Noise & Distortion in Microwave Systems

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 • Thermal Noise • 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）

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

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

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

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？

G B Te • Noise Figure

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

G1 F1 Te1 G2 F2 Te2 G F Te • Noise Figure of Cascaded（级联） Components

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。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

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

Nonlinear Device 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 DC output Linear output Squared（平方）output

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

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

Intermodulation Distortion

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

1dB • 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) DRl DRf Pin(dBm)

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

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。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

Noise & Distortion in Microwave Systems