Crosstalk - Xilinx Virtex-6 FPGA User Manual

Chapter 5: Board Design Guidelines
by several switching regulator component vendors. Normally, a switching power supply
regulation circuit requires a switching transistor element, an inductor, and a capacitor.
Depending on the required efficiency and load requirements, a switching regulator circuit
might require external switching transistors and inductors. In addition, these switching
regulators require very careful placement and routing on the PCB to be effective.
Switching regulators generate significant noise and therefore usually require additional
filtering before the voltage is delivered to the GTH transceiver analog power supply input
of the Virtex-6 FPGA. As mentioned in
should be limited to less than 10 mV
designed to attenuate the noise from the switching regulator to meet this requirement.

Crosstalk

A major contributor to degradation in the performance of a GTH transceiver is crosstalk.
The mechanisms for crosstalk are aggressor signals coupling into signal traces, coupling
into the GTH transceiver power supplies, or a combination of both. The latter is the most
common and often the most damaging. Noise coupled into the power supply can corrupt
the entire transceiver circuit rather than just a single lane—as in the case of coupling into
signal traces. Also, symptoms of noise coupled into the power supply are often more
difficult to interpret because the noise is convolved with the normal signals in the
transceiver. The result is degradation in the performance of the transceiver that reveals
itself as noise in the transmitter output and reduced jitter tolerance in the receiver.
To avoid performance degradation from crosstalk:
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•
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SelectIO Interface Usage Guidelines
Because the GTH transceiver's performance can degrade in an environment flooded with
SelectIO™ interface activity , it is important to have guidelines for SelectIO interface usage
that minimize the impact on GTH transceiver performance.
The pinout for the Virtex-6 FPGA package maintains a physical separation between the
GTH transceiver pins and the SelectIO interface pins. Because of this separation in the
package pinout, no SelectIO interface pins need to be excluded when using the GTH
transceivers.
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Exposure of power planes to other circuits on the board, such as data lines for
memory interfaces and processor buses, should be monitored.
Adequate filtering should be provided to the GTH transceiver power supplies near
the point of the load. The amount of filtering should be determined by the magnitude
and frequency of the signal from potential noise sources. Noise on the GTH
transceiver power supplies should be kept below 10 mV
80 MHz.
Return current paths of signal traces in the vicinity of the GTH transceiver power
distribution network should be monitored. Besides broadband and edge coupling of
traces on the same or adjacent layers, coupling from aggressor traces can occur if the
aggressor signal is propagating from one layer to another, each layer having a
different reference plane. As the signal propagates through the portion of the via that
does not have a return current path, it generates return currents in the next lowest
impedance structure on the board. A victim of this unintended return current path
could be a signal or power via for the GTH transceivers.
www.xilinx.com
Overview, page 139, the amplitude of the noise
. Therefore, the power supply filter should be
PK-PK
PK-PK
Virtex-6 FPGA GTH Transceivers User Guide
UG371 (v2.0) February 16, 2010
from 10 KHz to