Virtex-5 RocketIO GTP Transceiver User Guide
245
UG196 (v1.3) May 25, 2007
P/N Crossover Vias
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
compares the
S-parameter return loss for common-mode (SCC11) and differential (SDD11) responses.
From the graph in
, the common-mode response is 20 dB worse in terms of
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,”
provides additional examples of differential vias.
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
Figure 13-14:
Differential GSSG Via in 16-Layer PCB from Pins L11 and L6
Figure 13-15:
Simulated Return Loss Comparing Differential and Common-Mode
Losses for L11 and L6 GSSG Vias
UG196_c13_14_051406
From Pin L11, Exiting at Lower Layer
From Pin L6, Exiting at Middle Layer
UG196_c13_15_051406
1E9
1E8
1E10
-60
-40
-20
-80
0
Frequency, Hz
dB(Scc11_L11)
dB(Scc11_L6)
dB(Sdd11_L11)
dB(Sdd11_L6)
