RE: [SI-LIST] : IPC-D-317"**Maybe in reply to:**Khalid Ansari: "[SI-LIST] : Attenuation on a PCB trace"**Next in thread:**[email protected]: "Re: [SI-LIST] : Attenuation on a PCB trace"

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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**Next message:**Paul Levin: "[SI-LIST] : Attenuation and Delay on a PCB Trance"**Previous message:**Mike Saunders: "RE: [SI-LIST] : IPC-D-317"**Maybe in reply to:**Khalid Ansari: "[SI-LIST] : Attenuation on a PCB trace"**Next in thread:**[email protected]: "Re: [SI-LIST] : Attenuation on a PCB trace"

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