52 www.xilinx.com Spartan-6 FPGA PCB Design and Pin Planning
UG393 (v1.1) April 29, 2010
Chapter 5: Design of Transitions for High-Speed Signals
As shown from Figure 5-11 and Figure 5-12, clearing the ground plane under SMT pads
yields a significant improvement in the performance of an SMT pad transition. Excess
capacitance is reduced by 15x, and return loss is improved by 20 dB.
X-Ref Target - Figure 5-9
Figure 5-9: TDR Results Comparing 0402 Pad Structures
X-Ref Target - Figure 5-10
Figure 5-10: TDR Results Comparing 0402 Pad Structures
0
100
200
300
400
500
600
0.0 0.5 1.0 1.5
Time, ns
2.0 2.5 3.0
VtdrPlaneCleared, mV
VtdrPlaneNotCleared, mV
UG393_c5_09_091809
0.60 0.65 0.70 0.75 0.800.85 0.90 0.95 1.000.55 1.05
350
400
450
500
300
550
Time, ns
VtdrPlaneNotCleared, mV
VtdrPlaneCleared, mV
UG393_c5_10_091809
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