PROJECT DESCRIPTION, MOTIVATION, & RELATED WORK
Barium Strontium Titanate (BSTO) BaxSr1-xTiO3 (BSTO) Ferroelectric materials provide a low cost solution for effective phase shifter applications. Based on the advantages of using the multilayer Ferroelectric technology an effort was made to examine the feasibility of using coplanar waveguide structures to design and develop high performance phase shifters and other tunable microwave devices [1,2]. Among the desirable characteristics are: (1) low insertion losses, (2) large phase shift per unit length, (3) 50 Ohm impedance matching, (4) high figure of merit (°/dB), and (5) an insensitivity to manufacturing tolerances in conjunction with a practical biasing circuit.

For the simulation purposes we implemented two coplanar designs. One was based on one-sided loading with the multilayer dielectric material, while the other was based on two-sided implementation of the multilayer dielectric loading. It is suggested that the two sided loading may result in capturing more of the fringing fields and would, hence, enhance the performance of the phase shifter. It was found that although the two sided loading approach slightly out performed the one sided loading approach; the differences were quite small and may not warrant the complications in the fabrication process.

To demonstrate the advantages of using the multilayer dielectric approach, the one sided loading geometry was simulated and the results were compared with a case where the signal conductors lie on the Ferroelectric material [3]. Fig. 1 shows a cross section of a situation where the CPW conductors lie directly on the Ferroelectric (Fig. 1a), and the proposed design where the Ferroelectric was separated from the CPW conductors by a low dielectric, low loss material such as SiO2 (r ~ 3.8, tan ~ 1e-4) (Fig.1b).

Fig. 2 shows simulation results for the CPW architectures described in Fig. 2 for unbiased and biased Ferroelectric cases.

Fig. 1. Cross sections of multilayered co-planar waveguides for X-band phase shifter applications for (a) conductors directly on the Ferroelectric material [3] and (b) multilayer Ferroelectric design. Dielectric constant of SiO2 is 3.8 with tan of 1e-4 and the conductor thickness is 0.1 µm.