Sonnet User's Guide - Sonnet Software

Sonnet User’s Guide
When we vary the number of cells per wavelength, along the length of the line, we
see an inverse square relationship. Double the number of cells per wavelength
along the length of the line and the percent error decreases by a factor of four.
An equation which expresses the error as a function of subsectioning is:
16
E T ≅ ------- + 2 ⎛------
16 ⎞ 2
N
N ⎝ W
N ⎠ W ≥ 3 N L ≥ 16
L
where
N W = Number of cells per line width,
N L = Number of cells per wavelength along line
length,
E T = Total Error (DMAC) (%).
This equation estimates subsectioning error only. For example, any de-embedding
errors are added to the above error. This error estimate should be valid for any
electromagnetic analysis which uses roof-top subsectioning.
Notice that the quantities used for the error estimate are in terms of cells, not subsections.
Cells are the smallest possible subsections size. In Sonnet, subsections
in the corners of polygons are one cell on a side. Subsections along the edge of
polygons are one cell wide and can be many cells long. Interior subsections can
be many cells in both dimensions.
We have found that, for most cases, the cell size is the important parameter in determining
error. Or in other words, the smallest subsection size is important. For
example, the stripline benchmark geometry projects, mentioned before, are set to
make the lines 16 cells wide, even though those 16 cells may be merged into only
4 or 5 subsections. It is the 16 cells which determine the level of error, not the 4
or 5 subsections.
In performing this error evaluation, we also found that the error in characteristic
impedance due to N W is always high, never low. Also, there is very little variation
in the error for different impedance lines. The above equation can be very accurate
in evaluating error. And, finally, for N L above about 40 cells per wavelength, all
the error is in the characteristic impedance. The error in velocity of propagation is
essentially zero.
The above equation can be very accurate in evaluating error. With this precise
knowledge of the error, we can now do something about it.
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