A fast switching PLL frequency synthesizer with an on-chip passive discrete-time loop filter in 0.25-μm CMOS

A phase-locked loop (PLL) frequency synthesizer with an on-chip passive discrete-time loop filter is reported in this paper. The closed loop is robust stable, and a fast switching speed is achieved by creating a stabilization zero in the discrete-time domain. The circuit implementations and system-level analysis results of the proposed architecture are presented. Techniques and design considerations are presented to overcome several potential problems of the proposed architecture, such as finite lock-in range, translation of voltage-controlled oscillator noise into in-band phase noise, and spur degradation due to clock feedthrough of the sampling switch. A 2.4 GHz prototype frequency synthesizer for Bluetooth applications was developed in a 0.25-μm CMOS process. The measured results agree with theoretical predictions and demonstrate its high performance.