Advances in design, technology and materials science have pushed connectors to unprecedented performance. The demands of ever-faster data rates, as well as increasingly mobile applications, will continue to drive innovations in four major areas of connector design: miniaturization, straddle mounting, improved grounding and redundant contacts.
Miniaturization. As signals speed up, their rise and fall times become shorter. When the rise and fall time shrinks to a degree comparable to the unavoidable delay of signals passing through a connector, that connector exerts a noticeable influence on signal quality. At 100 MHz, for example, using a 2-nanosecond rise and fall time, the 50-picosecond delay of a signal pin only 0.6 inch long amounts to less than 2.5 percent of the signal rise and fall time. At that ratio of rise time to delay (40:1), the pin hardly affects signal passage. But as speeds soar beyond 1 Gbit/second, that same pin exerts a proportionately greater deleterious effect on signal propagation.
|Straddle mounting a connector drops the axis of connection down to a point coincident with the top surface of the board, thereby eliminating the signal-quality issues associated with multiple right-angle signal-path bends. This SMA connector straddles both sides of the pc board, with its signal pin lying directly on the top layer.|
In addition, connector performance may be negatively affected by the presence of low-grade plastic placed near the signal pins. The dielectric constant of the plastic material slows the propagation of signals, exacerbating the connector delay and worsening its impact on signal passage. Manufacturers of high-quality connectors choose low-dielectric-constant plastics, with low dielectric loss factors.
The most significant thing a designer can do to improve connector performance is deceptively simple: Just shrink the connector. Made 10 times smaller, the same connector geometry would automatically work to 10x higher speed. For example, the Rosenberger mini-SMP connector works up to 64 GHz (life-size). It's not made differently from any other RF connector, just smaller.
You can only shrink your connectors to a size commensurate with the mechanical tolerances of automatic pick-and-place assembly machines. It took about 20 years to transition the worldwide capital investment in pick-and-place machines from the DIP era, with holes on 0.1-inch centers, to SMT machines that handle mounting pads on 0.025-inch centers. During the same period, speeds generally increased by a factor of 100x. Clearly, assembly capabilities lag the need for miniaturization.
Straddle mounting. Consider a right-angle connector mounted on an ordinary pc board. Looking at the pc board resting on a tabletop, in a horizontal orientation, the right-angle connector must somehow bend the signals through 90Â° to change the vertical orientation of the mating pins of the connector into the horizontal orientation of traces within the pc board. It is difficult to maintain good signal quality in such an abrupt transition if the connector looms large compared with the rise and fall times of the signal conveyed. An ordinary edge connector suffers the same problem twice because it must incorporate two right-angle bends-one that brings the signals from the plane of connection down toward the board, and another that interfaces into the board at right angles.