A CMOS based low power digitally controlled oscillator design with MOS varactor

Abstract

With the shift from traditional analog circuit designs to an all-digital intensive approach, the all-digital Phase-locked loops (ADPLLs) have become more attractive in digital communication systems. Digitally controlled oscillators (DCO) are the key components of the ADPLL circuits. In this paper, a new low power DCO structure is proposed with NMOS transistor as the switching network and utilizing the NMOS varactor as shunt-capacitive loads for the delay cells. The new DCO is capable of producing much higher output frequencies and comprises of components that are fully digital. The proposed DCO structure is designed for three, five and seven stages in CMOS 0.18 µm technology. Variable capacitance is achieved by the use of control word which is applied through NMOS switches conditionally selecting combinations of capacitance and hence determining the delay of the circuit. A 3-stages digitally controlled oscillator shows output frequency variation from 1.986 to 3.526 GHz with a power consumption of 1.484 mW. In the 5-stages DCO, the output frequency varies from 1.154 to 2.210 GHz with a power consumption of 2.762 mW. For 7-stages DCO, the output oscillation frequency is in the range from 0.835 to 1.658 GHz with a power consumption of 4.04 mW. A 3-stages DCO shows a phase noise of − 100.06 dBc/Hz with the offset of 1 MHz with the corresponding figure of merit (FoM) of 165.37 dBc/Hz. Five and seven-stages DCO show phase noise of − 102.08 dBc/Hz and − 105.52 dBc/Hz at 1 MHz respectively. The figure of merit (FoM) for 5 and 7-stages is 160.92 dBc/Hz and 159.07 dBc/Hz respectively. The digital tuning range for 3, 5, and 7-stages DCO is 55.96%, 62.78%, and 66.05% respectively. Further, the results show that the designed DCO has a maximum supply voltage tuning range of 101.45% with the variation of VDD from 1 to 1.8 V. Comparison with earlier reported circuits has been made based on output frequency, power consumption, and phase noise.