AN-1541| Application Note

Driving Signals Over XAUI Backplanes Using DS42MB100/B200, or BR400

Driving Signals Over XAUI Backplanes Using DS42MB100, DS40MB200, or DS42BR400
1.0 Introduction
This paper demonstrates how National Semiconductor's DS42MB100, DS40MB200, and DS42BR400 equalize XAUI backplane FR4 traces and reduce data dependent jitter to within XAUI specifications. The XAUI data rate is 3.125 Gbps with a nominal unit interval (UI) of 320 ps. IEEE 802.3ae specifies the maximum jitter budget for the interconnect to be 0.20 UIp-p or 64ps (Clause 47.3.5, Table 1). The interconnect includes everything between the transmitter and receiver. For a backplane application, this includes short FR4 traces on line cards, line card to backplane connectors, and FR4 traces on the backplane. The maximum input jitter tolerance for the combined deterministic jitter and random jitter is 0.65 UIp-p (Clause 47.3.4.6, Table 1). In many backplane applications, high frequency attenuation causes data dependent jitter to exceed the interconnect jitter budget and the input jitter tolerance. This application note demonstrates that using National's signal conditioning devices reduces jitter to meet XAUI input jitte
r tolerance and interconnect jitter budget requirements. This study also illustrates the effects of under and over equalization.

National Semiconductor Application Note 1541 Leo Chang December 2006

TABLE 1. XAUI Jitter Budget Allowances. DJ (UIp-p) Driver Interconnect Other - crosstalk, noise Total 0.17 0.2 0.1 0.47 TJ (UIp-p) 0.35 0.2 0.1 0.65

This study uses a Tyco HM-Zd XAUI Test Backplane. Note that different backplanes may have different attenuation characteristics and jitter results may vary. The information provided in this application note provides a performance reference for FR4 backplane applications. The jitter numbers reported are statistically extrapolated to 1e-12 BER.

2.0 Characteristics of a XAUI Backplane
At 3.125 Gbps (XAUI datarate), the skin effect and dielectric loss on the transmission media results in high-frequency signal attenuation. Figure 1 shows the transmission loss for a 30 inch backplane at Giga-Hz frequencies. At the XAUI data rates, this equates to a 9 dB loss in operating bandwidth for the 30-inch XAUI backplane. In addition, the backplane connectors are weak links that introduce crosstalk. The crosstalk on the Tyco backplane is approximately -35dB.

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FIGURE 1. Attenuation and Crosstalk of the 30-Inch XAUI Backplane.

2007 National Semiconductor Corporation

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