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A new differential delay cell with a complementary current control to increase the control voltage range as well as the operation frequency is proposed for low-voltage operation. The new differential delay cell is employed in a four-stage voltage-controlled ring oscillator (VCRO). The VCRO is implemented using 0.18 mm 1P6M CMOS process and 1.8 V supply voltage. Measured results show that a wide operation frequency range from 5.36 to 3.03 GHz is achieved for the full range control voltage from 0 to 1.8 V. Measured phase noise is 2107 dBc/Hz at 1 MHz offset from the 5.22 GHz centre frequency.Introduction: The voltage-controlled oscillator (VCO) is the most crucial element in a phase-locked loop (PLL) circuit. The VCO can be built using a ring oscillator or LC resonator in CMOS process. Although the LC resonator possesses better noise performance and higher operation frequency, it has deficiencies of a larger on-chip spiral inductor and narrower tuning range. On the other hand, the ring oscillator does not require any inductance and can achieve a wide tuning range, but it has worse noise performance and lower operation frequency.The voltage-controlled ring oscillator (VCRO) has been continuously studied and enhanced because of increasing demand for low-voltage operation and high integration. The architectures of source capacitively coupled current amplifier (SC3A) [1] and multiple pass loops [2, 3] have been proposed to improve the operation frequency and phase noise. They adopt active components to control the operation frequency. However, as Fig. 1a shows, the control-voltage of a conventional VCRO cannot cover the full supply voltage range owing to the required turn-on voltage of the active components. Therefore, a reduced supply voltage will inevitably limit the linear voltage-frequency characteristic and the operation frequency of the conventional VCRO. To subdue the limitation of low-voltage operation, we propose a new complementary control technique on the delay cell of the VCRO for extending the control-voltage to cover the full supply voltage range, as the dashed line shown in Fig. 1b.

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