[SI-LIST] Re: PCB Trace impedance algorithms
- From: "Patrick Jabbaz" <pjabbaz@xxxxxxxxxxx>
- To: si.rules@xxxxxxxx
- Date: Mon, 22 Jan 2007 22:54:27 -0800
Hi Sam,
I used several layout tools including Allegro, and found that the impedance
tool in Allegro, seems very accurate at least in the Allegro expert series
610, I recommend however using POLAR impedance calculator, I guess now they
have the spreadsheet tool.
Most fab houses that is what they use. Very simple and easy to do what if
scenarios.
The numbers I get from Polar compared to Allegro are very similar.
Note in Allegro one must have the material properties set correctly in order
to get accurate results.
Anyway this is primarily use to get the nominal impedance you want nailed
down. The most important item is, having a table on your FAB drawing
indicating the LINE WIDTHS on each Layer and their corresponding impedance,
with this information the FAB house will be able to identify the target
impedance of concern, and make proper adjustments in order to meet them.
Another item of concern from FAB houses is that they use POLAR instruments
"TDR" to measure impedances, most of the time is just fine, however the rise
time on the instruments is very slow compare to High speed SERDES.
sample table
Layer Line width single diff Z space cent-2 cent
3 5 55
4 5 55
8 4.5 100 15
Regards,
Patrick Jabbaz
On 1/22/07, Sam Sam <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
>
> ---------------------------------
> 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
>
>
>
--
Patrick Jabbaz
408-621-6533
------------------------------------------------------------------
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
Other related posts:
