Power-Dependent Impedance of High Temperature Superconductor Thin Films: Relation to Harmonic Generation

We present measurements of the power-dependent microwave surface impedance of YBa₂Cu₃O_7– thin films, performed using patterned coplanar waveguide (CPW) resonators at 5.87 GHz and 76 K. We compare these resonator measurements with third-harmonic generation measurements performed on CPW transmission lines of the same geometry patterned onto the same thin-film sample at the same frequency and temperature. We find that the power-dependent surface reactance X_s(P_rf) is directly related to the magnitude of the generated third-harmonic signal, indicating a common origin for both of these manifestations of nonlinearity in high temperature superconductor (HTS) devices. These results are consistent with the nonlinear response generated by a current-dependent penetration depth (J), which provides a material limitation on the linearity that can be achieved in any practical HTS microwave device.     

Harmonic Balance Algorithms for the Nonlinear Simulation of HTS Devices

This paper describes the application of Harmonic Balance algorithms to predict nonlinear effects in planar High Temperature Superconductors (HTS) microwave circuits. The resulting algorithms are fast and efficient and can be used both for the characterization of the nonlinearities in the HTS material, and for the prediction of the behavior of an HTS circuit given the parameters of these nonlinearities (such as a dependence of the surface impedance on the current density). Most previously published nonlinear HTS models can be used, because the algorithms are not restricted to a specific model of HTS nonlinearities. Two different types of algorithms are described: (1) algorithms specific for one-dimensional resonators (transmission lines and TM₀₁₀ disk resonators) and (2) an algorithm based on the combination of Method of Moments and Harmonic Balance, applicable to 2D planar structures with few restrictions in their shape. Several cross-checks with theory and measurements are presented.     

Features of technology of high-temperature superconductor films for microwave filters

The characteristics of fourth-order bandpass microwave filters based on high-temperature super-conductor (HTS) films have been numerically simulated and experimentally measured for the devices based on YBCO films purchased from Theva GmbH (Munich) and manufactured at the Institute for Physics of Microstructures (Nizhni Novgorod). Experimental data were used to determine the parameters of the surface impedance model for the HTS films. The results show that HTS-based filters with high performance characteristics can be obtained using HTS films manufactured by different technologies.     

Investigation and Modeling Nonlinear Characteristics of High-Tc Superconducting Resonators

The high-quality resonators and pass-band filters based on high-T _c superconducting (HTS) films are of the most interest for practical applications. The industrial application of the devices is retarded by nonlinearity of the film surface impedance under microwave power of a high level. The phenomenological model of nonlinear characteristics of HTS resonators was developed and applied to different kinds of planar resonators: disk, parallel plate, and micro-strip resonator. A comparison of experimental and modeled characteristics of the resonators revealed the distinctive difference in the model parameters. Some limiting characteristics are discussed.     

Investigation and Modeling Nonlinear Characteristics of High-Tc Superconducting Resonators

The high-quality resonators and pass-band filters based on high-T _c superconducting (HTS) films are of the most interest for practical applications. The industrial application of the devices is retarded by nonlinearity of the film surface impedance under microwave power of a high level. The phenomenological model of nonlinear characteristics of HTS resonators was developed and applied to different kinds of planar resonators: disk, parallel plate, and micro-strip resonator. A comparison of experimental and modeled characteristics of the resonators revealed the distinctive difference in the model parameters. Some limiting characteristics are discussed.     

Monolithic HTS microwave phase shifter and other devices

We describe a monolithic high-temperature superconductor (HTS) phase shifter based on the distributed Josephson inductance (DJI) design integrated monolithically into a 10-GHz microstrip line. This microwave circuit incorporates >1000 HTS rf SQUIDS. Recent data demonstrate the performance of this broadband HTS circuit. We observed phase shifts greater than 150° in resonant structures, and 20° in broadband circuits. The nonlinear inductance of the superconducting transmission line can be used for other novel applications, including parametric amplification. A comparison of the DJI circuit to a series array of Josephson elements (used for pulse sharpening) will contrast these two new and exciting nonlinear transmission line circuits.     

High-temperature superconducting microwave devices: Fundamental issues in materials, physics, and engineering

High-T _c superconductivity has generated a great deal of interest because of the challenges it presents in the fields of material science, condensed matter physics, and electrical engineering, and because of the potential applications which may result from these research efforts. Thin-film passive microwave components may become the first high-temperature superconducting (HTS) devices available for widespread use and commercialization. In this article, we review aspects of material science, physics, and engineering which directly impact high-T _c superconducting microwave devices and discuss issues which determine the performance of these devices. Methods of growing HTS thin films on large-area substrates, techniques for fabricating single-level HTS passive microwave components, and the relevant properties of high-T _c superconducting films are discussed, with a focus on thin films of the HTS material YBa₂Cu₃O_7–. Several known mechanisms for microwave loss in both the superconductor and the dielectric substrate are presented. An overview of the general classes of superconducting passive microwave devices is given, and representative microwave devices which have been recently demonstrated are described in detail. Examples of a select few HTS active microwave components are also presented. Potential microwave applications are illustrated with comparisons to current technology.     

