244 www.xilinx.com Virtex-4 RocketIO MGT User GuideUG076 (v4.1) November 2, 2008Chapter 10: PCB Materials and Traces ROnce a verified configuration is obtained, begin by modeling the configuration in thefrequency domain by doing an AC sweep, and plot transmission line loss versusfrequency. If this matches the verified configuration, then test the time-domain accuracy ofthe model.To do this, apply a 2V sine source with a 50Ω source termination to the transmission linemodel. Add a 50Ω termination to the far end of the transmission line. Set the sine sourcefrequency at the data rate frequency. Then simulate the model with a stop time about 3.5times longer than the transmission line delay. Measure the peak-to-peak value (V 0,pk-pk) atthe far end of the transmission line and compute loss in dB as:Equation 10-2Repeat this process for 80%, 60%, 40%, and 20% of the data rate. These numbers should bewithin a few tenths of a dB of the validated AC model.CableCables are controlled-impedance transmission lines due to the constant physicaldimensions of conductor and dielectric along the length of the cable. The highest qualitycable shows little variation in these dimensions and also has a wide bandwidth with lowloss at high frequencies.ConnectorsThe connectors attached to cables should exhibit low parasitic inductance and capacitancefor high bandwidth operation. Most cables are very well shielded and therefore have verylow levels of crosstalk between signal pairs. However, there could be substantial levels ofcrosstalk within the connector region.Optimal Cable LengthXilinx provides HSPICE models of the transceiver. Designers can use these models withvendor-supplied cable models to select the appropriate cable for their systems.Skew Between ConductorsWhen selecting a cable, look for a specification of the skew between the conductors in acable. If the conductors are not length-matched, the skew can directly reduce the eyeheight. Indirectly — by means of crosstalk, power supply disturbance, or higher return loss— even smaller amounts of length mismatch can create common-mode signals that canhave deleterious effects on eye height.Vloss 20 V0,pk-pk2--------------------⎝ ⎠⎛ ⎞log=
