噪声图是RF接收器设计中的一个重要参数,它测量一个接收器给信号添加了多少噪声. 了解现实世界的例子有助于澄清噪声数计算如何应用在实际情景中.

例1:计算总噪声图

将RF接收器视为具有三个阶段. 第一阶段的增益为20 dB,噪音数字为2 dB. 第二阶段的增益为15 dB,噪音数字为3 dB. 第三阶段的增益为10 dB,噪音数字为4 dB. 要想找到总体噪声数字,请使用Friis的公式:

NF total = NF1 + (NF2-1)/G1 + (NF3-1)/(G1*G2) ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇

将收益转换为线性尺度 :

G1=10^(20/10)=100 (中文(简体) ).

G2=10^(15/10)=31.62 互联网档案馆的存檔,存档日期2013-09-02.

NF1=10^(2/10)=1.58 单曲

NF2=10^(3/10)=2.00 = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

NF3=10^(4/10)=2.51 单曲

计算总噪声数字 :

NF total = 1.58 + (2.00-1)/100 + (2.51-1)/(100 * 31.62) + 1.58 + 0.01 + 0.0005 + 1.5905

正在转换为 dB :

NF total dB=10 * log10(1.5905) ⁇ 1.01 dB

例2:单倍放大器的噪声图

RF放大器的增益为30 dB,噪声数字为1.5 dB. 为确定其线性噪声数字:

NF=10^(1.5/10) 1.41

这表明放大器在输入信号中增加了大约0.5 dB的噪声.

例3:低噪声放大器的影响

在接收器系统中,前端使用低噪声放大器(LNA),噪声数字为0.8 dB,增益为40 dB,后续阶段的噪声数字较高,LNA显著降低了系统的整体噪声数字.

计算 LNA 的线性噪声图 :

NF LNA=10^(0.8/10) 1.20 国 家

由于收益率高,LNA压制了以后各阶段的噪音贡献,提高了整体系统性能.