[SI-LIST] Re: Differential impedance

Return path discontinuities affect even mode energy.  If the geometry is 
perfectly symmetric, the odd mode would be unaffected. 

The 6mil plane will strongly dominate over the 22mil plane.

Steve
Lakshmi N. Sundararajan - PTU wrote:
> Hi Team,
>
> Suppose I run a differential trace through a signal layer bound by
> two solid planes top and bottom.
> Does the differential trace impedance vary if the top and bottom 
> plane are both Ground planes, 1 Ground and 1 Power, both Power planes?
>
> Secondly, if the layer is Gnd-Sig-Pwr,
> and Gnd plane is 6 mils away from the signal layer,
> but Pwr is 22 mils away from the signal layer.
> Then in this condition, I expect the return path to
> flow through Gnd plane. So, would the signal be affected
> if the power plane is cut. I am interested in high speed
> stripline traces. I do not expect so, because
> the high speed differential traces and its return path
> would reference the Gnd plane which is only 6 mils away, correct.
>
> Please can you provide some inputs here.
>
> Thanks,
> -LN
>
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