Virtex-4 RocketIO MGT User Guide www.xilinx.com 251UG076 (v4.1) November 2, 2008Differential ViasRFrom the graph in Figure 11-12, the common-mode return loss (SCC11) is about 20 dBworse when compared to differential return loss (SDD11). The much worse common-moderesponse relative to the differential response is the reason why it is a good idea to reduceP/N skew as much as possible before entering a transition. It is generally best to optimizefor the differential signal, so in many cases, the common mode performance can actuallyhave excess inductance.To quickly compute excess capacitance, the "60/40" rule of thumb states that a -40dB returnloss at 1 Ghz equates to approximately 60 fF, or 63 fF to be more exact, of excesscapacitance. Because excess capacitance is a single-pole response, simple extrapolationrules can be used. For example, a shift of +6 dB to –34 dB return loss doubles the excesscapacitance. The 60/40 rule of thumb is applied to determine that the excess capacitance isonly 60 fF for the via with the shorter stub. Due to the excellent performance characteristicsof GSSG vias, even long via stubs only double at most (less than a +6 dB shift) thedifferential via’s capacitance.Chapter 12, “Guidelines and Examples” provides additional examples of differential vias.Appendix D of the XFP SPecification [Ref 4] also provides example differential via designs.Figure 11-11: Differential GSSG Via in 16-layer PCB from Pins L11 and L6Figure 11-12: Simulated Return Loss Comparing Differential and Common-ModeLosses for L11 and L6 GSSG ViasUG072_c3_34_030106From Pin L11, Exiting at Lower Layer From Pin L6, Exiting at Middle LayerUG072_c3_35_1028051E91E8 1E10-60-40-20-800Frequency, HzdB(Scc11_L11)dB(Scc11_L6)dB(Sdd11_L11)dB(Sdd11_L6)
