Richard S. Withers

Current distribution, resistance, and inductance for superconducting strip transmission lines

A method for the calculation of the current distribution, resistance, and inductance matrices for a system of coupled superconducting transmission lines having finite rectangular cross-section is presented. These calculations allow accurate characterization of both high-T/sub c/ and low-T/sub c/ superconducting strip transmission lines. For a single stripline geometry with finite ground planes, the current distribution, resistance, inductance, and kinetic inductance are calculated as functions of the penetration depth for various film thicknesses. These calculations are then used to determine the penetration depth for Nb, NbN, and YBa/sub 2/Cu/sub 3/O/sub 7-x/ superconducting thin films from the measured temperature dependence of the resonant frequency of a stripline resonator. The calculations are also used to convert measured temperature dependence of the quality factor to the intrinsic surface resistance as a function of temperature for an Nb stripline resonator.<<ETX>>

High-power HTS microstrip filters for wireless communication

The performance of narrowband microstrip filters with low insertion loss and high power-handling capabilities made from YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) high-temperature superconducting (HTS) thin films is presented. Results are shown for two different designs that were chosen to optimize the power-handling capability. Both filters have a 2-GHz center frequency and 5 poles that incorporate coupled resonators with 10-/spl Omega/ internal impedances. They were made on 5-cm-diameter LaAlO/sub 3/ substrates. Both designs use parallel-coupled feed lines to avoid current crowding, The first design includes backward- and forward-coupled filters, has 1% bandwidth, and has handled over 25 watts of input power at 10 K with less than 0.25 dB compression. The second design has 1.2% bandwidth and uses only forward-coupled resonators. The dissipation loss is less than 0.2 dB at 45 K and it has a third-order intercept of 62 dBm. Another similar filter handled 36 watts of power at 45 K with less than 0.15 dB compression across the passband. We have developed a technique to visualize the power dissipation of the filter by observing the bubbles created by the filter when submerged in liquid helium, showing areas with local defects or where the current distribution is at its peak value. We also discuss several planar high-power filter issues, including material selection and fabrication, device configuration trade-offs, filter structure optimization, and design approaches to maximize power-handling capacity.

Thin-film HTS probe coils for magnetic-resonance imaging

Several planar coil designs, with integral resonating capacitors, are reported using large-area sputtered YBCO films. One design utilizes a distributed interdigital capacitor, while a second exploits capacitive coupling of two films on opposite sides of a dielectric. Devices of the first design made on LaAlO/sub 3/ have achieved Q>10/sup 4/ at 18.5 MHz, possibly limited by the loss tangent of this substrate. Identical coils made with silver films have Q=10. With the second design, Q>10/sup 4/ was achieved at the even lower frequency of 5.7 MHz.<<ETX>>

High-power high-temperature superconducting microstrip filters for cellular base-station applications

We report narrowband microstrip filters with low insertion loss and high power-handling capabilities made from YBa/sub 2/Cu/sub 3/O/sub 7/(-x) high-temperature superconducting films. One 5-pole filter on a LaAlO/sub 3/ substrate, consisting of backward-coupled and forward-coupled resonators, can handle over 27 W input power at 10 K. It has 1% fractional bandwidth and 10-/spl Omega/ internal impedance. The insertion loss increase in passband at 10 K, as the input power changed from a few mW to 27 W, is less than 0.25 dB. We also report a forward-coupled microstrip filter centered at 2 GHz, with a 1.2% fractional bandwidth, 10-/spl Omega/ internal impedance, and parallel-coupled feed lines. We applied over 10 /spl Omega/ to the filter at 45 K without noticeable degradation of the filter performance. The insertion loss at 45 K is less than 0.2 dB. The return loss is better than 12 dB.<<ETX>>

Narrowband lumped-element microstrip filters using capacitively-loaded inductors

Coupling between microstrip resonators decreases very slowly as a function of the resonator separation. Therefore it is difficult to realize narrowband filters (e.g. <0.1% bandwidth) in microstrip form due to the very weak coupling values required. In this paper, we have developed a class of lumped-element filters that uses capacitively-loaded inductors to give frequency-dependent inductance values. A novel frequency-transformation technique is used in the design process. Using this approach, strong coupling can be used in narrowband filter designs. The frequency-dependent inductance transforms the filter to a narrower bandwidth than the original circuit prototype, and does not require hard-to-realize weak coupling. We present a 0.3% bandwidth superconducting microstrip prototype filter. It was designed with the coupling of a 1% bandwidth filter, and then transformed to 0.3% fractional bandwidth using an appropriate inductance slope parameter, dL/d/spl omega/. Measurement showed good agreement with theory.<<ETX>>

Nuclear magnetic resonance probe coil with enhanced current-carrying capability

The conductive material in an RF coil disposed in the polarizing field of an NMR apparatus in miminized and the current density at each point in the coil kept constant by providing an inductive element and a set of tapered, interidigtated capacitors having a uniform gap therebetween. The invention maximizes the current-carrying capacity of the coil.

