Evaluating voltage-tunable materials for RF phase shifter technology

In this paper we developed a ferroelectric CPW phase shifters which will be integrated with wideband CTS antenna array. For fabrication of the ferroelectric CPW, the vacuum sputter has been used with a optimized sputtering parameters. The experimental RF performances of the developed phase shifters were compared with those from simulations using WIPL-D software. The results showed that while the approach of using thin Ferroelectric films may be successful in designing low cost tunable coplanar waveguide phase shifters, the amount of achievable phase shifts are small (less that 10°) unless Ferroelectric compatable substrates such as STO are used (phase shifts are 27°). Good agreement was also observed between simulations and measured results and this provides high confidence in the developed fabrication process‥ While our group has been successful in fabricating thin film ferroelectric phase shifter, work is still underway to help develop multilayer ferroelectric phase shifters as well as on the development of an integrated CTS and ferroelectric phased antenna array. As a next step, our group is now pursuing fabrication of a ferroelectric multilayer CPW integrated with the CTS antenna array.

Recent advances in the development of Barium Strontium Titanate ferroelectric composition has made possible reasonable performance of ferroeletric phase shifters to frequencies up to 10 GHz. These material improvements, coupled with phase shifter circuit design changes have resulted in phase shifts greater than 360 degrees with less than 6 dB insertion loss. In particular additives to the BaxSr1-xTiO3 composition have been shown to exhibit a consistent electrical phase shift verses DC potential over parameters of temperature and humidity. These ferroelectric material improvements and circuit design changes, included with the development of multiple ferroelectric phase shifters makes possible the fabrication of a low cost electronic scanning antenna. A single four element phase shifter was used with a one dimensional linear antenna array which was constructed on three layers and used an aperture coupled distribution technique. Individual elements of this multiple four element phase shifter were evaluated with respect to uniformity phase shift and insertion loss. The four element antenna was fed by four ferroelectric phase shifters and the phase shifters are corporately fed by the microwave source. The ferroelectric phase shifters are controlled via a dedicated microcontroller which calibrates out element phase variations and provides a real time scan capability for the antenna assembly.

A phased array antenna using ferroelectric coplanar waveguide (CPW) phase shifter active modules is presented based on a Ba0.6Sr0.4TiO3 (BST) thin film. The ferroelectric BST thin film was deposited on an MgO substrate by using pulsed laser deposition. The active module amplifies an RF signal with low noise and changes the phase of a received RF signal to control the main beam of the array antenna. It is composed of low noise amplifier (LNA), dc block, and ferroelectric CPW phase shifter. The prototype of 6×12 phased array patch antenna with the active modules using a ferroelectric phase shifter is proposed. The proposed array antenna continuously scanned its main beam in elevation plane to 32° from its broadside direction at 11.85 GHz by applying the continuous dc voltage to the ferroelectric phase shifter.

In this paper, a method is introduced to improve the performance of ferroelectric phase shifter. The influences of periodically inserting spiral inductor into ground of coplanar waveguide (CPW) are investigated. Three dimensional electro-magnetic simulation shows that at frequency of 25 GHz, the maximal phase differential will be 449°, which is 97° more than the conventional one with no spiral inductor loaded. The novel ferroelectric phase shifter is also fabricated. Comparing with the conventional phase shifter, additional 49° phase shift is obtained at 18 GHz.

Chapter 11 - Phase Shifters

A tunable capacitor based on a ferroelectric thin film grown on a dielectric substrate is a promising component for reconfigurable and tunable devices for numerous RF and microwave applications such as phase shifters, tunable filters, and tunable matching networks. The goal of this paper is characterization of the insertion loss in a reflection-type microwave phase shifter based on the ferroelectric tunable capacitor. The reflection-type phase shifter based on ferroelectric planar tunable capacitor was designed, manufactured, and tested. Its simulated and measured insertion loss and phase shift are compared. Two loss mechanisms in the phase shifter were distinguished: the loss in the tunable capacitor and the loss in the associated circuitry (metallization and substrate). The properly organized investigations of the dissipation of energy in metallization and dielectric substrate can result in a sufficient improvement of the quality of the devices. As a final result, one may anticipate that the figure-of-merit of a ferroelectric tunable or switchable phase shifter can reach 200 deg/dB

A reflection type phase shifter based on a tunable ferroelectric capacitor is under consideration. A design and characteristics of the one-bit ferroelectric phase shifter is proposed and discussed. The minimal insertion loss of the phase shifter is limited by the commutation quality factor of the capacitor and can be achieved in the case when the losses of the phase shifter in both states have the same value. The layout of the phase shifter based on a coplanar waveguide structure is shown. Measured characteristics of the phase shifter are compared with the predicted ones.

