6. Recommendations

Choose Voltage-Mode PHYs with Internal Termination and AC-Coupling Support:

• • Voltage-mode drivers are designed to drive a defined voltage swing across the differential pair, and they control their own common-mode output voltage, making them naturally compatible with AC-coupling.
  • Internal termination (e.g., 100 Ω differential impedance built into the PHY) simplifies board layout and ensures proper signal reflections are suppressed—critical in high-speed links like Gigabit Ethernet.
  • If AC coupling is not documented, the PHY may expect a fixed DC bias at its input, and coupling capacitors may lead to incorrect voltage levels or link failure.

• • Conclusion: This combination ensures clean signal operation without the need for biasing resistors or transformers, leading to reliable and compact PCB designs, especially in space, automotive, or backplane systems.

Use 1000BASE-KX or 1000BASE-CX PHYs for backplane and internal Ethernet:

• 1000BASE-KX (IEEE 802.3ap) is explicitly designed for backplane transmission over PCB traces, with AC coupling and transformerless operation defined in the standard. It mandates voltage-mode signaling and supports equalization and link training.
• 1000BASE-CX (IEEE 802.3z) uses shielded 150-ohm copper cable, often for rack-level interconnects, and is also based on voltage-mode drivers. While transformerless operation isn’t explicitly mandated, many implementations (especially SFP+ Direct Attach) omit magnetics in short-distance links.
• Conclusion: These standards and their PHY implementations are inherently suited to transformerless, space- and height-constrained designs (e.g., satellites, automotive modules, or high-speed control boards).

Avoid transformerless configurations with 1000BASE-T or current-mode PHYs unless the design is well characterized.

• 1000BASE-T (IEEE 802.3ab) is designed for long-distance twisted-pair cabling (up to 100 m) and requires:
  • Galvanic isolation via transformers (per IEEE safety requirements).
  • Current-mode drivers, which do not define a common-mode voltage and instead rely on external termination and biasing.
• Using capacitors alone with current-mode PHYs can lead to:
  • Floating input nodes (causing noise and false signaling).
  • Inconsistent common-mode levels, increasing bit error rates.
  • Potential damage if DC imbalance or overshoot occurs.
• If transformerless use is attempted with 1000BASE-T, it requires explicit design for biasing, termination, and link compatibility, often only done in closed systems with known PHY pairs.
• Conclusion: Without exhaustive testing, such configurations are electrically risky and non-compliant with Ethernet safety standards. Avoid them unless the system is fully characterized and operates in a controlled environment (e.g., custom internal links in space hardware).

Validate mixed configurations using simulation tools or prototype testing.

• Mixed-mode connections (e.g., voltage-mode PHY ↔ current-mode PHY) or non-standard configurations (e.g., AC-coupled 1000BASE-T PHY ↔ RJ45) involve:
• Differences in impedance, common-mode behavior, and termination expectations.
• Risk of link negotiation failures or silent data corruption.
• Signal integrity (SI) tools such as HyperLynx, ADS, or SPICE-based simulations can model AC coupling, line impedance, and signal swing.
• Prototype validation using real PHYs and test cables helps verify link training, auto-negotiation, and packet loss under real EMI and loading conditions.
• Conclusion: Even in voltage-mode-to-voltage-mode cases, signal reflections, board layout, and return loss must be verified. For mixed or edge cases, simulation and lab testing are essential for production confidence.