As we begin to expand usage of USB 3.0 to reap the many benefits, we must pay attention to how the connectors and cables affect circuit operation. Here, Omnetics’ Bob Stanton explains how to use USB 3.0 connectors and cables.
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As we begin to tap into the potential of USB 3.0 speeds that run up to 5.0Gb/s, we find the technology offers many benefits when moving large quantities of information from one place to another: Cloud transfer, portable instrument downloads, and high-speed digital camera applications all benefit, and surveillance equipment, from still-frame to continuous-scanning, can now send a constant stream of high-quality images with ease.
But as we begin to stretch the limits of the technology and speed up the data transfer rate so that it further evolves, we must pay careful attention to how the circuits operate as well as to the affects of the connector and cable interconnects involved on such. In moving ahead, we must look closely at the basics of cable and connector design. Much of our new high-speed data must travel on sets of differential twisted cable that splits the signal into two adjacent wires. Older versions of USB used one bi-directional path that both transmitted and received signals. Newer signal transmission protocols, such as USB 3.0, will require multiple signal paths, as they include separate paths for data transmit and data receive. The multiple paths will increase speed and signal capacity greatly.
In USB 3.0, each set of signal wires is twisted to avoid interference and requires its own drain wire. Each couple of twisted wires should also be shielded from other groups. Since we depend on fast digital signal processing, the rise-time of the circuit must be well managed. Most signal processors are designed to push the signal out to a set load, and that requires that the cable impedance should have a similar load which is specified at 90 ohms +/- 7 ohms.
The connector impedance is equally important, but somewhat less so, as the length is a minimal portion of the total cable impedance-match formula. At the location where the twisted pair cable is soldered to the connector, there will always be some impedance mismatch. Minimizing this transition area, mainly by keeping the shield on the twisted pair as close to the solder joint as possible, will help. If proper care is given to this wiring area, the impedance at the connector will be adequate and the connector assembly will work well.
An overall cable shield is often important and protects the rest of the circuit from adjoining noise. The jacket of the shield should connect tightly and wrap totally around a metal back-shell on the connector. During cable-to-connector assembly, each twisted wire set should also be attached at the same length on the connector lead. Different lead lengths cause a signal timing difference between the wires (called “skew”), and this causes jitter and noise in the signal transmission. With too much noise and jitter, we can lose the signal completely. This image transfer condition is what we call “pixilation” and is measured in the number of bit-errors or BER, bit error rate.
Many of the better cable and connector suppliers offer technical information regarding these critical specifications. More and more, we see cable performance information exhibited in a measurement called eye-diagram. These diagrams are simply an overlap of the signals from the two opposing wrapped wire pairs and should have a clean opening in the center of the displayed pattern. Eye patterns are coming of age as test and simulation equipment becomes more available to the general test industry.
Already, we see the extension of USB 3.0 in the release of USB 3.1 series connectors. We are now looking at up to 10Gb/s and beyond. A variation of the new USB 3.1 connector revolution includes reverse-compatible formats. Note closely which version you use, as we all evolve through different types. Fortunately, the USB 3.1 is made with blue insulators and is visible to the user on his equipment. More and more new connectors include both formats and work interchangeability.
Bob Stanton is director of technology at Omnetics Connector Corporation.