Electric field tunable ferroelectric materials have attracted extensive attention in recent years due to their potential applications for tunable microwave device such as tunable filters, phased array antennas, delay lines and phase shifters (Maiti et al. 2007a; Rao et al. 1999; Romanofsky et al. 2000; Varadan et al 1992.; Zhi et al. 2002). Ba1-xSrxTiO3 and BaZrxTi1−xO3 have received the most attention due to their intrinsic high dielectric tunability. However, the high inherent materials loss and high dielectric constant has restricted its application in tunable microwave device. Various methods have been investigated to lower the dielectric constant and loss tangent of pure ferroelectrics. Forming ferroelectric-dielectric composite is an efficient method to reduce material dielectric constant, loss tangent and maintain tunability at a sufficiently high level. For binary ferroelectric-dielectric composite (such as BST+MgO) (Chang & Sengupta 2002; Sengupta & Sengupta 1999), with the increase of dielectrics content, the dielectric constant and tunability of composites decrease. In order to decrease the dielectric constant of binary composite, it is necessary to increase the content of linear dielectric, and the tunability will decrease inevitably due to ferroelectric dilution. Replacing one dielectric by the combination of dielectrics with different dielectric constants and forming ternary ferroelectric-dielectric composite can decrease the dielectric constant of composite and maintain or even increase the tunability. This is beneficial for tunable application. The Ba0.6Sr0.4TiO3-Mg2SiO4-MgO and BaZr0.2Ti0.8O3-Mg2SiO4-MgO composites exhibited relatively high tunability in combination with reduced dielectric permittivity and reduced loss tangent (He et al. 2010, 2011). With the increase of Mg2SiO4 content and the decrease of MgO content in Ba0.6Sr0.4TiO3-Mg2SiO4-MgO composite, the dielectric constant decrease and the tunability remain almost unchanged. For BaZr0.2Ti0.8O3-Mg2SiO4-MgO composite, an anomalous relation between dielectric constant and tunability was observed: with the increase of Mg2SiO4 content (>30 wt%), the dielectric constant of composite decreases and the tunability increases. The anomalous increased tunability can be attributed to redistribution of the electric field. Ba1-xSrxTiO3-Mg2TiO4-MgO can also form ferroelectric (Ba1-xSrxTiO3)-dielectric (Mg2TiO4-MgO) ternary composite and the dielectric constant can be decreased. With the increase of Mg2TiO4 content and the decrease of MgO content, the tunability of Ba1-xSrxTiO3Mg2TiO4-MgO composite increase. The multiple-phase composites might complicate method to effectively deposit films, particularly if the dielectrics and ferroelectric are not compatible for simultaneous deposition or simultaneous adhesion with a substrate or with

Controllable microstrip transmission line phase shifters employing the membrane actuated ground plane are investigated. Simulated data are included for a uniform air gap transmission line phase shifter, as well. A single 10.4 mm diameter membrane had showed phase shifts of 37.07/spl deg/ and 55.48/spl deg/ at 11.01 GHz and 14.25 GHz, respectively, which can be further duplicated and put as an array to cover the desired length of the microstrip line, to achieve a desired phase shift in a particular application. This phase shifter may find applications in phased arrays, and as discrete phase shifter components.

A new photonic microwave phase shifter that can achieve a full 360 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> phase shift with very little RF signal amplitude variation, and which can operate over very wide frequency range, is presented. It is based on controlling the amplitude and phase of the optical carrier and the two RF modulation sidebands via the dc bias voltages to a dual-parallel Mach-Zehnder modulator (DPMZM). It has a simple structure, only requiring a single laser, modulator, and photodetector. A continuous 360 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> RF phase shift without changes in the RF signal amplitude is obtained by controlling the dc bias voltages of the DPMZM. Experimental results demonstrate a very wideband operation of the RF phase shifter over an 8:1 frequency range from 2 to 16 GHz, a continuous 0-360 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> RF phase shift with very low amplitude variation of <; 3 dB of over the entire frequency band, and with a phase deviation standard deviation of <; 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> .

