Phase-Lock Loop Frequency Acquisition Study

The ability of a phase-lock loop using a proportional plus integral control filter to acquire a noisy signal when the local oscillator is being swept was determined empirically by means of a low-frequency, GEESE¹ model of such a system. The effects of the damping factor and natural frequency on the frequency acquisition properties of linear loops (as distinguished from a loop in which the IF signal is limited) are considered in this study. In addition, consideration is given to a loop in which the IF signal is ``hard´´ limited and the loop designed to maximize the sweep rate under the constraint that the probability of acquisition is equal to or greater than 90 per cent for a given SNR. The rms phase jitter in the output signal was measured as a method of verifying the standard analytic approach to predicting phase jitter. The results of the study are as follows: 1) The range of damping factors from 0.5 to 0.85 yields near optimum acquisition performance. 2) Although a drop in loop gain produces lower phase jitter for a given (S/N)IF, it degrades the over-all ability of the loop to acquire and track a signal. 3) A ``hard´´ limiter in the IF can be effectively used as a gain control to enhance loop performance. 4) Using an empirical formula derived from experimental results, the VCO sweep rate for 90 per cent probability of acquisition can be predicted accurately, given the (SNR)IF and loop parameters.