Powering The Rocketio Mgts; Regulators; Filtering; Reference Clock - Xilinx Virtex-4 RocketIO User Manual

Chapter 9: Methodology Overview
•
At gigahertz signaling speeds, losses due to the transmission medium become significant
due to greater signal attenuation with increasing frequency. The attenuation of the high-
frequency components slow down the edge and reduce voltage swing, resulting in eye
closure. See
characteristic trace impedance design.
While there are several transitions in each channel, most or all transitions can be designed
for the least negative impact on performance. Because standard non-optimized PCB
structures tend to be capacitive at gigahertz frequencies, a convenient figure of merit for
transitions is excess capacitance. An ideal transition does not have excess capacitance or
inductance.
For each typical transition, techniques to limit excess capacitance and excess inductance
are provided to build a robust channel on the first pass. These design rules, techniques, and
examples are presented in
amount of performance lost due to transitions.
In general, minimizing the number of components and layer changes in the high-speed
serial PCB traces brings about the most benefit. Careful design of the traces, vias, and even
connector pads is required for gigahertz speeds.
Using the techniques described in this Guide, the ML421 board reference design
and other resources the specified transceiver performance can be attained.
Without previous experience with multi-gigabit designs, determining if via designs or
other transitions are adequate for target data rates can be daunting. Guidelines in
Chapter 11
adequate.

Powering the RocketIO MGTs

Regulators

Linear regulators are required for the RocketIO MGT power supplies. Although switching
regulators are an attractive option in applications requiring high-power efficiency, they are
unsuitable for use as they can introduce switching noise, even if care is taken to eliminate
the noise.

Filtering

The power distribution system guidelines in
the specified performance. The Virtex-4 transceiver characterization and demonstration
boards follow the same guidelines. Refer to

Reference Clock

Clock Sources

A high-quality crystal oscillator is essential for good performance. When using one of the
recommended oscillators, the manufacturer's power supply design guide must be
followed. Virtex-4 device characterization is based on the same recommended oscillators.
236
The impedance mismatch (to 50Ω) of a transition which can lead to reflections,
attenuation, and other artifacts in the signal.
Chapter 10, "PCB Materials and Traces"
Chapter 11, "Design of Transitions."
are provided to help the designer determine whether a board design is
www.xilinx.com
for guidelines on stack-up and
The goal is to minimize the
Chapter 6 must be strictly followed to achieve
Chapter 6 for design guideline details.
Virtex-4 RocketIO MGT User Guide
UG076 (v4.1) November 2, 2008
R
[Ref 3],