Reduced Size Frequency Agile Microwave Circuits Using Ferroelectric Thin-Film Varactors

Abstract

A new concept for tunable and reduced size microwave circuits is presented based on barium-strontium-titanate (BST) varactors. The proposed design methodology relies on substitution of quarter-wavelength transmission line segments with their equivalent low-pass structures. The ferroelectric BST-varactors are used as tuning elements and allow for a frequency agile behavior and size reduction of the circuit. Candidates for this methodology are quarter-wavelength based circuits such as Wilkinson power dividers and branch-line couplers. Prototype boards for both topologies were implemented and characterized through scattering parameter measurements. For the implemented Wilkinson divider a tuning range from 1.7 to 2.1 GHz is achieved. The insertion loss in both output paths varies from 0.6 to 1.2 dB while maintaining a worst case amplitude-and phase difference of 0.5 dB and 9 , respectively, for all operating cases. The isolation between the two output ports exceeded 25 dB over the whole tuning range. The implemented branch-line coupler exhibits a tuning range from 1.8 to 2.3 GHz with a maximum insertion loss of 2.7 dB while maintaining worst case amplitudeand phase difference of 0.4 dB and 90 5 , respectively. Port matching and isolation levels of more than 15 dB are exhibited at all bias conditions. Index Terms-Couplers, ferroelectric capacitors, power dividers, tunable circuits and devices.

Improved analysis of the intermodulation performance of ferroelectric thin-film and bulk-based varactors

“…1 and 2 are almost the same for all ferroelectric materials, there are some differences in the performance between thin-film and bulk ferroelectric devices. Thus, while the dependence of permittivity on bias voltage for thin-film BST; see for example [18] and bulk STO devices; see for example [17], have the same pattern, the permittivity of the thin-film BST has a relatively lower temperature dependence than that of the bulk STO devices. With these possible differences in mind, the characteristics of Figs.…”

“…This is attributed to their large dielectric constants that can be controlled using an external bias voltage, relaxed packaging requirements, fast tuning and the feasibility of fabrication on a variety of substrates using standard semiconductor manufacturing processes [6]. However, for all the ferroelectrics and the HTS/ferroelectric structures, the permittivity and the conductivity are inherently nonlinear [10][11][12][13][14][15][16][17][18] and follow almost the same pattern with the variations in the bias voltage and temperature. Figure 1 shows a typical DC bias dependence and temperature dependence of the permittivity and Fig.…”

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“…It is well established that electronically tunable subsystems can function optimally when conditions such as frequency of operation changes [7]. Depending on the…”

Compact Tunable Filtering Power Divider With Constant Absolute Bandwidth

“…The second category uses switches to adjust the number of output ports [21]. Thirdly, varactors are employed to tune the operating frequency [22], [23]. The above 0018-9480 © 2015 IEEE.…”