When Narrowband Matching Is Appropriate
- GPS L1 at 1575 MHz (only need 2–10 MHz bandwidth)
- Crystal oscillator load matching (Q=1000+, very narrow)
- Single-band narrowband filter port matching
- Specific IoT channels (433 MHz, narrow 200 kHz band)
Design Approach: High-Q L-Network
Higher Q = narrower bandwidth = higher peak efficiency
For a narrowband match, use the maximum Q L-network:
Q = √(Z_high/Z_low − 1) [not limited by bandwidth constraint]
Example: GPS LNA input matching at 1575 MHz
LNA input impedance: Z_in = 25 − j20 Ω
System: Z₀ = 50 Ω
Step 1: Cancel reactive part (+j20 → series C)
X_C = 20 Ω → C = 1/(2π×1575MHz×20) = 5.05 pF → pick 4.7 pF
Step 2: Match 25 Ω → 50 Ω with L-network
Q = √(50/25 − 1) = 1.0 (minimum Q for 2:1 ratio)
Shunt C = 50/Q/Z₀ × ... = 50/(1×1575MHz×50×2π) = 1.01 nH (series L)
Actually: X_shunt = Z_source/Q = 50/1 = 50Ω → C=2.02pF
X_series = Q × R_load = 1 × 25 = 25Ω → L=2.52nH
3 dB BW ≈ f₀/Q = 1575/1 = 1575 MHz (very wide despite "narrowband" design)
−10 dB S11 BW ≈ 400 MHz (more than enough for GPS L1 10 MHz channel)
For True Narrowband (GPS Interference Rejection)
If you want to also reject cellular signals at 1700–2200 MHz: Add a dedicated GPS SAW filter BEFORE the LNA SAW filter: 1575.42 ± 10 MHz passband, >35 dB rejection outside Do NOT try to make the matching network reject cellular — use the filter
In RF View
- Load GPS LNA .s2p → S11 Smith chart → read impedance at 1575 MHz
- Auto Match → set frequency 1575 MHz → synthesize L-network
- Verify: S11 < −15 dB at 1575 MHz, −10 dB BW covers GPS L1 ± 5 MHz
- Real Match → Murata 0402 C0G + LQP components for production
RF View Narrowband Match: Set target frequency precisely in Auto Match. For GPS, use 1575.42 MHz. RF View computes optimal L-network. Real Match substitutes Murata catalog components. Free on Android.