A Wideband Low-Phase-Noise CMOS VCO

A CMOS VCO has been designed and fabricated in a commercial 0.25 m CMOS process. Using a combination of switched binary-weighted capacitors and standard varactors, this VCO achieves a 28% tuning range with a control voltage ranging from 0-2 V, while maintaining a tuning sensitivity of less than 75 MHz/V over its entire frequency range. Compact choke inductors are used in place of resistors to provide a low noise bias point to the varactors. The choke inductors achieve more than 90 nH of effective inductance while consuming a die area of only 92 x 92 m. The measured single-sided phase noise is –127 dBc/Hz at a 600 kHz offset from a 1.24 GHz carrier when the VCO core is drawing 3.6 mA from a 2 V supply. Introduction Voltage controlled oscillators (VCOs) are essential building blocks of modern communication systems. The VCO performance in terms of phase noise, tuning range, and power dissipation determines many of the basic performance characteristics of a transceiver. The current trend to utilize multi-band multi-standard receivers and also very wideband systems is driving the effort to create new VCO topologies with wide tuning range, low phase noise, and low power consumption. Whereas relaxation oscillators easily achieve very wide tuning range (i.e. 100% or more), their poor phase noise performance disqualifies them in most of today’s wireless and wireline applications. Because LC VCOs have been successful in narrowband wireless transceivers, there is a growing interest to extend their tuning range. Recently, several wideband CMOS LC VCOs have been demonstrated using a variety of techniques [1-4]. The high intrinsic Cmax/Cmin of inversionor accumulation-type MOS varactors supports a very wide tuning range and their Q is sufficiently high that good phase noise performance can be maintained. However in practice, the overall phase noise performance is also highly dependent on the tuning sensitivity of the VCO, since noise from preceding stages of the frequency synthesizer is inevitably injected onto the VCO control input. Hence, aside from achieving a high raw tuning range, practical wideband VCO solutions must properly limit the overall VCO tuning sensitivity. Circuit Design An LC VCO topology is chosen mainly for its potential to achieve good phase noise performance, relative to ring oscillators or other types of relaxation VCOs. The LC tank consists of integrated spiral inductors, P+/N-WELL varactors allowing continuous frequency tuning, and an array of binaryweighted switched capacitors providing coarse tuning steps. Compact bias chokes are used to bias the anode-side of the varactors. This design is implemented in a 0.25 m bulk CMOS technology with a thick top metal layer. A. Frequency tuning scheme One of the main goals of this design is to concurrently achieve low phase noise and a wide frequency tuning range. A single varactor device with a steep C-V characteristic (i.e. a large Cmax/Cmin) can be used to achieve a wide frequency range and typically has sufficiently high Q so that it does not degrade the phase noise performance of the VCO [1,4-6]. However, this can result in an excessively high tuning sensitivity, KVCO. In practice, this is undesirable since the tuning line feeds substantial noise originating from preceding blocks of the frequency synthesizer. Noise present on the tuning line appears across the varactors and effectively modulates the device junction capacitance, resulting in phase noise sidebands about the carrier. To avoid this problem, the targeted frequency range is split into several sub-bands by means of a switched capacitor array [8]. Because the desired tuning range has been divided, a small varactor device with a shallow C-V characteristic is sufficient to cover each frequency sub-band. The capacitor array configuration is illustrated in Fig. 1. Capacitors Ca-Cb are implemented as high-quality metalinsulator-metal (MIM) capacitors. Minimal-length NMOS devices are used to switch each capacitor in and out of the tank. Because each MOS switch contributes additional loss to the tank due to its finite on-resistance, Ron, much effort has been expended in minimizing this penalty. On the other hand, the transistors cannot be made arbitrarily wide since in the off-state their parasitic overlap and drain-to-bulk capacitances limit the achievable tuning range. Simulations were used to establish a good compromise. This critical trade-off is one of many examples that reveal the conflicting nature of concurrently achieving low phase noise and a wide frequency tuning range. 4W/L 2W/L W/L Cb Ca 2Ca 4Ca