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Test considerations for USB 3.1 SuperSpeedPlus

10 Dec 2014  | Tami Pippert

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With revision 3.x, USB design and test shifted from the purely digital to requiring a combination of digital and RF design techniques. As well as the connector and cable physical design, designers must take care to minimise both radiated emissions that can interfere with other devices, and their device's susceptibility to external signals. Testing involves not only measuring traditional eye diagrams, but also RF analysis using spectrum analysers, time domain reflectometry, transmission characterisation using S-parameters, and the new requirement for loopback receiver testing.

USB started out as a replacement for the standard low-speed serial interfaces used in computer terminals and data modems. Data rates in the 10's to 100's of kilobits per second meant design, test and implementation were purely digital functions. With revision 3.0, and now 3.1, known as "SuperSpeed" and "SuperSpeedPlus" respectively, USB has moved to satisfy users' demands for high-definition streaming video services and near-instantaneous transfer of large data files.

External mass storage is a good use case: laptop and tablet development is moving towards using solid state drives (SSDs) instead of mechanical hard disc drives (HDDs), and SSD storage capacity will be significantly lower than for current HDDs. While business users can live with this limitation, personal users have family photos and videos, shot with higher and higher resolution, that create terabytes of data. Regular backup is essential and local backup and storage using an external HDD is much faster, easier and less expensive than cloud storage.

The USB 3.1 1.0 specification was published in July 2013 and introduced a new 10Gbit/s signalling rate in addition to the 5Gbit/s signalling rate defined in the USB 3.0 specification in November 2008. Though the signalling rate is doubled, theoretical throughput is almost 2.5 times higher due to its new data encoding scheme, with actual throughput, after taking into account overhead, expected to approach 1 Gigabyte per second, a significant increase from approximately 400 MB/s in a USB 3.0 system.

The high signalling rates can generate interference with other PC peripherals: wireless LAN adaptors, Bluetooth devices, wireless keyboards and mice, for example. USB 3.1 introduces new connectors with enhanced EMI contact zones that are designed to minimise RF leakage. Both radiated interference from the USB interface and susceptibility to other RF sources is improved.

The new connectors mate physically with the previous design for backward compatibility. Over time, the maximum length of cable supported has been decreased. While the USB 2.0 specification defined cables of up to 5m, the USB 3.0 specification assumes 2-3m cables will work when designed properly, and the USB 3.1 specification assumes that a 1m cable is sufficient.

Design and test considerations
The main measurement used to assess the quality of the USB interface is the eye diagram. Today's real-time oscilloscopes, such as the Keysight Infiniium 90000 X-Series, have the high bandwidth and picosecond rise-time specifications that make the measurement straightforward, given the correct fixturing. Figure 1 shows the specified transmitter requirements for both USB 3.0 and 3.1, which must be measured into a 50Ω single-ended load. With the increase in signalling rate, active effects such as timing jitter, oscillator phase noise and amplifier linearity, and passive effects such as reflections and delay variations within the board trace connectors and cables assume greater importance. Each of these effects will be present to some extent, causing the eye to close up, and leading to more uncertainty in demodulation and hence to transmission errors.

 USB 3.0 and 3.1 eye specification

Figure 1: USB 3.0 and 3.1 eye specification.

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