Xilinx Spartan-6 FPGA Series

50 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
The 2D field-solver example shows that close to 50Ω can be achieved if the ground plane
under the pad footprint is cleared out. A 3D field solver is then used to verify this result to
a greater degree of accuracy.
Figure 5-6 shows the ground plane cleared away exactly as it was for the 2D simulation.
Using frequency domain analysis within HFSS, there is a 20 dB (10x) improvement in
return loss using this technique.
Figure 5-7 shows the return loss comparison between 0402 pad structures with linear scale.
X-Ref Target - Figure 5-5
Figure 5-5: Transition Optimization
X-Ref Target - Figure 5-6
Figure 5-6: Ansoft HFSS Model of Pad Clear-Out
- L = 241 nH/m
- C = 89 pF/m
- Zo = 52Ω
28 Mil Pad
UG393_c5_05_091809
UG393_c5_06_091809
Z
Y
X