In previous columns (available from the online archives) I introduced a structure suitable for implementing a phase-locked loop in software - the Costas loop. Those articles described the loop's structure and operation, but I've postponed a discussion of how to "set the loop" until now.

The Costas loop implementation consists of a feedback structure where a derived error term forces the loop into a lock condition. As in any feedback arrangement, applying an incorrect amount can cause the loop either to not adapt (too little feedback) or blow up (too much feedback). In this installment I'll begin to investigate procedures for properly choosing the feedback coefficients for efficient operation.

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