Xilinx Spartan-6 FPGA Series Design And Pin Planning Manual

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Xilinx Spartan-6 FPGA Series Design And Pin Planning Manual Manual pdf 42 page image

42 www.xilinx.com Spartan-6 FPGA PCB Design and Pin PlanningUG393 (v1.1) April 29, 2010Chapter 4: PCB Materials and TracesLoss TangentLoss tangent is a measure of how much electromagnetic energy is lost to the dielectric as itpropagates down a transmission line. A lower loss tangent allows more energy to reach itsdestination with less signal attenuation.As frequency increases, the magnitude of energy loss increases as well, causing the highestfrequency harmonics in the signal edge to suffer the most attenuation. This appears as adegradation in the rise and fall times.Skin Effect and Resistive LossesThe skin effect is the tendency for current to flow preferentially near the outer surface of aconductor. This is mainly due to the magnetic fields in higher frequency signals pushingcurrent flow in the perpendicular direction towards the perimeter of the conductor.As current density near the surface increases, the effective cross-sectional area throughwhich current flows decreases. Resistance increases because the effective cross-sectionalarea of the conductor is now smaller. Because this skin effect is more pronounced asfrequency increases, resistive losses increase with signaling rates.Resistive losses have a similar effect on the signal as loss tangent. Rise and fall timesincrease due to the decreased amplitude of the higher harmonics, with the highestfrequency harmonics being most affected. In the case of 10 Gb/s signals, even thefundamental frequency can be attenuated to some degree when using FR4.For example, an 8 mil wide trace at 1 MHz has a resistance on the order of 0.06Ω/inch,while the same trace at 10 Gb/s has a resistance of just over 1Ω/inch. Given a 10 inch traceand 1.6V voltage swing, a voltage drop of 160 mV occurs from resistive losses of thefundamental frequency, not including the losses in the harmonics and dielectric loss.Choosing the Substrate MaterialThe goal in material selection is to optimize both performance and cost for a particularapplication.FR4, the most common PCB substrate material, provides good performance with carefulsystem design. For long trace lengths or high signaling rates, a more expensive substratematerial with lower dielectric loss must be used.Substrates, such as Nelco, have lower dielectric loss and exhibit significantly lessattenuation in the gigahertz range, thus increasing the maximum bandwidth of PCBs. At3.125 Gb/s, the advantages of Nelco over FR4 are added voltage swing margin and longertrace lengths. At 10 Gb/s, a low-loss dielectric like Nelco is necessary unless high-speedtraces are kept very short.The choice of substrate material depends on the total length of the high-speed trace andalso the signaling rate.What-if analysis can be done in HSPICE simulation to evaluate various substratematerials. By varying the dielectric constant, loss tangent, and other parameters of the PCBsubstrate material. The impact on eye quality can be simulated to justify the use of highercost materials. The impact of other parameters such as copper thickness can also beexplored.