Quasi-optical millimeter-wave band-pass filters using high-Tc superconductors

Quasioptical millimeter-wave band-pass filters using YBa₂ Cu₃O₇ high-T_c superconducting films were fabricated on MgO and LaAlO₃ substrates. Transmitted power through the filter was investigated in the 75 GHz to 110 GHz frequency range at temperatures ranging from 15 to 300 K. At 15 K the measured center frequency and the bandwidth of the superconducting filter were 92 GHz and 0.85 GHz, respectively. Measurements of YBa₂Cu₃O₇ filters were compared with similar filters fabricated using gold. At 15 K and 92 GHz, an improvement of 75% in the quality factor of the superconducting filter was obtained compared with a similar gold filter     

High-Tc superconducting monolithic phaseshifter

A high temperature superconducting (HTS) X-band phase shifter using a distributed Josephson inductance (DJI) approach was designed and fabricated. Phase swings of over 60° were measured at 65 K and below, with measurable phase shifts at temperatures above 77 K. High quality HTS films and superconducting quantum interference devices (SQUIDs) were deposited by laser ablation. A total of 40 HTS step edge SQUIDs were successfully integrated into a monolithic HTS circuit to produce a phase shifter in a resonant configuration. The magnitude of the Josephson inductance is calculated and a lumped element model is compared to measurements     

Field theory investigation of high-Tcsuperconducting coplanar waveguide transmission lines andresonators

An investigation is conducted of a coplanar waveguide structure using a partial wave synthesis taking into account the complex conductivity of the high T_c superconductor material according to the two-fluid model and the London theory. Micrometer transmission line dimensions are considered in the frequency range up to 100 GHz. The results obtained for superconductors are also compared with the results obtained for superconductors with real conductivity and the same geometry. The behavior of measured and published coplanar waveguide half wavelength resonators made of high-T_c superconducting thin films is also examined     

Surface resistance measurements of HTS films by means of sapphiredielectric resonators

A sapphire dielectric resonator with a copper cylindrical shield and two endplates replaced by high-temperature superconducting (HTS) layers was used for very accurate surface resistance measurements on laser-ablated YBCO films. A system using the TE₀₁₁ mode has a resonant frequency of about 18.1 GHz and parasitic-loss Q factor of about 120000. It allows 10 mm×10 mm samples to be measured with sensitivity of ±30 μΩ. Individual samples can be measured with somewhat lower accuracy. Using larger HTS samples, one can reduce parasitic losses of the system to an unsignificant level. The exact formulas presented for the resonant system allow for avoiding calibration procedures during the evaluation of the surface resistance     

Circulating power, RF magnetic field, and RF current density ofshielded dielectric resonators for power handling analysis ofhigh-temperature superconducting thin films of arbitrary thickness

In the current quest for HTS films with negligible power effects at high RF power levels for wireless communications, accurate calculations of a maximum RF magnetic field H_max and of a maximum RF current density J_max flowing on the surface of superconducting films is necessary to allow for any sensible conclusions and comparisons. As the dielectric resonator method is used most frequently for investigation of HTS losses, the authors discuss in this paper a dependence of the circulating power and of a maximum RF magnetic field H_max on dielectric resonators' geometry as well as of the maximum RF current density J_max flowing on the surface of superconducting films on the films' thickness, for a general case of a resonator shielded in a metallic cavity. The authors' results demonstrate that under the same input power levels the same HTS films may be exposed to differing RF power level conditions, depending on the cavity to dielectric radius ratio and thickness of superconducting films. This means that there may be a significant discrepancy between calculated and real power handling capabilities of HTS films tested in different dielectric resonators unless correct formulas are used     

