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This paper presents the design and the experimental measurements of a 24 GHz fully integrated fractional PLL, for ISM band, with a new low power prescaler. This circuit is implemented in a 0.25 μm SiGe:C process from STMicroelectronics (BiCMOS7RF). The PLL power dissipation is 170 mW and fulfills a 23.7 to 24.9 GHz frequency locking range, while exhibiting a phase noise of -100 dBc/Hz at 100 kHz from the carrier. The simulated PLL unity-gain bandwidth is 36 MHz, with a phase margin of 54°. The PLL uses a new latch-based prescaler (SRO) which exhibits a power dissipation of 0.68 GHz/mW.