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    Tech Spotlight Summer of FPGAs:

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    Introduction

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    The rapid increase of bandwidth requirements presents seemingly insurmountable challenges for effective heat management and signal routing, due to lossy printed circuit boards (PCBs), vias, and other components.  With a high-speed signal, the PCB's dielectric constant becomes an issue, and traces must be shaped and routed perfectly to avoid EMI/EMC, signal coupling, and crosstalk. Additional factors, such as impedance, attenuation, jitter, intersymbol interference, and reflections all impact signal integrity. As such, system designers use new system architectures to extend the signal reach and density. High-speed data rates are made possible by routing the signals through ultra-low-skew twinax cable assemblies or optical cable assemblies, instead of the potentially lossy traces in PCBs. Allowing data to “fly over” lossy board materials negates the need for layout complexities and limits signal degradation.

     

    In the FPGA world, transceivers support data rates up to 28/56 Gbps and beyond. However, transferring data at these rates over long lengths in lossy PCB materials can be tricky. The FPGA industry has thus adopted the approach of routing data via copper or optical cable assemblies. This Tech Spotlight will highlight the benefits and workings of Samtec Flyover® Technology, and also will explain how it’s a good fit for high-speed data connectivity in FPGA applications.

     

    Samtec Flyover® Technology

    System requirements with high data rates have approached the physical limits of traditional electronic hardware design elements. Developers strive to balance increasing throughput, scalability, and density demands with concerns such as power consumption, thermal dissipation, signal integrity (SI), time to market, and cost. Designers may use specialized PCB laminates to reach higher data rates. Unfortunately, these materials are expensive, and even with these exotic substances, at higher data rates trace lengths are limited. Figure 1 illustrates the practical limits of FR408 and MEGTRON 6 PCB laminates at specific data rates.

                                                                                           

    BANDWIDTH VS. TRADITIONAL & HIGH–SPEED MATERIALS
    FR408MEGTRON 6MICRO TWINAXOPTICS
    10 Gbps< 10"10"+10"+10"+
    14 Gbps< 5"< 10"10"+10"+
    28 Gbps< 2"< 5"10"+10"+
    56 Gbps0.0"< 2"10"+10"+
    28 Gbps0.0"0.0"< 10"10"+

    Figure 1: Comparison of SI of different materials at different trace lengths, -5dB Loss Target (Image Source: Samtec)

     

    Of course, design options are available to extend signal reach. Multiple clock retimers and data recovery circuits can be added every few inches along the signal path. This approach solves the SI challenges of lossy PCBs. However, additional ICs complicate PCB design and add to BOM costs. In short, it is tough to route contemporary high-speed signals in a PCB. Are other solutions available?

     

    Samtec Flyover® Technology breaks the constraints of traditional signaling substrate and hardware offerings, resulting in a cost-effective, high–performance, and heat-efficient answer to the challenges of 28/56 Gbps bandwidths and beyond. Signals traces route over the PCB via copper or optical cable assemblies, simplifying PCB design and minimizing thermal and latency issues.  This design approach enhances data rate performance from chip to chip, board to board, or even system to system. In addition, Flyover® Technology provides performance and cost advantages compared to PCBs. Reduced thermal challenges, simplified board layout, fewer PCB layers, and less expensive PCB materials are some benefits of this new architecture. Figure 2 shows the simulation result of a "traditional" switch compared to a Flyover® technology-enabled switch. The junction temperature (Tj) is 130° C for the conventional switch, while the Tj for Flyover® technology-enabled system is 98.5° C.

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    Figure 2: Simulation results of Flyover Technology vs Conventional Switch (Image Source: Samtec)

     

    Interconnects

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    A popular example of Samtec Flyover® Technology is the FireFly™ Micro Flyover System™. The interconnect provides flexibility to use micro-footprint optical and copper interconnects interchangeably with the same connector system. The FireFly™ system enables chip-to-chip, board-to-board, on-board, and system-to-system connectivity at data rates up to 28 Gbps (and soon 56 Gbps) per lane, via optical cable at greater distances, or copper for cost optimization (Figure 3). Some of the features of the FireFly™ Micro Flyover System™ are:

     

    • Highest Density: The micro footprint of FireFly™  frees up space on the mainboard for additional components and/or connectors. The highest 28 Gbps bandwidth is available with x12 bidirectional channels in 0.63 square inches.
    • Ease of Routing: The two-piece board level interconnect isolates the signal and power to help ease trace routing compared to array systems. Flyover® Technology simplifies PCB design and allows greater component density under the Flyover.
    • Ease of Assembly: The rugged two-piece socket system, with weld tabs, latch locking, and loading guides, provides simplified mating and un-mating compared to a compression system, using mechanical screw downs and hardware.
    • Signal Integrity: By taking data connections "off-board" with Flyover® Technology, the easier signal integrity design improves electrical performance.
    • End Option Flexibility: Multiple end options connectors are available, including MPO (MTP®), MT, MXC®, and U-SDI Interfaces.
    • End-to-End Support: The FireFly™ Micro Flyover System supports data center, HPC and FPGA protocols, including 10/40/100 Gb Ethernet, InfiniBand™, Fibre Channel, and Aurora
    • Heat Sinks: The system provides integral heat sinks in several default designs, including pin-finned, flat fiber grooves for multi-row configurations, and custom designs.

