Complete LNA S-Parameter Checklist
| Parameter | Typical LNA Spec | Measurement |
|---|---|---|
| Gain S21 at f₀ | +14 to +22 dB | Single marker at operating frequency |
| Gain flatness S21 | ±0.5 dB across band | Delta marker: max − min S21 in band |
| Input match S11 | −8 to −15 dB | BW Marker or single marker |
| Output match S22 | <−10 dB | Single marker at f₀ |
| Isolation S12 | <−20 dB | Single marker at f₀ |
| Stability K | K>1 ALL frequencies | Check S12 level relative to 1/|S21| |
Step-by-Step Analysis
- Load LNA .s2p → S21 dB view → Single marker at design frequency → read gain
- Add delta marker → sweep across operating band → read gain flatness ΔdB
- Switch to S11 → BW Marker → reads S11 bandwidth and return loss
- Switch to S22 → single marker → read output match
- Switch to S12 → verify <−20 dB (reverse isolation)
- Quick stability check: if S12 >−15 dB AND S21 >+20 dB → compute K-factor formally
Finding the Noise-Optimal Match Point
S11 of a noise-matched LNA ≠ −20 dB Expected: S11 = −6 to −12 dB (optimum VSWR ~2–4:1) Smith chart: S11 at f₀ is typically not at chart center → This is normal for noise-optimized LNA design → To achieve −20 dB S11: use conjugate match (loses 0.5–1 dB NF)
RF View LNA Analysis: Load manufacturer LNA .s2p. View all 4 S-parameters with one tap each. Auto Match synthesizes input network for 50Ω system integration. Free on Android.