Noise, spur characteristics and in-lock error reduction of DLL-based frequency synthesizers

Noise transfer characteristics of a delay-locked loop and the spur characteristics of a DLL-based frequency synthesizer are analyzed in this paper. Analysis shows that the DLL itself has no ability to suppress the external noise and the in-lock error greatly affects the output spur of a DLL-based frequency synthesizer. The technique for in-lock error reduction, which employs a highly symmetric charge pump phase comparator, is thus introduced and compared with a conventional one. This technique is verified by the measurement results.