AN 958: Board Design Guidelines

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ID 683073
Date 1/28/2022
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5.1.5.6.1. Inductive Discontinuity

Figure 53 illustrates a TDR voltage plot for two different SMA connectors, one side of the SMA connector is 50 Ω and the other is 58 Ω. The curve rises upwards due to the increasing inductance in the region.

The two plots in Figure 53 represent two different discontinuities due to SMA connectors. The curve with the higher peak represents a connector with higher inductive discontinuity of about 3.8 nH. The curve with a lower peak represents a connector with lower inductive discontinuity of about 2.6 nH. You can calculate the inductance for the discontinuity for both these curves from the graph.

Figure 53. Impedance Curves for SMA Connectors

Figure 53 shows a 3.125-Gbps signal transmitted through the two SMA connectors. The rise time of the signal is approximately 70 ps.

Figure 54 shows the eye opening plot when a signal passes through the lower-inductance (2.6 nH of discontinuity) SMA connector. The eye opening is 336 mV, and the jitter is 20 ps.

Figure 54. Lower-Inductance SMA Connector Eye Opening and Expanded View of the Eye Opening

An expanded view of the eye (Figure 54) provides a better jitter reading; the peak-to-peak jitter value is approximately 20 ps.

Figure 55 shows an eye opening plot of the same signal; however, this time the signal goes through 3.8 nH of inductive discontinuity due to the higher-inductance SMA connector. The eye opening is approximately 332 mV. When comparing the plots, the plot in Figure 55 has more jitter than Figure 54.

An expanded view of the eye (Figure 55) provides a better jitter reading; the peak-to-peak jitter value is approximately 24 ps.

Figure 55. Higher-Inductance SMA Connector Eye Opening and Expanded View of the Eye Opening

Jitter increases and the eye opening gets smaller when the wrong type of connectors are used or other forms of inductive discontinuities are added to the transmission path. Increasing jitter behavior becomes a significant problem with signals with faster rise times. Also, when the signals become more stressed (i.e., random), jitter is more pronounced.

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