Why are nonlinear microwave systems measurements so involved?

Performing nonlinear measurements on microwave devices is a complex task. This paper introduces, step by step, the key concepts that make the difference between linear S-parameter measurements and nonlinear measurements. The main goal here is to make nonlinear measurements more accessible to the practicing microwave engineer.     

HTS magnets: stability; protection; cryogenics; economics; current stability/protection activities at FBML

The unique features that distinguish a high-temperature superconducting (HTS) magnet from a low-temperature superconducting (LTS) magnet are: (1) operating temperature and (2) temperature span over which the magnet remains superconducting. Thus, for two magnets, one HTS and the other LTS, both satisfying the same set of field specifications, the operating temperature and the operating range of temperature of the HTS magnet are generally an order of magnitude greater than that of the LTS magnet. In this CHATS 2002 Workshop, four issues, i.e., stability, protection, cryogenics, and economic issues will be examined as they are impacted by the two unique features, first as a general introduction to this presentation, between HTS and LTS magnets. This is followed by discussion of economics of large-scale HTS devices, specifically of fusion; electric power devices; high-energy physics; research-purpose NMR; and high-B DC magnets. The paper concludes with a presentation of preliminary results of stability/protection research presently being performed at the FBML.     

Comparison of AC Losses of YBCO Circular Pancake Coils and Infinitely Long Stack Approximation

High-temperature superconducting (HTS) coils are key parts of many large-scale AC applications, such as transformers, superconducting magnetic energy storage, and motors. The estimation of AC losses of pancake coils is necessary for optimizing the design of HTS devices and cryogenic systems. To speed up the computation of AC losses, the numerical model of an infinitely long HTS stack is often utilized. An HTS stack is a good approximation of the circular coil only if the coil radius is sufficiently large, since AC losses will exhibit a stronger asymmetry along the radial direction for small values of coil radii. To assess the validation of an infinitely long stack approximation, the comparisons of AC losses between infinitely long stacks and circular coils with different radii are presented. The turn number varies from 10 to 80. We find that the AC losses of HTS circular coils will gradually increase to the same value as an infinitely long stack with increasing coil radii. A new parameter is proposed to quantitatively describe the correctness of infinitely long stack approximation. Finally, a method of AC loss estimation of HTS pancake coils is proposed.     

On the Power Handling Properties of High Temperature Superconducting Microwave Contiguous Duplexer

The power handling properties of a high temperature superconducting (HTS) microwave contiguous duplexer are measured in this letter. The HTS contiguous duplexer was fabricated using YBa₂Cu₃O₇ (YBCO) thin films on a 0.5-mm-thick LaA1O₃ substrate, operation frequencies of which are centered at 3.25 GHz and 3.75 GHz with 500-MHz-wide channels. The tested frequency response of power handling property of the HTS contiguous duplexer shows that there is a decreased power handling property on the crossover region; however, this microwave nonlinearity does not affect the engineering application of HTS contiguous duplexers and multiplexers.     

Conditions for and limitations of high-power handling capabilities of planar YBa2Cu3O7-x filters

The conditions for the design of compact high-power HTS filters are deduced from basic considerations, and related to the microwave properties of the HTS films and the substrates. Potential mechanisms limiting the linear microwave response of HTS films are analyzed systematically. Magnetic and thermal field breakdowns are found to confine the power handling capability to comparable field levels. While magnetic limitation is related to flux penetration at the lower critical field B_c1, thermally induced quenches occur dependent on the presence of defects and on the thermal conductivity of the substrate. The detrimental role of film inhomogeneities for the linear microwave response is confirmed experimentally for microwave fields at 19 GHz and for dc fields at 87 GHz.     

HTS Surface Wave Resonators

A novel class of surface wave high-temperature superconductivity (HTS) resonators was investigated. The resonator consists of HTS film and one or two dielectric plates. One of the dielectric plates may be the HTS film substrate. Theoretical analysis of the resonator structure was carried out using the partial region method, with the 20 lowest waves being considered in every region. The resonant frequencies, quality of oscillations, and the field and current distributions were calculated. It was shown that there is a high-density and homogeneous microwave current flowing on the film surfaces. Experimental measurements carried out in the 3-cm wavelength range demonstrate good agreement with simulated data.     

HTS Surface Wave Resonators

A novel class of surface wave high-temperature superconductivity (HTS) resonators was investigated. The resonator consists of HTS film and one or two dielectric plates. One of the dielectric plates may be the HTS film substrate. Theoretical analysis of the resonator structure was carried out using the partial region method, with the 20 lowest waves being considered in every region. The resonant frequencies, quality of oscillations, and the field and current distributions were calculated. It was shown that there is a high-density and homogeneous microwave current flowing on the film surfaces. Experimental measurements carried out in the 3-cm wavelength range demonstrate good agreement with simulated data.     

