S. Hirano

Design and Performance of Transmit Filters Using HTS Bulk Resonators for IMT-Advanced Applications

A transmit band pass filter using circular-disk high-temperature superconductor (HTS) bulk resonators was designed with a 5.0 GHz center frequency and a 100 MHz bandwidth for use in international mobile telecommunication (IMT) advanced applications. The frequency response and electromagnetic field were calculated using a three-dimensional simulator. The mode of the resonator was used because it had the smallest diameter. Using stripline (SL) resonators enhanced both the unloaded quality factor and the coupling between the input/output ports and resonators. The frequency response of the designed SL 3-pole Chebyshev filter meets the requirements of IMT-Advanced applications. Additionally, the maximum surface current of the HTS bulk filter was approximately 86% less than that of a conventional hairpin filter.

Design and Fabrication of 5 GHz Miniaturized Bandpass Filter Using Superconducting Microstrip Quasi-Spiral Resonators

We propose a miniaturized novel microstrip quasi-spiral resonator (QSR) system composed of rewound-type spiral resonator and c-type spiral resonator. We have also designed a 5 GHz superconducting 4-pole bandpass filter (BPF) using QSR for international mobile telecommunication (IMT)-advanced base station receiver applications. The filter has been fabricated using a single-sided MgB2 superconducting thin film deposited by thermal co-evaporation technique. The measured performance of the BPF has been found to show close agreement with the simulated one.

Frequency multiply circuit for superconducting A/D converter

A new frequency multiply circuit generating a 20 GHz sampling clock from an external 5 GHz signal for a lowpass sigma-delta modulator was proposed and designed. The multiply circuit was composed of a ladder circuit, a modified Josephson transmission line (JTL) and T-flip flop (T-FF). We confirmed by numerical simulation that the period jitter of SFQ pulse trains generated by the ladder circuit could be reduced to a value small enough to realize 14-bit resolution for 10 MHz bandwidth by utilizing the repulsion effect between SFQ pulses in the modified JTL. The multiply circuit was fabricated by a 2.5 kA/cm/sup 2/ Nb process, and its correct operation was confirmed.

Feedback-current-injection-type second-order lowpass sigma-delta modulator

In order to establish double-loop feedback design, we have fabricated a prototype superconducting double-loop lowpass sigma-delta modulator. For digital feedback gain, multiple flux quanta are produced by a ladder circuit and carried through Josephson Transmission Lines (JTLs) to the feedback driver. The feedback driver consists of a single Josephson junction or two serially-connected junctions. Direct injection of the pulse current from the JTLs through a low-value resistor drives the feedback driver. The sigma-delta modulator was operated at the sampling frequency of 1.1 GHz and the measured power spectrum showed the noise shaping roughly of 12 dB/octave, which is characteristic of the second-order sigma-delta modulator.

High-Tc SQUID magnetometers for use in moderate magnetically-shielded room

We have fabricated and characterized high-T/sub c/ planar SQUID magnetometers and first derivative gradiometers with directly-coupled pickup loops. The devices were made from single layer of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film on LSAT bicrystal substrate with 30/spl deg/ or 24/spl deg/ misorientation angle. Magnetic properties were investigated by applying a magnetic field B/sub 0/ for the SQUID magnetometer patterned with holes to reduce the maximum structural width. We found an increasing low frequency noise with cooling fields B/sub 0/ larger than 1.5 /spl mu/T. This value consists with the threshold field estimated from the maximum structural width. A magnetic field noise level of 500 fT/Hz/sup 1/2/ at 10 Hz was observed by using FLL electronics with a bias current reversal in a moderate magnetically-shielded room consisting of only two 1 mm thick layers of permalloy. Measurements of magnetocardiograms demonstrate the suitability of this magnetometer for biomagnetic applications. On the other hand, the gradiometer with two symmetric pickup loops was operated without any shielding. The performance obtained was a field gradient resolution of about 1 pT/cmHz/sup 1/2/ at 1 kHz and 10 pT/cmHz/sup 1/2/ at 1 Hz, with a baseline of 4 mm. The imbalance of this gradiometer was around 0.7%, limited by the sensitivity to homogeneous field of the SQUID itself.

