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PCB control impedance calculation

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DimaA

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impedance calculation

I have 6-layers stackup:

-----SIGNAL
-----GND
-----SIGNAL
-----SIGNAL
-----VCC
-----SIGNAL

the board is for PCI express cards, with differential lines, that must be impedance controlled to 100 Ohm differential.
For top layer and layer 3, the reference layer is the GND layer and I can use the microstrip model for calculationg the impedance.But how to calculate impedance for layer 4 and bottom layer? They don't have adjacent reference GND layer.
 

impedence control calculation

VCC can be considered as transmission line "GND" as well.
 

pcb control

But when the VCC layer consisted of many VCC islands,each island is different voltage, can it be still consisted as reference plane?
 

pcb impedence control caculation

Yes. It is still a "GND" for high frequency.
 

just don't run any critical signal across the gap in the planes. it will increase your return path for those signals and may ruin your noise budget.
 

Though you have different Power plane with different voltage level consider the PWr plane corresponding to the voltage level of traces.For example If USB traces are routed consider the 1.8v or 2.3v of USB supply voltage plane as reference and similarly For Ethernet it operates at 3.3V,so consider 3.3v plane as reference plane..


Regards
Rajan.k
 

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