     

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    Figure 3: Flyover® Technology Enables High Speed, Simplifies PCB Design (Image Source: Samtec)

     

    Flyover Systems in FPGA Applications

    FPGA manufacturers like Xilinx provide an array of characterization, development, and evaluation platforms targeted at application-specific tasks.  Several of their platforms can be connected at high data rates to deliver a smooth and seamless environment for product development.  Typically, these connections are made using various high-speed connectors or copper cable assembly options. The FireFly™ Micro Flyover System™ is another option that has increasingly gained traction for connecting multiple FPGA boards. It offers FPGA designers a choice of using either active optical engines or low-cost copper interconnects.

     

    Samtec has recently released a new development platform for the FireFly™ Micro Flyover System™ optical engine: the 25 Gbps (x12) FireFly™ FMC+ Module. This new development tool provides up to 300 Gbps full-duplex bandwidth over 12 channels from an FPGA/SoC to an industry-standard multi-mode fiber optic cable. This kit supports protocols including Ethernet, InfiniBand, Fiber Channel, and Aurora, typically found in video, embedded computing, instrumentation, HPC, 5G, FPGA development boards, and data center applications. As a VITA 57.4 FMC+ solution, the Samtec 25 Gbps (x12) FireFly™ FMC+ Module can be used for optical data communication on any FPGA development board supporting high-speed multi-gigabit transceivers.

     

    Figure 4a represents an image demonstration of the kit. The 25 Gbps (x12) FireFly™ FMC+ Module attaches to a Xilinx Virtex UltraScale+ FPGA VCU118 Evaluation Kit via the VITA 57.4 FMC+ connector. The 25 Gbps electrical signals travel from the FPGA on the VCU118 to the VITA 57.4 FMC+ connector set to the 25 Gbps (x12) FireFly™ optical into the transmitter. After the electrical to optical (E/O) conversion, photons travel from the FireFly™ through optical connectors. There are up to 100 meters of fiber optics that move back to the 25 Gbps (x12) FireFly™ optical engine receiver. After the optical-to-electrical (O/E) conversion, the electrical signals pass through the FireFly™, through the FMC+ connector, and back to the FPGA. Figure 4b represents the demonstration results based on the Vivado IBERT software. The BER (bit error rate) is roughly 7.69E-15, and eye patterns are wide open.

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    Figure 4a: 25 Gbps (x12) Optical FireFly™ FMC+ Module. 4b: Demonstration Result. (Image Source: Samtec)

     

    Product Examples

    The copper and optical cable assemblies of the FireFly™ Micro Flyover System™ provide the flexibility to achieve higher data rates and/or greater distance needs, while simplifying board design and enhancing performance. The optical version allows data over a longer distance of up to 100m, whereas the copper typically supports up to a few meters in length, assuming similar drive conditions. Optical links also provide electrical isolation, allowing for significant noise immunity. The copper FireFly™ is favored if cost is a factor. We will now discuss both cable systems in detail:

     

    Copper Cable System: The low-cost FireFly™ copper solution is based on Samtec's 100 Ω 34 AWG and 36 AWG Twinax ribbon cable. This cable is extensively used with existing high-speed cable assemblies. FireFly™ copper features performance of up to 28 Gbps with 4, 8, or 12 differential pairs. The positive latching feature is available for ease of engagement and disengagement. The cable system is pin-compatible with FireFly™ optical and assemblies are available in standard copper (ECUE), optimized copper (ECUE-2), and PCIe®-Over-FireFly™ copper (PCUE).

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    Figure 5: FireFly™ Micro Flyover Copper Cable System (Image Source: Samtec)

     

    Optical Cable System: FireFly™ optical engine technology, paired with high-speed interconnects (ECUO), can offer 14 Gbps, 16 Gbps, 25 Gbps, and 28 Gbps. Samtec's FireFly™ micro optical engines occupy the smallest overall footprint, consume the least amount of power, and enable fast, easy, and low-cost fiber termination. They feature an integral heat sink in several default designs, including pin-finned (14 Gbps only), flat, fiber groove for multi-row configurations, and customs. The extended temperature FireFly™, with a -40 ºC to +85 ºC range for military and industrial applications (ETUO), is also available.

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    Figure 6: FireFly™ Micro Flyover Optical Cable System (Image Source: Samtec)

     

    Evaluation and Development Kit Solutions: From concept and prototype to development and production, Samtec-designed and partner-designed kits and boards featuring the FireFly™ Micro Flyover System™ to simplify design and reduce time to market. The 14 Gbps FireFly™ FMC Module is one such example. Samtec's 14 Gbps FireFly™ FMC Module provides up to 140 Gbps full-duplex bandwidth over 10 channels from an FPGA to an industry-standard multi-mode fiber optic cable. The optical engines in FireFly™ provide adjustable power levels to support cable lengths up to 100m. As a VITA 57.1 FMC, the module can be used for optical data communication on any FPGA development board supporting high-speed multi-gigabit transceivers. It can run system data or BERT testing on all channels in parallel. This makes evaluation and development with FPGAs easier. It can be a good substitute for 28G test equipment.

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    Figure 7 :  14 Gbps FireFly™ FMC Module (Image Source: Samtec)

     

    Conclusion

    Flyover systems have emerged as a viable approach to reducing the use of PCB traces to conduct high-speed signals. They simplify board design and enhance performance by limiting signal degradation. The FireFly™ Micro Flyover System’s small footprint allows for greater density and closer proximity to the IC, enabling lower power consumption, system cost, and thermal dissipation. It is well suited to FPGA systems, and helps to address their interconnectivity needs via copper, optical cable systems, and specially designed development kits.

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