GHz top layer circuit board design

Hi,

I'm trying to design a board for a circuit working on few GHz (some parts 1GHz, other 8 GHz)
My question is, should I keep the (top) layer filled with copper among the traces, or I keep the traces only?
BTW, I'm leaving the (bottom) layer with copper to reduce the emission and coupling, right?

Thanks

please make sure that there are no islands formation (small unconnected, or thread like portions of copper) when you do copper filling in outer layers. This will sometimes show up as DRC warnings in your CAD tool and designers tend to ignore them. But these islands can be potential EMI sources. Secondly keeping such high speed traces in striplines would be much better than routing them in outer layers.

If you have enough GND planes in the card and all GHz lines are properly routed as differential traces, then you are much safe in terms of EMI.

Hope that Helps.

Thanks for your notes about "copper islands" and "differential traces"
I can conclude from your reply that you fill the top layer with copper.
But I still don't know why some circuits of GHz are not filled with copper like this one.
On the other hand here is a GHz circuit filled with copper.
So, which technique is better?

In a critical design is it better to have ground or ground filled top layer as it reduce RFI loop area. For a single circuit as in your pictures is this a minor problem as there are no other circuits to interfere with. Advantage in second picture with several ground layers and a lot of vias is better isolation and a bit less transmission loss. Isolation had been even better for buried transmission lines. For minimum losses must transmission line width be designed for correct impedance, which depends on how ground is designed.

Thanks for your reply
Yes, it's looks like filling the top layer depends on the circuit design and purpose as in this transceiver circuit which I think it includes antenna as well
As you said, the (general) idea is to fill the top layer and make a lot vias to reduce the losses and interference.

It is a typical RF/microwave circuit with "microstrip" lines, where you have a solid ground plane below. This provides a clean path for the return current, and is good design practice. Just make sure that it is indeed a solid ground, and there are no slots/cutouts underneath the signal line. That's what really counts, to have a well defined continuous ground. The question if it is underneath (microstrip) or side by side (coplanar/differential) is not so critical.

This means that additional ground on the upper level, connected with lots of vias, can be added if you wish. From my experience with PCB up to 24GHz, it's not required IF you have a well defined ground below (microstrip). Note that the line impedance will change if you add top level ground so close to the signal line, as shown in your second example. Use a transmission line calculator or EM analysis to calculate the required reduction in line width if you add that top level ground.

mmm.. some good points.. which made for me another new question here
thanks