Doug McKean wrote:
>
> Does anyone know if gridding a ground plane
> under a trace of controlled impedance increases
> or decreases by having to actually work with this?
>
> I can guess and theorize all day long...
> My guess is that since there's more distributed
> capacitance with a solid ground plane, that
> increasing the grid pattern of the ground will
> lower the distributed capacitance thus INCREASING
> the controlled impedance.
>
> big snip <
>
> Is this old stuff being hashed out again?
This kind of gridding has been around since some time, it's use seems to been
limited to mainframes. But it is possible that this gridding has some new
applications in MCM technology. The main reason for the perforation is to make
connections between different planes, although other reasons could be the
higher line impedance (which requires less driver power!) or a reduction of
required substrate planes (lines share the a plane with the power distribution).
Gridding (or perforation) does change the impedance and propagation velocity of
the signal lines. These signal lines must be treated as nonuniform transmission lines
with a periodicity due to the perforation. The gridding does not seem to attenuate or
does not introduce signifcant amounts of dispersion. Concerning the "leackage" of
RF energy from lines on perforated planes, the gridding is probably worse compared to a
continuos plane. However, personally I do not know of any published data and things
need not be that bad. Tthe higher line impedance translates in less line current
and possibly smaller radiated emissions. the main point is the size of the gridding,
i.e. how big is big?
You can find approximate analytic expressions for the parameters of these lines.
These methods are based on using "averaged" values over one period (or section
of the perforation). A second method calculates the line parameters of cascaded
sections of uniform lines. Other methods are based on 3-dimensional numerical techniques.
You can find information on the subject of signal lines on perforated planes in:
IEEE trans. on Microwave Theory & Techniques, IEEE trans. on Antennas & Propagation,
IEEE trans. on Components, Packaging & Manufaturing and the IBM Journal of Research &
Development (I am probibly missing others too). You can search or brows the IEEE web
site for abstracts on this topic (I think).
You can use following reference to get started:
"FDTD Analysis of the Electrical Performance for Interconnection Lins in Multichip
Modules (MCM) with Perforated Planes", Jin Zhao, Zhengfan Li, IEEE trans. CPM part B,
vol 20, nr 1, feb 1997, pp 34
It contains references to other numerical and approximate analytical techniques.
The same IEEE publication also contains a discussion of a MCM with perforated planes and
manu references:
"Modeling and experimental Verification of the Interconnected Mesh Power Systems (IMPS)
MCM Technology", Yee L. Low et al., IEEE trans. CPM part B, vol 20, nr 1, feb 1997,
pp 42
Good luck,
Jan Vercammen
PS. If anyone has additional info or knowledge of publications on this subject, especially
on the radiated "leackage", please put it on the this forum.
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