Tracking properties of phase-locked loops in optical communication systems

An attempt is made to investigate the tracking properties of a phase-locked loop (PLL) driven by the output of a photodetector in a direct-detection optical receiver. The objective is to determine the variance of the phase errors of the PLL as a function of circuit parameters and modulation characteristics. A theory is presented for describing and predicting the behavior of the PLL, with allowance for both pulse amplitude modulation and sinusoidal modulation. Theoretical predictions are in good agreement with experimental data obtained by computer simulation.