The noise in photonic true time delay systems based on broadband optical source and dispersion components is investigated. It is found that the beat noise induced by the optical source begins to dominate and grows far larger than other noise terms quickly, as long as the detected optical power is above some certain value $P_{\mathrm{thr}}$ . When the system dispersion is nonzero, the output carrier-to-noise ratio (CNR) will change periodically with the optical bandwidth due to the noise power increment and the dispersion induced radio frequency signal power degradation. The maximum CNR is the peak value of the first period. For a set of specified system conditions, the $P_{\mathrm{thr}}$ is calculated to be $-21\mathrm{dBm}$ , and the optimal optical bandwidth is $0.8\mathrm{nm}$ , at which the maximum CNR is $93.3\mathrm{dB}$ by considering the noise in a $1\mathrm{Hz}$ bandwidth. The results are verified experimentally.