In frequency-domain multiplexing (FDM) for TES signals, a magnetic field summation method utilizing a multi-input SQUID has the fundamental merit of small degradation of the signal-to-noise ratio. We formulated shifts of the operation point due to a common impedance and cross talk currents. These effects are evaluated for several FDM methods, and the requirements for the bandwidth and filters are summarized. The design parameters of multi-input SQUIDs and a flux locked loop driving circuits are also presented.

Collective transverse plasma modes in Bi₂Sr₂CaCu₂O_8+x intrinsic Josephson junctions (IJJs) can be excited by the moving fluxon lattices. Progressive transformation of the standing-wave-like fluxon-lattice configuration from a triangular lattice to a rectangular lattice takes place as the dynamic fluxon-lattice modes are in resonance with the collective transverse plasma modes. In this paper, we review the progress in terahertz-frequency-range electromagnetic wave generation from the IJJs using the resonance between moving fluxon lattice and the collective transverse plasma modes.

We describe three types of software-defined-radio (SDR) receivers based on superconducting technologies. The superconducting analog bandpass filters are essential for all types of the receivers. Another key component is an analog-to-digital converters (ADCs), which are required to have high resolution with a broad band width. The complementary Δ ADC based on the single-flux-quantum circuit is a promising candidate for the SDR receivers because it has a practical nature together with above-mentioned requirements. The experimentally obtained signal-to-noise ratio (SNR) and sensitivity, which are closely related to the resolution, are 34 dB and 20 µA for a quarter of the full-scale input with a band width of about 20 MHz. If we use the optimum decimation filter, the ADC is expected to have the SNR of 82 dB and the sensitivity of 300 nA. These values meet the requirements of the easiest type of the SDR receiver. After new fabrication process has been introduced and the architecture of the ADC has been improved, all types of recievers could be realized based on superconductors.

We have examined the improvement of filter properties using sapphire and nickel rod trimmers. We measured the resonance frequency of the hairpin resonator in the filter, and examined the difference between the simulated and measured values. When the measured resonance frequency was lower than the simulated frequency, we used a nickel trimmer to increase the resonance frequency, and when high, a sapphire trimmer to decrease the frequency. Our results showed that the use of sapphire and nickel rod trimmers is effective in improving the frequency response of HTS bandpass filters.

A system was developed to measure the microwave power dependence of the surface resistance superconductor films. The system uses a dielectric resonator method combined with a circle fit technique and a two-mode technique to measure the microwave surface resistance of superconductor films. For validation, this system was used to measure such surface resistance for superconductor films with different surface morphologies. Significant difference in microwave power dependence of surface resistance was observed. This measurement system proved suitable for evaluating superconducting films for passive microwave devices, including high power devices such as transmitting filters.

A dielectric resonator with a gap between the top plate and the rest has been useful for measuring the penetration depth (λ) of superconductor films, a parameter essential for obtaining the intrinsic microwave surface resistance (R_s) of thin superconductor films. We investigated effects of a gap on the microwave properties of TE_0ml-mode sapphire resonators with a gap between the top plate and the rest of the resonator. Regardless of a 10 µm-gap in TE_0ml-mode sapphire resonators, variations of the TE_0ml-mode resonant frequency on temperature (Δf₀) as well as TE_0ml-mode unloaded Q remained almost the same due to lack of axial currents inside the resonator and negligible radiation effects. The λ of YBa₂Cu₃O_7-δ (YBCO) films obtained from a fit to the temperature-dependent Δf₀ appeared to be 195 nm at 0 K and 19.3 GHz, which was well compared with the corresponding value of 193 nm at 10 kHz measured by the mutual inductance method. The intrinsic R_s of YBCO films on the order of 1 mΩ, and the tan δ of sapphire on the order of 10^-8 at 15 K and 40 GHz could be measured simultaneously using sapphire resonators with a 10 µm-gap.

A novel high temperature superconducting interdigital bandpass filter is proposed by using coplanar waveguide quarter-wavelength resonators. The CPW resonators are arranged in parallel, and consequently the filter becomes very compact. The filter is a 5-pole Chebyshev BPF with a midband frequency of 5.0 GHz and an equal-ripple fractional bandwidth of 3.2%. It is fabricated using a YBCO film deposited on an MgO substrate. The measured filtering characteristics agree well with EM simulations and show a low insertion loss in spite of the small size of the filter.

