Femtosecond laser frequency combs provide an effective and efficient way to take an ultra-stable optical frequency reference and divide the signal down into the microwave region. In order to convert optical pulses into a usable RF signal, one must use high-speed photodetection; unfortunately, excess phase noise from both technical and fundamental sources can arise in the photodetection process. In order to ultimately minimize the noise effects of the photodetector, we must first characterize some of the known sources for noise arising in these devices. In this paper, we will study two sources of excess noise in high-speed photodiodes—power-to-phase conversion and shot noise. The noise performance of each device will give us clues as to the nature of the sources, their effect on the output signal, and what design features of the photodiode minimize these noise effects.