The voltage-dependent dielectric constant (ε) of SrTiO3 (STO) thin films is the basis for developing cryogenic capacitors for tunable microwave applications. In this study, the effect of microwave signal level on nonlinear response at 1.7–1.9 GHz was examined by measuring the level of the third order intermodulation distortion (IMD) signal relative to the input signal level. Small signal dielectric properties such as capacitance, tuning, and loss (tan δ) were also measured at 1 MHz, 3 GHz, and 10 GHz, at temperatures from 4.2 to 300 K. Planar capacitors were comprised of highly (100)-oriented, 1 μm thick STO films deposited via magnetron sputtering onto CeO2-buffered (11_02)-oriented sapphire substrates, with 10 μm gaps between the electrodes. Deviations from the anticipated cubic dependence of the third order IMD product on incident power, for incident power ranges from −10 to 22 dBm, were attributed to conductivity nonlinearity. At incident power levels of 22 dBm and with no dc bias applied to the capacitor, the level of the third order IMD product was 21 dB below the fundamental signal level. Application of a 107 V/m dc electric field bias across the capacitor suppressed the third order IMD by an additional 10 dB. The nonlinear properties of thin film STO capacitors as a function of microwave voltage were determined by comparing the experimental and theoretical dependencies of the IMD products.
IntroductionThe increasing need for rapid steering of antenna beams has prompted significant research and development efforts in phased -array technology.Advances in solid -state power amplification now make it possible to construct arrays with individual amplifiers at each radiating element. Consequently, the power loss in the beam -forming network is greatly reduced, since it operates in the low -power regime.Traditionally, beam -forming and steering are performed by individually controlled phase shifters and variable gain amplifiers in the signal path of each array element. Waveguide power splitters distribute the signals to the input ports of the antennas.For large arrays, the hardware necessary to perform this beam -forming function becomes prohibitively complex and heavy. The high prime -power consumption of active phase shifters is a severe disadvantage in many applications.' In this paper, a novel technique will be described to form phased -array antenna beams.This technique can be implemented as a compact system, capable of rapid and high-resolution beam -steering with only azimuth and elevation commands, that is, without digital processing. An optical beam -forming systemThe far -field cross section of an antenna beam can be described as a two -dimensional Fourier transform (FT) of the antenna illumination.In the reverse direction, the illumination is obtained by an FT of Fourier transforming the antenna far -field distribution. This transformation is usually performed by digital computers that sequentially calculate the amplitudes and phases of the array elements across the antenna aperture.With an optical processor, a two -dimensional FT can be performed in real time by farfield diffraction of coherent light.2 Beam formation is therefore possible by coherent illumination of a mask that represents a scaled -down replica of the desired antenna beam, and by down-converting the spatial information of the optical far field into the spectral region where the antenna emits. This is shown schematically in Figure 1. The downconversion is possible with an array of photodetectors, one detector per element in the array antenna.
Multilayered structures of ferroelectric and superconducting films deposited on dielectric substrates are being developed for use in low-loss tunable microwave components for satellite and ground-based communications. In this paper, we report on the electrical characterization of BaxSrl_,TiOs (x = 0, 0.08 and 0.1 O)NBanCu307_s/LaA103 thin-film multilayer structures. These structures were formed in situ using a pulsed laser deposition method by the sequential deposition of the YBa2Cu307_s (YSCO) high-temperature superconductor (HTS) and the SrTi03 (STO) or Bao.loSro,goTi03 (BST) films on 20 mil (0.5 mm) thick (100) LaA103 single-crystal substrates and by using a metal organic deposition method to deposit the Bao.osSro.g2Ti03 films. These processes produced YBCO films with thicknesses of 300 nm and ferroelectric films with thicknesses of 250 and 500 nm. The YBCO films showed a transition temperature (TJ above 89 K after deposition and processing of the ferroelectric layers. Electrical characterization of these structures was performed by measuring the capacitance (C), relative dielectric constant (+), and loss tangent (tans) of the ferroelectric film in the temperature range from 300 to 40 K and at electric fields (E) from zero to 2.0 x lo5 V cm-'. For 500 nm and 250 nm thick BST films having the same stoichiometry, er values of 191 and 126, respectively, were obtained at 300 K, 1.0 MHz, and zero E . Corresponding tans values of 0.096 and 0.021 were measured for the 500 nm and 250 nm thick samples, respectively, at the aforementioned temperature, frequency and E . It was observed that the amount of variation of E , as a function of E was closely related to the microstructure of the films. Evidence of conductive pathways in the ferroelectric film as well as of ferroelectric domain 'lock-in' under a d.c. bias is presented.
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