By Danny Boesing, Product Marketing Director, Samtec, Inc.

Design Challenges Of High Data Rate
As data rate requirements approach and surpass 112 Gbps PAM4, developers are challenged with balancing increasing throughput, scalability, and density demands with concerns such as power consumption, signal integrity, time-to-market, and of course cost.

To achieve these data rates, designers often have to use special board laminate materials with lower dielectric constants and dissipation factors.  Unfortunately, these materials are often very expensive.

Another challenge, even with these exotic materials, is the trace lengths on the boards are still relatively short.  As shown in Figure 1, even using expensive, high speed PCB materials, it’s difficult to have trace lengths of any significant distance.

To combat this and allow for usable trace lengths at a higher data rate, multiple expensive “retimer” chips are often required every few inches along the signal path.  In short – routing today’s high-speed signals in a PCB is challenging and expensive.  See Figure 2 for a comparison of insertion loss by design option.

An Alternative Approach
System architects are now using an alternate approach which allows designers to 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.

In a typical application, a cable assembly is located next to an FPGA or processor, launching the signal, which travels via discrete or ribbon twinax cable, to another location on the board, or to an I/O connector.  See Figure 3.

There are many advantages of this approach:  First, customers can achieve those high data rates of 28, 56, and 112 Gbps and beyond.  It simplifies board design and it can reduce thermal challenges.

Expensive retimers are eliminated, PCB layer count is reduced (which results in additional cost savings), and designers don’t have to use more expensive PCB materials.

Many design elements contribute to the overall performance of the cable assembly system:  the high-speed connectors, the cable, transition boards, and PCB routing strategies.

Cable Is The Key
But the impact of the cable on system performance is huge.  Samtec Eye Speed® ultra-low skew cable technology uses co-extruded twinax cable, which improves signal integrity, bandwidth, and reach.  This eliminates the performance limitations and inconsistencies of the traditional design of individually extruded dielectric cabling,

Samtec twinax cable boasts the highest performance of any cable in the industry, typically four cable gauges faster than our competitors.

The connector system described in this article is Samtec Flyover®.  Other articles and links that may be of interest, pertaining to Samtec Flyover, include:

• A video presentation of this article
• A Live Product Demonstration Of Samtec Flyover
• Overview of Samtec Flyover
• Ultra-Low Skew Twinax Cable Technology

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