High-temperature superconducting delay lines and filters on sapphire and thinned LaAlO/sub 3/ substrates

The very low microwave surface resistance of high-temperature-superconductor (HTS) thin films allows the realization of microwave devices with performance superior to those made by conventional technology. Superconducting delay lines, for example, have very low propagation loss and dispersion. Long, low-loss, superconducting delay lines on both thinned LaAlO/sub 3/ and sapphire substrates are presented. Delay lines with 27- and 44-ns delay have been made, for the first time, on 5-cm-diameter 254- and 127- mu m-thick LaAlO/sub 3/ substrates, respectively. The insertion losses at 77 K and 6 GHz are 6 and 16 dB, respectively. Delay lines with 9-ns delay have, for the first time, been produced on M-plane sapphire substrates and demonstrate, at 77 K, an insertion loss of 1.0 dB at 6 GHz. A 2.5%-bandwidth 10 GHz four pole edge-coupled bandpass filter on M-plane sapphire substrates is also reported. The filter has minimum insertion loss of less than 0.5 dB at 9.75 GHz and 71 K.<<ETX>>

High-temperature superconductive devices on sapphire

The low-loss and uniform dielectric properties of sapphire make it attractive for high-performance microwave devices using high-temperature superconductors. YBa/sub 2/Cu/sub 3/O/sub 7/spl minus//spl delta// films have been deposited on oxide-buffered 5-cm-diameter wafers and demonstrated a surface resistance of 0.5 m/spl Omega/ at 10 GHz and 77 K. Long (9-ns) stripline delay lines have for the first time been produced on these substrates and have a measured insertion loss of 1.5 dB at 6 GHz and 77 K. Design techniques appropriate for the dielectric anisotropy of sapphire are discussed. >

Space-qualified superconductive digital instantaneous frequency-measurement subsystem

We have constructed a five-bit high temperature superconductive digital instantaneous frequency measurement (DIFM) subsystem capable of determining the frequency of unknown signals over a 500-MHz bandwidth, centered on 4 GHz, with a resolution of 17.8 MHz. The system consists of three parts: a limiting amplifier and filter, a comparator and data processing unit operating at room temperature, and a cryogenic subsystem containing five discriminator modules based on superconductive delay lines, GaAs mixers and power dividers. With a single-tone CW input between -40 dBm and +10 dBm, the frequency quantization boundaries of the subsystem are, on average, 3.1 MHz from their design values. This system was built for the High-Temperature Superconductor Space Experiment, Phase II (HTSSE II). It has passed all required qualification tests for vibration, shock and thermal cycling. The system is currently scheduled for satellite launch in fall 1996. Compared to a conventional system, the superconductive version of the DIFM is considerably smaller and provides enhanced resolution. In addition, the technology employed can be readily extended to a system with additional bits and wider instantaneous bandwidth. The system built demonstrates the potential for system-level applications of high-temperature superconductive electronics in instrumentation, communications, radar, and electronic warfare.

Development of a 13C-optimized 1.5-mm high temperature superconducting NMR probe

We report a 1.5-mm NMR probe based on high temperature superconductors operating at 14.1T optimized for (13)C detection. The probe has a total sample volume of about 35 microliters (μL) with an active volume of 20 μL and provides exceptional mass sensitivity for (13)C detection. The probe also has excellent (1)H sensitivity and employs a (2)H lock; (15)N irradiation capability can be added in the future. The coils are cooled to about 20K using a standard Agilent cryogenic refrigeration system, and the sample temperature is regulated near room temperature. The coil design considerations are discussed in detail. This probe is ideal for directly detected (13)C NMR experiments for natural products chemistry and metabolomics applications, for which 35 μL is an optimal sample volume. The outstanding (13)C sensitivity of this probe allowed us to directly determine the (13)C connectivity on 1.1mg of natural abundance histidine using an INADEQUATE experiment. We demonstrated the utility of this probe for (13)C-based metabolomics using a synthetic mixture of common natural abundance metabolites whose concentrations ranged from 1 to 5mM (40-200 nmol).