Reference plane For Return Current

[jack_pcb]

Hi All ,

I have a help request for the signal return current path .As we know it need a least impedance path for return path so a GND just below the Microstrip line works as its return path ,but when it is a Power Plane having relevant power of the signal does it affect the signal quality and how it return ,and If the Power plane is different power than the signal driver than how the return signal goes in it and how does it works .

Thanks in Advance

 

[marce]

If it is not a voltage the signal is derived from then the return path will find a way using caps or having to go all the way back to the PSU where the voltages are derived...
Generally though it will seek a path through the decoupling caps on a PCB....

 

[jack_pcb]

HI Marce ,
Thank you for the reply .
But what i understand is that , for the other voltage reference plane ,the return current have to travel through the Decaps ,it might be sometimes far or a long loop area . So doesnt it affect the signal quality . There is a confusion some of the writers say that signal does not know the Type of reference plane it only see a copper , low inductance path and so it doesn't affect either its power or GND .

 

[marce]

Yes it can create a large loop area, the return current doesn't know the reference planes voltage, but there does have to be a closed path so it has to return to source, so generally a GND plane is best as it provides the shortest loop- (provided there are no slots in the ground plane). basically once you get above 1MHz the return wants to find the path with the lowest inductance... If I was going to use a voltage plane for my return I would use the same voltage plane that the IC I/O is powered from.