This paper presents a wideband and low phase noise voltage controlled oscillator (VCO). The proposed tri-core tri-mode VCO was designed and fabricated in a 65nm CMOS process. The sixth-order resonant network of the tri-mode inductor is utilized to broaden the VCO operating frequency band; VCO output amplitude control is achieved through automatic amplitude control (AAC) to ensure good phase noise characteristics of the VCO; The frequency range has been further expanded through variable capacitors and 3-bit switched capacitor arrays; Phase noise performance is improved through Class-C operation. The proposed VCO can be tuned from 3.75GHz to 12.9GHz with an 109.9% frequency tuning range (FTR). The output amplitude is stabilized at 1.27V throughout the band with an error of no more than 2%, the phase noise at 1MHz frequency offset is less than -110.7dBc/Hz, the power consumption is less than 8.8mA, and the bandwidth-integrated optimum value FoMT is better than 200dBc/Hz.