Comparative Study of CMOS Voltage Controlled Oscillators

Abstract

Oscillators are integral part of many electronic systems. An oscillator is an electronic device used for the purpose of generating a signal. Applications range from clock generation in microprocessors to carrier synthesis in cellular telephones, requiring vastly different oscillators topologies and performance parameters. Robust, high performance oscillator design in CMOS technology continues to pose interesting challenges. This paper deals with the analysis and design of CMOS oscillators more specifically voltage controlled oscillators (VCOs). A VCO is an oscillator, where the control voltage controls the oscillator output frequency. VCOs are perhaps the most ubiquitous element in all communication systems, wired or wireless. In a wireless system the quality of the communication link is determined in large part by the characteristics of the VCO and in today’s wireless communication systems greater frequency range is required by the VCOs. Traditionally, relaxation oscillators using CMOS technology have been used for low frequency applications, but submicron processes have allowed CMOS oscillators to achieve frequencies in the gigahertz range. This range is made possible with the use of automatic swing control. The available transistors during the 1960's were in the 2 GHz range; today in our designs we use Bipolar transistors and FETS at frequency to 18 GHz. VCO can be built using many circuit techniques. Primarily, there are two methods of designing CMOS VCOs, one uses a ring oscillator and other uses a schmitt trigger. Though there are so many design requirements of a VCO, which are phase stability, large electrical tuning range, linearity of frequency verses control voltage, large gain factor, capability of accepting wideband modulation and low cost but the most important factor in designing the VCO is the linearity, on the basis of which the comparison between CMOS VCOs is described. With respect to digital phones that use these circuits, low power consumption, small size and low fabrication costs are important design factors. In order to achieve a higher quality factor, CMOS relaxation oscillators have been designed in the form of ring oscillators. Apart from communication systems, VCOs are an integral part of Biomedical Systems, especially in Pacemakers. In pacemaker, a low frequency VCO is required to control the frequency of the pulses coming from the heart, so as to obtain the desired frequency. Efforts have been made to achieve the frequency as low as possible, so that the VCO can be utilized for the pacemaker application.