Improved phase detector for electro-optical phase-locked loops

A phase detector with a reduced spurious output has been realised. Since spurious products at the output of a phase detector in an electro-optical phase-locked loop (PLL) can hardly be filtered out, this kind of a phase detector is recommended for such an application. The parallel operation of two conventional phase detectors (double-balanced mixers or fast-logic EXOR gates) is presented theoretically and then verified with a practical experiment. Maintaining a phase shift of 90° between the two phase detectors, the spurious output at twice the comparing frequency can be reduced significantly, resulting in a much cleaner output spectrum of the electro-optical PLL synthesiser.