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This paper presents a 28-GHz monolithic quadrature voltage-controlled oscillator (QVCO) realized in a preproduction 0.4-μm SiGe bipolar technology with 85-GHz transit frequency. QVCOs efficiently drive quadrature modulators and demodulators in receivers or transmitters. At 28.9 GHz, the circuit provides -14.7 dBm of output power and phase noise of -84.2 dBc/Hz at a 1-MHz offset. The two output signals are in quadrature with phase error of about 5°. Tuning of the QVCO may be done in the frequency range from 24.8 to 28.9 GHz with nearly constant output power. The circuit consumes 25.8 mA from the 5 V voltage supply.