ABSTRACT This paper proposes an LC-based oscillator structure which enables operation from a supply voltage as low as 0.85V, while being suitable for high-frequency RF applications. Two VCO prototypes were fabricated in a standard 0.18 μm CMOS process. The 8.7 GHz VCO operates from a supply voltage of 0.85 V, consumes 6 mW, and exhibits -100 dBc/Hz phase noise at 600 kHz offset. The 10 GHz prototype operates from a supply voltage of 1 V, consumes 9 mW, and has -98 dBc/Hz phase noise at 600 kHz offset. A tuning range of 400-450 MHz is achieved without using varactors. 1. INTRODUCTION A key and critical building block in both wireless and optical communications transceivers is the voltage controlled oscillator (VCO). The continuous increase in the operating frequencies of integrated circuits, driven by the need for wider bandwidths and higher data rates, and the quest for system-on- chip solutions, resulted in a remarkable growth of interest in fully- integrated LC-based CMOS VCO’s in recent years (e.g. [1]-[10]). Oscillating frequencies as high as 12.5 GHz have been achieved using standard digital CMOS processes, e.g. [1], [5], [10]. The structure of those VCO’s employed stacked PMOS and NMOS transistors sharing the same DC current, and therefore requiring relatively high supply voltages (2.5-3.5V). Driven by the reduction of the power consumption of digital circuits and the scaling of modern technologies, the supply voltages of integrated circuits continue to decrease towards sub-1V. New circuit architectures are needed, especially for analog signal processors, to cope with this trend [11]. The VCO topology proposed in this paper considerably reduces the supply voltage requirement, and consequently the power consumption. This is done by altering the structure of the conventional “complementary differential LC circuit” shown in Fig. 2(a) [5]-[6]. In addition to maintaining the features of the original topology (discussed in Section 2), the proposed architecture provides an alternative to overcome the limited tuning range of back-gate tuning (Section 3). Detailed circuitry and design guidelines for the proposed topology are presented in Section 4. Two VCO prototypes were implemented in a standard 0.18 μm CMOS process. They operate using 0.85 and 1-Volt power supplies, which is approximately one third the supply voltage Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. ISLPED'01, August 6-7, 2001, Huntington Beach, California, USA. Copyright 2001 ACM 1-58113-371-5/01/0008...$5.00. needed by the original topology (labelled k and l in Fig. 1). This is achieved while satisfying other requirements such as low phase noise, low power consumption (Fig. 1), and a reasonable tuning range. Measured results are reported and discussed in Section 5. 2. THE COMPLEMENTARY DIFFEREN- TIAL LC-STRUCTURE The complementary differential back-gate tuned VCO in Fig. 2(a) has been shown to allow very high frequencies of oscillation (9.8 - 12.5 GHz) [5], [10]. It uses NMOS and PMOS cross- coupled amplifiers along with a differential inductor L. The resonant tank is formed by the inductor and the parasitic capacitances of both amplifiers. Frequency tuning can be performed by controlling the PMOS transistors’ back-gate voltages. This configuration has several desirable features: 1) The differential excitation of integrated inductors yields 0 2 4 6 8 10 0.5 1 1.5 2 2.5 3 3.5 4 Frequency [GHz] Power supply [V] 0 2 4 6 8 10 5 10 15 20 25 30 35 40 Frequency [GHz] Power [mW] This work a c d e f g h i j k l b Fig. 1: Comparison of supply voltage and power consump- tion versus frequency to state-of-the-art VCO’s. a c d e f g h i j k l b This work A CMOS VCO Architecture Suitable for Sub-1 Volt High-Frequency (8.7-10 GHz) RF Applications Ahmed H. Mostafa and Mourad N. El-Gamal Microelectronics And Computer Systems Laboratory, McGill University 3480 University Street, Montreal, Quebec, Canada H3A 2A7 {ahmed,mourad}@macs.ece.mcgill.ca 237 247