SLAAET6 August   2025

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1S-Parameter Definition
    1. 1.1 Insertion Loss (S21)
    2. 1.2 Return Loss (S11)
  5. 2High-Speed Signal Design Example Of FPD-Link™ Serializer Body
    1. 2.1 Design Example Overview
    2. 2.2 Key Points in High-Speed FPD-Link Layout Design
  6. 3Factors Impacting Return Loss and Optimization Guidelines
    1. 3.1 Transmission Line Impedance Impact
    2. 3.2 AC Coupling Capacitor Landing Pad Impact And Optimization
      1. 3.2.1 Mitigation Strategy: Anti-Pad Implementation
      2. 3.2.2 Simulation Results With Ansys® HFSS
    3. 3.3 Through-Hole Connector Footprint Impact and Optimization
      1. 3.3.1 Through-Hole Connector Via Anti-Pad Impact
        1. 3.3.1.1 Simulation Results With Ansys® HFSS
      2. 3.3.2 Surrounding Ground Vias Impact
        1. 3.3.2.1 Simulation Results (Surrounding Ground Vias Impact)
      3. 3.3.3 Non-Functional Pad Impact
        1. 3.3.3.1 Simulation Results (Non-Functional Pad Impact)
    4. 3.4 Generic Signal Via Impact and Optimization
      1. 3.4.1 Simulation Results
    5. 3.5 ESD Diode Parasitic Capacitance Impact and Optimization
  7. 4Summary

1.2 Return Loss (S11)

The following list describes return loss (S11).

  • Measures reflected power due to impedance mismatches in the transmission path.
  • Expressed in decibels (dB).
Equation 2. S 11 ( d B ) = 20 × log 10 ⁡ ( V r e f l e c t e d V i n p u t )
  • Higher absolute values indicate better impedance matching and lower reflected power.
Note: The primary focus in this application note is maximizing trace impedance to enhance return loss (S11) performance. Insertion loss (S21) can be minimized through trace length reduction or adoption of low-loss dielectric materials. In contrast, S11 is more challenging to manage as S11 requires stringent impedance control across the entire signal path. Impedance control is the key of a successful design as this control not only directly enhances S11, but also indirectly optimizes S21 by mitigating reflections and energy dissipation.