[SI-LIST] Differential impedance

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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