A waveguide-based coupled oscillator array integrated with a voltage-tunable frequency selective surface for spatial power combining

Abstract

In recent years, research on quasi opt ical or spat ial power combining techniques becomes more and more attract ive due to the burgeon of wireless communicat ions, such as telecommunicat ion, satellite communicat ion and radar systems, which t rends to operate at higher frequency bands due to the exponent ially increasing demands on bandwidth and high data rate. Due to the physical limitations of semiconductor devices, power from many devices should be combined together to meet the system requirements. However, as a matter of fact , the loss of any transmission structures at such frequency bands is really a serious issue which limits the use of convent ional incorporating power combining approaches such as Wilkinson power combiner and compels the advent of new power combining methods. One of them is the quasi optical or spat ial power combining technique in either free space or a closed cavity filled with air or dielectric materials, for which the combining is conducted in a low-loss medium, employing act ive circuit element such as amplifier, oscillator or mixer. In this work, a spat ially combined oscillator array is discussed. For this waveguide-based design, oscillators are coupled together within a common cavity resonator for synchronizat ion and efficient ly combined at the output waveguide. Both input and output sides of the oscillators are coupled using wideband waveguide-to-microst rip t ransit ion array, to conduct the power combining in a spatial way in a standard WR90 rectangular waveguide. The oscillation cores are designed with the negative resistance method. The MESFETs are properly biased and terminated so that the negat ive resistance of the act ive devices is large enough to cancel the loss of the resonator to meet the oscillat ion condit ion in both small signal and large signal cases. In this research, the spatially combined oscillator array is synchronized using a common cavity resonator. To build a voltage cont rolled oscillator, a voltage-tunable frequency select ive surface (VTFSS) is integrated within the cavity as a reflect ion phase tuner so that the elect romagnetic characterist ics of the common cavity resonator are electrically controllable. This spatially tunable design is advantageous in terms of simplicity especially for the waveguide design where available space is rather limited, because all the oscillators are tuned by a single voltage bias instead of the applicat ion of bias tuning upon each one of them. To further enhance the output power level of the oscillators, a buffer stage is added at the outputs of the oscillator MESFETs. Not only does the buffer act as a gain block to the circuits, it also provides isolat ion, e.g. S12 of a BFP740F transistor is around -15 to 20 dB, between oscillators so that the only significant coupling between oscillators is the one conducted through the common resonator with voltage-tunable FSS integrated. The thesis is arranged as follows. Firstly, a brief introduct ion will be given in Chapter 1, followed by a research background with previous work by other researchers in Chapter 2. After the background informat ion on spat ial power combining, all the components of the system will be discussed in detail through Chapter 3-6. The tolerance and fabricat ion problems are covered in Chapter 7 for complet ion and conclusion will be drawn in Chapter 8.

Date of Award	15 Jul 2009
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chi Hou CHAN (Supervisor)