# 噪声系数测量的三种方法

## 噪声指数和噪声系数

NF = 10 * log10 (F)

## 定义

 Category MAXIM Products Noise Figure* Applications Operating Frequency System Gain LNA MAX2640 0.9dB Cellular, ISM 400MHz ~ 1500MHz 15.1dB LNA MAX2645 HG: 2.3dB WLL 3.4GHz ~ 3.8GHz HG: 14.4dB LG: 15.5dB WLL 3.4GHz ~ 3.8GHz LG: -9.7dB Mixer MAX2684 13.6dB LMDS, WLL 3.4GHz ~ 3.8GHz 1dB Mixer MAX9982 12dB Cellular, GSM 825MHz ~ 915MHz 2.0dB Receiver System MAX2700 3.5dB ~ 19dB PCS, WLL 1.8GHz ~ 2.5GHz < 80dB

* HG = 高增益模式，LG = 低增益模式

## 增益法

PNA = kTΔF,

T = 温度，单位为开尔文
ΔF = 噪声带宽(Hz)

NF = PNOUT - (-174dBm/Hz + 20 * log10(BW) + 增益)

NF = PNOUTD + 174dBm/Hz - 增益

-90dBm/Hz + 174dBm/Hz - 80dB = 4.0dB

## Y因数法

Y因数法是另外一种常用的测量噪声系数的方法。为了使用Y因数法，需要ENR (冗余噪声比) 源。这和前面噪声系数测试仪部分提到的噪声源是同一个东西。装置图见图3

ENR头通常需要高电压的DC电源。比如HP346A/B噪声源需要28VDC。这些ENR头能够工作在非常宽的频段(例如HP346A/B为10MHz至18GHz)，在特定的频率上本身具有标准的噪声系数参数。下表给出具体的数值。在标识之间的频率上的噪声系数可通过外推法得到。

 HP346A HP346B Frequency (Hz) NF (dB) NF (dB) 1G 5.39 15.05 2G 5.28 15.01 3G 5.11 14.86 4G 5.07 14.82 5G 5.07 14.81

ENR噪声头提供两个噪声温度的噪声源： 热温度时T = TH (直流电压加电时)和冷温度T = 290°K。ENR噪声头的定义为：

Y = G(Th + Tn)/G(290 + Tn) = (Th/290 + Tn/290)/(1 + Tn/290

## 总结

 Suitable Applications Advantage Disadvantage Noise Figure Meter Super low NF Convenient, very accurate when measuring super low (0-2dB) NF. Expensive equipment, frequency range limited Gain Method Very high Gain or very high NF Easy setup, very accurate at measuring very high NF, suitable for any frequency range Limited by Spectrum Analyzer noise floor. Can't deal with systems with low gain and low NF. Y Factor Method Wide range of NF Can measure wide range of NF at any frequency regardless of gain When measuring Very high NF, error could be large.