6.4. Voltage-Controlled Oscillator (VCO) 6.4. Voltage-Controlled Oscillator (VCO) Other applications of on-chip transformers include using them in voltage-controlled oscillators (VCOs) [103] because of their higher quality factor as compared to inductors of similar size [3]. In general practice, inductor-based LC tanks are used in VCOs as a resonating circuit. However, replacement of traditional LC tanks with transformer-based resonator circuits coupled with varactors revealed significant improvement in phase noise and sharper impedance response near-resonant frequency as compared to LC tanks [104]. Similarly, different design topologies for low noise VCOs incorporating transformer-based resonators extending up to mm-wave frequencies have been reported [3,103–107]. For instance, the use of on-chip transformers was exploited for developing a transformer-based VCO with low phase noise at 17 GHz [107]. A suggested design included a core VCO at 9 GHz followed by a frequency multiplier to achieve 17 GHz operation. Furthermore, a three-winding transformer design was suggested, serving as a resonator circuit and providing AC coupling between bases and collectors of VCO connected in cross-coupled fashion. In typical LC VCO designs, AC coupling is achieved through series capacitors which interact with varactors and lower the tuning range; and varactors of LC tanks are biased through high value resistors which increase noise. With the proposed transformer-based coupling, the need for AC coupling capacitors and high value bias resistors for varactors can be eliminated, and low noise VCOs with wide tuning range may be possible [107].