A BPSK Costas-type PLL circuit in which the circuit is positively prevented from falsely locking on to the input signal and in which the need for a complex lock detector is eliminated. The output levels of both the 0.degree. and 90.degree. sub-loops of the circuit are detected and a difference formed between the detected outputs. When the difference exceeds a predetermined reference level, it is determined that true locking has occurred, whereupon a sweep generator, used in achieving initial lock, is decoupled from the PLL circuit.