Backplane Architectures Proliferate
As system speeds increase to multi-gigabit levels and beyond, ensuring signal integrity in high-speed channels requires major backplane connector performance upgrades.
Backplane architecture has served the electronics industry well for many years. Circuitry embedded in up to 40 layers of the backplane can create custom point-to-point, broad parallel bus structures or full mesh networks, essentially making the backplane a very large connector. Daughtercards that plug into the backplane enable modular system design, as well as provide a simple process for upgrades and repair. The backplane can be populated with the same or a variety of connectors that are designed for specific functions including line card, switch, I/O, or power distribution, making those slots application-specific. Connectors can be fitted with keys to ensure cards mate with the proper slot.
As system speeds increased to multi-gigabit levels, ensuring signal integrity in high-speed channels required major backplane connector performance upgrades. Entirely new families of advanced backplane connectors specifically designed for differential signaling featured integrated ground planes, optimized PCB footprints, and smaller compliant pins. A new set of criteria, including tight impedance control, reduced crosstalk, attenuation, intersymbol interference, and skew, characterize this class of backplane connectors. The process of fine-tuning these connectors continues today with some manufacturers demonstrating 50+ Gb/s backplane channels.
System engineers are constantly looking for ways to increase data rates as well as system packaging density. That creates some challenges. As signal speeds increase, all the negative effects listed above intensify. As channel length increases, signal integrity of the channel suffers. Signal fidelity can be electronically restored, but those added devices consume space, draw more power, and add cost. Designing high-speed channels that minimize the physical distance between daughtercards became a priority, which led to the development of alternatives to traditional backplane architecture.
Systems designed with midplane architecture move the backplane toward the middle of the card cage, which allows daughtercards to plug in from both sides. With this design, the size of the backplane potentially can be cut in half, significantly reducing signal path lengths between daughtercards.
One step better is a system based on orthogonal midplane design where daughtercards oriented 90° front-to-back allow direct connection between all daughtercards. Front-side and back-side daughtercard connectors mate with modified midplane headers that achieve the 90-degree orientation.
Since the front and back daughtercards are oriented differently, orthogonal midplane systems often require two separate air cooling systems. Rear access to daughtercards on the back side may also be undesirable.
In order to solve these problems, connector manufacturers now offer orthogonal direct connectors that allow daughtercards to mate directly with each other. Eliminating the backplane addresses the airflow problem and also reduces signal degradation attributed to losses and signal distortion created by the plated through-hole in the backplane. The card cage must be designed to assure proper alignment of mating daughtercards.
In very large systems, high-speed signal management can be a particularly vexing problem. Systems with oversize daughtercards or racks with many daughtercards pose severe channel-length challenges. It may be impossible to reduce channel lengths sufficiently to assure adequate signal integrity. One solution is the adoption of cable backplane architecture where high-speed circuits embedded in the backplane are replaced with discrete shielded twinaxial cables. The backplane or midplane is used only for low-speed signals and power distribution.
Critical electrical characteristics such as impedance, crosstalk, skew, and attenuation can be much better controlled in cable than in a PCB backplane. Removing all high-speed lines from the backplane also may permit the use of lower-performance and lower-cost PCB laminates while greatly simplifying critical circuit layout. Daughtercards linked by cable backplanes can be joined to form very large virtual racks containing many daughtercards that may not be in close proximity.
Relocating all of the links of a full mesh network from a PCB backplane into discrete or flat cables can result in a mass of cables making cable management more of a challenge.
Fiber optic backplanes have been the subject of research for many years. Everything from discrete fibers linking through blind-mate optical connectors to optical waveguides laminated into the layers of a PCB have been investigated with few practical applications reaching production. That may slowly be changing.
High-end systems that utilize optical backplanes would feature greatly increased bandwidth with much-improved signal integrity. Channel length concerns would also be minimized.
The introduction of the MXC connector is focused on I/O as well as optical backplane applications. MXC is a blind-mate connector that can link up to 64 fibers in a single pluggable interface. It uses expanded-beam lensed technology to minimize sensitivity to end-face contamination. One proposed application would have optic fibers originating from mid-board optical transceivers going through MXC connectors at the backplane to enable high-speed links between daughtercards or racks.
The need for speed is pushing development of practical optical backplanes. For example, the ANSI-VITA 66.1 standard defines optical interfaces in hybrid VPX backplanes. Multi-gigabit copper connectors are coupled with blind-mate MT-type optical interfaces that can hold up to 24 optical fibers.
TE Connectivity has shown a developmental passive optical backplane using mid-board optical transceivers and optical flex circuits to deliver more than 900 Terabits per second of interchassis interconnectivity.
Modular system architecture continues to evolve in response to demands for ever-faster throughput. Traditional and modified backplane designs will be able to keep up with performance demands of most commercial applications for many years. Newer concepts including cable and optical backplanes provide a migration path to support next-generation equipment when traditional copper interconnects are no longer the most cost-effective solution.
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