A New Look at the Phase-Locked Oscillator

The uses of phase-locked oscillators are briefly reviewed. A simple automatic-phase-control (APC) system is analyzed as a servomechanism analog. Three major characteristics of the system are considered: the lock range, the capture range, and the filter bandwidth. The lock range is the total drift in the unlocked oscillator frequency which can be exactly compensated by the locked system. The capture range is the largest unlocked frequency difference at which synchronization, or lock-in, will occur. The filter bandwidth of the system expresses the performance of the system as a low-pass filter with respect to FM noise components existing in the input to the system and as a high-pass filter with respect to FM noise components generated within the output oscillator. The mutual interdependence of these characteristics and the various quantities affecting each one are discussed. The conditions for stable operation of the system are established. The unlocked, locking-in, and locked conditions of operation and the effects of the low-pass filter are discussed. Simple design criteria are established.