| In this study, we characterized the instability of an RF clock signal caused by free-space transmission of a frequency comb under typical lab conditions. The phase noise spectra showed the involvement of multiple random processes. For a 10m transmission, the rms timing jitter integrated over 1–10 5 Hz was 95 fs, and the root Allan variance was 4 x 10-13 with 1 s averaging. The measured Allan variance shows a τ-1 dependence to the averaging time as well as excellent agreement with the phase noise measurement. These results indicate that comb-based, free-space RF clock distribution over short distances is feasible under normal indoor conditions for transmission distances at least in the order of tens of meters. Further research, in this area should focus on testing comb-based, free-space RF clock distribution in outdoor, uncontrolled environments as well as studying the effect of longitudinal air flow and transverse flow. |
