Wideband Low Phase Noise CMOS VCO-free paper
A CMOS VCO has been designed and fabricated in a commercial 0.25μm 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 μm2. 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.
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 inversion- or 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.
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 0.25μm bulk CMOS technology with a thick top metal layer.