Introduction

   During the last decades, we have witnessed rapid progress and growth in wireless communication technology and markets. This innovation not only has provided easy access to communication and information through cell phones and wireless LAN but also is opening new possibilities for improving health care and transportation safety, along with exploring unknown world such as space and deep sea. Along with the recent rapid growth, wireless communication systems have demanded wider bandwidth for faster data transmission in order to handle multimedia data as well as voice data. To address this issue, the development of a high-performance wireless transmitter and receiver (transceiver) system has been accelerated in electrical engineering.

   Oscillators are key building blocks in communication transceivers. In a transceiver, oscillators generate reference sinusoidal signals for modulation and demodulation. The key issue in the oscillator design is how to minimize phase noise, which is caused by noise from active device and passive components and is a measure of the spectral purity of the oscillator signal. As shown in Fig. 1, the existence of phase noise broadens the output power spectrum of an oscillator around the carrier frequency.











                               
                                     Fig.1. Oscillator Frequency Spectrum

   Oscillator phase noise in wireless transceivers limits the overall performance of communication systems in a variety of ways. Phase noise directly affects short-term frequency stability, bit-error-rate, and adjacent channel interference. In wireless receivers, the phase noise of a strong adjacent channel signal interferes with a desired weak signal, thereby degrading the ultimate signal-to-noise ratio. Phase noise in wireless transmitters can also overwhelm the adjacent weak channels. Such degradations due to phase noise compromise the overall communication capability and put the stringent requirement on the performance of other transceiver blocks such as the noise figure of low noise amplifiers and the output power of power amplifiers .   Since the number of wireless subscribers and thus, the amount of RF interference continue to increase, modern communication standards demand better phase noise performance from local oscillators in transceivers.











                        Fig. 2. The effect of phase noise on receiver systems















                       Fig. 3. The effect of phase noise on transmitter systems

   In response to the above technology needs, my current research intends to demonstrate novel low phase noise oscillators. In particular, my research focuses on the development of multiple-device oscillators with low phase noise performance. In multiple-device oscillators, high-order circuit networks can be constructed with multiple transistors and passive components. The main idea of this research is to design low phase noise oscillators utilizing the mutual interaction between multiple devices and their interconnecting components in the high-order circuit network. This high-order circuit network has a number of circuit variables and can be modeled by complicated circuit equations. By manipulating the high-order circuit variables and equations, the best for low phase noise design can be found. Therefore, I am working on obtaining the optimal solutions in the high-order circuit network for specific types of multiple-device oscillators.

Low phase noise multiple device oscillator based on extended resonance technique

   A novel low phase noise design technique has been proposed for power combining multiple-device oscillators. The new multiple oscillator utilizes the extended resonance technique for power combining.  This technique compactly and effective places multiple solid state power devices in shunt. The extended resonance circuits are very compact since they eliminate the need for matching circuits for the individual devices. The novelty of the proposed oscillator circuit is that the extended resonance power combining circuit serves to lower phase noise as well as combine powers from multiple transistors. The solution for phase noise improvement in the oscillator is obtained by transforming the active device's impedance, based on a well-formulated high-order circuit analysis. The experimental result shows that the designed four-device oscillator at 6GHz shows approximately 160 times phase noise improvement over a single-device oscillator, whereas the phase noise reduction of a conventional four-device oscillator is only 4 times better than a single-device one.

           [1] J. Choi, A. Mortazawi, "A novel low phase noise multiple-device oscillator
           based on the extended resonance technique," in IEEE MTT-S Int. Microwave
           Symp. Dig. , San Francisco , CA , June 2006. (accepted)






















                                                    
                     Fig. 4. Four-device Extended Resonance Oscillator Layout

Design of push- and triple-push oscillators for reducing 1/f noise upconversion

   In this work, 1/f noise upconversion in push-push and triple-push oscillators was investigated and the design requirements for minimizing 1/f noise upconversion have been presented. The low frequency 1/f noise plays a dominant role in determining the close to carrier phase noise performance in oscillators. It is well known that 1/f noise is upconverted to the carrier frequency, resulting in a 1/f^3 region near the carrier frequency. 1/f noise upconversion is strongly dependent on the symmetry property of the oscillation waveform, according to a time-varying phase noise model. In push-push oscillators, the presence of the large second harmonic components may degrade the waveform symmetry of the oscillation waveforms, leading to the significant 1/f noise upconversion. To minimize 1/f noise upconversion in push-push oscillators, the phases of Fourier coefficients of all harmonics need to be equal. In triple push oscillators, the waveform symmetry can easily be achieved by eliminating all even harmonic components. The experimental results show that around 15 dB phase noise improvement at 100 kHz offset frequency can be obtained by satisfying the waveform symmetry condition in push-push and triple push oscillators. This work is the first reported approach that takes advantage of harmonic waveforms naturally generated in push-push and triple-push oscillators to optimize their phase noise performance. My work on this novel design technique resulted in my receiving a student paper award in 2005 IEEE MTT-S International Microwave Symposium.

            [1] J. Choi, A. Mortazawi, "Design of push-push and triple-push oscillators for
            reducing 1/f noise upconversion," IEEE Trans. Microwave Theory & Tech., vol. 53,
            no.11, pp. 3407-3414, Nov. 2005. [PDF]

            [2] J. Choi, A. Mortazawi, "Design of push-push oscillators for reducing 1/f noise
            upconversion," in IEEE MTT-S Int. Microwave Symp. Dig. , Long Beach, CA,
            June 2005, pp. 1531-1534. [PDF]


















                          Fig. 5. Designed push- and triple-push oscillator Layout


















                   Fig. 6. Phase Noise comparison between (a) push- and (b) triple-
                    push oscillators with symmetrical and asymmetrical waveforms