Second-order lowpass sigma–delta modulator with parallel multiple-flux-quantum feedback driver

Abstract We have made a prototype superconducting double-loop lowpass sigma–delta modulator with a parallel multiple-flux-quantum feedback driver. The feedback driver consists of multi-stage SFQ splitters and SQUID-gates connected in series. The multi-stage splitters forming a binary-tree structure produce multiple numbers of flux quanta for the feedback gain and switching of the SQUID-gates gives feedback to the first integrator. A simulation showed that the feedback duration of the SQUID-gate type was only 20 ps, whereas our earlier type with a ladder circuit of gain 20 requires 526 ps. This reduction of the feedback duration enables much higher speed operation. The fabricated modulator with SQUID-gates was operated at the sampling frequency of 1.2 GHz and showed noise shaping roughly of the second-order.

Design of superconducting band-pass sigma-delta modulators

Abstract We have been developing a superconducting band-pass sigma–delta modulator, which is the elementary circuit of the superconducting A/D converter. We have already obtained the noise-shaping characteristic of a superconducting band-pass modulator experimentally. In this paper we investigate the noise-shaping characteristic of the band-pass sigma–delta modulator from a circuit simulation, and compare it with the experimental results. We confirmed by the simulation that the designed band-pass modulator operated properly. The signal-to-noise ratio (SNR) of the superconducting band-pass modulator was improved by increasing the oversampling ratio. The SNR of 55 dB was obtained at the sampling frequency of 20 GHz and the bandwidth of 100 MHz from the simulation. It was found that the resonant frequency of the modulator decreased with the increase of the sampling frequency. For the design of the superconducting band-pass modulator, it is necessary to take account of this resonant frequency deviation.

SQUID Nondestructive Testing System With Vibrating Normal Pick-Up Coil

A vibrating normal pick-up coil coupled to a superconducting quantum interference device (SQUID) magnetometer as a nondestructive testing system was studied. The vibrating normal pick-up coil measures the magnetic field gradient at a point. Output of the SQUID magnetometer is lock-in detected at the vibration frequency. The frequency of an excitation magnetic field can be set low to detect deep structural flaws. The use of a normal coil vibrated at 20 kHz with output voltage amplified by a low-noise amplifier enabled detection of a cylindrical defect in an aluminum plate with an excitation magnetic field at 40 Hz.

Surface Resistance of YBCO Thin Films under High DC Magnetic Fields

We have studied the magnetic dependence of the surface resistance (Rs) of YBa2Cu3Oy (YBCO) thin films by changing the direction of an applied magnetic field by mean of a micriostrip line resonator method (MLRM). We measured Rs(0), Rs(90) and Rs(45) to which the direction of the applied magnetic field was respectively normal, parallel and at 45° to the film surface. In the low temperature region, (below 40 K), the Rs(0) had low magnetic dependence; however, the Rs(90) and Rs(45) had high magnetic dependence, even below 10 K. We examined the magnetic field dependence of Rs (90) and Rs(0) using the London equation, and found that Rs(90) in the higher temperature region could be mostly explained by the theory.

Superconducting high-order cascaded lowpass sigma-delta modulator

We propose a superconducting high-order cascaded lowpass sigma-delta modulator. The second-order cascaded modulator consists of two single-loop modulators. A part of an integrating inductor of the first-stage modulator forms a primary coil of a transformer. The secondary coil of the transformer forms an integrating inductor of the second-stage modulator. Output of the first integrator is thus magnetically fed to the second integrator through the transformer. Digital outputs from both modulators are input to a delay and a digital differentiator. The quantization noise of the first-stage modulator is canceled by choosing proper transfer functions for the delay and differentiator. This cascaded type modulator eliminates the difficulty of designing a feedback driver in a double-loop type modulator. Analytical calculation of the transfer function showed that the signal-to-noise ratio is comparable to an ideal second-order lowpass sigma-delta modulator at a given sampling frequency.