P/N Crossover Vias - Xilinx Virtex-5 RocketIO GTP User Manual

R
As a general rule, the P and N paths need to be kept at equal lengths through a transition.
Where possible, via stub length should be kept to a minimum by traversing the signal
through the entire length of the vias. The analysis shown in
S-parameter return loss for common-mode (SCC11) and differential (SDD11) responses.
From the graph in
return loss. The much worse common-mode response relative to the differential response
is the reason why it is a good idea to reduce P/N skew as much as possible before entering
a transition. The 60/40 rule-of-thumb is 40 dB of return loss at 1 GHz, which implies 60 fF
of excess capacitance. Because excess capacitance is a single pole response, simple
extrapolation rules can be used. For example, a shift to 34 dB return loss doubles the excess
capacitance. Due to the excellent performance characteristics of GSSG vias, even long via
stubs only double the differential via's capacitance at the most.
Chapter 14, "Guidelines and Examples,"

P/N Crossover Vias

Some transceivers offer the ability to independently switch the polarity of the transmit and
receive signal pairs. This functionality eliminates the need to cross over the P/N signals at
Virtex-5 RocketIO GTP Transceiver User Guide
UG196 (v1.3) May 25, 2007
From Pin L11, Exiting at Lower Layer
Figure 13-14: Differential GSSG Via in 16-Layer PCB from Pins L11 and L6
0
-20
-40
-60
-80
1E8
Figure 13-15: Simulated Return Loss Comparing Differential and Common-Mode
Figure 13-15, the common-mode response is 20 dB worse in terms of
www.xilinx.com
From Pin L6, Exiting at Middle Layer
1E9
Frequency, Hz
Losses for L11 and L6 GSSG Vias
provides additional examples of differential vias.
P/N Crossover Vias
UG196_c13_14_051406
Figure 13-15 compares the
1E10
UG196_c13_15_051406
245