We propose a design theory of the miniaturized high temperature superconducting (HTS) coplanar waveguide (CPW) bandpass filter (BPF), which is composed of meanderline quarter-wavelength resonator, J- and K-inverters. The J- and K-inverters are realized by using interdigital gap and meander-shape inductor. To evaluate the kinetic inductance of the K-inverter, we fabricate the YBCO resonator connected with K-inverters and redesigned the YBCO filter parameters. Finally, we designed and fabricated the YBCO CPW quarter-wavelength resonator BPF by taking account of the kinetic inductance of the K-inverter. The experimental results are in agreement with the design parameters.

A coplanar type lumped-element 6-pole microwave Chebyshev bandpass filter (BPF) of center frequency (f₀) 2.0 GHz and fractional bandwidth (FBW) 1.0 % was designed. For the design method, theory of direct coupled resonator filters using K-inverters was employed. Coplanar type lumped-element BPFs are composed of a meander-line L and interdigital C elements. The frequency response was simulated and analyzed using an electromagnetic field simulator (Sonnet-EM). Further, the changes in f₀ and FBW of the BPF were also realized by the mechanical tuning method.

This paper presents RF power properties of YBCO films on LaAlO₃ substrate examined using microstrip disk shape-patterned resonators for around 4 GHz. Characterization of the YBCO films for the resonators was carried out. For input-output power measurement, the resonators with the condition of strong I/O couplings were designed and fabricated. The RF power properties of the resonators at cryogenic temperatures were measured. The resonator condition to obtain the handling power over 10 W at 30 and 70 K was confirmed. The temperature dependence of the break-down powers at 4 GHz for the resonators showed two broad peaks. We discuss the unique phenomenon with magnetic flux model, as referred to the YBCO films characteristics results.

This paper describes the first experimental results for a waveguide-type all-NbN superconductor-insulator-superconductor (SIS) heterodyne mixer on an MgO substrate designed to operate over the gap frequency of Nb. The mixer consists of an NbN/MgO/NbN junction, which has a length of one wavelength at 880 GHz as a tuning circuit, an NbN/MgO/NbN microstrip as a λ/4 impedance transformer, and an RF choke filter. The mixer chip was designed using a high-frequency-structure simulator. Its return-loss and embedding-impedance characteristics were examined using a 180-times-scaled mixer model. By optimizing the cutting and polishing processes for the MgO substrate, we were able to fabricate the mixer chip with an accuracy of less than 5 µm. We succeeded in mounting the chip on a mixer block and in estimating the receiver noise temperature. The uncorrected minimum double-sideband receiver noise temperature was 740 K at 824 GHz. A comparison of the receiver noise temperature in a quasi-optical SIS mixer fabricated on the same wafer as the waveguide mixer showed that input noise was the major contributor to receiver noise in the waveguide mixer.

We report on the design and experimental results of a fix-tuned Superconductor-Insulator-Superconductor (SIS) mixer for Atacama Large Millimeter/submillimeter Array (ALMA) band 8 (385-500 GHz) receivers. Nb-based SIS junctions of a current density of 10 kA/cm² and one micrometer size (fabricated with a two-step lift-off process) are employed to accomplish the ALMA receiver specification, which requires wide frequency coverage as well as low noise temperature. A parallel-connected twin-junction (PCTJ) is designed to resonate at the band center to tune out the junction geometric capacitance. A waveguide-microstrip probe is optimized to have nearly frequency-independent impedance at the probe's feed point, thereby making it easy to match the low-impedance PCTJ over a wide frequency band. The RF embedding impedance is retrieved by fitting the measured pumped I-V curves to confirm good matching between PCTJ and signal source. We demonstrate here a minimum double-sideband receiver noise temperature of 3 times of quantum limits for an intermediate-frequency range of 4-8 GHz. The mixers were measured in band 8 cartridge with a sideband separation scheme. Single-sideband receiver noise below ALMA specification was achieved over the whole band.

