Connectors meet the needs of high speed communications system designers

4 mins read

Communications applications are seeing datarates creeping ever higher. In the telecomms world, major routes are beginning to see data rates of 40Gbit/s, with plans for even higher speeds, while in the equipment side of the business, interconnect rates continue to push towards – and sometimes beyond – 10Gbit/s.

But it's not just pure data rate; some emerging standards are looking to enable high speed serial data transmission. Work is continuing on developing a standard for 25Gbit/s serial data transmission over copper interconnects, but a complicating factor is these links could be chip to chip, chip to module or even system to system over backplane connections. A number of communication standards are set to encompass 25Gbit/s data rates, including Ethernet, InfiniBand, PCI Express and Fibre Channel. The systems in which these standards are applied are often demanding, not only when it comes to performance, but also when it comes to board space. Developed by Amphenol TCS, the XCede connector has been designed to meet and exceed the requirements of a range of communication standards. The interconnection system features an innovative 3d shield technology, designed to reduce crosstalk, and advanced materials to prevent high frequency resonance. Design features within the connector are said to remove skew within any differential pair, simplifying the board layout process. An innovative design features a resonance damping shield and an optimised footprint with double grounds, said to enable crosstalk of less than 40dB on any signal to 8GHz. According to Amphenol TCS, this level of electrical performance and design flexibility allows equipment manufacturers not only to meet current datarate requirements, but also to implement performance upgrades in the future. As system architectures become more complex and require higher levels of performance, interconnect packaging becomes increasingly important. The XCede platform offers an interconnect density of up to 82 differential pairs per inch. This, says the developer, makes the part suitable for use in the most difficult interconnect designs. High density benefits The connector's high density enables designers to meet the needs of architectures with multiple front or rear fabric slots, and blade systems with cooling straight through the backplane. Secondary routing channels in the connector backplane footprint mean escape is possible on all three sides of the connector module, allowing greater design flexibility to route congested areas and the ability to reduce the number of routing layers required. The XCede connector has also been designed to provide mechanical longevity and ruggedness. The part has wide backplane module shield contacts with a stiffness enhancing rib. XCede connectors are available in two to six pair versions, with integrated power and guidance. The product platform also includes bus bar power, stacker, cable and coplanar solutions. Interconnect specialist FCI, a licensed second source for XCede products, has recently released XCede vertical backplane headers and right angle daughter card receptacles designed to support 25Gbit/s performance. Vertical backplane header configurations provide two, four or six differential signal pairs per column wafer with four, six or eight wafers per signal connector module. The vertical header range also includes options with two, three or four wall connector shrouds. Amongst the three wall options are versions with integrated guide posts and polarisation key openings. "As evidence of both companies' commitment to second source partnering, FCI and Amphenol have successfully concluded extensive intermateability qualification testing of their respective four pair XCede header and receptacle products to assure acceptable performance, regardless of how backplane headers or daughter card connectors are interchanged by a customer," said Jeff O'Brien, FCI's global product manager. According to FCI, the XCede connector platform provides a clear long term migration path for equipment platforms that will need to evolve to enable higher speed signaling and higher levels of performance. XCede connectors also address requirements for higher linear signal density at the backplane or midplane interface. Connectors with six differential pairs per column fit 36mm minimum card slot pitch and provide 82.4 differential signal pairs per inch. Four pair connectors provide 54.9 signal pairs/in and can be used down to card slot pitches of 25mm, while the two pair connectors fit a slot pitch of 15mm, offering 27.5 signal pairs/in along the card edge. Looking to meet the requirements of those looking for connectors capable of handling datarates in excess of 12.5Gbit/s, FCI has developed the AirMax VS range. Suitable for use with single or differential signals, as well as power, the connectors are available in three, four and five pair configurations. The AirMax VS connector system eliminates the need for interleaving ground shields by using air as a highly efficient dielectric – VS stands for virtual shield. The connectors are also available in 2mm and 3mm centreline versions. AirMax VS eliminates the need for shields by offsetting the differential pairs relative to each other. The gaps between edges of the in column conductors are much smaller than the separation between the columns. Hence the field within a column is much stronger than it is between adjacent columns. Shifting the location of each differential pair relative to neighbouring pairs decreases the magnetic coupling significantly. In addition to reducing magnetic coupling, column to column staggering reduces noise. Because the connector is scalable backplane systems designed to handle data rates of up to 6.25Gbit/s can be upgraded to support 12.5Gbit/s without redesigning the basic platform. 25Gbit/s over a backplane FCI has partnered with National Semiconductor and Agilent at DesignCon 2011 to demonstrate 25Gbit/s differential signal transmission over a backplane link. The demonstrator featured the AirMax VSe backplane connector system, 28Gbit/s discrete quad channel retimer technology from National Semiconductor and a 28Gbit/s pulse pattern generator from Agilent. "The connector and chip technologies to enable 25Gbit/s lanes over copper backplane links are essential to accomplish narrower 4 x 25Gbit/s implementations of 100Gbit/s links for Ethernet or Infiniband," said David Sideck, FCI's global market manager for high speed and power products. "Moving beyond 10Gbit/s lanes on the backplane will also enable next generation data centre equipment designs to increase backplane capacities to support growing front panel I/O bandwidth demands while significantly reducing the number of required high speed signal traces." FCI's AirMax VSe connector provides a migration path to higher data rates while preserving the pin assignment flexibility of an open pin field design. The connectors feature backwards mating compatible interfaces to existing AirMax VS connectors with minimal changes to connector board footprints. Since launching the XCede range, Amphenol TCS has added the XCede 85 Ohm option for applications where matching lower system impedance is desired. Connectors with lower characteristic impedance increase the margin in high performance backplane systems by minimising the impedance discontinuity between the board footprint and the connector. XCede 85 Ohm has been designed specifically to meet the requirements of PCI Express 2.0 and 3.0, the Intel QuickPath Architecture and other applications where matching to an 85ohm system impedance is desired. Amphenol notes that the XCede connector platform provides designers with a readily available 85ohm solution without the challenges of retooling and requalification, in turn reducing the costs associated with meeting the latest specifications. AirMax VS is also available in an 85ohm version.