微波测量基础CH02_6噪声系数.pdf

2.6 噪声系数 现代接收机系统通常是对小信号进行处理，但是系统元器件引入的噪声 使得小信号失真。噪声系数不仅可描述整个系统处理小信号的能力，而且还 可以表征预放、混频器、IF放大器等元器件处理小信号的能力。通过控制每 个器件的噪声系数和增益，设计人员可以直接控制整个系统的噪声系数。低 噪声系数可以改善模拟接收机的信噪比，减小数字接收机的误码率。低噪声 系数的接收机系统可允许使用更小的天线和更小的发射机功率，达到相同的 系统性能，因此噪声系数的测量非常重要。 噪声系数的定义 V 2 4KTRΔf 噪声系数的定义 Given that for microwave frequencies, derive the following Rayleigh-Jeans Given that for microwave frequencies, derive the following Rayleigh-Jeans approximation: approximation: 2 v2 = 4kTBR v = 4kTBR Show that at room temperature (T = 290K), thermal noise power supplied Show that at room temperature (T = 290K), thermal noise power supplied by resistor R to a matched load is KTB or – 174 dBm/Hz. by resistor R to a matched load is KTB or – 174 dBm/Hz. Noisy R resistor Matched R load v = (4kTBR)1/2 Signal-to-Noise Ratio (SNR) Signal-to-Noise Ratio (SNR) SNR is the ratio of signal power to noise power. SNR is the ratio of signal power to noise power. S /N G S /N i i o o Input signal: low peak power, Output signal: high peak power, good SNR poor SNR ) m G B So/No d ( r e w o S /N P i i Noise floor Frequency (Hz) 对噪声系数物理意义的三种理解 噪声系数计算实例 级联网络的噪声