In this paper, a radio-over-fiber (RoF) communication system at 60 GHz based on passively mode-locked laser diode (PMLLD) is presented and the performance of different electrical down-conversion methods (coherent and non-coherent down-conversion) for this communication system is studied. Impact of PMLLD phase noise on error vector magnitude (EVM) is also observed and a complete theory and experimental survey on using non-coherent down-conversion (envelope detector) to reduce error of system caused by phase instability of PMLLD is provided. The performance of envelope detector to down-convert the signal with OOK modulation in this communication system is observed using eye diagram and quality factor (Q factor) measurement.