In new or custom applications, high-speed interconnects require industry-defined parameters on details like impedance, signal latency, propagation delay, and attenuation.
By Michael J. Karg
Many industry standards address the requirements of an interconnect for high-speed data communications, detailing the electrical and mechanical specifications of a cable to acceptable operation parameters. Understanding the requirements of the application and how they should be applied to the design through comprehensive application engineering is the key to providing a solution. High-speed data transmission standards like InfiniBand, Serial Attached SCSI, Serial ATA, and 10-Gigabit Ethernet are prime examples of high-speed datacom standards that have been established with well-documented requirements for the interconnect. Each standard establishes several key characteristics that ensure the interconnect functions as part of the entire system, and that the cable-assembly manufacturer provides a working product.
Electrical performance
Several electrical performance requirements are the key to ensuring a product will work within the system. Impedance is a key characteristic in high-speed data transmission. The most important attribute of the impedance of an interconnect is how well it matches the environment in which it is used. Mismatches between interconnect and system impedance result in unwanted signal reflections (also characterized by return loss), which interfere with data transmissions. Typical impedance values for high-speed data applications are 100, 105, and 110 Ω. Allowable variances for cable and connector can be anywhere from ±15 Ω to as little as ±5 Ω from nominal. It is important to know what impedance deviations can be tolerated by the system and translate this to the requirements of the interconnect.
Interconnect impedance is controlled through selection of appropriate materials and their dimensions, as well as consistent processing and proper mating of connector and bulk cable to minimize impedance discontinuities. Cable-to-connector termination methods, such as insulation displacement, crimp, soldering, or welding, can affect impedance variations in different ways. The termination method must be closely considered when impedance matching is critical.
Velocity of propagation and the corresponding time delay is an electrical characteristic that may need to be defined. In some data transmission applications where signal propagation speed is critical, this characteristic must be well-defined. These applications tend to be few and far between, but when required, this signal timing and the potential variations (time-delay skew) must be closely examined.
In these cases, special insulation types, such as highly expanded fluroupolymers or polyolefins with dielectric constants as low as 1.4, may be required to provide lower signal latency and propagation delays of 1.2 ns/ft. Furthermore, a finely controlled process may be necessary in order to reduce the amount of variation in the velocity of propagation of the dielectric material, thereby allowing signals on multiple data lines to arrive at or near the same point in time. Commonly, high-speed systems can tolerate 10 ps/ft or less.
Another key property is attenuation. System applications almost always dictate the signal loss through an interconnect that can be tolerated at known frequencies before the received signal is indiscernible. Therefore, signal loss or attenuation must be accounted for in the design. Attenuation is controlled by a number of factors, including conductor size, plating material, dielectric material, and cable construction, as well as the mechanics of the connecting hardware. Attenuation of the fundamental frequency is typically a requirement for the system. For instance, InfiniBand operates at a data rate of 2.5 Gbit/s and the specification dictates the maximum acceptable attenuation at the fundamental frequency of 1.25 GHz.
One method of examining signal loss and signal integrity as a whole is to examine the response of the interconnect to a known data-pattern stimulus which mimics the expected data of the system. This can be accomplished through the signal integrity analysis method known as eye-diagram analysis (see Fig. 1). When an interconnect is subjected to a data pattern stimulus, the result is a constellation diagram composed of all possible data pattern interactions and their impact on the interconnect. Analyzing these patterns at specific application data rates can give one a “big-picture” evaluation of the interconnect under real-world conditions.
Such an example is the eye-diagram analysis that is performed on the “BluStreak” InfiniBand bulk cable assemblies (see Fig. 2). These cables are stressed with a specific data stream of pseudo-random bit sequences (PRBS) at a data rate of 2.5 Gbit/s. The resulting eye diagram is analyzed by examining the amount of eye closure both in the horizontal (time) and vertical (voltage) axes. For instance, the requirement for eye closure in the InfiniBand industry standard is 316 mV in the vertical axis and 300 ps in the horizontal axis for the cable assembly. The standard defines the electrical signal in terms of an eye diagram that will allow the link to perform as intended.
Electromagnetic interference (EMI) will also affect the performance of the interconnect. The ability of the interconnect to both prohibit and withstand EMI radiation is important in high-speed signal applications. This is achieved through rigorous analysis of the interconnect’s ability to provide an acceptable level of shielding. The shield construction of both the connector and cable must be of such quality so as to permit suitable operation in noisy environments, as well as limiting the amount of noise that is transmitted from the interconnect itself. Issues that need to be considered include the type of connector enclosure, cable shielding materials, and the configuration employed.
Mechanical performance
While electrical performance requirements assure that the interconnect will operate within the system, mechanical constraints could potentially limit or disrupt operation altogether if they are also not also closely examined. The designer must be made aware of the intended physical constraints and purposefully design a cable with appropriate materials and suitable construction to provide for a robust solution.
Understanding the cable’s ability to operate within a predetermined bend radius will guarantee the cable will perform once connected and oriented within the physical environment of the system. For example, the InfiniBand specification dictates a minimum bend radius of 4" so that the cable can be properly routed. Cable constructions will vary depending upon the needs of the application. At times the way a cable must be put together to meet electrical characteristics may limit it ability to withstand too tight of a bend radius.
The manufacturer’s recommended minimum bend radius should be closely followed to assure that the special materials that make up such a high performance cable are not overly stressed so as to damage their integrity. Likewise, if an application’s environment is clearly understood, steps can be taken with certain designs to produce a cable construction which meets the electrical properties and as well stands up to the mechanical rigors of the application.
Mating cycles are one type of mechanical characteristic to consider. Typically, these types of interconnects are used in high-speed data transmission, in which they are “plug-and-forget.” That is, the cable is installed in the system and is seldom, if ever, removed. There are many applications, however, that may call for a robust-enough connector system required to withstand repeated mating and un-mating of the connector. The InfiniBand specification requires, for example, at least 250 mating cycles while still meeting the contact resistance requirement of 80 mΩ.
In this case, the cable designer’s experience will steer him to select a connector system that meets the number of expected mating cycles for this kind of application. If this is not taken into account, excessive wear in the mating surfaces can cause sufficient degradation in the connector to allow for mis-mating and signal integrity problems. Various types of contact mating configurations and materials can be designed in to meet a wide range of requirements.
These are just a sampling of the types of design considerations that must be taken into account for any high-speed data communication cable assembly application. Where no standard exists, a thorough examination of an application’s mechanical and electrical requirements will yield a interconnect solution that is right for the application. The end users need not be experts in all of these areas as long as they use an interconnect manufacturer well versed in such design considerations. Proper characterization and evaluation of high-speed interconnects based on the requirements of the application ensures solid cable assembly design, guaranteeing performance. Understanding the cable, connector, cable-to-connector interface, and their interactions provides for higher speed, longer distances, and superior signal integrity.
Michael J. Karg is the applications engineering manager at C&M Corporation, PO Box 348, Wauregan, CT 06387. Tel: (860) 774-4812; Email: MKarg@CMCorporation.com.
