Port Impedance Z₀
S-parameters are defined relative to a reference impedance Z₀ at each port. Standard is Z₀ = 50 Ω. Changing Z₀ changes all S-parameter values even for the same physical device — the S-parameter values depend on the reference, not just the device.
Touchstone options line declares Z₀: "# GHz S DB R 50" → Z₀ = 50 Ω (standard) "# GHz S DB R 75" → Z₀ = 75 Ω (video/CATV systems) If you don't know Z₀ of a Touchstone file: assume 50 Ω (default if R field missing)
Reference Plane
The reference plane is the physical location where the S-parameter measurement is calibrated to. Everything between the VNA ports and the reference plane is removed by calibration.
| Calibration Type | Reference Plane Location |
|---|---|
| SOLT at cable ends | At the SMA connector on the cable end |
| TRL on PCB | At the PCB trace, removing SMA connector and launch structure |
| On-wafer GSG probe | At the probe tips (removes probe parasitics) |
Moving the Reference Plane
Adding electrical length to move reference plane: S₁₁_new(f) = S₁₁(f) × exp(−j4πfτ) [τ = electrical delay in seconds] (Rotates S11 on Smith chart without changing magnitude) Useful for: removing connector or launch structure from measured data In RF View: use Frequency Change → Phase shift option to rotate S11 phase
RF View: RF View reads the reference impedance from the Touchstone R field and uses it for all calculations. Load .s2p files measured at 50Ω VNA directly — no renormalization needed for standard RF analysis. Free on Android.