Microstrip calculators inaccurate? - Page 1
Forum section: electronics / projects

author: PetAd
author_rank: Newbie
timestamp: October 26, 2014, 03:53:45 pm
content: Hi there!For a school project I need to calculate microstrip impedance and write a little essay. I checked the IPC-2141A guide for the calculations, and the errate too. I made an m file for Matlab to calculate the impedance, but I need to check the results. And here comes the problem. The internet is full of calculators, but with the same numbers the results are very different.My numbers are:Dielectric constant = 4.58Dielectric height = 10milSignal line width = 12 milCopper thickness = 1Oz = 1.4milI gathered the following results from different calculators:Saturn PCB Design Toolkit57.99Ohmhttp://www.microwaves101.com/encyclopedias/microstrip-calculator63.74Ohmshttp://www.multek.se/engelska/engineering/pcb-structures-2/microstrip-impedance-calculator-260.185Ohmshttp://www.finetune.co.jp/~lyuka/technote/ustrip/60.93Ohmshttp://chemandy.com/calculators/microstrip-transmission-line-calculator-ipc2141.htm60.26Ohmshttp://www.eeweb.com/toolbox/microstrip-impedance/46.6OhmsMy result is 69.28Ohms, more likely wrong

author: KJDS
author_rank: Super Contributor
timestamp: October 26, 2014, 04:29:26 pm
content: The high end microwave simulators use Hammerstad and Jensens equations.Covered in some detail here.http://brd4.braude.ac.il/~alomes/Introduction2EMC/Applicational_Notes/Single%20Microstrip%20Line%20by%20Stefan%20Jahn%202007.pdf

author: nctnico
author_rank: Super Contributor
timestamp: October 26, 2014, 05:17:15 pm
content: For general purpose use (to calculate 50 Ohm traces) I often use Wcalc:http://wcalc.sourceforge.net/cgi-bin/microstrip.cgiThe online calculators are only somewhat accurate for 50 an 60 Ohm traces. Beyond that you need the Hammerstad and Jensens equations KJDS pointed to.

author: KJDS
author_rank: Super Contributor
timestamp: October 26, 2014, 06:02:02 pm
content: IIRC Appcad uses H & J

author: TonyStewart
author_rank: Contributor
timestamp: October 26, 2014, 06:13:01 pm
content: 1. Dielectric constant is a large 10 or 20% depending on matl, so TDR testing at factory is advised.2. Dielectric constant not constant, declining slowly above where loss tangent starts to rise. e.g. 1GHz for FR43. Track thickness adds another tolerance to width called etchback factor.

author: Carrington
author_rank: Super Contributor
timestamp: October 26, 2014, 06:44:26 pm
content: Marked in red, without tolerances. Marked in dark blue, with a tolerance of a little more than 10% for Er:

author: PetAd
author_rank: Newbie
timestamp: October 26, 2014, 07:10:33 pm
content: QuoteI made calculations with Matlab and H&J methods, now i gotZ0 = 63.45OhmEffective permittivity = 3.325Eff. line width is 12.1259 milWcalc result is 60.4811 Ohm, closer with this method, but not the same again.QuoteWhere I work, there are some good DPO, maybe i can check some pcb-s.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 26, 2014, 09:09:37 pm
content: I would be more impressed with the assignment if it involved an analytical or computational approach to derive the formulas from theory, and/or actual experimental tests involving fabricated boards of various geometry (assuming of course you have access to the VNA fast enough and accurate enough to perform the requisite tests).Neither of which sounds like "short essay" material. Indeed, masters' theses have been written on less. So...On the plus side, for such a, well, weak paper -- on an otherwise rather advanced and involved topic -- whether your conclusion is ultimately correct or not, probably won't matter very much, just as long as you've presented your case clearly from thesis to conclusion, based upon the references used to inform that conclusion. Or to put it slightly differently: even if you use shitty references, as long as your conclusion is right, it's only partially your fault (choice of reference, or failure to sufficiently research the subject, is still your fault, but it's not usually as big a part as doing the writing itself).Tim

author: westfw
author_rank: Super Contributor
timestamp: October 27, 2014, 05:47:13 am
content: When I was looking at some TI C3000 designs, I couldn't get any of the online calculators to match up with the dimensions that TI had in their reference design, either :-(