Sideband false-lock performance of squaring, fourth-power, and quadriphase Costas loops for NRZ data signals

This paper derives the sideband false-lock margins of the short squaring, short fourth-power, long fourth-power, and quadriphase Costas loops for NRZ synchronous data streams. The false-lock margins in the short fourth-power and quadriphase Costas loops are found to be 5-10 dB worse than those in the short squaring and biphase Costas loops. The extremely low false-lock margins of 3 and 4 dB in the short fourth-power loop at typical design values of the prefourth-power bandpass filter bandwidth were verified by experimental results. Also described are several experimentally tested methods for false-lock prevention.