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48 The PCB Design Magazine • February 2014 Advances in semiconductor manufacturing have paced the development of electronics for six decades. The smallest transistor geometries are one thousandth the size possible 40 years ago, and that scaling, along with innovations in materials and architecture, has resulted not only in the integration of whole systems on chips, but device speeds that now challenge the limits of carrying digital signals over copper traces on FR-4 for distances as long as a desktop computer motherboard. I venture that progress in electronics now hinges not so much on semiconductor advanc- es as it does on a transition to PCB laminates that have better dielectric properties than FR- 4. Consider the latest USB chip sets. A consor- tium of semiconductor, PC, and software com- panies drafted the original Universal Serial Bus standard in the 1990s to provide a common interface for connecting all sorts of peripher- als to personal computers, one that could sup- port the disk-drive data rates at the time and also supply power to external equipment. That standard specified two communication modes: a "full-speed" mode with a 12-Mb/s maximum signaling rate and a low-speed mode for such devices as a keyboard or joystick. The USB to- pology consists of a central host residing in a PC, and multiple ports downstream of that host controller, which can connect with peripherals that may in turn serve as hubs for communica- tion with other equipment. The original standard was amended within a few years by USB 2.0, which upped the maxi- mum signaling rate by 40 times to 480 Mb/s. That version was superseded in 2008 by USB 3.0, which increased the maximum rate to 5 Gb/s. USB 3.0 is backward-compatible with the preceding standard, but includes a new high- speed bus in parallel with the USB 2.0 bus. In 2013, the standard was again upgraded, adding an even faster transfer mode whose ceiling is 10 Gb/s. The first chip sets to support the latest version of the standard, USB 3.1, will be intro- duced this year. A peripheral that would communicate in the fastest mode can be connected to the host by a cable up to a meter long. The total allowable by Amit Bahl SieRRA CiRCuiTS DESIGN FoR MANUFACTURE What's Driving High-Speed PCB Design? feature column

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