 effect of adding more amplifiers upon this ratio. However it should be Signal to noise ratio is the ratio of the required signal power to the noted that this will only hold true for amplifiers of identical noise noise power, as in Equation 10.37 figures. A similar graph may be produced for other combinations of amplifiers. SIN = Required signal power Noise power Required signal power (10.37) Noise power In some systems it is useful to compare the input signal to noise with the output signal to noise ratio sits S The performance of a system may be judged in terms of the signal to noise ratio, and for the best results it must be as large as possible. The theoretical maximum data rate of a transmission medium is related to the signal to noise ratio and can be determined using a Number of amplifiers in cascade formula attributed to Shannon and Hartley. This formula, known as the Shannon-Hartley Law is given in Equation 10.38, where C is the Figure 10.17 Effect on S/N of multiple amplifier data rate, B is bandwidth in Hz, and N is the random noise power in connections watts. (10.38) C = B log (1+'N second 10.11 Noise factor 10.10.2 Effect of amplification 10.11.1 Definition of noise factor (noise figure) The effect of amplification upon the signal to noise ratio is to The noise factor, usually of an amplifier, is defined as the ratio of degrade it at the output of the amplifier as compared to its input. the signal to noise ratio at the input to the signal to noise ratio at the 10/14 Signals and noise output of the amplifier- stage. It indicates the "noisiness" of the total = Min + 10 log 2 = (10.45) amplifier, as in Equation 10.40. In a similar manner, for system with X amplifiers, the noise input noise ratio at input (10.40) Noise factor at the xth amplifier (N) will be given by Equation 10.46. Signal to noise ratio at output Nx = Nin + 10 logx (10.46) As the units on the top and bottom of this equation are the same then it will be seen that the noise factor is just a number. It is convenient to work in decibels and