5 GHz high-temperature-superconductor resonators with high Q and low power dependence up to 90 K

The authors have fabricated high-temperature superconducting films made of TlBaCaCuO (2212) and YBaCuO (123) by postdeposition annealing techniques on (100) LaAlO₃ substrates. These films, especially the TlBaCaCuO(2212), exhibit high temperature operation, high Q (low surface resistance), and low power dependence. Both types of films have measured surface resistances which are better than 1/10 that of copper to 20 GHz. Microstrip resonators with a fundamental resonance frequency of 5 GHz were fabricated from these materials. The performance of the best resonator at 90 K (loaded Q>20000 at 5 GHz) was 50 times better than an analogous copper resonator (also measured at 90 K) and can handle more than 10 W of peak power in the resonator with only a small degradation of the Q. In addition, the shift of the resonator frequencies with temperature was fit to a two-fluid model     

Tunable Solidly Mounted Thin Film Bulk Acoustic Resonators Based on BaxSr1−xTiO3 Films

Electrically tunable solidly mounted thin film bulk acoustic resonators based on Ba_xSr_1-xTiO₃ films are reported for the first time. The films are acoustically isolated from the silicon substrate by a Bragg reflector stack. Applying DC bias induces piezoelectric effect and an acoustic resonance at approximately 4 GHz. Under 10 V applied DC bias the resonance frequency of the resonators based on Ba_0.25Sr_0.75TiO₃ films is tuned 1.2% to lower frequencies. The Q-factor of these resonators is approximately 120, and the electromechanical coupling coefficient is 0.5%. The resonant frequency of the BaTiO₃ based resonators shifts upwards 1.3% under 10 V DC bias, and the -factor is approximately 30, with an electromechanical coupling coefficient of 6.2%.     

Theory of digital phase shifters based on high-Tcsuperconducting films

A theory of digital phase shifters based on transition from a superconducting (S) state into a normal (N) state in high-T_c superconductor (HTS) films is developed. It is shown that the insertion-loss level in the phase shifters is determined by the commutation quality of the HTS film and does not depend on the S-N switching element shape and size. The procedure for designing the S-N phase shifter with minimum insertion loss is equivalent to designing a matching circuit for the termination Z=√R_SR_N (R_S and R_N are the resistances of the switching element in the S- and N-states, respectively). Different versions of S-N phase shifters are suggested and discussed. High-quality S-N phase shifters can be successfully designed in a 1-10 GHz frequency range and find application on the receiving end of microwave systems     

Laser processing of high-Tc superconductingthin films

High-T_c superconducting thin films can be deposited and processed by pulsed and CW lasers, and a respectable materials technology for the Y-Ba-Cu-O superconductor is rapidly emerging. The pulsed laser deposition technique is simple because it produces films with compositions nearly identical to those of the target pellets. A larger variety of substrates can be used, compared to other deposition technologies, because of the relatively low temperature requirements. The laser deposition mechanism has been investigated. As-deposited superconducting films, epitaxial films with smooth surfaces, and multilayer structures with abrupt interfaces have been produced. The electrical transport properties can be changed locally using a focused argon-ion laser by modifying the oxygen stoichiometry. This laser writing can be erased by room-temperature exposure to an oxygen plasma. Other laser patterning methods such as material removal, melt-quench, and direct pattern transfer are being developed     

A low-loss bandpass filter using electrically coupled high-Q TM01δ dielectric rod resonators

For a TM_01δ mode dielectric rod resonator placed coaxially in a TM₀₁ cutoff circular waveguide, characteristics such as the resonant frequency, its temperature coefficient, the unloaded Q, and the other resonances are discussed on the bases of accurate calculations using the mode-matching method. The results show that this resonator compares favorably with a conventional TE_01δ mode dielectric resonator, particularly for realization of a high unloaded Q. Analytical results also verify that interresonator coupling between these two resonators can be expressed equivalently by a capacitively coupled LC resonant circuit. A four-stage Chebyshev filter having a ripple of 0.035 dB and an equiripple bandwidth of 27 MHz at a center frequency of 11.958 GHz was fabricated using these resonators. Its insertion loss is 0.5 dB, which corresponds to an unloaded Q of 17000, and no spurious response appears in the frequency range below 17 GHz     