Influence of the calibration kit on the estimation of parasitic effects in HEMT devices at microwave frequencies

In this paper, we investigate how critical the calibration kit is in an accurate estimation of microwave device parasitic elements. The semiempirical cold FET method has been applied to the extraction of the small signal equivalent circuit of several HEMT devices. Two different measurements were made on the same devices by using two calibration kits. The first kit is a commercial one based on the LRM method, whereas the second kit was designed by the authors and fabricated on the same chip of the devices. The discrepancies found in the calculated parasitic elements provided information on the sensitivity of the elements with respect to the calibration kit, and, therefore, on the physical origin of the parasitics. These discrepancies. show that it is possible to evaluate the influence of the contact pads on the electrical behavior of on-chip semiconductor devices by making measurements with different calibration standards.     

Recent Progress of HTS Microwave Applications in Satellite Receiver, Meteorological Radar, Mobile Communication and Radio Astronomy

The applications of high-temperature superconductor (HTS) filters or subsystems to radio astronomy, the third generation (3G) mobile communications, meteorological radar, and satellite are introduced. A wideband HTS filter for the Miyun 50-m radio telescope in Beijing was presented, and a novel resonator and a new style interleaved coupling structure were proposed for the design of the wideband filter. An HTS transceiver subsystem was constructed. A field trial in commercial TD-SCDMA base station network was carried out in Beijing and remarkable improvements were achieved for RF performance in both transmitting and receiving chains. A demonstration HTS meteorological radar station was set up in Beijing and it showed great enhancements in sensitivity and anti-interference ability in urban area wind profile measurements. An experimental HTS microwave receiver front end for satellite applications had been constructed and passed all the space qualification tests. As a payload of an experimental satellite, it was ready for launch.     

Current Distribution and Nonlinearity of Open-Ends and Gaps in Superconducting Microstrip Structures

Superconducting devices are known to produce nonlinear effects. In planar structures, these nonlinearities depend on the current distribution on the strip, which definitely depends on the structure of device. This paper used a numerical method based on 3D-FEM to obtain the current distribution in the open-ends and gaps in the superconducting microstrip structures. This is used to present the nonlinear distributed circuit modeling of these discontinuities and its impact on the nonlinear phenomenon. This nonlinear circuit model is used in the Harmonic Balance (HB) method to analyze nonlinearity in the superconducting microwave devices. Therefore, this simple accurate enough nonlinear circuit model is warmly welcomed to retire the seemingly inevitable use of time- and memory-consuming numerical techniques for nonlinear analysis of discontinuities in superconducting microwave structures. As an example, we analyze a microstrip superconducting end-coupled band pass filter (BPF). These results are very useful for optimizing the resonators of the superconducting microwave filters in order to minimize its nonlinear distortions.     

Advanced High-temperature superconducting components for microwave systems

The application of high-temperature superconducting thin films to microwave systems is expected to lead to major volume and weight savings as well as improved performance. To take full advantage of the properties that the new materials have to offer and justify the additional cooling equipment that accompanies the introduction of superconductivity, many HTS components will have to be integrated. In this paper some of the key microwave circuits that show great promise in this respect, such as multiplexers, circulators, and very narrowband filters, will be discussed and experimental results presented.     

Substrate Contribution to the Surface Impedance of HTS Films on Si

We study the effects of semiconductor substrates on the surface impedance of high-T _c Superconductor (HTS) YBa₂Cu₃O_7−δ (YBCO) films. The characteristic impedance of silicon (Si) (for different doping levels and for different charge carrier scattering times) is evaluated. In particular, the most relevant features of Si electrodynamics are highlighted by the introduction of suitable normalized quantities. The effective surface impedance of the YBCO films on Si substrates is then calculated and discussed for different temperatures and frequencies in the microwave range, comparing the obtained results to their limiting expressions for bulk and thin-film HTS. Our analysis shows how the widely used thin-film approximation for the surface impedance can fail, critically highlighting the conditions it requires to be correctly used. We show that substrate contributions can heavily influence the overall response.     

Recent Progress in Microwave HTS Technologies in Korea and Japan

Activities in fields of microwave HTS technologies in Korea and Japan are introduced on the basis of efforts for commercializing HTS filters, improving the quality of HTS films grown for microwave applications, and setting up the surface resistance measurement methods for quality control of large HTS films. In both countries, receiver front-ends with HTS filters operating at cryogenic temperatures are about to be introduced into the market place for Personal Communication System and International Mobile Telecommunication-2000. Results from field tests of the receiver front-ends as well as attempts to improve microwave properties of HTS films and microwave characterization technologies are described.