Design of various VCO topologies and performance comparison at 2.4 GHz

Abstract

RF transceiver architecture design is critical in the realization of monolithic chip solutions. Low noise and low power designs are necessary for high performance wireless transceivers. Voltage controlled oscillators are among the key building block for the RF communication systems. They are used as local oscillator in up and down converters. Therefore, the performance of voltage controlled oscillator is important to determine overall performance of the transceiver.Power consumption, phase noise and tuning range are some important design parameters for voltage controlled oscillators. There are many different circuit structures to improve these parameters. However, many of these structures require on-chip inductors. There are several downsides to use IC inductors in design such as high cost and difficult to accurate design.It is also possible to design an oscillator without using inductors. One of them implements inductor using active circuits. However, noise performance is very poor for these structures. Another approach is ring oscillator topology. Although, it is still worse phase noise performance compared to LC oscillators, they are used in RF systems because of their high tuning range and easy integration. Another oscillator topology without IC inductor is Wien-bridge structure. This topology is based on band-pass filter structure. An operational amplifier is used there to provide stable oscillation. However, this structure isn?t suitable for high frequency oscillations because it is limited by operational amplifiers gain-bandwidth product. If an operational transconductance amplifier is used, this structure can also provide high frequency operation. Relaxation oscillators are another alternative to LC oscillators. These kinds of oscillators operate by alternately charging and discharging a capacitor between two threshold voltage levels.In this thesis, four different voltage controlled oscillator structures are designed in the same 2.4 GHz oscillation frequency. 2.4 GHz is one of the most popular frequency band that is used in Bluetooth, cordless telephones, car alarms and Wi-Fi networks. 0.18µm process and 1.8V supply voltage is used in all designs to compare fairly each other.The design with LC structure has best phase noise performance (-120 dBc/Hz @ 1MHz) as expected. However, relaxation oscillator topology provides also good phase noise performance especially using quadrature structure (-108 dBc/Hz @ 1MHz). Some applications in RF communication systems such as image rejection mixer topologies require quadrature local oscillators and it is very critical to have signals with 900 phase shift to reject image band accurately. Quadrature relaxation oscillators can be alternative to LC oscillators for these applications because LC oscillator phase noise performance decreases in quadrature structure. Ring oscillator and RC BPF based oscillator structures are also analyzed and designed in this work. The phase noise performances are obtained about 95.8 dBc/Hz @ 1 MHz for RC BPF based topology and 97 dBc/Hz @ 1 MHz for ring topology. Their phase noise performances are worse than others but they have also some advantages such as high tuning range, easy design and implementation.Each design is compared with similar structures in literature at the end of chapters. The best alternative without using on-chip inductor is the quadrature relaxation oscillator structure. Quadrature up conversion mixer is designed as an application for quadrature relaxation oscillator. It is used there as local oscillator. Obtained results are satisfying that quadrature relaxation oscillator can be used effectively in image reject up and down conversion systems.At the end of thesis, these four structures are compared and discussed in detail. Their advantages and disadvantages are manifested according to each other.