Design of a 3/2-Order Phase-Locked Loop for Improved Laser Probe Resolution

A 3/2-order phase-locked loop (PLL) was realized in order to increase the ultimate resolution of a heterodyne laser probe. The essential features of this PLL include: large-scale static-gain tuning, phase margin of 45°, and constant transfer function Q-factor (√2). A linearized theory was developed, pointing out the advantages of the 3/2 order. The PLL developed for the laser probe can be used for many other applications when rejection of low-frequency phase fluctuations of a signal is needed.