Hello, i need to deign a multilayer microstrip with more then two layers and i need to calculate the charateristic impedance(Z0)of the line. i'm looking for a model or a software that can make this calculation. thanks for help yoni

If you know all the details of your materials, and you are doing differential pairs, then this script might help (note there is a known bug in the signal velocity calculation that I haven't bothered to fix).

This requires you to know the thicknesses and dielectric constants of the board material and the prepreg (varies widely) for the stack you are using.

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Invoke with perl

# top-level options

# Globals

$Pi = 3.1415926535;

# main print "\\n\\n"; print "Impedance calculator\\n\\n"; print "Source material:\\n Johnson & Graham, "; print "\\"High-Speed Digital Design.\\" 1993. Appendix C.\\n"; print "Differential impedance calculations based on empirical data\\n"; { my ($choice); do { print "\\nEnter S for stripline, M for microstrip, X to exit : "; $entry = ; chomp $entry; $choice = uc $entry unless $entry eq ""; print "\\n";

if ($choice eq "S") { STRIPLINE(); }

if ($choice eq "M") { MICROSTRIP(); } } until ($choice eq "X") }

Why ? Usually the microstrip active line is on the top (where the RF components are soldered) and ground on the bottom. Every via means a parasytic inductance. Knowing the epsilon of the board, the trace width, the copper/gold thickness and isolations between the line and ground you may design exactly the line impedance. The line lenght gaves you the phase of the signal measured from the reference point. You need to get microwave office or other RF design program.

estimate effective E and run a regular microstrip program

or

use a real field-solver. ATLC would work and is free, if a bit of a pain to use. There are others around, maybe demo/student versions. Sonnet Lite might work, but I'm not sure it will let you mix layers like this. Otherwise expect big bucks.

What are the dielectric constants here? Are the traces long enough, and the frequency high enough, that transmission line effects matter? Distributed amplifier or something like that?

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