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carolyn
Canada
4 Posts |
 Posted - Aug 01 2014 : 18:06:55
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| Hello Enrico, Is there some way to specify some conductor as ground, like if I wanted to model a coplanar waveguide? |
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Enrico
550 Posts |
Posted - Aug 04 2014 : 19:17:37
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Actually this is a matter of how you treat the resulting capacitance matrix. FasterCap will always give you a full Maxwell matrix, this is the most complete information. You can 'restrict' to a scenario with some grounded conductors by playing with the values you have.
In a nutshell, if you consider one conductor grounded, then ignore its self-capacitance, and sum the mutual capacitances to the self-capacitances of the other conductors respectively (this capacitance goes in parallel to the self capacitance that is the capacitance to infinity, i.e. the zero potential reference). In a two-conductor system, you will easily recognize that this is exactly one row (or column) of the Maxwell capacitance matrix, i.e. your calculation is already done.
Best Regards,
Enrico
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Enrico
550 Posts |
Posted - Aug 28 2014 : 18:47:12
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I realize I've been probably making a long explanation short.
A bit more formal, but less intuitive way to see it is using directly the Maxwell capacitance matrices. Let's take a three conductor set, two of which (say 1 and 3) are grounded. If you set the voltage to 1V for the conductor 2 and 0V for the others, then the charge accumulated on 2 is your capacitance (by definition). So:
|C11+C12+C13 -C12 -C13 | | 0 | | -C12 |
|-C21 C22+C21+C23 -C23 | * | 1 | = | C22+C21+C23 |
|-C31 -C32 C33+C31+C32| | 0 | | -C32 |
Since the charge is C22+c21+c23, this is your value.
More generally, if you have a set of m conductors, out of which you ground n, you simply extract from the Maxwell capacitance matrix the sub-matrix composed by the row / cols corresponding to your non-grounded conductors, getting another Maxwell capacitance matrix for your system.
Please be aware that in cases like this one, FasterCap saves you most of the above trouble, since you can connect conductors together with the '+' symbol at the end of the conductor definitions. So if you connect conductor 1 and 3 together, you get a 2x2 Maxwell capacitance matrix, from which you need to delete only ONE row / col (and this is independently of the number of grounded conductors, as long as you short them together)
I hope to have been clearer now.
Best Regards,
Enrico |
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ground planes