The analysis, construction, and performance of compact surface wave ferroelectric phase shifters suitable for operation in the100-to1000-MC frequency range are described. Although this ferroelectric loaded parallel-plane structure is a very low impedance structure, a satisfactory terminal impedance matching technique has been devised. Kilovolt level voltages are needed; but since the current required to maintain or rapidly shift phase is low, the over-all control power requirements are at least an order of magnitude less than those for comparable ferrite phase shifters. One of these room temperature operable phase shifters provided 348/spl deg/ of phase shift at 207 Mc. It had an insertion loss which varied from 3.7 to 2.2 db over a zero- to 4000-volt range of applied dc control voltage.

Recent advances in ferroelectric and high temperature superconductive technology have enabled new phase shifters incorporating ferrite, ferroelectric, and high temperature superconductor materials to be developed. Each material offers important and unique properties which may be combined with one of the other materials to obtain useful phase shifters. Ferrite materials with tunable magnetic permeabilities have classically been used for phase shifters. Ferrites offer large phase shifts with small changes in insertion loss. Ferroelectric materials employing variable dielectric constants can also be used as phase shifters. One advantage of ferroelectric phase shifters over ferrite phase shifters, ultra-wideband performance, enables coverage of the entire band from 2 to 18 GHz. Superconductors offer low losses and unique properties including the kinetic inductance effect and SQUIDs. The physics of the three materials will be reviewed briefly, and the difference between ferroelectricity and ferromagnetism will be discussed. Combination of magnetic and superconducting materials is possible, despite the fact that magnetic fields suppress superconductivity. A comparison of variable time delay and phase shift devices will show interesting contrasts between these devices and MMIC components. Superconducting phase shifters using only superconducting properties, and phase shifters combining superconductivity with ferrites or ferroelectrics will be described.

The design and microwave characteristics of a ferroelectric thin film slotline-waveguide phase shifter operating in the millimeter wavelength range (f∼60 GHz) are presented. In this frequency range, the phase shifter possesses a figure of merit F=32 deg/dB and provides for a continuous phase shift in the interval from zero to 255 deg. The ferroelectric films are characterized by a dielectric loss tangent of tan δ=0.04 at a tunability factor of K≈1.7, which are promising parameters for the given frequency range.

A space-fed X-band phased array antenna (PAA) is presented. The space-fed PAA uses physically autonomous individually addressable unit cells as the building blocks of an arbitrarily large array antenna. The X-band PAA unit cell consists of both a feed facing printed antenna element and a target facing printed antenna element. These elements are related by an RF phase shifter placed in the RF signal path between them which introduces relative phase delay and enables analog beam forming. Unit cell autonomy is realized by wirelessly transferring power and command signals required to operate a microcontroller (MCU) and the RF phase shifter at each unit cell. Each unit cell includes RF to DC conversion circuitry as well as an MCU designed to interpret a common control signal and command unit cell phase delay. The wireless phased array unit cell can be replicated in configurations of size and shape that are independent of RF, power, and array module control distribution networks. To demonstrate feasibility a four-element wireless PAA is constructed and antenna patterns for various control states are shown.

The present study discusses the design of microelectromechanical systems (MEMS)-based phase shifters and their application in phased array antenna (PAA) realization. Commercially available MEMS Single-Pole-Double-Throw (SPDT) switches have been employed for the phase shifter design at a frequency of 5.7 GHz (ISM band). These phase shifters are in turn integrated with the feed lines of a 4 × 1 planar patch antenna array such that electronic mode of antenna beamsteering is resulted. It has been observed that a progressive phase shift of 45° between successive patch antenna elements would generate 11° beam tilt with good directivity and a high gain of 9.5 dB. The entire PAA structure along with the antenna elements, phase shifter modules, and the corporate feed network are integrated on the same PCB laminate, Rogers Ultralam 2000™ having permittivity e r = 2.5, thickness h = 30 mil (0.762 mm), and a dissipation factor tan δ = 0.0019 in order to facilitate simplicity in fabrication process.