Key LNA Metrics from S-Parameters
| Metric | S-Parameter | Typical LNA |
|---|---|---|
| Forward gain | S21 at f₀ | +14 to +20 dB |
| Gain flatness | S21 variation across band | ±0.5 dB |
| Input return loss | S11 at f₀ | −8 to −15 dB |
| Output return loss | S22 at f₀ | <−10 dB |
| Reverse isolation | S12 at f₀ | <−20 dB |
| Stability (K) | from all 4 S-params | K > 1 across all freq |
Step 1: Load LNA S2P File and View S21
- Load the LNA manufacturer's .s2p file into RF View
- Select S21 display → verify gain level (e.g., 18 dB at 1.8 GHz)
- Add BW Marker → read −3 dB bandwidth (e.g., 1.5–2.7 GHz = 1200 MHz)
- Note: gain should be flat within ±0.5 dB across operating band
Step 2: Check S11 (Input Match)
LNA S11 is often NOT minimum for maximum power match. The amplifier is typically noise-matched: Γ_s = Γ_opt (noise opt) This makes S11 = −6 to −10 dB (acceptable, not ideal) Switch to Smith chart view: see where S11 locus is at design freq If S11 is at −5 dB (VSWR = 3.6): LNA input may be noise-matched → Design input matching network to bring S11 to <−10 dB
Step 3: Verify Stability Using S12 and S21
Rule of thumb check: if |S12| << 1/|S21|, likely stable For 18 dB gain (|S21|=7.9): need |S12| << 0.13 (−17.7 dB) If S12 > −15 dB: compute K-factor rigorously K = (1 − |S11|² − |S22|² + |S11·S22 − S12·S21|²) / (2·|S12|·|S21|) Must be K > 1 at ALL frequencies, not just operating band Check 100 MHz to 10× f₀ for out-of-band instability risk
Step 4: Design Input Matching Network in RF View
- Open Matching tab → select Port 1 (input)
- Set target frequency (e.g., 1.8 GHz)
- Select "Power Match" for maximum gain, or "Noise Match" for minimum NF
- Tap Auto Match → L/C network synthesized
- Switch to Real Match for Murata component values
- Run Monte Carlo at ±5% tolerance → verify 90%+ yield
RF View for LNA: Load LNA .s2p, view S21 gain, S11 input match, S22 output match, and S12 isolation in one screen. Auto Matching synthesizes the optimal input network. Free on Android.