Here's my relevant 6-layer stackup:
L2: GND, L3: Power, PP: 37mil, L4: Signal, core: 4mil, L5: GND
I have a 200MHz signal on L4 and my assumption is that most return current will go through L5 and some will return through L3. But on L3, the power planes are split. Should I put separate caps on these split power planes and place them right next to each other so that the return path can continue through the caps? My confusion is whether the caps will reduce the impedance and let more return current flow through and make EMI worse. Or should I not put these caps and somewhat force the return current to flow through L5 GND. Greatly appreciate your response. Thanks.