Re: [SI-LIST] : Dual Stripline impedance

Fred Balistreri (fred@apsimtech.com)
Wed, 15 Apr 1998 11:48:10 -0700

Vinu Arumugham wrote:
>
> For 1.4 mil Cu, 5 mil lines and a dielectric constant of 4.5, the
> Synopsys (Viewlogic ADG ) field solver gives
>
> Zdiff ~ 97 ohm for:
>
> Plane
> -----------------------
> 14 mil
>
> --------
> | L1 |
> --------
> 10 mil
> --------
> | L2 |
> --------
>
> 14 mil
> -----------------------
> Plane
>
> and,
>
> Zdiff ~ 104 ohm for:
>
> --------
> | L1 |
> --------
> 10 mil
> --------
> | L2 |
> --------
>
> Vinu
>
> Magnus Homann wrote:
>
> > Hi all,
> >
> > We're thinking about using a dual stripline for a differential
> > pair. The diff. impedance should be 100ohm.
> >
> > Have anyone used this before? Or seen it in suggested? Come to think
> > of it, it does sound like a good idea (not mine). Now, the hard part
> > is getting the right geometry... Any help from various programs or
> > books? Any references would very helpful, the only one I found is in
> > "Transmission Line Design Handbook", and it is not very clear. Can you
> > analyze this with a field solver?
> >
> > The concept goes like this (cut-through view of the board):
> >
> > Plane
> > -----------------------
> >
> >
> > --------
> > | L1 |
> > --------
> >
> > --------
> > | L2 |
> > --------
> >
> >
> > -----------------------
> > Plane
> >
> > The Zdiff should be between L1 and L2.
> >
> > Homann
As a bit of further proof ApsimRLGC gave a value of 99.72 for diff
impedance for Vinu's case. Two different field solver concur that this
stackup is correct (more or less) for 100 ohms. By the way the first
response to seperate the traces by 5 mils yeilds a 72 ohm diff result.
That would be way off the mark.

Best regards,

-- 
Fred Balistreri
fred@apsimtech.com

http://www.apsimtech.com