This letter demonstrates that, under certain condition, the harmonic content of a rectangular pulse train is reduced by a considerable extent in the presence of another equal frequency pulse train of opposite polarity. The condition for maximum harmonic rejection is derived. It is also shown that this technique can, very effectively, be applied to reduce the harmonic content of a sequential phase detector (PD) output. This letter also presents the experimental performance of a sequential PD, incorporating this technique, in a single-loop synthesizer.