GPS L1 Frontend Requirements
GPS L1 signals at 1575.42 MHz are extremely weak (−130 dBm received typical). The frontend LNA must have very low noise figure (<1 dB) and high gain (+15 to +20 dB) to overcome downstream losses. Out-of-band rejection is critical — cellular TX at 1710–1980 MHz must be attenuated >40 dB to protect the LNA from saturation.
GPS L1 Frontend S-Parameter Specifications
| Component | Key Spec | Typical Value |
|---|---|---|
| SAW pre-filter (L1) | IL at 1575 MHz | <1.5 dB |
| SAW pre-filter | Rejection at 1800 MHz | >35 dB |
| GPS LNA | Gain at 1575 MHz | +16 to +20 dB |
| GPS LNA | Noise Figure | <1.0 dB |
| GPS LNA | Input Return Loss (S11) | −6 to −12 dB (noise-matched) |
| GPS LNA | Output Match (S22) | <−10 dB |
GPS LNA Matching in RF View
Typical GPS LNA input impedance (noise-optimal):
Γ_opt ≠ conjugate(S11) → NF-optimized match
Load GPS LNA .s2p (e.g., MAAX-010922) → Smith chart S11 at 1575 MHz:
z = 0.5 − j0.4 (normalized) → Z = 25 − j20 Ω
Auto Match for power match (max gain):
Shunt C: 3.9 pF at 50 Ω input side
Series L: 4.7 nH at LNA input
For noise match: RF View can target Γ_opt if noise parameters available
Out-of-Band Rejection Verification
- Load GPS SAW filter .s2p → S21 view
- BW Marker at 1575 MHz → reads IL (target <1.5 dB)
- Place marker at 1800 MHz → reads rejection (target >35 dB)
- Check 800/900 MHz cellular rejection: marker at 900 MHz
- Compare Murata SAFC1575D35MT vs competitive filters on same chart
RF View GPS Analysis: Load GPS SAW filter and LNA .s2p files, verify passband IL, rejection at cellular frequencies, and design optimal input matching. Free on Android.