[SI-LIST] Re: Couplin capacitance
- From: "Jeremy Plunkett" <jeremy@xxxxxxxxxxxxxxx>
- To: ARIAZI@xxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Mon, 24 Feb 2003 22:24:40 -0800
Abe, Anshuli,
I'd just like to point out that the actual crosstalk voltage created by =
the coupling capacitance will depend on both the mutual capacitance and =
the capacitance to ground of the victim transmission line(both per unit =
length). If they both are scaled by the same amount, the amplitude of =
crosstalk due to the capacitive coupling will remain the same =
(neglecting changes in the inductance of the victim as the width is =
varied). =20
This is because for each unit length of the transmission lines, the =
mutual capacitance and capacitance to ground of the victim line form a =
voltage divider. As the capacitance of one element of the divider grows =
larger relative to the other, the impedance of that element drops and a =
larger portion of the noise voltage appears across the other element. =
Noise voltage that is dropped across the mutual capacitance (due to a =
small value of mutual capacitance creating a high impedance) does not =
appear as noise on the victim.
In your results, the ratio of the coupling capacitance to the =
capacitance to ground becomes slightly smaller as the trace width is =
increased while the spacing is held constant, indicating that total =
capacitive crosstalk is also decreasing. This is what I would expect =
based on similar simulations I've done using the Hspice 2D field solver. =
However I would check your geometry and units, because the amount of =
coupling capacitance is very high compared to what I would expect.
regards,
Jeremy
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Abe Riazi
Sent: Saturday, February 22, 2003 10:21 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Couplin capacitance
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 =3D 0.0000254 meter.
1 inch =3D 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 =3D 1.4 mils)
The two trace files for Typical case (width W =3D7 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 =3D 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 =3D 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 =3D 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 =3D 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 =3D 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 =3D 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 =3D 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 =3D 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 =3D 4 mils ):
Cm =3D 1.065 pF
Typ case ( W =3D 7 mils ):
Cm =3D 1.176 pF
Max case ( W =3D 10 mils ):
Cm =3D 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
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