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. > > > > >------------------------------------------------------------------ >To unsubscribe from si-list: >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > >or to administer your membership from a web page, go to: >//www.freelists.org/webpage/si-list > >For help: >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > >List technical documents are available at: > http://www.si-list.org > >List archives are viewable at: > //www.freelists.org/archives/si-list >or at our remote archives: > http://groups.yahoo.com/group/si-list/messages >Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu