SNLA498 August   2026 DS90UB948-Q1 , DS90UB954-Q1 , DS90UB960-Q1 , DS90UB984-Q1 , DS90UB988-Q1

 

  1.   1
  2.   Abstract
  3. 1Fundamentals of S-Parameter Theory
    1. 1.1 Characteristic Impedance Explained
    2. 1.2 Reflection Coefficient Explained
    3. 1.3 Insertion Loss Explained
    4. 1.4 Return Loss Explained
    5. 1.5 Two-Port and Four-Port S-Parameters Explained
  4. 2S-Parameter Measurement Methods and Calibration Techniques
    1. 2.1 Introduction to Vector Network Analyzer Principles
    2. 2.2 TDR Measurement Explained
    3. 2.3 VNA Calibration Operating Procedure
    4. 2.4 Test Methods for S11 and S21
  5. 3FPD-Link Channel Specification Analysis
    1. 3.1 Overview of Total Link, PCB, and Cable Channel Specifications
    2. 3.2 Channel Specification Analysis
    3. 3.3 Key Design and Test Considerations for the FPD-Link Channel Specification
  6. 4Conclusion
  7. 5References

1.4 Return Loss Explained

Return Loss (S11) characterizes the degree of impedance matching on a transmission line. It is the dB representation of the reflection coefficient (Γ) and directly reflects the amount of signal energy reflected back to the source during transmission.

The relationship between return loss and reflection coefficient Γ is:

Equation 5. R L = 20 ⋅ l o g 10 ( | Γ | )   d B

A larger absolute value of S11 (i.e., a more negative value, for example, −20dB is better than −10 dB) indicates less reflection and better matching. Voltage Standing Wave Ratio (VSWR): Another common representation of return loss, defined as the ratio of the maximum to minimum voltage on the transmission line:

Equation 6. V S W R = 1 + | Γ | 1 - | Γ |

A VSWR closer to 1 indicates better impedance matching.

In automotive environments, connector mating/unmating, component pads on PCBs, PCB manufacturing tolerances, and cable bending all introduce impedance discontinuities. The FPD-Link Channel Specification typically requires that the overall link S11 be better than −10 dB within the target frequency band (−12dB recommended for PCB) to ensure efficient signal energy transfer to the receiver and to avoid intersymbol interference (ISI) caused by multiple reflections.

S-Parameter DiagramFigure 1-1 S-Parameter Diagram