Slotline ring resonator test method for high-Tcsuperconducting films

A novel method of testing high-T_c superconducting (HTS) films by means of a slotline ring-like resonator is presented. This method does not require any metallic contact with the HTS films and ensures that the tests have quick turnaround times. Small samples can be tested very easily. The resonator circuit used has been studied analytically using Sonnet Inc.'s em software. The parameters of the HTS films are obtained by fitting the theoretical results to the experimental ones. YBa₂Cu₃O_7-δ films on LaAlO₃ have been tested and the theoretical and experimental results are presented and compared. The results match very well. This method is useful for quick screening HTS films since no patterning of the film is required     

High-temperature superconducting delay lines and filters onsapphire and thinned LaAlO3 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 LaAlO₃ 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-μm-thick LaAlO₃ 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     

YBCO superconducting ring resonators at millimeter-wave frequencies

Superconducting microstrip ring resonators operating at 35 GHz have been fabricated from laser ablated YBa₂Cu₃O _7-x (YBCO) films on lanthanum aluminate substrates. The circuits consisted of superconducting strips over normal metal ground planes. The circuits are measured from 20 K to 90 K and with microwave input powers ranging from 0.25 mW to 10 mW. The superconducting resonators show significant improvement in Q over identical gold resonators at 20 K, but only marginal improvement at 77 K. No variation in the superconductor performance is observed with varying input power. The lowest microwave surface resistance of the superconductors at 77 K is 9 mΩ. The change in the resonant frequency with temperature is analyzed and a value for the penetration depth computed. Double resonances were observed in some superconducting ring resonators and an explanation is advanced. Factors limiting millimeter-wave high-temperature superconductor circuits are explored and potential performance levels calculated based on current reported values for high-temperature superconductor surface resistances     

Miniature Dual-Band Filter Using Quarter Wavelength Stepped Impedance Resonators

A miniature dual-band filter using quarter wavelength (lambda_g/4) stepped impedance resonators (SIRs) is proposed. Short and open SIRs are coupled together to realize lower and upper passbands, respectively. Miniaturization is achieved due to the use of lambda_g/4 resonators and a combline coupling structure. Two transmission zeros in a mid-stopband and one in each lower and upper stopbands are achieved. In order to see the capability of this structure to achieve different second passband frequencies, two dual band filters at frequencies of 2.45/5.25 GHz and 2.45/5.75 GHz are realized. Measured insertion losses are 1.3 dB and 2.3 dB and return losses are better than 17 dB and 18 dB at the first and second passband frequencies, respectively, with a mid-stopband attenuation better than 30 dB. The size of the filter is as compact as 19.0 times 5.2 mm² on a RO 4003C (epsiv_r = 3.38, h = 0.81 mm) substrate.     

Application of high Tc superconductors asfrequency selective surfaces: experiment and theory

The use of YBa₂Cu₃O_7-x and Tl₂CaBa₂Cu₂O₈ high-temperature superconducting thin films to fabricate frequency selective surfaces (FSS) at millimeter-wave frequencies (75-110 GHz) is discussed. An analytical/numerical model was applied, using a Floquet expansion and the method of moments, to analyze bandstop superconducting frequency selective surfaces. Experimental results were compared with the model, and showed agreement with resonant frequency prediction with an accuracy of better than 1%. The use of the superconducting frequency selective surfaces as quasi-optical millimeter-wave bandpass filters was also demonstrated     

Compact dual-band bandpass filter using trisection hairpin resonator for GPS and WLAN applications

A compact dual-band bandpass filter using trisection hairpin-shape resonators has been designed and fabricated. Measured results are in good agreement with electromagnetic simulation results and verify that two central frequencies of the fabricated filter are located at about 1.57 GHz (GPS application) and 2.40 GHz (WLAN application). The measured return losses in both 1.57 and 2.40 GHz bands are higher than 16 dB while the measured insertion losses are about 1 dB in the two passbands. The circuit size of the proposed filter occupies only 26 times 26 mm (sime0.22lambda_g times 0.22lambda_g).     

