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This paper presents a dual-mode voltage-controlled oscillator (DMVCO) and a DMVCO-based wideband frequency synthesizer for software-defined radio applications. The DMVCO allows the synthesizer to leverage single-sideband (SSB) mixing, a power efficient approach, for high-frequency local oscillator (LO) signal generation, without the need of poly-phase filter or quadrature voltage-controlled oscillator (QVCO). When compared to the QVCO approach, the DMVCO solution allows the synthesizer to provide continuous LO signals without frequency gaps. The synthesizer is implemented in a 0.13-μm CMOS technology, occupying an active area of 2.2 mm2 and consuming 34-77 mW of power. It provides in-phase and quadrature-phase LO signals over the frequency bands of 0.4-3- and 5-6 GHz and differential LO signals from 0.4 to 6 GHz, supporting major wireless standards including DVB-T, GSM, WCDMA, TD-SCDMA, WLAN802.11 a/b/g, and Bluetooth. The measured phase noises are -135 and -124 dBc/Hz at 3-MHz offset under 1.8- and 5.15-GHz carriers, respectively. The measured spurious tones are less than - 42 dBc at the SSB mixer output.