The Study On Miniaturized Passive Devices Design Of RF Front-end

At present, along with the development of wireless communication, wireless communication equipments focus on compact, low cost, low power and multifunction. Miniaturization and high performance of passive devices is a prerequisite for this goal. At the same time, multi-path fading, frequency spectrum crowding and noise interference have negative impact on data rate and reliability in wireless channel. The diversity antenna provides a new way for implementing miniaturization and high reliability of system. In this thesis, the miniaturized polarization-diversity antenna, micro-strip balun and dual-frequency micro-strip balun are studies. The dissertation includes the following contents.Firstly, diversity technology and miniaturization technology in wireless communication are introduced. Also, basic theory of antenna and balun are described. Further more, the balun measurement method and the antenna numerical methods such as Method of Moments, Finite difference Time-Domain method and Finite Element Method are introduced.Secondly, a miniaturized polarization diversity antenna for wireless communication is presented in the third chapter. The antenna has a simple and compact structure, and it has two vertical T antenna elements, which can provide polarization diversity. To further increase the isolation between the two ports and achieve the miniaturized antenna, two L-shape slots are etched on the ground. Compared with the conventional antennas, the size of the proposed antenna is reduced about 57%; the total size of the antenna is 48mm×39mm. The simulated and measured results show that the proposed antenna works at 2.4GHz, and the relative bandwidth for S11≤-10 dB is over 30% and the isolation less than -20dB on the two ports. The maximum gain of 4.345dBi can be achieved.Thirdly, a dual-frequency micro-strip balun adopting branch approach is presented in the fourth chapter. The higher frequency of balun is controlled by the most wide micro-strip line at the center and its length equals to a quarter wavelengths. While, low-frequency is controlled by a branch added in the input port. The simulated and measured results show that the proposed balun works at 1.57GHz and 2.4GHz, and the relative bandwidth for S11≤-10 dB is over 55%. S21 and S31 are equal amplitude and opposite phase signals in the work frequency.At last, a miniaturized micro-strip balun is proposed. To reduce the size, a short-circuit probe is loaded on the patch. The short-circuit probe breaks the balance between the two outputs. In order to eliminate this imbalance, a rectangular slot is etched on the port2 ground. Two baluns operate at 2.4GHz and 5.2 GHz are simulated. The total size of the 2.4GHz balun is 17.5mmx 18mm. The simulated result shows that the impedance bandwidths for S11≤-10dB reach up to 1GHz-3.42GHz, and the relative bandwidth is 100.4%. The differences of amplitude and phase for two ports are 0.03dB and 176.98 degrees, respectively. It reaches the requirements of balanced outputs. The total size of the 5.2GHz balun is 9.5mm×10mm. The simulated result shows that the impedance bandwidths for S11≤-10dB reach up to 2.96GHz-6.68GHz, and the relative bandwidth is 71.7%. The differences of amplitude and phase for two ports are 0.05dB and 179.8 degrees, respectively.