Publications (10)9.71 Total impact
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ABSTRACT: This paper evaluates the RF tuning and the microwave nonlinear response of SrTiO<sub>3</sub> thin films by measuring the third harmonics and thirdorder intermodulation (IMD) products of several coplanar waveguide transmission lines fabricated directly on SrTiO<sub>3 </sub> thinfilm samples. From these measurements, we obtained the distributed nonlinear capacitance per unit length as a function of RF bias voltage C(V<sub>rf</sub>) using an accurate equivalent circuit to model the spurious signal generation. A unique value of C(V<sub>rf</sub>) is used to describe the thirdharmonic generation at frequencies 3f<sub>1</sub> and 3f<sub>2</sub>, and the IMD products at frequencies 2f<sub>1</sub>+f<sub>2</sub> and 2f<sub>2</sub>+f<sub>1</sub>, where f<sub>1</sub> and f<sub>2</sub> represent the fundamental frequencies of the twotone excitation signal. We compare C(Vrf) with the distributed nonlinear capacitance measured as a function of the dcbias voltage C(V<sub>dc</sub>) and obtain excellent agreement at 50 and 76 K. From these results, we conclude that full tunability can be achieved on nanosecond time scales, and that spurious signals generated in SrTiO<sub>3</sub> ferroelectric transmission lines at microwave frequencies can be modeled based on dcbiased measurements 
Article: Unified Description of Nonlinear Effects in High Temperature Superconductor Microwave Devices
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ABSTRACT: We experimentally demonstrate the equivalence of different manifestations of nonlinear response in high temperature superconductor (HTS) microwave devices. Using a combination of analytical and numerical analysis, we show that the results of intermodulation distortion measurements, harmonic generation measurements, and powerdependent resonator measurements of different coplanar waveguide structures patterned onto the same HTS thinfilm sample all yield approximately the same values for the nonlinear penetration depth. The extraction of an underlying nonlinear material parameter that is independent of the specific device geometry and experimental configuration will allow our results to be quantitatively compared with other nonlinear measurements, and will therefore help in determining the dominant source(s) of nonlinear response in HTS microwave devices. 
Conference Paper: Measurements and Analysis of Microwave Nonlinearities in Ferroelectric Thin Film Transmission Lines
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ABSTRACT: This work evaluates the microwave nonlinear properties of ferroelectric BaSrTiO thin films by measuring the frequency response of several coplanar transmission lines and interdigital capacitor structures as a function of the applied electric field from 150 Hz to 40 GHz. From these measurements, we obtain the distributed nonlinear capacitance C(V<sub>dc</sub>) as a function of dc bias. We also measure the harmonic generation at microwave frequencies in ferroelectric transmission lines, and use an accurate circuit model to obtain C(V<sub>rf</sub>), the nonlinear capacitance as a function of RF bias. Information about the tuning speed of the film is obtained from a comparison between the two nonlinear capacitances. Characterization of this mechanism is also required to assess the spurious signal generation in ferroelectricbased devices 
Article: A CurrentDensity Scale for Characterizing Nonlinearity in High $rm T_rm c$ Superconductors
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ABSTRACT: In this paper, we characterize microwave nonlinearity in a high temperature superconducting (HTS) thinfilm by measuring a geometryindependent currentdensity scale j<sub>o</sub>. The quantity j<sub>o</sub> specifies the strength of a materialdependent nonlinearity, and can be used to calculate the nonlinear microwave response of planar superconducting transmissionline devices. Our procedure for determining j<sub>o</sub> involves microwave measurements on superconducting coplanar waveguide devices patterned onto the HTS sample. In a YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7x</sub> (YBCO) sample, we obtained a maximum experimental value for j<sub>o</sub> of approximately 3.26×10<sup>8</sup> A/cm<sup>2</sup> at 25 K. From the measured temperaturedependence of j<sub>o</sub> in our YBCO sample, and assuming a theoretical pairbreaking current density j<sub>c</sub> equal to 3×10<sup>8</sup> A/cm<sup>2</sup>, we calculated the temperature dependence of the quasiparticle factor b(T). The curve of the experimentally obtained b(T) matched well with a theoreticallypredicted behavior in the scenario of dwave symmetry in the superconducting order parameter.  [Show abstract] [Hide abstract]
ABSTRACT: We present new frequencydependent measurements of both the magnitude and phase of the nonlinear response in superconducting thin films at microwave frequencies obtained using a largesignal network analyzer. Our measurements show that the nonlinear inductance dominates the nonlinear response in thin YBCO films at 76 K, and our analysis yields two currentdensity scales corresponding to the real and imaginary components of the nonlinear response. The currentdensity scale associated with the dominant inductive response likely results from intrinsic pairbreaking, while the currentdensity scale associated with the nonlinear resistive term is smaller than the expected value due to pairbreaking, and could originate from vortex motion or other extrinsic effects.  [Show abstract] [Hide abstract]
