[SI-LIST] Re: PCB Trace impedance algorithms

The Allegro Tool uses a BEM field solver to  solve Maxwells equations and 
calculate the trace parameters/impedances and is not dependent on closed form 
equations. Polar also use the BEM method for their tools. If you use closed for 
formulae note the limitations of applicability, many authors fail to quote 
these, despite them being in the original sources .

Polar because of their heritage have good relationships with PCB fabricators 
and so have good correlation to the manufacutured PCBs.  The biggest problem 
with all tools is the validityof the laminate relative permittivity 
measurements. This is discussed in other threads.

Regards

John

si.rules@xxxxxxxx wrote:
> Dear si-list members,
>    
>   I am learning tool support for pcb designs. I have some questions regarding 
> calculating impedance of a traces in PCB. I use allegro's built impedance 
> calculator. I am also aware that there are various other calculator tools 
> from UltraCAD, Polar Instruments etc. I am wondering how efficient and 
> accurate these calculations are. I guess most of them use some kind of 
> assumptions and have simplified closed form formulas to qucikly extimate the 
> impedance. But can you people guide me as what is the exact technique or 
> algorithm to calculate the impedance of a pcb trace say for a microstrip 
> structure. Any papers or links to this study would be appreciated. In 
> specific, since i am using allegro 's calculator i would like to know how 
> they calculate the impedance and what are the assumptions they take. I have 
> seen most calculator allow single ended and differential trace calculations. 
> Is it possible to extend these techniques to multiple traces. More 
> importantly the accuracy of the
>  formulas is of concern to me. When compared to full wave results these 
> formulas from different tools give different result. So i am looking to learn 
> what is the background behind these? Please advise me on this. Thanks in 
> advance. Looking forward for your answers....
>    
>    
>   Sam
>               
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