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Design and implementation of a millimeter-wave dual-band frequency synthesizer, operating in the 24 GHz and 77 GHz bands, are presented. All circuits except the voltage controlled oscillators are shared between the two bands. A multi-functional injection-locked circuit is used after the oscillators to simplify the reconfiguration of the division ratio inside the phase-locked loop. The 1 mm times 0.8 mm synthesizer chip is fabricated in a 0.18 mum silicon-germanium BiCMOS technology, featuring 0.15 mum emitter-width heterojunction bipolar transistors. Measurements of the prototype demonstrate a locking range of 23.8-26.95 GHz/75.67-78.5 GHz in the 24/77 GHz modes, with a low power consumption of 50/75 mW from a 2.5 V supply. The closed-loop phase noise at 1 MHz offset from the carrier is less than -100 dBc/Hz in both bands. The frequency synthesizer is suitable for integration in direct-conversion transceivers for K/W-band automotive radars and heterodyne receivers for 94 GHz imaging applications.