Design of quadricorrelators for automatic frequency control systems

The authors investigate the performance of an automatic frequency control loop operating in a receiver for linearly modulated digital signals. The loop uses a balanced quadricorrelator as a frequency detector. The design of such a detector for optimum loop tracing performance is considered. This problem involves the analysis of the disturbances generated in the quadricorrelator, which are conveniently divided into self-noise and thermal-noise-induced components. It is shown that the quadricorrelator can be designed to make self-noise negligible in the steady state, both with staggered and nonstaggered modulations. This leaves the thermal-noise-induced components as the sole obstacle to loop operation in the tracking mode. The level of such components is then minimized by further selection of the quadricorrelator filters. Numerical examples are worked out to assess the advantages of optimized quadricorrelators over more conventional schemes. It is found that tracking errors can be substantially reduced with no penalty for frequency acquisition times of the loop.>