[SI-LIST] Re: Couplin capacitance
- From: "Abe Riazi" <ARIAZI@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Sat, 22 Feb 2003 10:21:25 -0800
Anshuli Goel Wrote:>
>
> I have a question . What wil happen to coupling capacitance on the
> coupled transimission lines if I keep the separation same but increase
> width of the lines.
>
> Regards
> Anshuli
>
>
Dear Anshuli and Others:
I have attempted to analyze this problem using fastCap.
My assumptions, methodology, results and conclusions are presented below.
I. ASSUMPTIONS
Let us assume that the geometry includes two coupled microstrip lines, each
trace being 1.0 inch long and 0.72 mils thick (0.5 oz copper weight).
Three different values of trace widths ( 4.0, 7.0 and 10 mils) are
considered; however, the edge-to-edge separation between the two conductors
is maintained constant at 6 mils.
Substrate thickness is 4 mils, with relative dielectric constant of 4.25.
The ground layer is 1.5 by 2 inches, with thickness of 1.44 mils
(1.0 oz cu weight).
All dimensions are converted to metric units by applying:
1 mil = 0.0000254 meter.
1 inch = 0.0254 meter.
II. METHODOLOGY
The FastCap field solver program is utilized for computations as it is 3D,
accurate and free!
The MIT cubegen utility is also employed to generate the objects/panels
representing the trace and ground plane structures required for the FastCap
input files.
The ground plane file is created by the command:
>cubegen -xo-0.019 -xh0.0381 -yh0.051 -zh0.0000366 -p -t -nx10 -ny10 -nz10 -
e1 > plane.in
(Note: xh0.0381 and yh0.051 relate to dimension of plane which are 0.0381 by
0.051 meter or equivalently 1.5" by 2.0". zh0.0000366 means thickness of
plane is 0.0000366 m = 1.4 mils)
The two trace files for Typical case (width W =7 mils ) called trace1_typ.in
and trace2_typ.in are produced via:
>cubegen -xo-0.000165 -xh0.00018 -yh0.0254 -zo0.00014 -zh0.000018 -nx3 -ny3
-nz3 > trace1_typ.in
>cubegen -xo0.000165 -xh0.00018 -yh0.0254 -zo0.00014 -zh0.000018 -nx3 -ny3 -
nz3 > trace2_typ.in
Here we note that -xo0.000165 and -xo0.000165 imply center to center
separation of these two 7 mils wide traces is 0.00033 meter ( 13 mils)
translating to edge-to-edge separation of 6 mils.
Furthermore,
xh0.00018 implies that width of trace is 0.00018 meter ~ 7 mils.
yh0.0254 defines length of each trace 0.0254 meter = 1 inch.
zh0.000018 indicates that thickness of trace is 0.000018 meter ~ 0.71 mils.
Zo0.00014 takes into account distance between trace relative and plane (e.g.
thickness of dielectric substrate) of 4 mils plus plane thickness of 1.4
mils.
For the minimum case ( trace width 4 mils = 0.00011 meter), the trace files
trace1_min.in and trace2_min.in are generated by means of:
>cubegen -xo-0.000127 -xh0.00011 -yh0.0254 -zo0.00014 -zh0.000018 -nx3 -ny3
-nz3 > trace1_min.in
>cubegen -xo0.000127 -xh0.00011 -yh0.0254 -zo0.00014 -zh0.000018 -nx3 -ny3 -
nz3 > trace2_min.in
Similarly, for the maximum case when trace width is 10 mils (0.000254 m),
the required
files trace1_max.in and trace2_max.in are created by commands:
>cubegen -xo-0.0002 -xh0.000254 -yh0.0254 -zo0.00014 -zh0.000018 -nx3 -ny3 -
nz3 > trace1_max.in
>cubegen -xo0.0002 -xh0.000254 -yh0.0254 -zo0.00014 -zh0.000018 -nx3 -ny3 -n
z3 > trace2_max.in
The three FastCap input files consist of:
First input file:
* coupled_ustrip.in when trace width W = 4mils
C trace1_min.in 4.25 0.0 0.0 0.0
C trace2_min.in 4.25 0.0 0.0 0.0
C plane.in 4.25 0.0 0.0 0.0
Second input file:
* coupled_ustrip.in when W = 7 mils
C trace1_typ.in 4.25 0.0 0.0 0.0
C trace2_typ.in 4.25 0.0 0.0 0.0
C plane.in 4.25 0.0 0.0 0.0
Third input file:
* coupled_ustrip.in when W = 10 mils
C trace1_max.in 4.25 0.0 0.0 0.0
C trace2_max.in 4.25 0.0 0.0 0.0
C plane.in 4.25 0.0 0.0 0.0
III. RESULTS
After executing FastCap, the following matrices are obtained:
Result for Min Case W = 4 mils
CAPACITANCE MATRIX, picofarads
1 2 3
1%GROUP1 1 2.106 -1.065 -1.017
1%GROUP2 2 -1.065 2.106 -1.019
1%GROUP3 3 -1.017 -1.019 9.427
Result for TYP case W = 7 mils
CAPACITANCE MATRIX, picofarads
1 2 3
1%GROUP1 1 2.351 -1.176 -1.148
1%GROUP2 2 -1.176 2.361 -1.159
1%GROUP3 3 -1.148 -1.159 9.692
Result for Max case W = 10 mils
CAPACITANCE MATRIX, picofarads
1 2 3
1%GROUP1 1 2.586 -1.29 -1.265
1%GROUP2 2 -1.29 2.611 -1.294
1%GROUP3 3 -1.265 -1.294 9.939
The above matrix elements show capacitance between the conductors where,
GROUP1 represents trace1, GROUP2 trace2, and GROUP3 the ground layer.
IV. CONCLUSIONS:
The matrices reveal the coupling capacitance Cm between the two coupled
microstrip lines for various trace widths W when the edge-to-edge separation
is fixed
at 6 mils.
Min case ( W = 4 mils ):
Cm = 1.065 pF
Typ case ( W = 7 mils ):
Cm = 1.176 pF
Max case ( W = 10 mils ):
Cm = 1.29 pF
Subsequently, for the conditions analyzed
the coupling capacitance increases with the increasing trace width.
The matrices further indicate that capacitance between each trace and
the ground (e.g. ~ 1.018 pF for Min, ~ 1.153 pF for Typ, and ~ 1.279 pF for
Max cases)
also varies directly with the trace width.
A logical next step is to check these FastCap results using another field
solver.
Kind Regards,
Abe Riazi
ServerWorks
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