A new high performance phase shifter using BaxSr1-xTiO3 thin films

In this paper, a new device topology has been proposed to implement parallel plate capacitors using Ba_xSr_1-xTiO₃ (BST) thin films. The device layout utilizes a single parallel capacitor and minimizes conductor losses in the base electrode. The new design simplifies the monolithic process and overcomes the problems associated with electrode patterning. An X-band 180° phase shifter has been implemented using the new device design. The circuit provided 240° phase shift with an insertion loss of only 3 dB at 10 GHz at room temperature. We have shown a figure of merit 93°/dB at 6.3 GHz and 87°/dB at 8.5 GHz. To our knowledge, these are the best figure of merit results reported in the literature for distributed phase shifters implemented using BST films at room temperature     

Characteristics of smooth and periodic HTS coplanar resonators on ferroelectric/dielectric substrates

Characteristics of smooth and periodic (consisting of sections with different dimensions) high temperature superconducting (HTS) thin film coplanar waveguide (CPW) resonators with a ferroelectric component were calculated. Frequency tunability and microwave (MW) losses of the structure were analyzed using the obtained formulas. It was shown that for the ferroelectric layer thickness less than 100 nm, the MW losses are determined by the losses of HTS electrodes, while for thicker layers, the MW losses are completely determined by the loss tangent of the ferroelectric. Experimental aspects of the problem were investigated in the example of a YBCO/Ba/sub x/Sr/sub 1-x/TiO/sub 3/ smooth CPW resonator at 77 K and information about the permittivity and the loss tangent of the ferroelectric was obtained.     

A hybrid phase shifter circuit based on TlCaBaCuO superconductingthin films

A superconductor-semiconductor hybrid reflection-type phase shifter circuit has been designed, fabricated, and characterized for 180° phase bit with center frequency of 4 GHz and bandwidth of 0.5 GHz for operation at 77 K. All of the passive components of the phase shifter circuit such as input/output feed lines, 3 dB Lange coupler, impedance matching networks, and transmission lines consisted of thallium based superconducting TlCaBaCuO thin films of 4000 Å thickness on lanthanum aluminate substrate. Metal-Schottky field-effect-transistors (MESFET's) on GaAs semiconductor were used as active devices for switching action (on-state and off-state) in the phase shifter circuit. The phase shift and insertion losses were investigated as a function of frequency from 3.6 to 4.6 GHz at 77 K. The circuit exhibited a fairly flat response of 180° phase shift with a maximum deviation of less than 2° and a maximum insertion loss of 2 dB for on-state and 2.2 dB for off-state conditions over 0.5 GHz bandwidth at 4 GHz. The insertion losses were also fairly flat within the bandwidth. The insertion losses were constant between 50 and 80 K, giving the circuit a large range of operation at or below 77 K. The performance of this circuit as compared to a gold microstrip-semiconductor circuit designed identically was superior by a factor of 1.5, and may be due to lower conductor losses and lower surface resistance in the superconducting microstrips     

Design of high-temperature superconducting low-pass filter forbroad-band harmonic rejection

A new type of low-pass filter based on a coupled line and transmission line theory is proposed for suppressing harmonics and spurious frequency. The seventh-order coupled-line low-pass filter with strip line configuration was fabricated using a high-temperature superconducting (HTS) thin film on MgO(100) substrate. The seventh-order low-pass filter has five attenuation poles in the stop band. Its cutoff frequency is 1 GHz with a 0.01-dB ripple level. The fabricated filter exhibits a characteristic stop band in the range from 1 to 7 GHz. A measured insertion loss at the pass-band of the HTS low-pass filter is 0.2 dB at 77 K, while the maximum return loss is 17.1 dB. These results match well with those obtained by the EM simulation. The new low-pass filter structure is shown to have attractive properties such as compact size, wide stop band range, and low insertion loss