VCO sweep-rate limit for a phase-lock loop

Phase-lock loops (PLLs) serve important roles in phase-lock receivers, coherent transponders, and similar applications. For many of these uses, the bandwidth of the loop must be kept small to limit the detrimental influence of noise, and this requirement makes the natural PLL pull-in phenomenon too slow and/or unreliable. For each such case, the phase-lock acquisition process can be aided by the application of an external sweep voltage to the loop voltage controlled oscillators (VCOs). The goal is to have the applied sweep voltage rapidly decrease the closed-loop frequency error to a point where phase lock occurs quickly. For a second-order loop containing a perfect integrator loop filter, there is a maximum VCO sweep-rate magnitude, denoted here as Rm rad/s2, for which phase lock is guaranteed. If the applied VCO sweep rate is less than Rm, the loop cannot sweep past a stable phase-lock point, and it will phase lock. On the other hand, for an applied sweep-rate magnitude that is greater than Rm, the PLL may sweep past a lock point and fail to phase lock. In the existing PLL literature, only a trial-and-error approach has been described for estimating Rm, given values of loop damping factor ζ and natural frequency ωn. Furthermore, no plots exist of computed Rm/ωn2 versus ζ and Rm/BL2 versus ζ (BL denotes loop-noise bandwidth). These deficiencies are dealt with in this paper. A new iterative numerical technique is given that converges to the maximum sweep-rate magnitude Rm. It is used to generate data for plots of Rm/ωn2 and Rm/BL2 versus ζ, the likes of which have never appeared before in the PLL literature.