[SI-LIST] Re: Difference betweem single stripline & dual stripline
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <whizplayer@xxxxxxxxx>
- Date: Fri, 2 May 2003 07:14:17 -0700
Hi Bob,
Here's a quick synopsis of a short study I did using XFX to model 4 =
topologies:
1) True "stripline"
2) Asymmetric stripline
3) Embedded (or I'm coining the phrase "homogenous") microstrip - =
meaning there's only a single reference plane, and the trace is embedded =
far enough in the dielectric that it sees only a homogenous dielectric
4) Microstrip, with only a thin dielectric covering the trace.
Findings:
True Stripline has conductor(s) sandwiched equidistant from 2 reference =
planes (Symmetric), and has homogenous dielectric. This is contrary to =
a previous understanding that I had - I thought stripline implied only a =
homogenous dielectric.
Of the 4 topologies studied - Symmetric and Asymmetric Striplines, =
Homogenous (embedded) Microstrip, and Microstrip -=20
FEXT is 0 for Symmetric Stripline
FEXT is very low (0.002) for traces on the same layer of Asymmetric =
Stripline and Homogenous Microstrip
FEXT is fairly high (0.016) for traces on different layers of =
Asymmetric Stripline
FEXT is highest (0.04) for traces on Microstrip
NEXT is lowest (~0.03) for Symmetric Stripline and traces on =
different layers of Asymmetric Stripline
NEXT is highest (-.13) for both Microstrips and traces on the same =
layer of Asymmetric Stripline
Losses are approximately equal, except in case where trace widths are =
reduced for symmetric stripline (to obtain correct Z0) - in that case =
losses are higher
Conclusions:
Given choice between Asymmetric Stripline or Microstrip:
Microstrip Traces will have significantly more FEXT
Microstrip Traces will have the same NEXT as Asymmetric Stripline =
traces on the same layer
Losses are similar
Traces on different layers of Asymmetric Stripline are susceptible to =
FEXT - should be routed orthogonally
Symmetric Stripline (once trace widths are reduced to obtain the same =
Z0) are more lossy than other topologies
Another interesting note: XFX calculates forward and reverse crosstalk =
as:
Fwdx =3D (Se - So)/(0.5(Se + So))=20
Rvsx =3D (Ze - Zo)/(Ze + Zo); not sure why units are given as v/v=20
I'd love to hear where these equations were derived. The XFX manual I =
have doesn't say.
This was a quick study, and I wouldn't deem it conclusive. I'll be =
interested to hear what others have found...
My study was also only for single-ended traces. The same concern about =
increased loss for symmetrical (edge-coupled) stripline would hold for =
differential, if you had to narrow the traces to attain the correct Z0, =
versus broadside-coupled.
Jeff Loyer
-----Original Message-----
From: Bob Patel [mailto:whizplayer@xxxxxxxxx]
Sent: Thursday, May 01, 2003 2:19 PM
To: si_list
Subject: [SI-LIST] Difference betweem single stripline & dual stripline
Hi!
Is there a difference in terms of electrical
parameters i.e. say loss, etc when using single
symmetric stripline layer and dual stripline layer.
I guess I am asking beside the cost & geometry is
there a harm if I use dual stripline layer over single
stripline layer where adding more routing layer is
necessary for me.
I ran cases to look at differences if I route edge
coupled differential traces on single symmetric
stripline or if I use dual stripline. I did not any
difference at least in loss or impedance etc.
Any input is appreciated.
THanks
Bob
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