ds. (10.47) 10.11.3 Noise factor and noise temperature These measures are used to indicate the amount of noise generated in a network. In both cases the higher the value the more noise is (10.48) generated in the network. 2kT - 20 The relationship between noise factor and noise temperature are given by Equations 10.43 and 10.44, where F is the noise factor and F = 201 R (10.49) T is the noise temperature in degrees Kelvin. T==(F-1)290 (10.43) 10.12 Noise waveforms (10.44) 10.12.1 Mathematical model Thermal noise is a combination of a very large number of random Note that for low noise systems. where F is small, then it is more events and satisfies the conditions of gaussian distribution, as in convenient to use T for calculations which is larger in both range Figures 10.19 and 10.20, and Equation 10.50. and magnitude. 10.11.4 Amplifier input noise in cascaded amplifiers The concept of equivalent input noise of an amplifier may be used that the noise level at the input of the second amplifier is given by (10.50) to calculate the noise level in cascaded amplifiers. It may be shown Equation 10.45. Bibliography 10/15 the midband frequency, modulated in amplitude by low frequency wave whose highest frequency is dependent on the bandwidth of the noise. (See Figures 10.21 and 10.22.) 0.5/0 Figure 10.19 Gaussian density function Figure 10.21 Rayleigh density function Figure 10.20 Gaussian distribution function 10.12.2 Band limited white Gaussian noise In most practical applications where white noise is produced it will Figure 10.22 Rayleigh distribution function be passed through a band limiting filter. Filtering uncorrelated white noise produces correlated band limited white noise or coloured noise. The autocorrelation function is given by Equation 10.51. 10.13 Bibliography sin ( 2 IT BT) (10.51) RT T = N B o 2 r BTT Bell (1979) Transmission Systems for Communications, Bell Tele- phone Laboratories. Freeman, R.L. (1989) Telecommunication System Engineering, 10.12.3 Narrowband Gaussian noise John Wiley. Lee, E.A. et. al.