Broadband radio frequency active quasi-circulator


Student thesis: Master's Thesis

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  • Wai Yin MUNG

Related Research Unit(s)


Awarding Institution
Award date16 Feb 2009


Wireless communication standards can cover several frequency bands spread over quite a wide range, e.g. GSM, WiFi, GPS, Bluetooth, etc. With global roaming, it becomes necessary to use wideband RF front-end circuits to cover these different global standards. Applications of wideband directional components in particular circulators will be an avenue to explore for future wireless communication components because it enables simultaneous signal transmission and reception at different frequencies due to good port to port isolation. The conventional ferrite circulator, however, suffers from narrow bandwidth, large size and weight, which make these circulators impractical. In this thesis, two different wideband equalization techniques (self-equalization technique and phase equalization technique) are proposed to enable wideband operation in active quasi-circulator so that the RF front-end circuit components will be fewer, smaller and, more importantly, the unit will be transparent to changing standards such as the proposed future RF architecture called software defined radio. A self-equalization technique is proposed for the distributed quasi-circulator, which is comprised of a distributed balun and combiner. The self-equalization is necessary in order to realize wideband isolation. Experimental results show that the distributed quasi-circulator can operate over the frequency range 0.8-2.2 GHz. The minimum isolation obtained between ports 1 and 3 is around 20 dB using the self-equalization technique, with greater than 20 dB between the other ports due to the FET unilateral characteristic. The phase equalization technique is proposed for a simple active quasi-circulator, which it is formed by configuring three transistors together with a complimenting phase shifter. Experimental results show that it can operate over the frequency range 0.8-2.2 GHz. The return losses of the three ports are all greater than 10 dB while the minimum isolation between ports 1 and 3 is 15 dB, and with greater than 30 dB between the other ports over the frequency range of 0.8-2.2 GHz. The insertion losses, S21 and S32 are around 0 ± 1.5 dB. Past works on the design of the active quasi-circulator was limited to narrow band operation. The main contribution of the research presented in this thesis is to increase the bandwidth of the quasi-circulator using active devices in a way that it maintains its advantages of small size, light weight and compatibility with monolithic microwave integrated circuit (MMIC) technology compared to previously presented work on active and passive circulators. The simultaneous operation at several different frequency bands, therefore, will be adaptable to the current multi-band standards and foreseeable standards in future.

    Research areas

  • Circulators, Wave-guide, Broadband communication systems