Silicon-Based Distributed Voltage Controlled Oscillators for Super-High-Frequency Applications

Hui Wu (wu@caltech.edu)

 

The ever-increasing demand for larger bandwidth in the wired and wireless communication markets is pushing integrated circuits to higher operation frequencies. Traditionally, these microwave circuits have been implemented in compound semiconductor technologies (GaAs and InP). However, silicon-based RFIC’s have become very competitive in this arena with comparable performance, lower cost and better system-on-a-chip (SoC) capabilities. Since silicon suffers from larger parasitic elements both in active and passive devices, new circuit techniques and design methodologies are needed at such high frequencies.

Voltage-controlled oscillators (VCO’s) are essential building blocks for frequency synthesizers and clock-and-data recovery loops. Monolithic ring and LC oscillators have been commonly used in such systems. Ring VCO’s have larger tuning range and can easily generate quadrature signals, but have inferior phase noise performance which disqualifies them in wireless and timing-critical wired applications. LC VCO’s offer better phase noise performance, but it becomes more difficult to achieve all the desired VCO specifications simultaneously as the operation frequency approaches the self-resonance frequency of on-chip inductors and the cutoff frequency of transistors. More specifically, to operate at higher frequencies, the tank’s LC product should decrease. However, the inductor loss, parasitic capacitances of transistors, and loading from the output buffers do not scale at the same rate. So L should decrease further, which results in larger power dissipation for a given oscillation amplitude, and more severe constraints on the tuning capability. These limitations make it desirable to pursue alternative approaches, such as distributed oscillators.

Distributed oscillators originate from distributed amplifiers (a.k.a. traveling-wave amplifiers), which have been widely used in wide-band applications. A distributed amplifier achieves a higher gain–bandwidth product by absorbing the parasitic capacitances of transistors into artificial transmission lines. Correspondingly, a distributed oscillator can potentially operate at higher frequencies than a lumped one. Also, through a proper choice of number of stages, a distributed oscillator can generate low-noise quadrature signals without using dividers or poly-phase filters like in current implementations.

In order to realize a fully-functional distributed voltage-controlled oscillator (DVCO), new tuning techniques are needed since conventional varactor tuning would defeat the goal of high-frequency operation and degrade the phase noise performance. Also, a systematic and analytical approach to the design of DVCO’s is required to be able to make accurate a priori predictions of frequency and amplitude.

Based on our detailed analysis, the general oscillation condition of distributed oscillators has been derived, resulting in analytical expressions for the frequency and amplitude. Two novel tuning techniques for DVCO’s have been invented, namely, the inherent-varactor tuning and patented delay-balanced current-steering tuning. In the latter approach, the effective length of transmission lines is varied by changing the signal path. CMOS and bipolar DVCO’s have been designed and fabricated in a 0.35-μm BiCMOS process. A 10-GHz CMOS DVCO achieved a tuning range of 12% (9.3–10.5 GHz), a phase noise of -103 dBc/Hz at 600 kHz offset, and output power of 4.5 dBm without any buffering, drawing 14 mA of dc current from a 2.5-V power supply. A 12-GHz bipolar DVCO consuming 6 mA from a 2.5-V power supply has also been demonstrated. It has a tuning range of 26% with a phase noise of -99 dBc/Hz at 600 kHz offset.