[SI-LIST] Re: Diff Z for asymmetric coupled striplines

Hi Mike,
Thanks to you and everyone else for their suggestions. 

I tried 2 open source field solvers on the same configuration. MMTL gave a 
diffZ of 91.5ohms, and mdtlc gave 93.57ohms. These results seem reasonable, and 
both tools were very easy to use.

Structure Parameters
---------------------
line width = 3mils
line spacing = 4mils  (edge coupled)
trace thickness = .7mils (1/2 oz cu)
h1 = 3.9mils (dist to upper ref plane)
h2 = 7mils (dist to lower ref plane)
er = 4.3
Zo = 53.5ohms (obtained from a formula for a single asymmetric stripline)


Thanks for your help,
Jim
-------------- Original message from "Kotson, Michael" <mkotson@xxxxxxxxxxx>: 
-------------- 


> Hi Jim, 
> Give MMTL a try. I have had pretty good luck with the results, and 
> it's free. 
> 
> http://mmtl.sourceforge.net/ 
> 
> Best of luck, 
> -Mike 
> 
> -----Original Message----- 
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] 
> On Behalf Of otter30@xxxxxxxxxxxxx 
> Sent: Monday, October 20, 2008 10:16 AM 
> To: si-list@xxxxxxxxxxxxx 
> Subject: [SI-LIST] Diff Z for asymmetric coupled striplines 
> 
> Hello, 
> I need to determine the differential impedance of pair of coupled 
> stripline traces. They are edge coupled and are asymmetric in the sense 
> that the distances to the 2 reference planes are different - both traces 
> are 
> 4 mils from the upper plane and 7mils from the lower. 
> 
> Our pcb fabricator told us that this configuration would have a diff Z 
> of 100 ohms, but I have gotten a value from the cross section calculator 
> in Cadence Allegro of 79 ohms. I'd like to find another method to settle 
> this, as our board does have SI problems. 
> 
> The diff Z formula for this configuration isn't in any of the well known 
> SI or TL texts, although the symmetric case (H1 = H2) is. Can anyone 
> either: 
> 1) Point me toward the correct equation 
> 2) Suggest a free field solver that would let me calculate this value? 
> 
> I've downloaded the student version of Ansoft Maxwell, and am trying it 
> out. If it can extract the mutual 
> capacitance and inductance, is there a general method of getting the 
> diff Z from these? 
> 
> Any help is greatly appreciated, 
> Jim 
> 
> 
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