Industry White Papers

A Connector Solution to Achieve 112 Gbps PAM4 Data Rates

August 31, 2020 by Samtec

Examine an alternative approach used by system architects to extend signal reach and density to achieve next-gen speeds by routing signals through low-loss, ultra-low-skew twin axial cable instead of through expensive, lossy PCBs.

Industry observers have noted that emerging system requirements of 28 Gbps and 56 Gbps, with an eye towards 112 Gbps, are nearing the physical limitations of traditional electronic hardware design elements, in particular traditional PCB substrate and component solutions. Even those who feel that rates of 112 Gbps PAM4 and beyond are achievable understand that developers are challenged with balancing increasing throughput, scalability, and density demands with concerns about power consumption, thermal dissipation, signal integrity, time to market, and—of course—cost.

To achieve these data rates, designers are often required to use special PCB materials with lower dielectric constants and dissipation factors, and, unfortunately, these materials are often very expensive. Another challenge posed by higher data rates is that the signal integrity (SI) channel performance budget significantly compresses the allowed trace lengths. Even when more expensive high-speed PCB materials are used, it’s difficult to have trace lengths of any significant distance.

 

Trace lengths comparison of PCB materials, twinax cable, and optics

Figure 1. Trace lengths comparison of PCB materials, twin axial cable, and optics

 

To combat this and allow for usable trace lengths at a higher data rate, multiple expensive retimer integrated circuit (IC) chips are often required every few inches along the signal path. In short, routing today’s high-speed signals in a PCB is both challenging and expensive.

 

Comparison of insertion loss for PCB materials vs. micro twinax cables.

Figure 2. Comparison of insertion loss for PCB materials vs. micro twinax cables.

 

An Alternative Approach Using Twinax Cables

System architects are now using an alternative approach to extend signal reach and density to achieve next-gen speeds by routing signals through low-loss, ultra-low-skew twinax cable instead of through expensive, lossy PCBs.

In a typical mid-board application, a cable connector is located next to a field-programmable gate array (FPGA) or an application-specific integrated circuit (ASIC). The signal launches through discrete or ribbon twinax cable, over a minimum trace length across the lossy PCB, to another location on the board or to an input/output (I/O) connector, such as a quad small form factor pluggable (QSFP).

 

Designers can extend signal reach with low-loss, ultra-low-skew flyover twinax cable, instead of routing signals through the lossy PCB.

Figure 3. Designers can extend signal reach with low-loss, ultra-low-skew flyover twinax cable, instead of routing signals through the lossy PCB.

 

High-performance cable systems allow designers to connect one board to another within a rack, establishing a flexible backplane architecture, and can also be used as a rack-to-rack interconnects. Both approaches retain the traditional backplane architecture while reducing PCB complexity and achieving higher performance targets – and that’s not all.

There are many advantages to this approach: First, designers can achieve those higher data rates of 56 Gbps, 112 Gbps, and beyond. This strategy also simplifies board design, allows for a more flexible, modular architecture, and supports thermal dissipation by eliminating retimers and increasing airflow. In addition, it minimizes the need for expensive PCB material and higher layer counts and reduces costs.

 

Connector Design

Many design elements contribute to the overall performance of the cable assembly system discussed above, including direct-attach high-speed connectors, twinax cable, and PCB routing strategies.

The direct-attach approach often eliminates transition boards in cable assembly connectors. For example, contact systems must be intentionally designed to accept the direct-attach geometry of ultralow-skew twinax cable. This is not merely a slight modification to an existing contact, where the leads are extended to allow the cable to attach. The direct attach contact system is designed to minimize impedance variation throughout the cable transition region to mitigate resonance within the channel.

 

Contact systems are designed to optimize signal integrity performance.

Figure 4. Contact systems are designed to optimize signal integrity performance.

 

Using the Correct Cable is the Key

The impact of cable on system performance is critical. Samtec Eye Speed ultra-low-skew cable technology uses coextruded twinax cable and best-in-class process control. This combination improves cable loss and critical intra-pair skew. Co-extrusion is superior to the performance limitations and inconsistencies of conventional twin-ax designs; it eliminates performance limitations such as higher loss, suck-out, and higher skew and resonances.

 

Construction of the twinax cable is critical to overall system performance.

Figure 5. Construction of the twinax cable is critical to overall system performance.

 

Samtec Flyover Systems for High Speeds

High-speed cabled backplane designs provide a good example of this solution. Three common challenges designers face with high-speed backplanes include loss, routing density, and skew. Loss is a concern because backplane signal paths are often long. Even when using more expensive high-speed PCB materials, it’s difficult to have trace lengths of any significant distance. In addition, these long paths require a considerable amount of valuable PCB space due to required retimers, which may require larger boards (which is often not an option) or adding layers (which is an expensive alternative).

Glass-weave skew is an increasingly important problem with even higher data rate serial links. The problem is the additional time-delay skew introduced on one line of a differential pair due to local variations in the dielectric constant (Dk). This causes a collapse of the eye from the increasing loss of the differential signal with frequency and the creation of a common signal, which may introduce additional problems. These effects also become more impactful with higher data rates

Alternately, innovative approaches like Samtec Flyover® Technology allow designers to create more flexible system architectures and extend signal reach and density to achieve next-gen speeds by routing signals through ultra-low-skew twinax cable instead of through expensive, lossy PCBs.

 

Direct-attach Flyover QSFP28 cable assemblies route critical high-speed signals through ultra-low-skew twinax for improved and extended signal integrity.

Figure 6. Direct-attach Flyover QSFP28 cable assemblies route critical high-speed signals through ultra-low-skew twinax for improved and extended signal integrity.