The effect of additive noise on the signal phase noise in radio over fiber links was studied and the theory model of the output signal phase noise was proposed. The theory model was verified by experiments. The results suggest that the signal phase-noise performance decreases due to enhancing the average photocurrent of the photodiode. In practical applications, in order to reduce the deterioration of reference's phase noise, it is necessary to improve the average photocurrent when the opto-electro modulator works at quadr...