[SI-LIST] Re: Finding the impedance of a PCB trace

Graham, the issue is that each calculator uses closed form formulas that 
use various curve fit approximations.  Particularly for microstrip you will 
tend to see divergence from actual field solver results provided by tools 
like Polar Instruments www.polarinstruments.com( SI8000 about $1000. the 
last I checked but may have gone up, or more expensive tools like Ansoft 2D ).

I have found Tx-Line provides pretty good correlation to actual field 
solver results and is great for quick estimates.  But if you really want 
the answer, get a field solver.  This topic of field solver versus 
closed-form calculators is treated in considerable detail in Dr. Eric 
Bogatin's book "Signal Integrity Simplified".   Dr. Bogatin's recurrent and 
important theme is that field solvers model the actual E/M field 
distribution in all the materials and therefore has the means to provide an 
accurate answer.  That's a tough trick for closed form approximations, 
particularly in microstrip where so much of the fields distribute through 
both the board dielectric and air.

I just checked Polar's web site and they now have a hour by hour rental of 
SI8000 for $20.  Polar uses a BEM method which I believe is also the type 
of method used by InCases, that represents Dr. Poltz's work.  I haven't 
tried the latest version, but that rental looks like a terrific idea for 
someone in your position.  If you place any value on your time, I would 
give that a spin rather than trying every calculator you can find in Google 
and wondering about the results.  Your other two alternatives are to go 
through the learning curve of FastHenry / FastCap which are free from MIT, 
or hire a consultant(s).

Good luck.

Steve.
At 09:28 PM 4/29/2005 +0000, Graham Davies wrote:
>This is, of course, related to my search for a good 6-layer stackup.
>
>What is the impedance of a PCB trace (microstrip) with the following
>parameters:
>Width ......................... 6.00 mil
>Height above return plane ..... 3.20 mil
>Thickness ..................... 1.35 mil (1 ounce copper)
>Dielectric rel. permittivity .. 4.2
>
>These are the answers I get from various tools:
>
>41... ohms (http://www.icd.com.au/)
>41.69 ohms (http://www.emclab.umr.edu/pcbtlc/microstrip.html)
>41.69 ohms (TraceSim version 1.0.0)
>41.69 ohms (http://www.technick.net/public/code/cp_dpage.php?
>aiocp_dp=util_pcb_imp_calculator)
>41.7 .ohms (http://www.sunmantechnology.com/resources/cal_cat00.shtml)
>46.14 ohms (ZTOOL Impedance Calculator V1.2)
>47.31 ohms (http://www.rogers-corp.com/mwu/mwi_java/mwij_vp.html)
>50.89 ohms (http://www.csgnetwork.com/boardrunimpcalc.html)
>51.27 ohms (TxLine version 1.1)
>
>Does anyone have an idea as to why these calculators give results
>that vary by 20% and which are more reliable?
>
>Graham.
>
>
>
>
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