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In this paper, a 9.1 to 11.5 GHz four-band PLL is presented. In the proposed PLL, both an improved multi-modulus (MM) divider and an adaptive four-band LC VCO are depicted. The MM divider provides division ratios of 6 to 455 depending on the division mode of the six-mode prescaler. The LC VCO generates four bands of oscillation frequencies covering 9.1-11.6 GHz, including an adaptive cross-coupled MOS array. The cross-coupled MOS array was devised to reduce the area of the capacitor array and compensate for the oscillation power. The PLL achieves phase noises of -98 dBc/Hz from 11.4 GHz and -102 dBc/Hz from 9.61 GHz, at an offset of 980 kHz. The PLL consumes 32 mA at 1.2 V. It was fabricated using 130 nm CMOS process technology.