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Coplanar structures and how to pour a ground plane:

Question asked by JERRY_MARKETOS on Apr 7, 2005
Coplanar structures and how to pour a ground plane:

COPLANAR STRUCTURES:
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From time to time, someone will deploy a coplanar model and view the layout,
then ask the question "why does the ground plane NOT appear along side the coplanar structure?"

The linear model for the coplanar element (in the schematic) is indeed modeling coplanar with
ground plane existing at the specified gap distance on either side of the entity
(and also ground plane underneath if the model happens to be grounded coplanar...)

The reason why ground plane cannot automatically appear in the layout is because the full boundary
of the ground plane is not defined by the coplanar entity-  only one border of the groundplane is
actually defined by the coplanar entity, and the remaining (usually 3) borders of the groundplane
remain to be defined by whatever may exist on the rest of the circuit layout.  For this reason, the
ground plane is not automatically generated and must be created later (usually after layout orientation
is finished).

"OK, so I finished orienting my layout and I want the groundplane aside the coplanar structures.  Now what?"

Most often, this is done using the polygon "pour" feature in Genesys....


HOW TO POUR A GROUND PLANE
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First, draw a metal rectangle or polygon that covers the entire area where ground plane construction is desired.
Often, this can mean drawing a rectangle that covers the entire periphery of the layout, while other times
this can mean just drawing a rectangle over a certain portion of the layout.  (But yes, it means to draw a large
metal entity that lies on top of other existing objects such as SMD footprints, transmission lines etc. on the same layer.)

Click once on the new rectangle or polygon to make it "current"

Find the icon in the layout toolbar that looks like a paint can.  (mouseover shows "pour polygon)  and click that icon.

(Often, the keepaway distance is specified same as the gap for coplanar waveguide)

The "tolerance" should typically be set to about 10% of keepaway distance - this is what determines precision
around corners, etc.

Depending on specific application and setting of the keepaway dimension, you may obtain undesired webs
of metal (often between pads of SMD footprints).  To deal with this....  Once a pour is performed, a "scissors"
icon will appear.  This can be used to trim webs of poured metal.  Scissors is used by clicking the icon, then
drawing a multiple point polygon (must be at least 3 points, usually 4 points, sometimes many more than 4 points).
The last point clicked needs to be same as the first point- this is what closes the polygon and completes the "cut".
It's best to zoom in before doing this for better resolution of the grid.

Note: If you are using EMPOWER to simulate Coplanar waveguide, use the "SLOT MODE" setting in the
EMPOWER Layers tab of the layout properties to reduce the simulation time.  

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