We fabricated Josephson vortex flow transistors (JVFTs) with a parallel array of Josephson junctions that were prepared using c-axis-oriented 100-nm-thick YBa₂Cu₃O_7-δ (YBCO) thin films grown on 24bicrystal MgO (100) substrates. We observed clear modulations of the critical current and the flow voltage with DC current input to the control line that was inductively coupled to the array of junctions. From the results, we estimated the parameters of the device, e.g., the mutual inductance and the self-inductance, and calculated the operation frequency at which the device potentially exhibited these parameters. Moreover, the current gain and the transresistance were evaluated and found to be 0.5 and 0.15 Ω, respectively. In addition, we observed the high-frequency responses of the JVFT to the input AC current of the sine wave or the square pulse wave. A clear oscillation of the output voltage could be observed with a 1 MHz sine wave and 250 kHz square pulse wave. We also discussed the feasibility of higher frequency operation by using it as an input interface for a single flux quantum (SFQ) logic circuit.

Double-sided YBa₂Cu₃O₇ (YBCO) films were successfully prepared on 50-mm-diameter CeO₂-buffered sapphire substrates by metalorganic deposition (MOD) process using an acetylacetonate coating solution. Mapping analysis of superconducting current densities (J_c) at 77.3 K revealed that J_c values of the double-sided films indicated in excess of 2 MA/cm² in the center parts with a small decrease of J_c at the outer side of the specimens. The J_c values of one side (A) are higher than those of the other side (B). Microwave surface resistance (R_s) of sides A and B of the film exhibited 0.57 and 0.60 mΩ, respectively, at 70 K (12 GHz). The difference in the R_s values should be attributed to the slight difference in the J_c values, which arose from the surface morphology of the CeO₂ buffer layer and heat treatment conditions during the firing process in MOD.

The 5-cm-diameter double-sided YBa₂Cu₃O₇ (YBCO) films were prepared by metal organic deposition (MOD) using a commercially available metal-naphthenate coating solution. Firstly, YBCO film was prepared by MOD on one side of a double-side-polished 5-cm-diameter LaAlO₃ substrate. Secondly, another side was similarly coated with YBCO by MOD. After the latter processing, degradation of average J_c value in the first side was not observed; but the fluctuation of critical current density J_c slightly increased. The double-sided YBCO films showed average J_c values of 0.8-1.0 MA/cm² at 77 K and microwave surface resistances R_s(12 GHz) of 0.86-1.07 mΩ at 70 K.

We have developed a novel type of holey fiber which has a conventional raised-index core surrounded by two layers of air holes with different sizes. The fiber has single-mode operation and shows a low bending loss even for an extremely small bending diameter and a low splicing loss for fusion splicing to a conventional single-mode fiber. The structure and the properties of the fiber are reported in this paper.

Novel microstrip dual-band bandpass filters with controllable fractional bandwidths and good in-between isolation are presented and implemented. A half-wavelength stepped-impedance resonator is firstly characterized, aiming at producing the two resonant frequencies at 2.4 and 5.2 GHz. Two types of coupled microstrip lines in the parallel and anti-parallel formats are then investigated in terms of unified equivalent J-inverter network. Extensive results are derived to quantitatively show their distinctive frequency-distributed coupling performances under different coupling lengths. The coupling degrees of these two coupled lines at the two resonances are properly adjusted to achieve the dual-passband response with varied or tunable bandwidths. In addition, the parallel coupled line is modeled to bring out a transmission zero between the two resonances so as to achieve the good in-between isolation. The three two-stage bandpass filters are initially designed to exhibit their dual-band response with changeable dual-band bandwidths. A three-stage dual-band filter is in final optimally designed and its predicted performance is confirmed in experiment.

A low-noise, low-power 10-GHz CMOS VCO was developed using cost-effective 0.18-µm CMOS technology. A complementary cross-coupled topology was employed to decrease the power dissipation and phase noise. The fabricated VCO demonstrates a low phase noise of -106 dBc/Hz at an offset frequency of 1 MHz and a low power dissipation of 4.4 mW.

A new wide-dynamic-range CMOS image sensor with pixel-level analog-to-digital (A/D) converters is proposed. The pulse-counters in the sensor are arranged outside the pixel area in order to reduce pixel size, which is a key requirement for making high-definition cameras. A new scheme called variable threshold operation is also presented as a suitable readout operation method for the sensor. Experimental measurements on a prototype sensor show that the prototype can obtain linear output response proportional to illumination by applying the new readout operation.