Cascaded RF Receive Chain
An RF receive chain consists of several stages between the antenna and the baseband processor. Each stage contributes gain (or loss) and adds noise. The total system performance is determined by the cascade of all stages.
Example 4G LTE Receive Chain (Band 3, 1.8 GHz)
| Stage | Component | Gain (dB) | NF (dB) |
|---|---|---|---|
| 1 | Antenna switch | −0.5 | 0.5 |
| 2 | SAW pre-filter | −2.0 | 2.0 |
| 3 | LNA (SiGe) | +18.0 | 1.5 |
| 4 | Cable/trace | −1.0 | 1.0 |
| 5 | Mixer | −8.0 | 8.0 |
Cascade Gain and NF Calculation
Total gain = −0.5 − 2.0 + 18.0 − 1.0 − 8.0 = +6.5 dB
Friis NF cascade:
F₁ = 10^(0.5/10) = 1.122 G₁ = 10^(−0.5/10) = 0.891
F₂ = 10^(2.0/10) = 1.585 G₂ = 10^(−2.0/10) = 0.631
F₃ = 10^(1.5/10) = 1.413 G₃ = 10^(+18/10) = 63.1
F₄ = 10^(1.0/10) = 1.259 ...
F₅ = 10^(8.0/10) = 6.31
F_total = F₁ + (F₂−1)/G₁ + (F₃−1)/(G₁G₂) + (F₄−1)/(G₁G₂G₃) + …
= 1.122 + 0.585/0.891 + 0.413/(0.891×0.631) + …
≈ 1.122 + 0.657 + 0.734 + small contributions
≈ 2.53 → NF_sys = 10·log10(2.53) = 4.03 dB
Sensitivity at 10 MHz BW, SNR_min = −1 dB:
= −174 + 4.03 + 70 + (−1) = −100.97 ≈ −101 dBm
Optimization Strategy
- Moving the filter from Stage 2 to after the LNA (Stage 4): NF drops by ~2 dB (filter no longer before LNA)
- Using a lower-NF LNA (NF=0.8 dB vs 1.5 dB): system NF improves by ~0.7 dB
- Higher LNA gain (+22 dB): suppresses mixer NF contribution further
RF View NF Cascade Calculator: Enter NF and gain for up to 4 stages in the Utilities tab to compute system NF and receiver sensitivity. Available offline, free on Android.