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Copper Balancing
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T3sl4co1l:

--- Quote from: Yansi on February 07, 2019, 06:45:30 am ---4layer board is not made out of two glued together 2layer PCBs! A common miss-conception.

It is made out of an internal 2layer substrate, and two outer prepregs - which for RF PCBs may be of different (lower loss) dielectric.


OP: Fill the 3rd layer also with GND.

--- End quote ---

It is both, however you want to order it.

Standard proto is done this way however, and you will most likely pay full-custom price for the pair-stack build. :)


Yeah, you say 3rd layer is "VCC", but there's only four nets routed on it?  If you don't need to pour VCC, why not add a few more vias and route everything 2-layer?  Fine, that's probably awful for RF.  Then pour VCC on the 3rd layer, like the name suggests.  It's only better that way (or, it should be anyway)!

Tim
dmills:
Yea it is the L2/L3 imbalance they are complaining about, flood L3 with a VCC polygon, it will fix this and lower your supply impedance at the same time.

I am guessing some sort of Doherty amplifier?

Regards, Dan.
Nikos A.:
vealmike thank you for your reply and your comments.

Yes layer 3 is power routing. Why you suggest to fill with power plane instead of ground?

In general  I avoid doing vias on pads. The only vias I placed are under SMA connector pads.

I do not have any RF signal goes from layer 1 to layer 4. All the RF signals are in layer 1. At Layer 4 are the control signals which are driven to layer 1 through vias.
vealmike:

--- Quote from: Nikos A. on February 08, 2019, 09:49:27 am ---vealmike thank you for your reply and your comments.

Yes layer 3 is power routing. Why you suggest to fill with power plane instead of ground?

In general  I avoid doing vias on pads. The only vias I placed are under SMA connector pads.

I do not have any RF signal goes from layer 1 to layer 4. All the RF signals are in layer 1. At Layer 4 are the control signals which are driven to layer 1 through vias.

--- End quote ---

You're welcome.
You will generally get better signal quality if you route both power and ground as planes. Putting power on that plane gives you a solid plane. If you fill with ground, the plane will have cuts and islands to allow for your power routing. From an ac perspective power and ground are the same thing, provided you have plenty of decoupling. So why would you fill with ground, and have to stitch the islands together with inductive vias when you could fill with power and get a solid plane?

Return currents flow in the nearest plane and  (where they can) track the signal current. Think of the return current as lazy. If it flows in a loop, the loop becomes an aerial, which radiates power. So to avoid doing that, the return current follows the path of the signal current as closely as possible.
As all your high speed is on 1 then all the high speed return currents will be on 2. As 2 is a solid plane it will shield 3 & 4 from the signal / return currents, so you could argue that it doesn't matter what you do with 3 in this case. But as has been previously said, creating a solid power plane lowers your supply impedance. This is because you get a free capacitor with layers 2&3 acting as big low impedance plates.



Via in pad - fine. you can do it, it just gets more expensive to ensure good design for manufacture, or for a hobby board makes soldering harder. 

Rf signals jumping layers - great, if they're all on 1 you don't have to worry about this either.
Nikos A.:
Thank you vealmike, that was a great explanation!!
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