Thesecondpartdealswithampli?cationproblemsandtherecovery ofrepetitivesignalsburiedinnoise. Thelastpartisdevotedtosolvingthe problemsconnectedwithreachingtheultimatedetectionlimitorquantum limit. Thisisdoneforheterodynedetectionandphotoncounting. Although VIII Preface heterodynedetectionyieldstheultimatesensitivity,itsspatialmodesel- tivityand,ingeneral,thelowspectralpowerdensityofthesignalrequire sophisticatedprovisions. Thisisdiscussedindetail. Theinherentproblemsare analyzedandappropriatetechnicalsolutionsaredescribedtoreachtheul- matesensitivityfordetectingincoherentradiationandcommunicationsignals thatarerandomlyDopplershifted. Theresultsareillustratedwithexamples ofspacecommunication. Hengelo(O),January2006 W. J. Witteman Contents 1 RandomFluctuations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1. 1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1. 2 ThermalNoiseofResistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1. 3 ShotNoise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. 3. 1 SpectralDistribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1. 3. 2 Photons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1. 4 FlickerNoise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1. 5 Generation¿RecombinationNoise. . . . . . . . . . . . . . . . . . . . . . . . . . 10 1. 6 ThermalRadiationandItsFluctuations. . . . . . . . . . . . . . . . . . . . 13 1. 7 TemperatureFluctuationsofSmallBodies. . . . . . . . . . . . . . . . . . 18 1. 7. 1 AbsorptionandEmissionFluctuations. . . . . . . . . . . . . . . 20 2 Signal¿NoiseRelations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2. 1 SignalLimitation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2. 2 Backgrou