ABSTRACT: We propose a modified twotone method that could be used for sensitive measurements of the intrinsic microwave surface impedance (Z<sub>S</sub>) of thin superconductor films and the tanδ of a lowloss dielectric. An opengap resonator scheme is used to measure the penetration depth (λ) of thin superconductor films and extract the intrinsic Z<sub>S</sub> of the superconductor films from its measured R<sub>S</sub><sup>eff</sup> and λ. We use a very small gap of 10 μm between the top plate and the rest parts of the resonator. The tanδ of rutile in the low 10<sup>7</sup> range and the dielectric constant as high as ∼110 are observed at temperatures below 10 K at ∼15.2 GHz, which enable to measure the R<sub>S</sub> of the 10 mmindiameter YBCO films as low as ∼100 μΩ at the same frequency (f). The discrepancy between the R<sub>S</sub><sup>eff</sup> at ∼15.2 GHz and that at ∼8.5 GHz scaled to ∼15.2 GHz appears less than 2% when the relations of R<sub>S</sub> ∝ f<sup>2</sup> and tanδ ∝ f are used. We describe usefulness of our measurement method for measuring the intrinsic microwave properties of various superconductor samples.  [Show abstract] [Hide abstract]
ABSTRACT: This report describes the design and initial noise floor measurements of a radiometric calorimeter designed to measure therapeutic medical radioactive sources. The instrument demonstrates a noise floor of approximately 2 nW. This low noise floor is achieved by using high temperature superconducting (HTS) transition edge sensor (TES) thermometers in a temperaturecontrol feedback loop. This feedback loop will be used to provide absolute source calibrations based upon the electrical substitution method. Other unique features of the calorimeter are (a) its ability to change sources for calibration without disrupting the vacuum of the instrument, and (b) the ability to measure the emitted power of a source in addition to the total contained source power. 
Conference Paper: A superconducting microwave power limiter for highperformance receiver protection
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ABSTRACT: We report on the development of a microwave power limiter based on hightemperature superconductor technology. The power limiter takes the form of a 50 Ω coplanar waveguide transmission line that can be reversibly driven from the lowloss superconducting state into the highloss normal state as microwave currents within the device exceed a critical value. This device has demonstrated very low insertion loss (<0.5dB/cm at 70K, 40GHz) and extremely wide bandwidth (constant impedance up to 40 GHz) in the signalpass state, with variable attenuation in the signalblock state. Switching times for transitions from the signalpass state to the signalblock state are estimated to be on the order of a nanosecond or less. Reversible operation has been demonstrated for continuouswave (CW) signals up to 10 W at 3GHz, and for 100 μs transient signals up to 100W. This device should be valuable for protecting highperformance receiver components from highpower transients encountered in realworld applications.  [Show abstract] [Hide abstract]
ABSTRACT: We describe the design, fabrication, and testing of a superconducting passive nonlinear reference device that has a calculable phase relationship between the fundamental RF drive signal and resulting higher order harmonic components. We show this passive device to be a useful standard for verifying the phase calibration of nonlinear vector network analyzers, as the magnitude and phase of the thirdorder product are found directly from independent measurements of the fielddependent surface impedance. Microwave powerdependent measurements of coplanar waveguide (CPW) resonators fabricated from thin film highT<sub>c</sub> superconductor materials yield the transmission line resistance and inductance per unit length as a function of rf current. With these values and the line geometry we computed both the nonlinear surface impedance of the material and the phase and magnitude of the thirdorder product of a traveling wave in a quasilinear transmission line. To demonstrate our CPW reference device, we measured both the magnitude and the phase of thirdharmonic components generated in a number of 133 mm long meander transmission lines using a commercial nonlinear vector network analyzer. We demonstrate agreement to within 10 degrees between the measured and the predicted phase for thirdharmonic signals relative to the fundamental.  [Show abstract] [Hide abstract]
ABSTRACT: The origin of the detrimental nonlinear response in high T<sub>c</sub> superconductor (HTS) microwave devices is currently not well understood. In order to help elucidate the origin of these nonlinear effects, we have developed a description of the nonlinear response in superconductors in terms of a currentdependent complex conductivity. We demonstrate that such a treatment can consistently describe the results of powerdependent surface impedance measurements in resonator geometries as well as harmonic generation and intermodulation distortion effects in transmission line geometries. This approach yields a deviceindependent quantity that describes the nonlinear response of the superconducting material itself, which is suitable for comparisons of different materials and for material optimization. A further benefit of this description of nonlinear effects is that the relative importance of the nonlinear resistive and inductive components of a superconductor can be examined. We use this approach to predict the phase of the nonlinear response in HTS planar transmission lines, and compare our predictions with new phasesensitive measurements made using a nonlinear vector network analyzer.
Publication Stats
103  Citations  
9.71  Total Impact Points  
Top Journals
Institutions

2006

Polytechnic University of Catalonia
 Department of Signal Theory and Communications (TSC)
Barcino, Catalonia, Spain 
National Institute of Standards and Technology
Maryland, United States
