Fully integrated voltage-controlled LC-oscillators for direct-conversion radio-on-a-chip applications in CMOS technology
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The development of a 3G/GSM dual-system Radio-On-a-Chip for mobile handsets in CMOS technology depends on the GSM transmitter’s compliance with the receive-band noise mandated by the standard. An integrated direct conversion solution eliminates filtering at the intermediate frequency stage, requiring a low-noise transmitter chain. The transmitter synthesizer is the main noise contributor due to the high phase noise of the voltage controlled oscillators integrated in CMOS technology. While the integrated LC-oscillators have a better phase noise performance than the ring-oscillators, the high loss of the integrated inductor has prevented the fully integrated solution in GSM transmitters so far. The compromise solution consists of an integrated active circuit coupled with an off-chip, discrete resonator. The thesis motivation was to implement novel design techniques and achieve completely integrated oscillator in CMOS technology, capable of meeting the strict receive-band noise requirements of GSM mobile handset. An integrated oscillator operating at frequencies sufficiently higher than the transmit carrier frequency has two main benefits: the immunity from power-amplifier output injection locking, and higher Q-values of integrated inductors. The oscillator phase noise at large offsets depends on the resonator Q-value, which in turn is dominated by the integrated inductor Q-value. In addition, the phase noise is inversely proportional to the power in the resonator, achieved by a large voltage swing across the differential nodes. However, a large voltage swing across the resonator varactor modulates its capacitance due to the nonlinear voltage-to-capacitance function. This effect increases the phase noise at close offsets through the AM-to-FM noise conversion mechanism. An optimum solution is an oscillator achieving the phase noise requirements at all offsets. The problem of varactor capacitance modulation due to the large voltage swing is fairly new phenomena, expected to be encountered more frequently in the future as the supply voltages become lower and oscillator tuning ranges become larger. The expressions of the effective capacitance and modulation are derived for a variety of C(V)-curve-linearization techniques, aimed at reducing the modulation while keeping the varactor tuning range intact. Novel technique of anti-parallel varactor branches, which are non-symmetric about the tuning line, is presented and confirmed in a prototype design. The varactor elements of each branch have different reactance, and therefore experience different voltage amplitude. When optimized, the varactor element having a more nonlinear C(V) function will experience lower voltage amplitude while a more linear element will experience a larger amplitude. This will lead to an overall lower capacitance modulation. The branches are placed in anti-parallel manner across the oscillator differential nodes, maintaining the large differential swing and symmetry, while lowering the AM-to-FM noise conversion, i.e. the close-in phase noise. In order to keep the far-out phase noise low integrated inductor is optimized by trading its physical size for higher performance. The performed research indicates that an integrated inductor of large physical size and low inductance value operated at higher frequency can achieve Q-values 2-3 times larger than those at GSM transmit carrier frequency. It is desirable that the designs are portable across different foundries, i.e. maintain the performance independent of the semiconductor process chosen. The inductor losses can be grouped into ohmic, i.e. conductor losses, and losses due to the electromagnetic radiation and coupling, dominated by the substrate loss. Methodology for fast identification of loss mechanisms in integrated inductors is developed, based on the element values of standard single-π network inductor model. The presented theory is practically confirmed through a test chip. The close-in phase noise of a reference oscillator design incorporating a traditional varactor is compared to the oscillator design with the new varactor topology. The close-in phase noise profiles measured on a test chip compare closely with the derived theory. The novel oscillator experiences less varactor capacitance modulation, and has a flatter phase noise across the tuning range. The reference oscillator suffers from strong capacitance modulation, and its close-in phase noise profile corresponds to that predicted by the theory and previous works, having a non-monotonic shape across the tuning range. The resonator of the reference oscillator has a higher Q-value and therefore has a better far-out noise performance at same biasing conditions. Therefore, the inductor characteristics dominate the far-out phase noise, and the varactor characteristics dominate the close-in phase noise in the large voltage swing oscillator. The last part of this thesis presents the results of a case study of the enhanced LC-tank resonators. The topology is modified such that oscillator overharmonics are either filtered or boosted within the resonator. The result is a lower phase noise and a possibility of an overharmonic oscillator. This novel technique relies on a harmonic analysis of enhanced LC-tank, and is an attractive topic for further research.