Why Analyze Inductor S-Parameters?
SMD inductors for RF matching networks are not ideal components. They have finite Q factor (loss), self-resonant frequency (SRF beyond which they behave as capacitors), and package parasitics. Analyzing the manufacturer's .s2p file reveals these real-world limitations before PCB assembly.
Key Inductor S-Parameter Metrics
S11: reflection at one terminal → converts to Z for R and X extraction S21: transmission through a series-connected inductor in test fixture From S11 → Z (using S11→Z calculator): Z = R + jX = ESR + j(ωL - 1/(ωCp)) [full model with parasitic Cp] Q factor = |X| / R = ωL / ESR [at frequencies below SRF] Self-resonant frequency: |X| = 0 → ωSRF = 1/√(L·Cp) At SRF: Z = ESR (minimum impedance for parallel resonance)
Example: Murata LQP15MN3N3B00 (3.3 nH, 0402)
| Frequency | |Z| (Ω) | ESR (Ω) | Q | Behavior |
|---|---|---|---|---|
| 500 MHz | 10.4 | 0.15 | 69 | Inductive |
| 900 MHz | 18.7 | 0.32 | 58 | Inductive |
| 2.4 GHz | 52.0 | 1.6 | 32 | Inductive |
| 4.8 GHz | Max | ≈SRF | ≈1 | Near SRF |
| 6 GHz | Decreasing | — | — | Capacitive |
Analysis Procedure in RF View
- Download inductor .s2p from SimSurfing.com
- Load into RF View → S11 view → switch to Smith chart
- Observe trace: starts inductive (upper half) → moves through open circuit point → becomes capacitive at SRF
- Place marker where trace crosses the real axis (top of Smith chart) → this is the SRF
- Use S11→Z calculator at operating frequency → read R and X → compute Q = |X|/R
RF View Inductor Analysis: Load inductor .s2p from manufacturer's website. Smith chart view immediately shows inductive region, SRF location, and capacitive behavior above SRF. Essential for selecting the right inductor for your operating frequency. Free on Android.