A study of the theoretical calculations of noise in a CATV system and the methods to measure the noises is presented. This paper is focused on the effect of phase noise on the signal-to-noise ratio (S/N). It is well known that excessive phase noise will degrade the S/N but lack of understanding of the exact relationship between phase noise and S/N causes unneccessary confusion about the importance of phase noise. It is the purpose of this paper to clarify the significance of phase noise and provide a detailed calculation and measurement method for phase noise and its conversion to S/N.