Dear all, Thanks for all your answers. I will definitely go over the references you mentioned to help me more. John, I understand full wave field solvers provide accurate answers compared to formulas at the cost of the computational time. How is the impedance calculated from the field solutions? If i calculate my scattering parameters, is it possible to find the impedance of the trace from that? As you mentioned allegro uses BEM field solver, they should be able to calculate the impedance matrices to do that? Steve, When you say curve fitting technique is used for closed form formulas, what do they curve fit to calculate the impedance. Can you plz elaborate a little on that?Also from John's comments polar also uses full wave technique, why does that be more effcient or attractive that ansoft tools. Also people,I am thinking what is the impact of frequency dependence. That is when calculating impedance do these tools include frequency dependent effects or they calculate at one frequency point? thanks again for your earlier responses, Sam "Dodd, Ian" <ian_dodd@xxxxxxxxxx> wrote: Sam, Formulas are available for the calculation of trace characteristic impedance (and velocity of propagation) for simple trace topologies. Many are reasonably accurate over specified ranges of geometric ratios. Enterprise PCB design tools tend not to use formulas because the geometric range limitations. Instead they use 2D plus finite element fields solvers. They are called 2D plus because they assume the trace segment or trace segments being analyzed have a uniform cross-section, so they calculate the inductance, capacitance, resistance and conductance per unit length for the representative cross-section, then multiply the values by the actual length. These types of solvers can usually handle systems of many coupled traces. Obviously there is a lot more information. The reference kindly provided by Dave Instone is certainly good. You might also try Signal Integrity Simplified by Eric Bogatin for a wider view. I hope this helps Ian Dodd Mentor Graphics Inc =20 -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of David Instone Sent: Tuesday, January 23, 2007 3:46 AM To: si.rules@xxxxxxxx Cc: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: PCB Trace impedance algorithms Sam, A good place to start would be 'Transmission Line Design Handbook' by =3D Brian C Wadell published by Artech House ISBN 0-89006-436-9 which = has=3D20 various formulae for just about any type of transmission line = you'll=3D20 ever come across. Dave Instone Oxford Semiconductor Ltd 25 Milton Park Abingdon Oxon ox14 4sh UK www.oxsemi.com +44 (0)1235 824963 Sam Sam wrote: > Dear si-list members, > =3D20 > I am learning tool support for pcb designs. I have some questions=20 > reg=3D arding calculating impedance of a traces in PCB. I use allegro's built im=3D pedance calculator. I am also aware that there are various other calculat=3D or tools from UltraCAD, Polar Instruments etc. I am = wondering how efficie=3D nt and accurate these calculations are. I guess most of them use some kin=3D d of assumptions and have simplified closed form formulas to qucikly exti=3D mate the impedance. But can you people guide me as what is the exact tech=3D nique or algorithm to calculate the impedance of a pcb trace say for a mi=3D crostrip structure. Any papers = or links to this study would be appreciate=3D d. In specific, since i am using allegro 's calculator i would like to kn=3D ow how they calculate the impedance and what are the assumptions they tak=3D e. I have seen = most calculator allow single ended and differential trace =3D calculations. = Is it possible to extend these techniques to multiple trace=3D s. More importantly the accuracy of the > formulas is of concern to me. When compared to full wave results=20 > these=3D formulas from different tools give different result. So i am looking to =3D learn what is the background behind these? Please advise me on this. Than=3D ks in advance. Looking forward for your answers.... > =3D20 > =3D20 > Sam > =3D09 > --------------------------------- > All new Yahoo! Mail =3D20 > --------------------------------- > Get news delivered. Enjoy RSS feeds right on your Mail page. > > ------------------------------------------------------------------ > 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 technical documents are available at: > http://www.si-list.net > > List archives are viewable at: =3D20 > //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 > =3D20 > > > =3D20 ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net List archives are viewable at: =20 //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 =20 ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net 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 --------------------------------- All new Yahoo! Mail --------------------------------- Get news delivered. Enjoy RSS feeds right on your Mail page. ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net 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