236 www.xilinx.com Virtex-4 RocketIO MGT User GuideUG076 (v4.1) November 2, 2008Chapter 9: Methodology Overview R• The impedance mismatch (to 50Ω) of a transition which can lead to reflections,attenuation, and other artifacts in the signal.At gigahertz signaling speeds, losses due to the transmission medium become significantdue to greater signal attenuation with increasing frequency. The attenuation of the high-frequency components slow down the edge and reduce voltage swing, resulting in eyeclosure. See Chapter 10, “PCB Materials and Traces” for guidelines on stack-up andcharacteristic trace impedance design.While there are several transitions in each channel, most or all transitions can be designedfor the least negative impact on performance. Because standard non-optimized PCBstructures tend to be capacitive at gigahertz frequencies, a convenient figure of merit fortransitions is excess capacitance. An ideal transition does not have excess capacitance orinductance.For each typical transition, techniques to limit excess capacitance and excess inductanceare provided to build a robust channel on the first pass. These design rules, techniques, andexamples are presented in Chapter 11, “Design of Transitions.” The goal is to minimize theamount of performance lost due to transitions.In general, minimizing the number of components and layer changes in the high-speedserial PCB traces brings about the most benefit. Careful design of the traces, vias, and evenconnector pads is required for gigahertz speeds.Using the techniques described in this Guide, the ML421 board reference design [Ref 3],and other resources the specified transceiver performance can be attained.Without previous experience with multi-gigabit designs, determining if via designs orother transitions are adequate for target data rates can be daunting. Guidelines inChapter 11 are provided to help the designer determine whether a board design isadequate.Powering the RocketIO MGTsRegulatorsLinear regulators are required for the RocketIO MGT power supplies. Although switchingregulators are an attractive option in applications requiring high-power efficiency, they areunsuitable for use as they can introduce switching noise, even if care is taken to eliminatethe noise.FilteringThe power distribution system guidelines in Chapter 6 must be strictly followed to achievethe specified performance. The Virtex-4 transceiver characterization and demonstrationboards follow the same guidelines. Refer to Chapter 6 for design guideline details.Reference ClockClock SourcesA high-quality crystal oscillator is essential for good performance. When using one of therecommended oscillators, the manufacturer’s power supply design guide must befollowed. Virtex-4 device characterization is based on the same recommended oscillators.
