A low power, low phase noise, square wave LC quadrature VCO and its comprehensive analysis for ISM band

This paper presents a phase-noise reduction technique for voltage-controlled oscillators (VCOs) using a harmonic tuned (HT) LC tank. The phase-noise suppression is achieved through almost rectangular-shaped VCO oscillating signal which effectively maximizes oscillating signal slope at zero crossing points resulting in-phase-noise degradation. In addition, by shortening down converted noise power around oscillating signal second harmonic, more phase-noise suppression has been achieved. A comprehensive analysis for frequency and amplitude deviations as high as 20% for third harmonic and its effect on output phase-noise suppression has been discussed. In the followings, a comprehensive analysis on time-variant theory of phase noise where a more simplistic time-invariant approach fails to explain numerical simulation results even at the qualitative level has been exposed. General closed-form formulas are derived for the phase noise generated by LC tanks losses and MOS transistors noisy current. It is also shown that by using third harmonic tank on VCO and steering coupling and coupled section current sources by quadrature signals, total phase-noise improvement will be as high as 9 dB compared to conventional structures. Designed harmonic tuned LC Quadrature VCO has been fabricated using 0.18 um 1P6M CMOS technology operating at 1.8 V for frequency band of 2.4–2.6 GHz with achieved phase noise of −136 dBc/Hz at frequency offset of 3 MHz. Total current drawn by VCO is 3.9 mA making the power consumption as low as 7 mW with the silicon area of 500 um × 500 um. Implemented HT VCO figure-of-merit (FOM) is −186 dB making the implemented VCO superior compared to the recently published VCOs. An extensive spectreRF simulation covering a wide range of operating conditions has been used to verify the theoretical analyses.