Frequency Detectors for PLL Acquisition in Timing and Carrier Recovery

A significant problem in phase-locked loop (PLL) timing and carrier extraction is the initial acquisition. Very narrow loop bandwidths are generally required to control phase jitter, and acquisition may depend on an extremely accurate initial VCO frequency (VCXO) or sweeping. We describe two simply implemented frequency detectors which, when added to the traditional phase detector, can effect acquisition even with very small loop bandwidths and large initial frequency offsets. The first is the quadricorrelator, previously applied to timing recovery by Bellisio, while the second is new, and called a rotational frequency detector. The latter, while limited to lower frequencies and higher signal-to-noise ratios, is suitable for many applications and can be implemented with simpler circuitry.