Current State of Integrated Oscillator Design

Different methods to build integrated resonators and frequency tuning elements are reviewed. Two new figures of merit are defined and used to compare voltage-controlled oscillators (VCO) reported by various authors. It is shown that bipolar and CMOS VCOs can achieve similar performances. Introduction A large number of integrated resonator-based oscillators have been reported during the last few years [1]-[19]. Different designers have tried to optimize different parameters using different circuit design techniques. These oscillators have been implemented using different process technologies (such as CMOS, BiCMOS and bipolar), operating at different frequencies (ranging from 900MHz [3][4] to 10GHz [19]). The power dissipation, tuning range and phase noise vary significantly from one design to another. It is therefore difficult to compare these designs unless a few parameterindependent figures of merit are defined to compare different oscillators. In the next two sections, various resonators and tuning methods used in these designs will be reviewed. Next two different figures of merit are defined that will be used to compare the performance of various oscillators. Resonators High-frequency resonators can be built in many different ways. Here we will focus on resonators that require no explicit external component and are based on the elements that can be built in standard IC process technologies: A. Spiral On-Chip inductors To this date, spiral on-chip inductors are the most reliable, repeatable resonator available in standard IC process technologies. Recently, a lot of attention has been paid to accurate modeling [20], improvement [22] and optimization [21][23] of this kind of inductors. On-chip spiral inductors are reasonably repeatable and robust to changes in external parameters, however, they suffer from high loss (low quality factor, Q). B. Wirebond Inductors Bond wires can be used as inductors with reasonably high Q [5][10][14]. Although they offer good quality factors, they are more vulnerable to mechanical vibrations and have questionable repeatability due to the inaccuracy of wire bonding process. At higher frequencies, shorter lengths of wirebond is used, thereby further degrading the tolerance of the total inductor. Also the varactor Q becomes the limiting factor at higher frequencies, canceling the effect of higher inductor Q. Based on these observations it seems that bond wire inductors will have very limited use in the future. C. Transmission Line Delay Elements As the frequency of oscillation increases, on-chip transmission lines can be used as delay-lines to form a distributed oscillator [24]. The delay-based oscillators have better phase sensitivities since the passive delay elements introduce very little noise for a given delay forming a low noise ring oscillator [25]. Frequency Tuning Frequency tuning is an essential feature in voltage-controlled oscillators. In discrete VCOs, frequency tuning is usually achieved by using high-Q abrupt or hyper-abrupt tuning diodes that can easily provide Cmax/Cmin ratios in excess of 4 which is enough for tuning over an octave. Unfortunately, such devices are not available in standard CMOS and bipolar process technologies and the designer is limited to a few sub-optimal options. In this section, we will review some of these tuning methods. D. Junction Diode Tuning The capacitance of a junction diode is a nonlinear function of the reverse bias given by [26]