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component grounding problem..

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top gnd stackup sequence

Dear all:
In 4 layer PCB, Layer sequence from top to bottom is top--gnd--vcc--bottom.
1. what's the difference if component ground connect to layer 2 and
to layer 4 ?
2. when we do RF front end PCB layout , why we need to dig out layer 2 and
layer 3 ?

Thanks.
 

Sorry , I description it more clearly.
In 4 layer PCB, Layer sequence norminal from top to bottom is top--gnd--vcc--bottom.

1. what's the difference if ground in layer 2 and
ground in layer 4 ?

2. when we do RF front end PCB layout , sometimes
we need to dig out layer 2 and layer 3 .
I don't know why we need to do that ?

Thanks.
 

For RF the normal practice is it should see the GND immediately.
If you use the above said signal --Gnd--VCC---signal method then you can use RF on both sides because VCC is also a GND for RF.
This way you will get more room to play with RF circuit with signal integrity.

You can also do the GND layer to layer 4 with the cost of signal integrity. In this case you will play only on one side of the board. needs much care and the problem is complicated to solve as it will interact with the digital signals if any.
SO better go in simple and easy mode.

2) In my view dig out is understood as RF via from top to bottom creating island on GND and VCC.

Let me know is my understand is correct?
 

    S

    super

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Sometimes the stack up is very thin for the layers. If you want to design a microstrip of a certain impedance, it might be too thin to make. So you remove the referencing ground on the layer below it and put the ground it references on a lower layer. The spacing to ground is longer so the trace is now wider and easier to make.
 

    S

    super

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Dear toonafishy ,
your point is right , but exclude impedance it have another
reason ?

Dear kspalla ,
in item 2 , toonafishy have a great point .
in item 1, thanks for your clearly explain, but I don't understand
one thing.
C=e0*A/d R=Rsqr*L/A
if GND connect to layer 2 , it will get more cap and less Res
if GND connect to layer 4 , it will get less cap and more Res
so it seems connect to layer 4 is better , why layer 2 is best choose ?

Thanks yours help.
Many thanks.
 

Your point is right but the transmission line width is chosen such that it is 50 ohm impedance for RF.
This will avoid the case of more cap and less res as pointed by you.
if chosen layer 4 then the 50 ohm impedance will be realized with higher trace width then the 2 layer case.
More trace width means you have less room to play.

If layer 4 is meant for RF gnd then you can not use layer 2 and 3 for routing purpose as it interfere with RF. This will limit though you have 4 layers but actually two layer only. This way you are not getting any benefit instead increased cost by going 4 layer.
 

    S

    super

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I know that you said , but these are not I ask.
sorry, I redescribe it again.
my question is
if GND connect to layer 2 or layer 4 have what difference (don't care impedance and routing just only consider grounding problem) ?

Thanks.
 

If we do not care for impedance then with proper placement of via and its diameter will determine the Grounding problem no matter which layer it is located.

I do not understand why you do not care for impedance. Probably I did not understood the question my apologies.
 

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