Jeha Kim

Characteristics of high power capability for high temperature superconducting microwave multiplexer

A high temperature superconducting (HTS) multiplexer consisting of two branch line hybrids and two identical parallel-coupled-line type bandpass filters was fabricated with a center frequency of 10.9 GHz and a bandwidth of 100 MHz. We used single sided YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) epitaxial thin film on MgO substrates prepared by using in situ pulsed laser deposition. Electric connection between adjacent multiplexers was easily done with a microstrip transmission line at the terminations so that the size and weight of the multiplexer could be significantly reduced. The multiplexer showed approximately 0.8 dB insertion loss and 100 MHz bandwidth in a range of -10 dBm to 30 dBm. We observed the third order intercept at 94 dBm and the fifth order intercept point at 86 dBm by using intermodulation distortion (IMD) measurements at T=80 K. Our results demonstrate the possibility of realizing a narrow band HTS multiplexer for high performance.

Millimeter-wave self-radiation from high-Tc Josephson junction arrays

We report the strong microwave Josephson radiation from an array of high-T/sub c/ junctions on a MgO bicrystal substrate from centimeter- to millimeter-wave ranges. The dc bias current was fed to the junction array having parallel geometry with the pair of junctions shunted by superconducting loops. The configuration of bias leads was a series of interlocking dc SQUIDs geometry which guaranteed the oscillation of all junctions at the same frequency. The phase differences between adjacent junctions with different I/sub c/R/sub n/ products could be controlled by an external small magnetic field. For a five-junctions array, we observed the coherent output power of about 13 pW without external magnetic flux, which was nearly five times higher than that of a single bicrystal junction. We observed the Josephson linewidth of the self-radiation in the coherent state to be less than 1 GHz by adjustment of the external flux.

Microwave characteristics of Ag-doped YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films grown on MgO

We prepared the Ag-doped YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (Ag-YBCO) bulk pellets of undoped, 10 wt.%, and 20 wt% and grew high quality thin films on MgO (100) substrates from the pellet targets by pulsed laser deposition. The films are highly c-axis oriented with T/sub c//spl ap/85 K. At T=44 K, the observed critical current density, J/sub c/ was 2.70/spl times/10/sup 6/ A/cm/sup 2/ and 1.17/spl times/10/sup 6/ A/cm/sup 2/ for 10 wt.% and 20 wt.% Ag-YBCO films, respectively. The critical current density, J/sub c/, of the Ag-YBCO/MgO films was improved by 65% with Ag-doing with respect to that of the undoped film. Using meanderline microstrip resonators, we measured the low temperature microwave surface resistance, R/sub s/. The observed R/sub s/ value of the 20 wt.% Ag-YBCO film was as small as 0.71 m/spl Omega/ for at 36 K and 8 GHz. XRD and RBS analysis showed that no silver was in the Ag-YBCO films even for the largest composition of Ag (20 wt.%) in the target pellet.

HTS Multipole Bandpass Filters with Arbitrary Parallel Coupled-Line Length

We report the design, fabrication, and characterization of new high-temperature superconducting (HTS) multipole bandpass filters (BPF) with arbitrary coupled-line length. We fabricated the 2-pole, 4-pole, and 6-pole BPFs using superconducting YBCO epitaxial thin films grown on MgO (100) substrates. The measured insertion losses were -0.5 dB and -1.0 dB at the center frequencies of 12 GHz and 11.5 GHz for the 2-pole and 4-pole filters, respectively. For 6-pole filter, the insertion loss measurements showed no obvious characteristics of a bandpass filter that was qualitatively agreed with the simulation results.

Dependence of microwave properties of HTS parallel coupled-line type 4-pole bandpass filters on enhanced electrodynamic properties of Ag-doped YBCO films

We designed and fabricated high temperature superconducting (HTS) 4-pole Tchebyshev bandpass filters (BPFs) fabricated on Ag (5 wt.%, 10 wt.%, and 20 wt.%)-doped YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films. The measured insertion losses of the filters appeared to interrelate strongly with Ag-doping while center frequency and bandwidth exhibited little change. The filter on Ag (10 wt.%)-doped YBCO film exhibited the lowest insertion loss of 0.22 dB at 37 K at the center frequency of 11.6 GHz. The bandwidth was 400 MHz.

HTS 4-pole bandpass filters with closed loop resonators and coupled lines using YBCO thin films for PCS

We designed, fabricated and characterized new type high temperature superconducting (HTS) 4-pole bandpass filters. We prepared epitaxial YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films on MgO substrates by using a hollow cathode sputtering method. To reduce the physical size of the filter pattern, we used closed-loop resonators and parallel coupled-lines on the basis of a microwave circuit design theory. We observed that the HTS bandpass filters exhibited a center frequency of 3.9 GHz, a low insertion loss of about 0.42 dB, and an high skirt selectivity of 200 dB/GHz. The value of third order intercept measured at 80 K and at center frequency of 3.9 GHz was about 95 dBm.

Measurement on electrical tunability of microstrip line resonators using YBa 2 Cu 3 O 7-δ /SrTiO 3 /MgO multilayer thin films

We have fabricated YBa2Cu3O7-(delta ) (YBCO) superconducting line resonator and tunable line resonator on ferroelectric Sr0.5Ba0.5TiO3 (SBTO) buffered MgO(100) substrate and discussed the frequency shift mechanism of superconductor as a function of temperature and bias voltage, respectively. The resonators were designed using superconducting YBCO epitaxial thin films. Optimized resonator shown the resonant frequency of 10 GHz at 77 K. The YBCO films were grown in situ by pulsed laser deposition technique at 750 degree(s)C and oxygen partial pressure of 200 mTorr. The resonators have linear microstrip line separated by a gap of 5 micrometers and 0.5 mm, respectively. A gap is intentionally introduced to generate mainly a capacitive series reactance. The equivalence circuit of line resonator is a II network consisted of three capacitances. As the series capacitance C12 of SBTO ferroelectric thin films was changed by a bias voltage applied on the strip conductors including the gap, resonance frequency was shifted about 20 MHz from the unbiased center frequency of 10 GHz. The variation of resonance peak could be explained by a serial capacitance model. To find a central frequency mechanism depending on temperature, we fit the raw data using f(T)/f(10 K) and simple power law model. The shifting of the resonant frequencies due to temperature was fit to a two-fluid model, BCS theory and empirical formula. Also the surface impedance of superconducting YBCO films as a function of temperature at 10 GHz has been estimated by a transmission line method.

Miniaturized microstrip multipole low-pass filters using high-temperature superconducting YBa2Cu3O7-x thin films

We have designed the microstrip-type multipole (7-pole and 9-pole) lowpass filters consisting of both transmission lines and open stubs. The filters were fabricated on high temperature superconducting (HTS) YBa2Cu3O7-(delta ) (YBCO) thin films grown on MgO(100) substrates by pulsed laser deposition. For 7-pole lowpass filter, the measured insertion losses were within 0.5 dB, and up to 8 GHz the passband shows very flat with ripples of less than 0.05 dB. For 9-pole filter, we observed the insertion loss of 5.0 dB and the ripples of 0.64 dB. The skirt became steep and the off-band rejection increased large as the number of poles increases.