The structural scheme of a pulse-code modulator (PCM) based on high-temperature superconductors and Josephson junctions is proposed. For the implementation of comparators superconducting quantum interferometers are used based on Josephson junctions, which give a possibility to process weakly intense signals in the terahertz frequency range. Analytical dependences of blurring of the threshold characteristic from the critical current of Josephson junction and temperature are obtained. It is established that the amount of blurring increases directly proportional to square root of the product of critical current and the junction temperature.The analytical dependences of the PCM sensitivity from the critical current, temperature and characteristic voltage of Josephson junction were obtained. It is shown that the sensitivity increases directly proportional to the square root of the product of the critical current and the junction temperature and decreases in proportion to the square root of the characteristic voltage of Josephson junction.Research results show that devices based on low-temperature superconductors are characterized by a lower threshold of sensitivity in comparison with similar devices based on high-temperature superconductors.The threshold of sensitivity are ones of nA which is much lower than similar devices based on semiconductors and makes it possible to use such PCMs to process weakly intense signals in the terahertz frequency range.
The structural scheme of a pulse-code modulator (PCM) based on high-temperature superconductors and Josephson junctions is proposed. For the implementation of comparators superconducting quantum interferometers are used based on Josephson junctions, which give a possibility to process weakly intense signals in the terahertz frequency range. Analytical dependences of blurring of the threshold characteristic from the critical current of Josephson junction and temperature are obtained. It is established that the amount of blurring increases directly proportional to square root of the product of critical current and the junction temperature.The analytical dependences of the PCM sensitivity from the critical current, temperature and characteristic voltage of Josephson junction were obtained. It is shown that the sensitivity increases directly proportional to the square root of the product of the critical current and the junction temperature and decreases in proportion to the square root of the characteristic voltage of Josephson junction.Research results show that devices based on low-temperature superconductors are characterized by a lower threshold of sensitivity in comparison with similar devices based on high-temperature superconductors.The threshold of sensitivity are ones of nA which is much lower than similar devices based on semiconductors and makes it possible to use such PCMs to process weakly intense signals in the terahertz frequency range.
Analog-to-digital converters of radio signals, which are based on the phenomenon of superconductivity and the Josephson effect, are proposed to use in order to increase the efficiency of digital antenna arrays. Digital beamforming systems were analysed. Application of the proposed approach was shown to promote processing signals directly at carrier frequencies in the SHF and EHF bands. This approach allows to exclude frequency mixers, local oscillators and intermediate frequency filters from a receiving module. A parallel scheme with comparators on superconducting quantum interferometers is proposed for constructing a superconducting ADC. A digital Gray code is formed at the output of the ADC. The proposed scheme being utilizing, when the input signal increases, each comparator passes through several thresholds, and if Gray codes are applied, only one of the output bits changes between consecutive numbers. Therefore, Gray codes are less sensitive to errors that arise due to small differences in comparator’s thresholds. This promotes reduction of the bit error probability. As an outcome the performed analysis, it was shown that generators of single-quantum pulses on single Josephson junction with a hysteresis-free current-voltage characteristic can be used for constructing comparators, that provides obtaining a high time resolution. The dependence of time resolution on Josephson junction parameters, namely dielectric thickness, relative dielectric permeability and critical current density, was investigated. The resolution was shown to be equal units — tenths of a ns, and this allows using such devices for signal processing at frequencies of several hundred GHz. The time resolution changes with changes in the critical current density of a Josephson junction and in the dielectric thickness and it is almost independent on the relative dielectric permittivity. The sensitivity of the comparator on a single Josephson junction was evaluated. An analytical expression was obtained, it can be applied to estimate how parameters of the Josephson junction equivalent circuit, namely, electrostatic capacity, nonlinear conductivity, and critical current impact on the sensitivity of the comparator. The conducted studies showed that the sensitivity can be units of nA, which exceeds significantly the sensitivity of similar circuits based on semiconductors.
This chapter discusses the high sensitivity superconducting mixers, i.e. detectors used in heterodyne receivers. Today, there are essentially two very successful concepts of superconducting mixers, or frequency down-converters, the superconductor-insulator-superconductor tunnel junction SIS mixer and the superconducting hot-electron bolometer mixer. Moving up to higher frequencies in the microwave and millimeter wave electromagnetic spectra makes it more and more difficult to retrieve a good signal-to-noise ratio with ordinary solid-state amplifiers. The driving force for using superconducting mixers is their excellent down-conversion efficiency and capability to down-convert RF signals from microwave frequencies up to the terahertz region. The main purpose of a receiver is to pick up and amplify a weak signal, which can then be accurately determined in amplitude and phase. The mixer noise temperature and conversion gain are the most important parameters for the mixing device. The Josephson junction mixer was one of the first suggested devices based on superconducting point contacts and later tunnel junctions.
