Analysis and design of injection-locked LC dividers for quadrature generation

Injection-locked LC dividers for low-power quadrature generation are discussed in this paper. Modeling the circuits as regenerative frequency dividers leads to very simple analytical expressions for the locking band, phase deviation from quadrature and phase noise. Maximizing the ratio between the injected and the biasing current is beneficial to all the above parameters whereas reducing the tank quality factor improves locking band and quadrature accuracy, though at the expense of current consumption, for given output amplitude. To validate the theory, experiments have been carried on a 0.18-/spl mu/m CMOS direct conversion IC, embedding an injection-locked quadrature generator, realized for the Universal Mobile Telecommunication System. Frequency locking range as large as 24% and phase deviation from quadrature around 0.8/spl deg/ are measured while each divider consumes 2 mA. The phase noise of the quadrature generator is determined by the driving oscillator phase noise because the dividers contribution is easily made negligible up to hundreds of megahertz offset.

I. Introduction

Highly integrated transceivers for wireless applications have drawn attention, in the recent past, because they lead to potential cost savings. The best suited architectures are direct conversion and low-IF. On the other hand, a power consumption penalty is paid versus traditional multichip superheterodyne solutions. The used technology and the adopted circuits solutions play key roles in maintaining consumption within acceptable levels. Among receiver building blocks, local oscillator (LO) generation circuits deserve particular attention, when CMOS technology is adopted. In fact, besides quadrature accuracy and low phase noise, they are required to drive the mixer LO devices capacitance, typically large (ranging between hundreds of femtofarads to 1 pF) for minimum noise contribution. Fig.1. Receiver front-end block diagram showing where injection-locked LC dividers fit in the overall system.