Ruigu Electronic

USB (Universal Serial Bus) data signals exhibit specific electrical properties to ensure reliable communication between hosts and peripherals. These characteristics vary slightly across USB versions (e.g., USB 2.0, USB 3.x, USB4) but share core principles. Below is a summary of key parameters:

1. Signal Voltage Levels

• USB 2.0 (Low-Speed/Full-Speed/High-Speed):
  • Differential signaling uses two lines: D+ and D-.
  • For Low-Speed (1.5 Mbps) and Full-Speed (12 Mbps):
    • Idle state: One line is pulled to 3.3V (via 15 kΩ pull-up resistors), the other to 0V (via pull-down resistors).
    • Active state: Voltage swing between 0V and 3.3V, with a differential voltage (V_D+ – V_D-) of at least 200 mV (min) and up to 3.3V (max).
  • For High-Speed (480 Mbps):
    • Uses differential signaling with a nominal amplitude of 400 mV (peak-to-peak) to reduce EMI and improve speed.
• USB 3.x (SuperSpeed, 5 Gbps/10 Gbps):
  • Introduces a separate set of differential pairs (Tx+/- and Rx+/-) for high-speed data.
  • Differential voltage swing: Typically 800 mV (peak-to-peak) for 5 Gbps, with reduced swing (e.g., 400 mV) for 10 Gbps to minimize crosstalk.
• USB4 (up to 40 Gbps):
  • Based on Thunderbolt 3/4 electrical specifications, using differential pairs with even lower voltage swings (e.g., ~200 mV peak-to-peak) and PAM-3 (Pulse Amplitude Modulation) encoding for higher data rates.

2. Impedance

• All USB data lines require controlled impedance to prevent signal reflections:
  • USB 2.0 differential pairs: 90 Ω ± 15% (single-ended: ~45 Ω).
  • USB 3.x/USB4 differential pairs: 90 Ω ± 15% (higher frequency operation demands stricter impedance control).

3. Signal Integrity

• Rise/Fall Times:
  • USB 2.0 Full-Speed: Rise/fall time (10%-90%) between 4 ns and 20 ns.
  • USB 2.0 High-Speed: < 500 ps (10%-90%) to support 480 Mbps.
  • USB 3.x/USB4: Sub-nanosecond rise/fall times (e.g., < 200 ps for 10 Gbps) to maintain signal integrity at high frequencies.
• Jitter:
  • Timing jitter is tightly constrained (e.g., < 20% of the unit interval for USB 3.2) to avoid bit errors.
• Crosstalk:
  • Minimized through twisted-pair cabling and shielding (common in USB 3.x/USB4 cables) to reduce interference between adjacent signals.

4. Power and Ground

• USB data lines operate with a 3.3V supply (derived from the USB bus power or a local regulator).
• A common ground reference between host and peripheral is required to ensure stable differential signaling.

5. Error Correction

• USB uses differential signaling to reject common-mode noise (e.g., EMI/RFI).
• Cyclic Redundancy Check (CRC) and retransmission mechanisms further enhance reliability, especially in high-speed modes (e.g., USB 3.x).

These characteristics ensure USB’s versatility across devices (e.g., smartphones, printers, external drives) while maintaining backward compatibility across versions.