Figure 1: This plot represents the differential insertion-loss profile for a 20" FR-4 microstrip trace |

In the previous post in this series, we looked at the loss characteristics of a 20-inch-long FR-4 microstrip differential trace, which we'd simulated using a 3D field solver. Figure 1 reproduces the mixed-mode S parameters for that trace in the form of a plot of losses vs. frequency.

Figure 2: These eye diagrams show the relationship between Nyquist and insertion losses |

When we increase the data rate to 4 Gb/s and a Nyquist frequency of 2 GHz, the plot shows our signal at about -8 dB, which yields a more discernible effect on the eye opening. Moving up to 7 Gb/s, our 3.5-GHz Nyquist frequency results in a -12 db signal. At this point, the eye is barely open. For this particular channel, this is where the insertion losses become overwhelming. Just to drive home the point, the eye diagram is completely collapsed at 9 Gb/s with no discernible opening.

Note that none of these eye diagrams benefited from application of equalization techniques at the receiver. We'll discuss transmit and receive equalization in depth in upcoming posts.

What exactly do these sort of losses do to your signal? Some effects are painfully obvious. For example, there is a monotonic increase in insertion loss; as frequency rises, so do the losses. That's frequency-dependent degradation.

Figure 3: The greater attenuation of higher-frequency signal components results in rise-time degradation |

The maximum data rate drops precipitously for a channel with higher losses. Without equalization, you can have up to -12 dB of insertion loss. However, with the application of equalization, it's possible to have as much as -25 dB insertion loss at Nyquist and still have a link that performs quite well.

Next, we'll work through the effects of inter-symbol interference (ISI) or, as it's sometimes known, ISI jitter.

Previous posts in this series:

Introduction to Debugging High-Speed Serial Links

A Look at Transmission-Line Losses

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