Page 6 - ConnectorSupplier.com Mil/Aero Handbook

Connector Considerations for High-Speed Mil/Aero Applications
Ryan Satrom, Signal Integrity Engineer, Omnetics Connector Corporation
High-speed systems create several new challenges for system designers to overcome. In addition to the ruggedized and size, weight, and power (SWaP) requirements of many mil/aero connector designs, high-speed connectors must also be able to successfully pass the high-data-rate signals that are increasingly common in modern mil/aero applications.
Connectors with high-speed signals require more than just a mechanically robust solution, and should also be balanced with an electrically optimized design. Higher signal speeds make it critical for both connector manufacturers and their customers to establish a thorough understanding of the many new considerations impacting these designs, including: pin selection, cable selection, SWaP management, pin-to-pin spacing optimization, proper termination to pins, and proper shield connections. These considerations are based on important design decisions that have been used in the successful deployment of high-speed mil/aero connectors currently operating in the eld at speeds up to 5Gbps.
Pin Selection
A number of variables are to be considered when determining the type of pin used for any given application. For high-speed systems, it is bene cial to use smaller pins whenever possible because they typically have a shorter impedance mismatch that yields better performance through the connector. For example, nano contacts generally provide about a 10 – 20% reduction in loss through the path when compared to micro contacts. This reduction in loss corresponds to a proportional improvement in data rate through the connector.
Cable Selection
Cable is a critical part of the signal path. Designing a cable without proper consideration for the high-speed requirements of the path may lead to product failure. For low-speed signals (generally signals below around 100Mbps), the impedance of the path is not critical. However, as signal speeds increase, the impedance and the intra-pair skew of the critical twisted pairs become key variables in the design.
Matching the impedance requires controlling the thickness of the wire insulation, tightly twisting the pairs together, and including a shield around each pair if possible. Controlling the wire insulation thickness and twisting the pairs alone will likely provide a relatively good impedance match, but including the foil on each pair is typically the only way to maintain a tight impedance tolerance throughout the cable. Our general recommendation is to use unshielded twisted pairs to control the impedance at 100Mbps and higher and to use shielded twisted pairs to control the impedance at 1Gbps or above.
Minimizing intra-pair skew (i.e., the time difference between electrical signals within a pair) requires a tightly controlled design. If the surrounding environment is not identical for the two lines, then skew is introduced. This can be caused by asymmetries in the wire extrusion, pairs that are too loosely twisted, or slight variations within the extruded plastic. In short, minimizing skew requires a good cable design and a tightly monitored manufacturing process.
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