New wideband Distributed Voltage Controlled Oscillator with a coarse-fine tuning Francesco Cannone, Gianfranco Avitabile and Nicola Lofù Department of Electrical and Electronic Engineering (DEE) — Polytechnic of Bari, Bari, Italy, Via Ezio Orabona 4, 70125 Abstract — The modern telecommunication services require increasingly wide tuning bands and improved spectral purity in order to support the growing demand for high data rate in the new generation wireless standards. The resulting VCOs have to deal with a difficult trade-off between wide tuning bandwidth and phase-noise. The paper introduces a new VCO architecture with a double tuning control, the former coarse to select the band the latter fine to precisely tune the frequency. The VCO is organized in an innovative distributed arrangement. Experimental results validating the theory are reported and discussed. Index Terms — Tuning, Voltage Controlled Oscillators. I. INTRODUCTION The modern TLC systems support multiple standards, as in the case of mobile telephony. This kind of situations implies that the frequency synthesis must cover wide bandwidths usually divided in sub-ranges, each of them being determined by the reference standards (GSM, PCN, EDGE, etc.). The wanted VCOs have to deal with a difficult trade-off between wide tuning bandwidth and phase-noise, normally requiring low K VCO s and as linear as possible for PLL use. Distributed VCOs (DVCO) have been proposed as an alternative approach to ring oscillators and L-C tank architectures in order to obtain a wide tuning range and good phase noise performances at the same time[1]. The DVCO is arranged around a distributed amplifier in a phase shift configuration, in this way the DVCO can benefit of the extremely wide bandwidth of the amplifier. Moreover the double tuning control allows to obtain low and very linear K VCO s, this features are very important for the PLL design. The literature proposes several examples of such organization, but none of those examples exploits the potentiality of such class of oscillators. The proposed topology [2] allows a very wide tuning range VCO and, at the same time, an effective fine–coarse tuning to be achieved. In this paper the section II describes the basic operation of the Distributed Voltage Controlled Oscillator. Starting from the analysis of the classical DVCO, the proposed topology is introduced and discussed. The section III deals with the design and experiments. The first prototype is described and experimental results validating the theory are reported and discussed. II. DVCO BASIC OPERATIONS A. Classical Distributed Voltage Controlled Oscillator The DVCO design [3] bases its wideband tuning range on the phase-shift topology. Assuming equal propagation properties for both transmission lines in the distributed amplifier, the oscillation frequency, f o , for this type of oscillator is expressed by o 1/ f 2 2 2 phase c v f LC nl nl n π = = = (1) where v phase is the phase velocity along the transmission lines, l is the length of the single cell, n is the number of transistors, L and C are the inductance and the capacitance per unit length and , f c , is the cut-off frequency of the loaded transmission lines in the distributed amplifier. According to (1) there are two approaches for achieving the frequency tuning: 1) to change the phase velocity and/or 2) the effective length of the transmission line. Anyway the DVCO operates at an oscillation frequency , f o , related to the line cut-off frequency, f c , and to the cell count, n as expressed in (1) all over the tuning range. Moreover the number of cells used in the architecture is limited by losses in the distributed amplifier, mainly due to active devices input and output impedances, and usually does not exceed 3-4 cells [3]-[4]-[5]. Accordingly, the distributed amplifier operates at a frequency close to f c , introducing several problems: • the transmission lines’ attenuation due to impedance mismatch increases; • the characteristic impedance of transmission line strongly varies with frequency; • the noise figure increases. In addition, in classical DVCOs the tuning frequency is changed by changing the bias point of each active devices and, thus, varying either the phase velocity of the lines or their effective lengths. These tuning techniques give rise to a limited tuning range because too many parameters in the active devices are changed simultaneously when changing the resonating frequency, leading to a fast switching off of the oscillation. Proceedings of the 9th European Conference on Wireless Technology 2-9600551-5-2  2006 EuMA September 2006, Manchester UK 302