Applied Superconductivity最新文献

Nuclear magnetic resonance (NMR) experiments at ultralow temperatures place high demands on superconducting quantum interference device (SQUID) amplifiers. We have developed a suitable direct-coupled readout SQUID electronics system, which utilizes additional positive feedback (APF), and have achieved a system bandwidth of 5 MHz, maximum slew rate of 1.3×10⁷ Φ₀/s at 14 kHz and a white noise level of 3.4 μΦ₀/$\text{Hz}$. We have also investigated improving the signal-to-noise ratio by using a low noise series array of dc SQUIDs and by placing the SQUID amplifier closer to the ultralow temperature experiment. We will discuss how we have configured our SQUID amplifiers for operation at T≪1 K.

Low noise dc-SQUID systems allow noninvasive measurements of magnetic fields generated by electric currents in human peripheral nerves. High-resolution magnetoneurography techniques were used to detect the magnetic fields of stimulated leg nerves with amplitudes of only 10–20 fT over the lower spine. A signal-to-noise ratio of about 10 was achieved after a special designed signal processing routine which was sufficient for an analysis of the source current, i.e. in particular the estimation of its location, strength, and spatial extent.

We designed and measured a high-Tc superconductor sampler circuit based on ramp-edge junctions with an upper-layer groundplane. Undesirable current distributions as a result of parasitic inductance occurred in the circuit. We experimentally observed that about 30% of signal and feedback current distributed to the control line of the read-out SQUID. During sampler operation, we avoided the effect of distributed current by selecting a suitable detection timing for output voltage and we successfully measured a signal current waveform using the circuit at 50 K.

The sigma-delta architecture is the method of choice for designers and manufacturers of analog-to-digital converters (ADCs) for high dynamic range applications. This architecture uses oversampling and precise feedback to generate a shaped spectral distribution of the quantization noise. Subsequent digital filtering suppresses out of band quantization noise, yielding a large signal to in-band noise ratio. This permits the use of quantizers with only a few bits of resolution, most applications use single-bit quantizers. A unique advantage of superconducting electronics is the availability of the flux quantum which can be used to provide quantum mechanically accurate feedback at GHz rates. Josephson digital technology extends the realm of sigma-delta ADCs from MHz sampling rates to GHz sampling rates, from kHz signal bandwidths to MHz signal bandwidths, with comparable or better dynamic range when compared to semiconductor implementations. This paper presents circuits for Josephson sigma-delta ADCs, including single-loop, and double-loop modulators, circuits for quantized feedback, and digital data processing. Experimental results of a double-loop modulator sampling at 1.28 GHz are reported.

We have measured a single electron transistor (SET) using a transimpedance amplifier which increases the bandwidth of the SET by two orders of magnitude compared to the conventional voltage sensitive amplifier. Using this amplifier to measure the properties of the SET we find a bandwidth of 6.2 kHz and measure the SET noise density up to 1 kHz. The noise is δQ_n≈3.9×10⁻⁵ e/$\text{Hz}$ at 300 Hz.

We have implemented a low-temperature electro-optic sampling system for non-invasive, nodal testing of superconducting Nb integrated circuits. With submillivolt sensitivity and a subpicosecond temporal response, this system has been used to perform nodal analysis on rapid-single-flux quantum (RSFQ) devices and superconducting microstrip interconnects. Here we demonstrate that by measuring the propagation of 6-ps-wide pulses at various test nodes, we are able to fully characterize a superconducting microstrip waveguide the size of an entire chip. The transmission line was selected not only to perform the first complete characterization of a superconducting microstrip, but also to demonstrate full nodal testing of a foundry-fabricated RSFQ integrated circuit. Finally, our results provided much-needed feedback for improving computer simulations of superconducting digital circuits.

The authors report the design, fabrication and test results of a 12-bit NbN SFQ counting A/D converter operating at 9 to 10 K and its insertion into a test IR focal plane array sensor system. The NbN IC is based on a linearized SQUID front-end which generates SFQ pulses at a frequency proportional to the signal. A gated SFQ counter integrates the signal over the sample time and the data is driven off chip through a serializing latching voltage state logic (MVTL) output shift register. The TRW A/D converter chip has been packaged and inserted into an IR focal plane array sensor test facility, or test bed, at the NASA Jet Propulsion Laboratory. The entire system has been successfully demonstrated producing IR images at 100 frames/s with the NbN A/D converter operating at 9 K, dissipating 0.3 mW. Performance of the A/D converter chip, the package including magnetic shielding and medium/high speed signal I/O, and the integrated test bed system are discussed.

Thin-film HTS SQUIDs operated at 77 K and exposed to weak magnetic fields exhibit significant excess low-frequency noise arising from thermally-activated hopping of flux trapped in the superconducting film. We report an investigation of the dependence of this phenomenon on SQUID design and fabrication, measurement conditions and magnetic field history. The level of excess noise was directly related to the amount of flux penetrating the SQUID, and consequently was worse in large SQUIDs than in small SQUIDs due to the greater flux focussing of the larger SQUID. In SQUID fabrication, good film quality (high J_c) was found to be essential to minimize low frequency noise and careful patterning was required to avoid degrading the film. The method of cooling the SQUID was found to strongly affect the level of excess noise, with cooling in the magnetic field in which the SQUID was to be operated being preferable to zero-field cooling. The excess noise was typically 10 pTHz^−1/2 at 1 Hz for 150 pH rf washer SQUIDs having a white noise floor of about 1 pTHz^−1/2 operated in an applied field of 50 μT.

We report design, implementation and testing of a superconductive rapid single flux quantum (RSFQ) shift register based on a data-driven self-timed (DDST) architecture, and demonstrated the validity of this asynchronous design approach. In the DDST architecture, a clock signal is localized within the basic modules, and complementary data signals are used between the modules to transmit timing information. A larger system is simply an array of the basic modules and no extra timing consideration is required. Monte Carlo analysis on a 4-bit DDST shift register has shown that a 40-kbit shift register operating at 20 GHz can be built by using the present Nb Josephson technology. We have observed fully correct operation of a cascade of two 4-bit DDST shift registers with dc bias voltage margin of ±15% at low frequency and ±10% at 20 GHz.