Mohsen Sazegar

Beam Steering Transmitarray Using Tunable Frequency Selective Surface With Integrated Ferroelectric Varactors

A tunable frequency selective surface (FSS) with beam steering capability is presented. The FSS is used as a transmitarray with a bandpass characteristic in Ku-band. The periodic sub-wavelength (λ0/25) unit-cells are composed of capacitive and inductive structures creating a bandpass for an incident wave. By patterning the capacitive elements on a screen-printed barium-strontium-titanate (BST) thick-film ceramic, the resonant frequency of the FSS can be tuned. This technology offers a simple and cost effective way for integrating varactors into the FSS and is particularly attractive for microwave circuits with a high varactor density. A prototype is fabricated including two capacitive layers with an overall size of 40 mm × 40 mm. Each layer includes a total of 1600 integrated BST varactors that are fabricated by using a patterning and metallization process. A transmitted wave passing through the FSS will experience a phase shift, which can be tuned by tuning the passband. This allows steering the transmitted wave to a certain direction by applying a phase gradient along the FSS interface. A phase shift range of 360° can be covered by cascading several FSS panels. The prototyped tunable FSS demonstrates the feasibility of the proposed technology and its potential for beam steering.

Low-Cost Phased-Array Antenna Using Compact Tunable Phase Shifters Based on Ferroelectric Ceramics

A low-cost phased-array antenna at 10 GHz is presented for a scan angle of ±50°. The array employs continuously tunable phase shifters based on a screen printed barium-strontium-titanate thick-film ceramic. Due to the use of artificial transmission line topology, the proposed phase-shifter design has a very compact size (3 mm × 2.8 mm) for 342° total phase shift. In the frequency range from 8 to 10 GHz, it exhibits a figure of merit >;52°/dB, which is among the best of phase shifters based on ferroelectric thick films. In a prototyped phased array, the RF circuit consists of a feeding network, phase shifters, and antenna elements, which are integrated into one planar metallization layer. Furthermore, a simple way for routing bias lines for phase shifters is demonstrated using high resistive electrodes. Using screen printed thick films and applying a simplified fabrication process for the RF and bias circuitry can reduce the total expense of phased arrays considerably.

Compact Substrate Integrated Waveguide Tunable Filter Based on Ferroelectric Ceramics

This letter presents the design and the realization of a compact tunable filter integrated in ferroelectric ceramic substrate. The bandpass filter bases on an evanescent-mode dielectric cavity, which is loaded by a pair of tunable mushroom-type complementary split-ring resonators. Tunable impedance matching networks are additionally implemented to improve passband insertion loss across the tuning range. A prototype has been realized in a 12.5 mm × 9.5 mm × 0.8 mm planar module. Measurements confirm a frequency coverage from 2.95 to 3.57 GHz, a 3 dB fractional bandwidth below 5.4%, with an insertion loss between 3.3 dB and 2.6 dB.

Tunable impedance matching networks for agile RF power amplifiers

The efficiency is one of the most critical parameters in the design of RF power amplifiers. For classical power amplifier (PA) the efficiency drops dramatically in the back-off region. One possibility of enhancing the properties of the PA is the implementation of a load modulation as used in the Doherty topology. The idea of using an adaptive impedance matching network at the output of the transistor is presented and analyzed in this paper. The topology of the matching network enables the connection of the matching circuit directly to the transistor. The matching network has been designed using the minimum mean square error method and realized on a BST thick film substrate. The measured input impedance is in the range of several ohms for a load of 50Ohm as required for power amplifiers.

Compact Tunable Phase Shifters on Screen-Printed BST for Balanced Phased Arrays

Two designs for compact continuously tunable phase shifters are presented employing integrated ferroelectric varactors based on screen-printed barium-strontium-titanate thick film. The circuits are based on left-handed transmission-line topologies that allow to realize very compact modules. The proposed designs have total lengths of 2.1 and 3.8 mm and provide a differential phase shift of more than 360° and a figure of merit of 51°/dB at 10 GHz. Furthermore, the phase shifters are in coplanar strip configuration, making them attractive for systems composed of balanced components.

Barium strontium titanate thick-films: dependency between dielectric performance and their morphology

The paper is focused on the influence of the morphology of barium strontium titanate (BST) thick-films on their effective dielectric properties. Therefore two BST thick-films are made with a minor deviation in sinter temperature, and hence their mean grain size of the films and their sintering necks. By modeling the meso-structure using a Nonlinear 3D Finite-Difference Time-Domain field solver the assumed effect of the morphology on the effective dielectric properties are confirmed. The simulation results show the potential for a further improvement of the dielectric tunability of BST thick-films.

Compact artificial line phase shifter on ferroelectric thick-film ceramics

An artificial transmission line based on Barium-Strontium-Titanate (BST) thick-film is used as a phase shifter in this work. The benefits of artificial transmission lines are exploited to realize a very compact phase shifter with a length of 3mm for 342° differential phase shift. The prototyped phase shifter exhibits a figure of merit (FoM) of 52°/dB in the frequency range 8–10 GHz. The proposed circuit is fabricated in only one photolithography process cycle on a low cost screen printed thick-film ceramic.

Ferroelectric thick-film varactors based on Barium-Strontium-Titanate for high power applications

Tunable coplanar interdigital capacitors (IDC) based on Barium-Strontium-Titanate (BST) thick-films are evaluated for their suitability for high power applications, such as tunable impedance matching networks of power amplifiers. The design process has been reviewed with respect to the nonlinearity of the varactors. Various devices have been fabricated and their performance has been verified by measurements. The realized devices show a high IP3 around 60 dBm.

Electrically tunable composite right/left handed transmission-line based on open resonators and Barium-Stronium-Titanate thick films

This paper is focused on the study of the tunability of planar composite right/left handed (CRLH) transmission lines based on the combination of open split ring resonators (OSRRs) and open complementary split ring resonators (OCSRRs), and its application to microwave devices. The tunability is achieved by means of Barium-Stronium-Titanate (BST) thick films, which change the electrical properties of the structures through the application of an static electric field. Some potential applications such as phase shifters or tunable band pass filters are presented, where a frequency tunability of 36.6% is obtained.

Tunable dielectrics for microwave applications

This paper gives a short introduction into tunable dielectrics for microwave applications based on nematic liquid crystal and Barium-Strontium-Titanate (BST) thin- and thick-films. Due to their tuning voltage dependent electromagnetic properties, both are very promising material candidates for tunable passives in the microwave and millimeter-wave regions. For applications, such as frequency-agile communications and sensor systems we illustrate the challenges of optimization BST films based on a multi-scale view for material, processing, device and application design.