AnsweredAssumed Answered

Substrate height stripline

Question asked by JERRY_MARKETOS on Oct 30, 2005
Latest reply on Oct 30, 2005 by JERRY_MARKETOS
Substrate height stripline

Let's say we wish to construct an analysis that includes
stripline transmission lines.

We begin by defining a substrate, let's call it SUBS1.
For example, let's say the substrate height entered is 50 mils.

For linear analysis, using a schematic with stripline models,
we place stripline models on the schematic and in their "properties"
boxes, we direct those stripline models to use substrate SUBS1.
(We also fill in width and length of the substrate line segments...)

The linear stripline models (as well as the vast majority of linear models
that utilize a substrate) will automatically pick up the substrate height
from the substrate they are told to use.  Thus, on the schematic,
within the "properties" box for the linear model, the substrate height
entry will be labeled "optional".

INFO #1:
In the vast majority of applications, it is correct to leave no entry in the
linear model's "substrate height" entry box, and let the linear model
get its substrate height automatically from the substrate entred.

INFO #2
In this example case, where we have a linear stripline model and
a substrate height of 50 mils,  the stripline model understands
this to mean 50 mils between ground planes, so the distance between
the stripline RF trace and each ground plane would be 25 mils.

Next, let's transition to making a layout of the circuit we're analyzing,
and performing an EM analysis of that layout....

The schematic models produce a corresponding footprint in the
layout (such as rectangular entities of length and width for tlines),
and we define the layer stackup for the layout in the layout properties
dialogue box (general layer).  For layout of the stripline circuit example
used here, the proper layer setup would include dielectric layer
on the bottom, then (working upward) the metal stripline layer,
then dielectric again on the top.  So in the layout properties
"general layer" tab, we would need to include at least a metal layer
with a substrate layer above and below it.

Then, during setup for EM analysis,  (the "empower layers" tab of the
layout properties dialogue box) we need to make use of an important
convenience feature provided in Genesys;

Proper layer setup in "empower layers" for this stripline analysis would be:

Top cover
substrate above
metal
substrate below
Bottom cover

All layers including top cover and bottom cover should be
pointed to the substrate we've defined in the workspace, same
substrate that was used for the linear analysis.

At this point the astute reader might chime in and say:
"BUT WAIT !  We defined upper and lower substrate for the EMPOWER
analysis both to call the substrate we defined in the workspace, which
has height of 50 mils.  Doesn't that make my stripline structure now
have 50 mils above and 50 mils below, for a total ground-to-ground
height of 100 mils ?  That's not same as the linear analysis !!"

The answer is YES.  Because EMPOWER allows the user to define
the layer stackup, and because we've called the same substrate now
for upper and lower dielectric in the stripline, the above statement
is true.  But there's an important convenience feature we're about
to invoke that solves this and makes the EM layer stackup equivalent
to that used in the linear analysis:

INFO #3
When doing stripline analysis in EMPOWER, note on the "empower layers"
tab the column for "use 1/2 height".  For the dielectric layers above
and below the stripline metal layer, check the "use 1/2 heigh" boxes.
This simply causes Empower to use half the height entered for the
substrate it's referencing.  (you'll see the greyed out numbers change
accordingly)

(continued in first reply below)  

Outcomes