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Improving Frequency Acquisition of a Costas Loop

A Costas loop for tracking a BPSK signal does not acquire when the initial frequency error is comparable to the loop bandwidth. This paper describes methods for deriving control proportional to frequency error so as to improve the frequency acquisition capability by an AFC augmentation. A composite AFC/Costas loop is realized by combining the individual loops. Pull-in from a frequency error much greater than the Costas loop bandwidth is now feasible. The increase in phase error due to the AFC capability is evaluated by a linearized analysis. If the bandwidth of the AFC portion is sufficiently narrow, the degradation is negligible. Improved frequency acquisition can also be realized by a simple modification of the Costas loop to remove the low-pass filter on the quadrature phase detector channel, with negligible degradation to phase tracking.