We've always used solid ground plane for controlled impedance work here
at Motorola. To answer your question though, I think the impedance will
increase. To be sure, ask your resident sparkey what he or she thinks.


Steve

*******************************************************

On Mon, 12 Jan 1998, 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.
>
> I'm using the following for my line thinking ...
>
> Ideal Lossless Transmission Line
>
> Z0 = [ L0/C0 ]^0.5
>
> L0 = inductance per unit length
>
> C0 = capacitance per unit length
>
> Basic equation for characteristic impedance
> over a solid ground plane.
>
> The method to my madness is that if I'm restricted
> by adjusting dimensional constraints for increasing
> or decreasing impedance, that I can somehow fiddle
> with gridding of the associated ground plane to
> adjust for the impedance I need. Now there's alot
> of other considerations that may involve current
> distribution and such but right now, I'm not
> worried about that.
>
> Is this old stuff being hashed out again?
> Or, have I stumbled onto a fabing secret?
>
> Regards,  Doug McKean
>
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