Zhuyongfa,
the effect of the power plane split depends on the risetime or
frequency of the signal. A first-order model is that the signal in
propagating along a 50 ohm line, then a short length of 100 ohm line,
and then 50 ohm line. For a slowly rising edge (I.E. 250 ps) a fairly
wide split, with your stackup, is not that bad. I ran a SONNET
simulation of a 12 mil power plane split to check my model described
above. Putting this into SPICE we see that the risetime degradation
just is a few picoseconds. There is additional crosstalk between
traces. I can sim this for you if you'd like given your particular
dimensions.
The split power plane issue does not appear to be that bad. Keeping
the plane splits small (150 um) will help at the expense of increased
power plane-to- power plane coupling. All of this can be easily
simulated using SONNET LITE (free) or another 3-D field solver.
Dave Cuthbert
Consultant
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