Seungku Lee

Wideband Branch-Line Couplers With Single-Section Quarter-Wave Transformers for Arbitrary Coupling Levels

Wideband branch-line couplers with arbitrary coupling levels are demonstrated. By integrating single-section quarter-wave transformers at each port, wideband characteristic with excellent coupling flatness is achieved. Also, a method is demonstrated that introduces intentional mismatch for a further enhancement in bandwidth. Superior structural and design simplicity outperform previous wideband couplers. Branch-line couplers at 3 GHz are demonstrated experimentally that maintain the coupling level within 0.5 dB from the specified 10 dB in as much as a 50.9% bandwidth, with minimum return loss and isolation of 16.3 and 18.7 dB, respectively.

Large-Signal Performance and Modeling of Intrinsically Switchable Ferroelectric FBARs

This paper presents the large-signal performance of intrinsically switchable ferroelectric thin-film bulk acoustic resonators (FBARs), as well as their modeling procedure. There has been a growing interest in ferroelectric FBARs due to their electric-field-dependent permittivity and electric-field-induced piezoelectricity. Ferroelectric barium-strontium-titanate (BaxSr(1-x)TiO3) FBARs are intrinsically switchable, namely, they have resonances that switch on with the application of a dc-bias voltage. In this paper, the large-signal performance and nonlinear behavior of ferroelectric BST FBARs are investigated. Measurement results show that the device nonlinearity can be reduced by applying higher dc-bias voltages. Moreover, a large-signal model that accurately describes the dc-bias voltage, as well as RF power-dependent performance of BST FBARs is developed. Large-signal simulation results obtained from this model at different bias voltages and RF power levels show good agreement with the measurement results.

Intrinsically switchable, high-Q ferroelectricon-silicon composite film bulk acoustic resonators

This paper presents a voltage-controlled, high-quality factor (Q) composite thin-film bulk acoustic resonator (FBAR) at 1.28 GHz. The composite FBAR consists of a thin layer of barium strontium titanate (BST) that is sandwiched between two electrodes deposited on a silicon-on-insulator (SOI) wafer. The BST layer, which has a strong electrostrictive effect, is used for electromechanical transduction by means of its voltage-induced piezoelectricity. The silicon layer, with its low mechanical loss, increases the Q of the resonator. The composite FBAR presented here exhibits Qs exceeding 800 with a resonance frequency and Q product (f × Q) of 1026 GHz.

Linearity analysis of intrinsically switchable ferroelectric FBAR filters

This paper presents the linearity analysis of intrinsically switchable ferroelectric thin-film bulk acoustic resonator (FBAR) filters for the first time. The FBARs are based on the barium-strontium-titanate (BST) ferroelectric thin-film material. BST FBAR filters composed of two series FBARs and one shunt FBAR are designed, fabricated, and measured. Input-referred third-order intercept point (IIP3) of more than 26 dBm for a 1.6-GHz filter with an insertion loss of 4.1 dB is obtained.

An Intrinsically Switchable Ladder-Type Ferroelectric BST-on-Si Composite FBAR Filter

This paper presents a ladder-type bulk acoustic wave (BAW) intrinsically switchable filter based on ferroelectric thin-film bulk acoustic resonators (FBARs). The switchable filter can be turned on and off by the application of an external bias voltage due to the electrostrictive effect in thin-film ferroelectrics. In this paper, Barium Strontium Titanate (BST) is used as the ferroelectric material. A systematic design approach for switchable ladder-type ferroelectric filters is provided based on required filter specifications. A switchable filter is implemented in the form of a BST-on-Si composite structure to control the effective electromechanical coupling coefficient of FBARs. As an experimental verification, a 2.5-stage intrinsically switchable BST-on-Si composite FBAR filter is designed, fabricated, and measured. Measurement results for a typical BST-on-Si composite FBAR show a resonator mechanical quality factor (Qm) of 971, as well as a (Qm) × f of 2423 GHz. The filter presented here provides a measured insertion loss of 7.8 dB, out-of-band rejection of 26 dB, and fractional bandwidth of 0.33% at 2.5827 GHz when the filter is in the on state at a dc bias of 40 V. In its off state, the filter exhibits an isolation of 31 dB.

Large signal performance of ferroelectric FBARs

Ferroelectric thin film bulk acoustic wave resonators (FBARs) have been of growing interest due to their electric field dependent properties. Resonators based on the ferroelectric barium strontium titanate (BaxSr(1−x)TiO3, BST) are intrinsically switchable, namely they have resonances that switch on with the application of a dc bias voltage. In this paper, the large signal performance and nonlinear behavior of BST FBARs are investigated. Measurement results show as the dc bias voltage increases, the nonlinear behavior due to high RF input power decreases. For better understanding, measurement results of BST FBARs are fitted to a nonlinear modified Butterworth-Van Dyke (MBVD) model with respect to dc bias voltage and RF input power.

Application of Stepped-Impedance Technique for Bandwidth Control of Dual-Band Filters

This paper demonstrates application of the popular stepped-impedance technique to control the bandwidths of dual-band filters independently. Unlike previous dual-band stepped-impedance filters, the role of the stepped-impedance technique is to control the bandwidths of the two bands rather than to provide dual-band performance. The exact relationship between stepped-impedance ratios and the ratio of the two absolute bandwidths is obtained through rigorous analysis. Moreover, since the proposed filters can be synthesized with popular transmission lines and coupled lines that are very-well characterized, time-consuming full-wave simulation can be minimized. Along with the complete set of design equations, experimental results for transmission-line and coupled-line dual-band filters are provided that verify the proposed approach.

An Intrinsically Switchable, Monolithic BAW Filter Using Ferroelectric BST

This letter presents an intrinsically switchable monolithic filter based on laterally acoustically coupled bulk acoustic wave resonators. The filter is based on a barium strontium titanate-on-silicon (BST-on-Si) composite structure which can be turned on and off by the application of a dc bias. In its off state, the filter provides an isolation of 40 dB. In its on state, the filter exhibits an insertion loss of 7.1 dB and greater than 30 dB of out-of-band rejection. The center frequency and bandwidth of the filter are 748 MHz and 13.8 MHz, respectively. The mechanical quality factor of the coupled resonators is calculated to be 99.

Temperature dependent characteristics of intrinsically switchable ferroelectric composite FBARs

This paper presents temperature dependent characteristics of intrinsically switchable ferroelectric composite FBARs. Ba0.5Sr0.5TiO3 and Si are used to form a composite FBAR. Measurement results show that both series and parallel resonance frequencies decrease with the temperature. The temperature coefficient of frequency is approximately -35 ppm/°C. The variation of quality factor and electromechanical coupling coefficient is less than 6.5% and 6.2% above 20 V bias.

Intrinsically switchable ferroelectric bulk acoustic wave filters based on barium strontium titanate thin films

Single-standard wireless transceivers are of limited use as multi-mode devices become ubiquitous. Adaptive and reconfigurable RF circuits are necessary to decrease size, cost, and power consumption as well as increase reliability and robustness of wireless devices. In this paper, the design, fabrication, and performance of acoustically coupled and electrically coupled filters based on the ferroelectric barium strontium titanate are discussed. In their off state, the input and output ports are isolated from one another. When switched on with the application of a dc bias voltage, a bandpass filter response is obtained.