Addressing high-speed connector requirements

October 14, 2014 OpenSystems Media

The past several years have seen a significant increase in the amount of data that’s transferred between various interfaces, creating new challenges for all types of electronics – and the connector is no different. Connectors and cables are tasked with being electrically transparent, or as invisible as possible to the electrical signal. This can be a challenge at low speeds, and becomes increasingly more difficult as speeds cross the gigabit barrier.

At low speeds, a connector/cable assembly can adequately be viewed as either a perfect short or small resistor. But as speeds increase, a connector/cable assembly becomes increasingly lossy and signal reflections can become problematic. Signal reflections occur whenever the instantaneous impedance at any point in the path differs from the system impedance.

The most helpful way I’ve found to understand impedance is to consider fluid flowing through a simple pipe. If the pipe diameter changes, some of the fluid will reflect back to the source, unable to reach its destination. This is analogous to how impedance impacts high-speed signals; if the impedance changes, a portion of the signal will reflect back to the source, and the signal that ends up reaching the destination is reduced in magnitude and likely distorted. In addition, the fluid flow rate is important. If the rate is low, changes in the pipe diameter likely won’t matter. However, as the rate increases, the impact of the diameter changes will increase accordingly.

High-speed challenges are changing some of the ways that connectors are designed. Here are three design features that are required to improve high-speed performance:

  • Optimized pitch. The spacing of most connectors is rarely dictated by electrical performance. This most often yields a poor impedance match through the connector. However, as speeds reach the gigabit regime, the importance of the electrical performance increases such that it must dictate the spacing. Optimal spacing will ensure the best impedance match, which will in turn guarantee optimal performance.
  • Advanced manufacturing techniques. High-speed connectors require a very meticulous design. This is because very small changes in design may cause significant changes in performance. Thus, strip lengths must be minimized, solder joints and crimps must be carefully made, and tolerances must be well managed.
  • Reduced size. The size of hardware continues to decrease, and real estate is at a premium for many hardware boxes. This, of course, leads to smaller connectors. Fortunately, smaller connectors most often yield better high-speed performance. Since most connectors introduce some level of reflections into the high-speed path, smaller connectors perform better because they minimize the amount of these reflections.

By developing a thorough understanding of high-speed interconnectors, using state-of-the-art software and hardware to adjust designs, and by understanding these challenges laid out here, connector manufacturers are both ready to design for today’s challenges and prepared for the future challenges that will undoubtedly arise as speeds increase, sizes decrease, and the room for error in our designs gets smaller.

Ryan Satrom holds a BSEE from Marquette University. He is currently a Signal Integrity Engineer at Omnetics Connector Corp., where he focuses on high-speed characterization and design for all connectors.

Ryan Satrom, Omnetics Connector Corp.
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