Research On New Ultra - Wideband Balun And Its Application

Recently, with the rapid development of wireless communication system, ultra-wideband, compact, and low lost have become the key points in the design of RF front-end circuits. Thus, new types of microwave passive components which have the properties of ultra-wideband and easy for integration, such as ultra-wideband balun, have been attracted more and more attention at present. This paper introduces two kinds of ultra-wideband baluns based on wideband transition structures. Two different forms of microstrip-to-slotline transition structures are used in the microstrip-slotline balun to achieve the functions of balun, i.e., good magnitude balance performance and 180 degree phase difference. And its integration with a Vivaldi antenna is designed and fabricated to validate the feasibility of the new approach. In the microstrip-CPW balun, two different types of transition structures, i.e., the microstrip-to-CPW (coplanar waveguide) transition and the microstrip-to-CPS(coplanar stripline) transition, are adopted for the two balanced output ports design to achieve 180 degree phase difference. To verify the design concpet, a patch antenna under radiation of tri-mode resonance is designed and integrated with the proposed microstrip-CPW balun. In addition, a new microstrip tri-mode balun bandpass filter (BPF) with good filter-type and balun-type functions is presented in this paper. The balun BPF not only utilizes the transition structures of the microstrip-CPW balun for balun-type function but also introduces multi-mode stepped-impendence resonator (SIR) and parallel coupled structures to realize the tri-mode BPF performance.By using EM simulation software ANSOFT HFSS 13, the two ultra-wideband balun and their applications were simulated. Bisides, they were fabricated and tested. For the microstrip-slotline balun, simulated and measured results show that a bandwidth of 3.6 to 10.7 GHz with amplitude imbalance better than 0.5 dB and phase difference better than 180° ° 6° can be achieved. And the Vivaldi antenna integrated with balun can work from 3.6 to 11.2 GHz with good radiation performance. For the microstrip-CPW balun, simulation and test results indicate that a bandwidth of 0.2 to 5.2 GHz (26:1) with amplitude imbalance better than 0.4 dB and phase difference better than 180°± 2°. The patch antenna integrated with balun has a wide bandwidth up to 45%. In addition, the wideband balun BPF exhibits not only high selectivity with two transmission zeros locating at 1.86 GHz and 3.7 GHz but also good balance performance with 0.5 dB magnitude imbalance and 5 degree phase imbalance between the balanced outputs. The two baluns exhibit the properties of wide bandwidth, low insertion loss, compact structure and good phase/amplitude imbalance. With all these good features, the proposed baluns and their applications can be well-suited for RF communication systems.