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Abstract: The design challenges are increasing with the increase in demand for low power, high-frequency wireless systems. The reasons are poor phase noise performance of integrated VCO and large area taken by the loop filter. Many recent reporting show that with careful design, on-chip VCO can be made to give comparable phase noise performance with off-chip VCO. In this paper we have presented a design of a ring oscillator which is implemented in CMOS.
Keywords: Ring oscillator,CMOS.
1. INTRODUCTION
The VCO circuit is a fundamental part in a PLL system. It could provide a controllable and stable frequency signals for the system, so it has wide application in the electronic measurement equipments. At present, the most frequently used VCO circuit fabricated in CMOS process is LC oscillators and ring oscillators. Although LC oscillators perform well in phase noise suppression, it also have many disadvantages, such as large layout area, the difficulty of designing inductance with a high Q value in present CMOS process, and a narrow tuning range etc. On the contrary, the CMOS ring oscillators have been more widely used since it have smaller area, wide tuning range, excellent linearity, and require no external device while it could be easily designed and fabricated in CMOS process Among the CMOS ring oscillators, the differential ring oscillators are generally adopted in the application without a high requirement for the bandwidth since it have an excellent linearity and perform well in the common mode noise, the environmental noise and the power supply noise suppression while have a tiny phase noise.The VCO designed in this paper need to operate in the frequency range of 2GHz and has a high demand in phase noise suppression. So the differential ring structure is used. Firstly, this paper combines the. The VCO circuit in this paper adopts differential ring structure and the whole circuit design includes the VCO unit circuit, the bias circuit for the tail current source, the start-up circuit, and the wave shaping circuit. In addition, an improved VCO unit structure is adopted to lower the phase noise effectively and the start-up circuit is designed to ensure that the VCO could start to oscillate quickly. In...