Linearly frequency-tunable and low-phase noise ring VCO using varactors with optimally-spaced bias voltages

A linearly frequency-tunable and low-phase noise ring voltage-controlled oscillator (VCO) is presented. Using multiple varactors, of which the bias voltages can be optimally controlled, the voltage-to-frequency gain, K-VCO, can be kept constant over control voltages in all frequency bands. Since the noise contribution and the power consumption of the proposed frequency-tuning method are minimal, the ring VCO also can achieve low phase noise and an excellent figure-of-merit (FOM). The prototype ring VCO was fabricated in a 180-nm CMOS technology. In measurements, the variation of K-VCO over the full range of control voltages was restricted to less than +/- 12%, even in the worst case. The 1-MHz phase noise and the FOM of a 1.22-GHz signal were -108.7 dBc/Hz and 161.3 dB, respectively, when the power consumption was 8.5 mW.