USB3.0来了

编辑笔记：

USB flash drives such as the Kanguru FlashBlu now hold up to 64 Gbytes of data. Courtesy of Kanguru.
With 64-Gbyte flash drives and terabyte external hard drives now available, USB 2.0 has become a data bottleneck. USB 3.0, with its 5-Gbps bit rate, will change that. If you plan to develop USB 3.0 products and you’re not currently testing other high-speed buses such as PCIe (PCI Express) Gen2 or SATA-3 (Serial ATA III), start making a case to your manager for a faster oscilloscope.
USB 3.0 will leverage technology from PCIe Gen2 (5-Gbps) and SATA-3 (6-Gbps) serial buses. It will use 8b/10b data encoding, which reduces the data throughput rate to a maximum of 4 Gbps. “In practice,” said Mike Engbretson, USB test solution marketing manager at Tektronix, “the data throughput may be around 3 Gbps.” Still, the 5-Gbps bit rate is more than 10X faster than USB 2.0. (Click here to read a discussion with Engbretson, covering USB 3.0 technology in more detail.)
Like PCIe Gen2, USB 3.0 will use a 2.5-GHz fundamental clock frequency. So, you’ll likely need a 12.5-GHz oscilloscope if you need to see a signal’s fifth harmonic. USB 3.0 will also require you to measure a signal at the receiver, which isn’t necessary for USB 2.0. With the higher bit rate, USB 3.0 receivers will need equalization at the receiver because the signal eye will be closed after traveling through PCB traces, connectors, and cables.
Just how much oscilloscope bandwidth you’ll need is still open to debate. Jim Choate, product management engineer at Agilent Technologies, suggests that using a 10-GHz oscilloscope at the receiver will reduce the amount of jitter and noise you’ll see because some noise is generated by the instrument itself. “There’s very little high-frequency content at the receiver,” he said.
Engbretson countered that you’ll need 12.5 GHz to capture the signal’s fifth harmonic and that using a lower-bandwidth oscilloscope will reduce the amplitude of the measured signal. Engbretson, Choate, and Mike Micheletti, senior product marketing manager at LeCroy, agree that you’ll need to use a 12.5-GHz oscilloscope at the transmitter.
Sample rate will be equally important because of the USB 3.0 signal’s rise time. “At the transmitter,” noted Choate, “edges will rise in less than 70 ps.” You’ll need a sample rate fast enough to measure the signal’s rise time.
“The USB 3.0 specification could specify a 50-ps minimum rise time, although actual hardware may not reach that speed,” said Steven Sanders, product development engineer at LeCroy.
USB 3.0 should have a specification version 1.0 in time for a USB developer’s conference in November. Initial USB 3.0 interface ICs and consumer products should appear in mid-2009, with widespread deployment in 2010. You can expect the first USB 3.0 products to be data-storage devices such as flash drives, external hard drives, digital music players, and digital cameras. After that will come video products and, eventually, data-acquisition systems that need the high data throughput.

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