A wide-tuning-range X-band voltage-control oscillator (VCO) with built-in envelope detectors is designed and measured. The VCO is implemented using a 0.18-μm CMOS technology. The VCO core is composed of a cross-coupled pair and a parallel-LC resonator. The variable capacitance is implemented using MOS varactors. Large varactor capacitance and small transistor size are used to reduce the tuning-range degradation due to the parasitic capacitance of the cross-coupled pair and buffer amplifier. Each single-ended output of the differential buffer amplifier is connected to a Greinacher voltage doubler, which serves as an envelope detector that senses the output power of the VCO and translates it into a dc voltage. The chip area of the VCO is 1.25 × 0.6 mm2. The measured frequency tuning range and FoMT (at 1-MHz offset) are 31.7% and -192.5 dBc/Hz, respectively. The relation between the VCO output power and detector output voltage is characterized. Using the relation, the envelope detectors can be used for real-time monitoring of the power level at any location in a chip, providing built-in self-test capability.