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This paper presents a low-power frequency shift keying (FSK) transmitter using a direct voltage-controlled oscillator (VCO) modulation scheme integrated in a 0.13-μm CMOS technology. The carrier frequency is set with a finite-modulo fractional-N phase-locked loop (PLL) in order to achieve a low start-up time of 5.5 μs. The LC-based VCO is directly modulated using simple pMOS transistors to change the tank capacitance. Measurements verify that the carrier frequency and the frequency deviation meet the requirements of the Bluetooth low energy standard up to 85°C even considering supply voltage variations of 1.0 V ± 10%. The FSK synthesizer only consumes 600 μW and drives a power amplifier (PA) with a differential class-E output stage. An integrated step-up transformer is used to up-convert the antenna impedance to a load impedance of 840 Ω seen by the PA. The overall transmitter consumes 8.9 mA and delivers +1.9 dBm to the antenna, which means a total power efficiency of 17.4%.