Noise & Distortion in Microwave Systems

Noise: -> Small Signal Signal-to-Noise (S/N) Bit Error Rates (BER) Dynamic Range Minimum Detectable Signal Level Distortion:-> Large Signal (Nonlinear Systems) Mixer & Amplifier Thermal（热） Noise by Components & Devices Atmosphere and Interstellar（星际的） Radiation Man-made Interference External Environment Internal Circuit Noise : Random Process!

实际放大器的动态范围

1 Thermal Noise Random motion of charge carriers Passive circuit element contains Loss Atmospheric attenuation & Interstellar Background Radiation Shot（散粒） Noise Random motion of charge carriers in Electron tubes & solid-state devices Flicker （闪烁）Noise（1/f Noise） In Solid-state & Vacuum（真空） tube Quantum（量子） Noise In quantized nature of charge carriers & photons（光子） Plasma（等离子体） Noise In ionized（电离的） gas or sparking electrical contacts（火花放电）

Noise Voltage and Power of Resistor R R: Resistor at T: Temperature R Mean Value RMS（均方根） Value Available Noise Power: Maximum Power can be delivered from Noise Source to load Two-side Power Spectral Density of thermal noise -B +B

白噪声源用一个等效噪声温度来表征，Te表征元件或系统，B是元件或系统的带宽 理想带通滤波器 2 Noise Temperature & Noise Figure Equivalent Noise Temperature: 任意 白噪声源用一个等效噪声温度来表征，Te表征元件或系统，B是元件或系统的带宽

Noisy Amplifier Noiseless Amplifier

Direct measurement: difficult to measure small level of noise Measurement of Te: Direct measurement: difficult to measure small level of noise Y-factor Method: T1 (hot) N1 G,B,T T2 (cold) N2

例1. X波段放大器有增益20dB和1GHz带宽，其等效噪声温度通过Y因子法测量，得到：对于T1＝290K，有N1＝-62dBm；对于T2＝77K，有N2＝-64.7dBm。确定该放大器的等效噪声温度。若该放大器使用等效噪声温度Ts＝450K的源，其输出功率为多少dBm？

Noise Figure G B Te

Noise Figure of a Lossy Line :For Thermodynamic Equilibrium （热平衡方程）

Noise Figure of Cascaded（级联） Components Te1 G2 F2 Te2 G F Te

例2.对图示的无线接收机前端，计算其总的噪声系数。假定从馈送天线来的输入噪声功率是Ni＝kTAB，其中TA＝150K；求输出噪声功率。假定要求接收机输出处的最小信噪比为20dB，问能加到接收机输入处的最小信号电压应为多少?设定系统在温度T0下，其特征阻抗50欧，中频带宽为10MHz。 Ff＝1 dB

3 Noise Figure of Passive Networks For Thermodynamic Equilibrium

Application to a Mismatched Lossy Line

Application to a Wilkinson Power Divider 1. Terminate Port 3 with matched load If at room temperature If lossless & at room temperature

4 Dynamic Range & Intermodulation Distortion（交调失真） Nonlinear（引起非线性响应的原因）: At very Low Power Level due to Noise Effects. At very High Power Level due to device nonlinearities Nonlinear Device DC output Linear output Squared（平方） output

Gain Compression（增益压缩） Nonlinear Device DC output Linear output

Voltage Gain: 1 dB Compression Point: Pout(dBm) 1dB Output P1dB Pin(dBm) Pout(dBm) 1 dB Compression Point: 1dB Output P1dB 1dB Compression Point Input P1dB

Intermodulation Distortion

IP3: Third-Order Intercept（截断） Point: Where the first order and third order powers are equal Input IP3 Output IP3

Dynamic Range Input P1dB Input IP3 Output P1dB Output IP3 Linear Dynamic Range ( DRl ) SFDR: Spurious-free Dynamic Range ( DRf ) 无杂散动态范围（寄生响应最小） Pout(dBm) 1dB DRl DRf Pin(dBm)

Intercept（截断） Point of Cascaded Components G1 IP3’ G2 IP3’’

G1 IP3’ G2 IP3’’

例3.图示为一低噪声放大器和混频器。放大器有增益20dB和3阶截断点22dBm（以输出作为参考），而混频器有6dB变换损耗和3阶截断点13dBm（以输入作为参考）。求该级联网络的截断点。

5 Passive Intermodulation（无源交调 PIM） Generated by passive nonlinear effects in any Metal-to-metal Contact: Cable（电缆）, Connectors, Antenna, etc… Caused by Poor mechanical contact Oxidation（氧化） Junctions between Ferrous-based（铁基） metals Contamination（污染） of conducting surface at RF junctions Use of Nonlinear Material, such as Fiber composites（纤维复合物） or Ferromagnetic material（铁氧体材料） PIM only significant（明显） when HIGH Power Cellular Telephone BS Transmitter（蜂窝电话基站发射台） Pout=30-40 dBm , PIM < -125dBm