Re: gEDA-user: Trace Impedance with wcalc

[Dan McMahill <dan@xxxxxxxxxxxx>]

Marcel.Schmedes.EXTERN@xxxxxxxxxxxxxxxxxx wrote:
> Hello @all,
> I done a Mainboard Schematic with gEDA, now I have to layout the hole 
> thing,
> because I need LVDS,GigE,... I need to care about trace impedance.
> Because of this, I used wcalc to calculate trace impedance, but now I have 
> some (general) questions about impedance calculations.
> 
> For GigE e.g. I need 55Ohm Single- and 100Ohm Diff. Impedance.
> I used wcalc "Microstrip Analysis" and PCB Values to get the Single-Imp 
> (55Ohm).
> (w=0,55mm;l=20mm;h=0,38;Er=4,6;Tand=0,015;Tmet=35um,Rho=1,72e-8;100MHz)
> 
> But now I want to get the diff. Impendance with "Coupled Microstrip 
> Analysis", the only Variables I can influence are w,s,l.
> If I using w=0,55Ohm (from Single Imp.) it's impossible to get 100Ohm 
> diff. Imp..
> Minimal Spacing and Trace width is 0.15mm, if I'm using this, I get nearly 
> 100Ohm diff., but then my single Imp. is also 100Ohm.
> 
> So, please - just give me a hint. Do I completly misunderstood wcalc!?
> I really need to get this pcb to fab... - Any tips would be greatly 
> appreciated.


What you want is to use the coupled microstrip calculator and ask for 55 
Ohms for the even mode impedance and 50 for odd mode.  At least this is 
I think what you are asking.

Even mode impedance is the impedance of one of the lines when the other 
line is driven by an identical source.  Odd mode impedance is the 
impedance of one of the lines when the other lines is driven by an out 
of phase source.

So certainly you want 50 Ohms for Z0o (odd mode impedance) since that 
produces a 100 Ohm differential impedance.  The question is what you 
really mean by "single impedance" since the lines are coupled.

In general though if you are designing a coupled line, you should only 
use the coupled line calculator.

Hope this helps a little.

-Dan




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