[SI-LIST] Re: Couplin capacitance

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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