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.

  • 1
    \$\begingroup\$ Is it really only 4mil from the L4 signals to L5? \$\endgroup\$ – user1844 Nov 23 '16 at 22:27
  • \$\begingroup\$ The ESR of the ceramic cap ought to be << load and located to reduce the current loop on the board. Power feed to board may also need NiZn ferrite bead to prevent cable radiation. Dont forget dielectric tolerance >10% effects on R.L. for trace width. consider coplanar RF and copper Return with more suitable trace sizes that match Zo needs \$\endgroup\$ – Sunnyskyguy EE75 Nov 23 '16 at 22:30
  • \$\begingroup\$ incompliancemag.com/article/… you can estimate plane impedance based on square size and microvias as well as Cap inductance based on distance to planes and loop size \$\endgroup\$ – Sunnyskyguy EE75 Nov 23 '16 at 22:46

Caps at the split will not "encourage" more (high-frequency) current to flow in that plane, because the whole point about the problem with splits is that at the time the current sets-off into the 'bad' plane it doesn't yet know about the split - it's when it arrives at the split that the problems start.

But if there really is a ~10:1 difference in spacing between L3-L4 and L4-L5 then I doubt it's a big deal, as most of the return current will be L5 anyway.

  • \$\begingroup\$ keep loop currents to RF at a minimum with coplanar and some between L4&L5 to minimize edge egress. \$\endgroup\$ – Sunnyskyguy EE75 Nov 23 '16 at 22:53

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