A ring VCO with wide and linear tuning characteristics for a cognitive radio system

This paper presents a novel voltage-controlled ring oscillator fabricated in TSMC 0.18-mum CMOS technology. In order to obtain a linear frequency-voltage characteristic over a wide tuning range while maintaining good phase noise performance, a transmission gate was adopted in a saturated-type ring oscillator. The resistance tuning capability of the transmission gate was then theoretically and experimentally analyzed. The proposed ring oscillator achieved a wide operating frequency range from 20 MHz to 807 MHz covering TV channel bands for the cognitive radio spectrum sensing system. The measured phase noise is -108 dBc/Hz at 1-MHz offset from 630 MHz. The power consumption is 22 mW, and core chip area is 60 mum times 51 mum.