USB 3.0, whose USB speed mode is termed "SuperSpeed", is the third major revision of the Universal Serial Bus (USB) standard. Its main technical specifications include: support for full-duplex communication, the use of transmit lists for data packet transmission, a power delivery standard of 900mA, and a transfer speed of 5 Gbps (Gigabits per second). USB 3.0 is designed to be compatible with USB 2.0 and USB 1.1 versions and employs tiered multi-level power management technology, allowing different power management schemes for various devices.
USB 3.0 utilizes a new packet routing transmission technology. Its cables are designed with 8 internal conductors. Besides VBus and GND serving as power lines, the remaining 3 pairs are all data transmission lines. Among these, the D+ and D- lines are retained for USB 2.0 compatibility, while new SSRX (SuperSpeed Receive) and SSTX (SuperSpeed Transmit) lines are added specifically for the new standard. Consequently, USB 3.0 features several additional contacts compared to USB 2.0. The USB 3.0 Standard-A connector maintains the same physical dimensions as earlier versions. The plug features a blue color (whereas the USB 2.0 Standard-A plug is typically black), but the internal contacts are changed; the new contacts are positioned behind and adjacent to the existing 4 contacts. Spread spectrum clocking (SSC) technology is introduced to reduce electromagnetic interference (EMI) emissions.
The eXtensible Host Controller Interface (xHCI) specification published by Intel supports USB 3.0 interfaces and is backward compatible with USB 2.0 interfaces. Windows 8 and subsequent Windows operating systems include native USB 3.0 support.
Electromagnetic Interference (EMI) Improvements in USB 3.0
USB uses differential signaling for data transmission. Taking USB 2.0 as an example, to achieve 480 Mbps, the differential signal must operate at 240 MHz. USB 3.0 requires a 2.5 GHz differential signal frequency to achieve its 5 Gbps transfer rate. To mitigate EMI generated by this high operating frequency, USB 3.0 incorporates spread spectrum clocking (SSC). This technique spreads the energy originally concentrated at 2.5 GHz into a sine wave with an absolute value distribution centered around 2.5 GHz, thereby reducing the peak energy density at that frequency. Consequently, the next significant energy concentration point becomes the third harmonic at 7.5 GHz (which is why USB 3.0 cable specifications include requirements targeting 7.5 GHz).
However, the 2.5 GHz operating frequency is very close to the ISM band (Industrial, Scientific, and Medical band), specifically the 2.412–2.462 GHz range used by Wi-Fi and Bluetooth. Combined with the effect of spread spectrum clocking, which broadens the originally single 2.5 GHz signal energy into a bandwidth spanning from DC up to several GHz, USB 3.0 operation can cause its EMI signals to overlap and interfere with signals in the ISM band. This interference cannot be easily filtered out. As a result, on the same circuit board, USB 3.0 often cannot operate reliably near Bluetooth or 2.4 GHz band WLAN (Wi-Fi) devices. The USB Implementers Forum (USB-IF) primarily addresses this issue by requiring manufacturers to implement robust electromagnetic shielding, including grounding, throughout the entire path – from the USB port and cable to the external device itself – to minimize radiated emissions. Practical testing has confirmed that when USB 3.0 devices are near Bluetooth or 2.4 GHz Wi-Fi devices, issues such as dropouts, connection interruptions, and significantly degraded performance occur. Implementing proper shielding at the USB 3.0 connection point, or using extension cables to physically separate Bluetooth/2.4 GHz Wi-Fi devices from USB 3.0 devices, demonstrably improves coexistence and reduces interference problems.
These USB 3.0 EMI issues with the ISM band have also indirectly led many mobile devices (such as smartphones) to omit support for USB 3.0 speeds, though some smartphones do support it (e.g., the Samsung Galaxy Note 3).
