Study Of Properties Of BPST Ceramic And Design Of Ferroelectric Phase Shifter

Barium strontium titanate(BaxSr1-xTiO3, BST),which is an perovskite structure(ABO3) ferroelectric materials , have employed as tunable material in microwave devices due to its very strong non-linear dielectric characteristic, that is its permittivity will reduce when bias field increase. These microwave devices includes phase shifter, voltage controlled oscillator and tunable filter. This paper has studied the material and device of microwave phase shifter.For the study of material, PbO was chosen as'A'site adulterant to BST in order to improving the performance of the compound. A series of BPST(Ba1-xPbySrxTiO3,x=0.4-0.6,y=0-0.4) ceramic targets were made through traditional solid reaction method. Microstructure test and analysis were made to the crystal structure and surface of these three serial ceramics. The relations between permittivity/dielectric loss and temperature had been measured around -40℃-80℃. The X-ray diffractometry (XRD) patterns analysis reveals that: BPST is belong to perovskite structure and is good solid dissolve as same as BST. With the increasing of the lead content, the length of crystal lattice'a'will reduce (from 0.3958nm to 0.3945nm, x=0.4). At room temperature, the transition of paraelectric cubic phase to ferroelectric tetragonal phase in crystal has happened. The ESEM photo show: As given sinter temperature, proper lead can improve the forming of crystal, large and regular the grain. But too much lead will increase the number of holes in target and weaken the performance. The measure of permittivity and loss show: The Curie temperature increase from -25℃to above 80℃when increasing the lead content. The peaks of loss usually happen near the Currie temperature. The loss of paraelectric state is less than that of ferroelectric state and decrease when the temperature increasing. Both peaks of permittivity and loss of BPST are flatter than that of BST. So the characteristic of stability to temperature is improved. At room temperature, the relative permittivity is about between 1000 and 2000.And the loss is usually below 0.01 and the best performance is 0.002 of Ba0.4Sr0.6TiO3.For the device, using High Frequency Structure Simulator (HFSS) software, the structure design and simulation of co-planar waveguide (CPW) ferroelectric phase shifter were studied. The characteristic impedance, differential phase and insert loss of three structure of CPW were simulated. These three structures are BST-substrate phase shifter, BST-film phase shifter using sapphire substrate and the unit of period load capacitance CPW phase shifter. The result of simulation show: for the BST-substrate phase shifter, its characteristic impedance is low, and phase shift is large. Have the low permittivity demand for the material. And it is suit for low frequency application (<8GHz). As for BST-film phase shifter, is suitable for the high frequency application (10-20GHz). For the period load capacitance CPW phase shifter, the demand of bias voltage is lowest, but the proper working frequency is more linked with its structure. The figure of merit (FOM) of three structure are 1700/dB,500/dB,590/dB respectively. The goal of phase shift was reached.