Controlling Skew
A major cause of skew is the variation in electrical length due to differences in the dielectric constant. For skew control, in-line measurement of time delay is important, but not always available from cable manufacturers. Figure 1 shows data from a process producing 38 AWG coax within an 8-ps/ft. tolerance.
Various system-level issues limit the practicality of higher speeds. These include the transmission distance, required signal-to-noise ratio, tolerance to signal degradation, system bandwidth, and packaging requirements. Within these constraints (and when carefully optimized), ribbon interconnect systems can support signal transmission rates of 2 GHz or higher (Fig. 2).
Fortunately, maintaining signal integrity and controlling EMI in coaxial systems tend to be complementary functions. Shielding with uniform coverage not only reduces susceptibility and radiation effects in signal lines, but provides more consistent impedance.
Short cable terminations maintain a complete shielded signal environment as long as possible.They also minimize a system's EMI windows. Full 360° shield terminations provide mechanical strength, as well as a more continuous and lower-inductance path to ground for shielding effectiveness. Conversely, inadequate shielding opens the signal path to incident EMI and can cause the interconnect system to radiate.
Other means can be used to help control EMI. Filters may be introduced at the signal receiver end to remove noise at frequencies appreciably higher or lower than that of the signals. Or, signal rise times can be slowed at the signal source to minimize the high-frequency components most prone to radiate.
Usually, cable shields should be grounded at both ends. By grounding the shield at one end only, the cable becomes susceptible to frequencies with wavelengths less than six times the cable length. In certain applications, single-ended grounding may be advantageous to eliminate ground loops--especially when EMI susceptibility is out of the signal band.
Maintaining signal integrity over time can be challenging, especially in applications requiring repeated cable flexing or exposure to harsh conditions. Cabling-system environments can be characterized as static, repetitive, or freely dynamic. Static conditions are typically found inside instrumentation to join circuit elements sharing a common structure.
Routing requirements for installation, however, can impart significant stresses to the cabling system. And even static conditions may include vibration, mechanical shock, and other environmental stresses during transportation, storage, and usage. Board-to-board jumpers would fit this description.
Repetitive conditions can be found in applications like those encountered in articulating mechanisms, hinged structures, or devices with highly predictable movements. For example, ribbon coax can be used in mobile electronics, where size and weight prove critical (Fig. 3)>.
Dynamic systems include probes in which an operator or machine can freely move one end of the cable to any desired location within its length limits. Test probes are examples (Fig. 4).
Vibration, repeated bending, and gross mechanical shocks can cause short- or open-circuit conditions. Conductor choices must be made carefully, since material degradation of the conductors themselves will eventually cause variable resistance, high impedances, or cracking at points of flexural stress. Choices of alloys typically require tradeoffs in terms of dc resistance, tensile strength, flex life, flexibility, and cost. It's also important to control and qualify the thin dielectrics and jackets incorporated into miniature coax so that shorting between conductors is prevented during the product's life.
To help reduce electrical and mechanical degradation, designers should use encapsulants to protect termination regions. Cable bend radii should be kept as large as practical (at least 10 times the coax diameter). And materials must be reviewed for compatibility with all anticipated usage conditions.
Interconnection points also must be protected against mechanical shock using suitable flex/strain relief mechanisms. Cable assemblies have to be rugged enough to withstand any mechanical stresses characteristic of the application, such as crush forces or abrasion. These stresses need to be anticipated to assure adequate performance over the life of the product.
Make sure the operational and storage requirements are thoroughly understood. These mainly include temperature, humidity, corrosive conditions, and exposure to processing or cleaning solutions. Choosing materials with the appropriate mechanical properties and compatibilities can help ensure that the cable interconnect system sustains its ability to perform over the product's intended life. Materials and assemblies may undergo relatively subtle changes during the course of repeated use. Because of this, designers can help avoid the risk of large numbers of downstream field failures through sufficient up-front qualification testing.
Smaller coaxial conductors can provide high-speed performance at spacings of 0.6 mm or smaller. A common design error is the failure to incorporate interconnect pad geometries to fit an optimized termination process, leading to unnecessary labor content and operator dependence. Usually, transitions to circuit geometries smaller than the coax can be most easily accomplished through printed circuits.
Conductor sizes may be mixed within the same ribbon in applications requiring varied power, signal, and shielding requirements. Ribbons can also be made to virtually any width to support the transmission of high-count data channels. These structures support demanding applications in which size, weight, flex life, the ability to route, and signal integrity are key considerations.
With the ability to apply mass-termination techniques, ribbon coaxial structures offer advantages in assembly. Ribbon structures support organized packaging geometries that are free of crossovers and make effective use of space.