The FC1500-250 and the FC8004 Optical Frequency Synthesizers require a radio frequency reference, usually a Cs clock, a GPS disciplined quartz oscillator or a rubidium clock. To complete the Comb product line, we offer a GPS disciplined quartz oscillator, if such a reference is not yet available in the labs. The choice of the frequency reference is critical, since the accuracy and stability that can be achieved by a frequency comb system is currently only limited by the quality of the frequency reference used as the input.