Josephson junctions made from high temperature superconductors (HTS) are active high frequency devices which can be used as oscillators or frequency mixers in the submillimeter-wave region. The frequencies of these electromagnetic waves range from 300 GHz up to 3 THz. Up to now, this wave band is a relatively unexploited region of the electromagnetic spectrum because it is very difficult to produce and detect submillimeter-waves. However, submillimeter-wave technologies are becoming important in many practical scientific applications such as radio astronomy, atmospheric research, plasma diagnostics and THz-imaging, recently. In this work, the behaviour HTS Josephson junctions have been investigated experimentally and theoretically with the aim to THz-applications. In the first part the behaviour of intrinsic Josephson junction stacks are measured, and the possibility of phase locking is discussed by the theoretical analysis of two different shunt technologies depending on the parameter spread of the junction and the shunt parameters. It could be shown that stacks made from sputtered BSCCO layers does not act as intrinsic Josephson junction arrays because of their many micro shorts resulting from the layer growth. However, the stacks made from sputtered TBCCO show clear intrinsic Josephson junction behaviour. But their quasiparticle branches in the current-voltage characteristic deviate from the behaviour predicted by the common used RCSJ model. These branches can be discussed within a tunneling model using d-wave superconductor density of states. A temperature dependent current distribution and a zero bias anomaly was found. Furthermore, the coherent behaviour is studied for intrinsic arrays with additional side-wall shunts. The existence of thresholds of phase locking at especial junction and shunt parameters are demonstrated and discussed in regard to THz-oscillator applications. It was found that the Josephson junctions in a stack can be phase locked even if they have a parameter spread in the critical current up to 15% using a resonant shunt. In the second part of this work the mixing properties of HTS bicrystal Josephson junctions as wave guide mixers for 115 and 345 GHz have been investigated. (orig.)
We have studied Josephson junctions with intrinsic energy ħIc/2e (Ic being the critical current) comparable to thermal energy kT and examined their potential for electromagnetic signal conversion. In this case, moderate thermal fluctuations are sufficient to destroy Josephson oscillations but do not entirely suppress the Josephson dynamics. We have numerically calculated the DC I–V curves, their derivatives dnV/dIn, and the DC and harmonic voltage responses Vn of such Josephson junctions to the AC currents Iωsinωt. We have found that Josephson junction with γ = 2ekT/ħIc ≥ 0.5 is a nonlinear resistance at frequencies ω below characteristic Josephson frequency ωc = 2eIcRn/ħ and all current biases I0. Frequency conversion in such junctions occurs due to the classical mechanism on the nonlinearity of the DC I–V characteristics at low signal frequencies. With increasing the signal frequencies to the characteristic Josephson frequency ωc = 2eIcRn/ħ and higher, the Josephson mechanism by frequency modulation of the Josephson current gradually replaces the classical one. The classical conversion mechanism demonstrates higher conversion efficiencies than the Josephson mechanism, although it operates within a narrower bandwidth. At zero DC bias i0 and in the range of γ-values from 0.1 to 2, we have found odd harmonics Vn emerging from low signal currents. Zero-bias values of the third harmonic V3 maximize at 0.18IcRn for JJs with γ = 0.5 and Iω = 2.5Ic. Based on these findings and high-Tc Josephson junctions, a family of low-noise square-law detectors, harmonic multipliers, and linear heterodyne receivers can be developed for emerging terahertz applications.
We report a novel terahertz (THz) detector with a high- T c superconducting (HTS) Josephson junction. This detector was fabricated using a YBa 2 Cu 3 O 7-δ grain boundary junction. Clear THz signals induced by THz-pulse radiation from an ultrafast photoconductive antenna excited by femtosecond optical pulses were observed. We investigated the characteristics of the detector by changing the parameters of THz radiation such as pump laser power for the THz emitter and THz polarization. The results indicate that the detector mainly detects the THz electric fields. This is the first demonstration of a detection of pulsed THz electric fields using an HTS Josephson junction.
For sensitive wideband spectroscopy at TeraHertz frequencies one needs a wide-range electrically tunable THz source and a sensitive detector. In this paper a superconducting normal metal cold electron bolometer (CEB) was used as a broadband sensor. Bolometers were integrated with broadband log-periodic antenna designed for 0.2-2 THz frequency range and double-dipole antennas designed for 300 and 600 GHz central frequency. A Josephson junction was used as a wide band electrically tuned terahertz cryogenic oscillator. Bicrystal YBaCuO Josephson junctions demonstrated a characteristic voltage IcRn of over 4 mV that corresponds to characteristic frequency about 2 THz. The bolometer chip is attached to a Si substrate lens at 260 mK and the oscillator chip is attached to the sapphire substrate lens at 1.8 K, with lenses facing each other at the distance of few centimeters. High signal was measured in the whole frequency range up to 1.7 THz by simple changing the bias voltage of Josephson junction from zero to 3.5 mV. A voltage response of the bolometer up to 4*108 V/W corresponds to an amplifier-limited technical noise equivalent power of the bolometer NEP=1.25*10-17 W/Hz1/2. Combining a Terahertz band Josephson junction, a high-sensitive hot electron bolometer, and a sample under test in between, makes it possible to develop a cryogenic compact Terahertz-band transmission spectrometer with a resolution below 1 GHz corresponding to the linewidth of Josephson oscillations. For frequencies below 600 GHz a conventional Nb shunted SIS junction can be used as Josephson oscillator.