An UWB RF system can be used in the area of signal processing. Phase Locked Loop (PLL) is one of the important key technologies for broadband communication and signal processing. The phase-noise of output voltage of the oscillator with PLL should be suppressed for the performance. The low phase-noise characteristics as well as the broadband and high-frequency use are very important factor for the UWB application. This study reveals basic considerations for low phase-noise PLL. In order to develop a low phase-noise PLL, we newly attempt to establish a method for designing the phase comparator with high sensitivity. A phase comparator is consisted of XOR with CMOS transistor. The W (Channel Width)/L (Channel Length) ratio related to the delay of CMOS transistor in the phase comparator was designed to improve the sensitivity. As ratio of size of each stage decreases, detection sensitivity increases. Especially, the improvement effect of the W/L ratios of the MOS transistor in 2nd and 3rd stage on detection sensitivity is significant. The proposed phase comparator is built into PLL. It is demonstrated by circuit simulation that the proposed phase comparator has a possibility for improving phase-noise of the PLL.