Achieving signal integrity in 25-Gbit/sec backplanes

An examination of various high-density connector performance levels with and without NEXT and FEXT

BY CHAD MORGAN

To address the ever-increasing need to widen network bandwidth, signal integrity (SI) engineers continue to push backplane links toward 25 Gbits/sec. When designing links, SI engineers often begin with a specific connector platform and then modify system variables, such as trace length, dielectric materials, and equalization settings to achieve acceptable performance. This paper takes a different approach by setting various system and equalizer parameters to slightly aggressive values, then studying the effect of various connector performance levels. Results demonstrate the need for a new level of connector performance for successful 25-Gbit/sec transmission across modern backplanes.

Tyco Electronics’ STRADA Whisper connector, shown here, was the Gen3 technology used in the study.Click here to enlarge image

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System parameters and setup

The goal in choosing base system parameters is to create lossy channels that allow 25-Gbit/sec signals to transmit successfully with margin when no connector reflections or noise are present. The primary goal of the paper is then to examine how real connector (and footprint) imperfections affect the final 25-Gbit/sec signal. For this study, the system parameters diagramed in Figure 1 are used. These values represent realistic backplane channel parameters that are slightly aggressive yet achievable in next-generation 25-Gbit/sec systems.

FIGURE 1. This illustration details the system parameters put in place for the study of Gen1, Gen2, and Gen3 connectors in a 25-Gbit/sec backplane system.Click here to enlarge image

In this study, two system lengths are examined using highly reliable and validated models. The first system length of 14.8 inches consists of two, 3-inch daughtercard traces, a 6-inch backplane trace, and two, 1.4-inch “perfect connection” models that are lossy, reflectionless, and noiseless. The second system length of 30.8 inches consists of two, 4-inch daughtercard traces, a 20-inch backplane trace, and two, 1.4-inch “perfect connection” models. The 1.4-inch length for the “perfect connection” model comes from the addition of a 0.250-inch backplane footprint, a 1-inch connector, and a 0.150-inch daughtercard footprint.

“Perfect” channel results

As already stated, the goal in selecting system parameters is to choose aggressive yet realistic values that allow 25-Gbit/sec signals to be transmitted across both a short (14.8-inch) and a long (30.8-inch) system when no connector (or footprint) reflections or noise are present. These channels include loss and dispersion, but there is no noise in the system, and the only reflections come from 0.14 picofarad (pF) chip parasitics. For the 14.8-inch system, only 2 baud-spaced pre-emphasis taps are required to transmit 25-Gbits/sec data successfully with a 31.8% open eye. The 30.8-inch system, on the other hand, requires 5 baud-spaced pre-emphasis taps and 7 baud-spaced decision feedback equalizer (DFE) taps to transmit 25-Gbit/sec data successfully with a 16.4% open eye.

Real channel results

Without including any real connectors (and footprints) in the “perfect connection” channels, it is clear that successful 25-Gbit/sec transmission is a challenge requiring tight jitter control, small chip parasitics, and aggressive equalization techniques for longer channels. Even so, it is clear that 25-Gbit/sec transmission is possible. The next task is to introduce varying levels of connector (and footprint) technology into the channels to examine the impact on received data.

FIGURE 2. These graphs show the time-domain response for each connector’s impedance profile, worst-case asynchronous NEXT, and worst-case asynchronous FEXT.Click here to enlarge image

In this paper, three generations of presently existing high-speed, high-density connectors are introduced into the short and long channels to examine their impact on 25-Gbit/sec data throughput. For each channel, a specified connector generation’s electrical model replaces an ideal 1.4-inch “perfect connection” model. It is important to note that the connector model includes the 0.250-inch through-the-board backplane footprint, the 1-inch long connector differential pair, and the 0.150-inch through-the-board daughtercard footprint. It is also important to note that all three generations of connectors and footprints currently exist. Each connector generation represents an advancement in connector technology, with Gen3 performance being the best.

To quantify the performance of each connector generation, the graphs above shows the time-domain response for each connector. Note that the Gen1, Gen2, and Gen3 100-Ω connectors are simulated in a 100-Ω environment, but the Gen3, 85-Ω connector is simulated in an 85-Ω environment.

Given the four connectors and footprints, the next step is to examine the effect of inserting each of them into the 14.8-inch system. Table 1 shows all 14.8-inch system 25-Gbit/sec eye-pattern simulation results. The first three rows of data give results for the eye patterns without the presence of crosstalk. The second three rows give eye-pattern values when full near-end crosstalk (NEXT) is included. The final three rows give eye-pattern values when full far-end crosstalk (FEXT) is included.

Click here to enlarge image

It is clear from Table 1 that both Gen3 systems successfully transmit 25-Gbit/sec signals, even in the presence of NEXT or FEXT. Both the Gen1 and Gen2 systems had failed without the presence of noise, so the addition of noise does not change that.

The next step is to examine the effect of inserting four connectors and footprints into the 30.8-inch baseline system. Table 2 shows all 30.8-inch 25-Gbit/sec eye-pattern simulation results. Results are displayed the same way they were in Table 1. The first three rows of data give results for the eye patterns without the presence of crosstalk. The second three rows give eye-pattern values when full NEXT is included. And the final three rows give eye-pattern values when full FEXT is included.

Click here to enlarge image

Table 2 makes it clear that both Gen3 systems successfully transmit 25-Gbit/sec signals, even in the presence of full FEXT?the Gen3 100-Ω system barely fails the receiver eye mask while the Gen 3 85-Ω system passes. Once again, the Gen1 and Gen2 systems had failed without the presence of noise, and the addition of noise does not change that result.

This study has shown that noiseless systems with reflections only from low-capacitance chip parasitics can successfully transmit 25-Gbit/sec signals across backplanes up to 30.8 inches in length with proper equalization. It has been shown, however, that 25-Gbit/sec signal transmission is difficult or impossible to achieve when Gen1 or Gen2 connectors, available from a number of vendors today, are inserted into the backplane systems. It has also been shown that the Gen3 connector allows successful 25-Gbit/sec signal transmission even in the presence of full NEXT or FEXT.