Port Tutorial Series Coplanar Waveguide CPW HFSS v8 Training HFSS v8 Training This presentation is one in a series of Port Tutorials intended to help users better understand the nuances of model excitation With incorrect inputs the entire 3D field solution will be incorrect Therefore proper attention to port definitions can make the difference between a successful and unsuccessful HFSS analysis In this tutorial presentation the user will be presented with guidelines for setting up ports on bot......
Port Tutorial Series: Coplanar Waveguide (CPW)
HFSS v8 Training
This presentation is one in a series of Port Tutorials, intended to help users better understand the nuances of model
excitation. With incorrect inputs, the entire 3D field solution will be incorrect. Therefore, proper attention to port
definitions can make the difference between a successful and unsuccessful HFSS analysis.
In this tutorial presentation, the user will be presented with guidelines for setting up ports on both grounded and
ungrounded coplanar waveguide (CPW) transmission line structures. Recommendations for both
wave
and
lumped
ports will be outlined. General advice will be provided for initial port sizing, followed by diagnostic
procedures to use to evaluate whether or not a user’s ports are appropriate for their specific configuration. Pictorial
examples will be provided of grounded, ungrounded, and limited-side-ground CPW port mechanizations
throughout the tutorial.
1
Coplanar Waveguide: Basic Review
HFSS v8 Training
Structure
Coplanar Waveguide is a transmission line system consisting of a central current-carrying trace atop a substrate,
coplanar with side grounds extending beyond a symmetric gap to either side of the trace. There are different kinds of
CPW transmission lines used in RF and microwave applications
Grounded
CPW (GCPW) will have an additional ground plane on the underside of the substrate. In practice
this plane needs to be sufficiently distant from the trace as compared with the side grounds that the system
carries a CPW mode rather than a lossy microstrip mode
Ungrounded
CPW is more standard, in which the side grounds coplanar to the strip itself provide the only
return current path, and the underside of the substrate is unclad.
Finite Ground
CPW (FG-CPW) traditionally refers to an ungrounded CPW in which the side ground
metalization is of limited width, often not more than 2 – 3 times the width of the center trace itself, due to space
considerations.
Advantages
CPW has the same advantage as microstrip, in that the signal is carried on an exposed surface trace, on which
surface-mount components can be attached. Being a surface signal carrier it also lends itself well to testing via
ground-signal-ground type probes
Unlike microstrip, CPW (at least in the ungrounded form) has little parasitic losses between surface mounted
components and an underlying ground plane
Disadvantages
The primary disadvantage of CPW is that it is harder to design with: features such as open and shorted stubs are not
as simple as they are with microstrip or stripline. Additionally, CPW is not well supported even by many modern
transmission line calculators and circuit simulators
While obtaining the necessary dimensions for a CPW of a desired characteristic impedance is possible, often the
dimensions output by transmission line calculators are impracticable given etching constraints
If the ‘aspect ratio’ of a CPW (the ratio of its gap to trace widths) becomes too high or too low, the desired CPW mode
can be supplanted by parasitic microstrip modes or parallel slotline modes, resulting in poor performance
2
Coplanar Waveguide: Dimensioning
HFSS v8 Training
CPW Dimensions
CPW is generally defined by center strip width
w,
gap width
g,
substrate height
h,
and substrate dielectric material
Metal thickness is also important, especially when metal thickness
t
0.1w or
t
0.1g
For FG-CPW, the width of the side grounds,
S,
must also be considered in port design
3
HFSS Ports: General Requirements
HFSS v8 Training
Purpose
A
Port
is a 2D surface on which the fields will be solved according to Maxwell’s Equations to determine appropriate
RF modal excitations into the 3D model volume. Think of a port as an “aperture” in which the field distribution and
orientation is known for the steady-state finite element solution
Wave
ports solve actual field distributions in transmission line cross-sections.
Lumped
ports excite simplified field
distributions to permit S-parameter outputs where wave ports are not feasible.
Characteristics
Port surface area takes on the material characteristics of the materials which touch its face
Wave Port boundaries take on the boundary characteristics of the boundaries which share its edges
Edges touching
perfect_e
faces, such as ground planes, become
perfect_e
edges for the port computation
Edges touching
perfect_h
faces become
perfect_h
edges for the port computation
Edges touching
symmetry
faces take on the definition of the appropriate
perfect_e
or
perfect_h
type
Edges touching
radiation
faces, however, default to
perfect_e
conductive boundary conditions!
The environment variable ZERO_ORDER_ABC_ON_PORT = 1 can set them to 377 ohms instead
Due to the port bounding edges, which may not match boundaries on field behavior in the full 3D volume around the
transmission line past the port plane, proper port sizing and location is crucial
A microstrip port at left has
sufficient surface area for fringing
field behavior, while the one at
right forces field attachment to the
port side walls, even if the
surrounding area was designated
as a
radiation
boundary
4
CPW Wave Ports: Starting Recommendations
HFSS v8 Training
Wave Port Size
The standard recommendation for most CPW wave ports is a rectangular aperture
Port
width
should be no less than 3 x the overall CPW width, or 3 x (2g +
w)
Port
height
should be no less than 4 x the dielectric height, or 4h
If no convenient ‘solid face(s)’ meet the above sizing requirements, draw a 2D rectangle for the Port.
Wave Port Location
The wave port should be centered horizontally on the CPW trace
If the port is on GCPW, the port bottom edge should lie on the substrate bottom ground plane
If the port is on ungrounded CPW, the port height should be roughly centered on the CPW metal layer
Wave Port Restrictions
As with all wave ports, there must be only
one
surface normal exposed to the field volume
Port should be on exterior model face, or
capped
by a perfect conductor block if internal
The wave port outline
must
contact the side grounds (all CPWs) and bottom ground (GCPW)
The wave port size should
not
exceed lambda/2 in any dimension, to avoid permitting a rectangular waveguide
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