链路预算计算器
计算无线通信系统的链路预算和信号余量。
Cascaded Noise Figure
1.80 dB
Cascaded Noise Figure vs Stage 1 Noise Figure
公式
## Friis Noise Figure Formula ### Formula **F_total = F1 + (F2 - 1)/G1 + (F3 - 1)/(G1 x G2) + ...** All values in linear (not dB). The first stage dominates overall noise figure. Use a low-noise amplifier (LNA) first for best sensitivity.
计算示例
LNA (1.5 dB NF, 20 dB gain) followed by mixer (10 dB NF, 15 dB gain) and IF amp (15 dB NF).
- 01F1 = 10^(1.5/10) = 1.413
- 02F2 = 10^(10/10) = 10, G1 = 10^(20/10) = 100
- 03F3 = 10^(15/10) = 31.62, G2 = 10^(15/10) = 31.62
- 04F_total = 1.413 + (10-1)/100 + (31.62-1)/(100x31.62) = 1.413 + 0.09 + 0.0097 = 1.513
- 05NF = 10 x log10(1.513) = 1.80 dB
常见问题
Why is the first stage so important?
Noise from later stages is divided by the gain of preceding stages. High first-stage gain makes subsequent noise negligible.
What is a good LNA noise figure?
Below 1 dB for satellite receivers. 1-3 dB for typical RF systems. Above 3 dB is considered noisy.
Loss before the LNA?
Any loss (cable, filter) before the LNA adds directly to the noise figure. A 3 dB cable loss adds 3 dB to system NF.
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