Renormalization of Coulomb interactions in s-wave superconductor Nax_xCoO2_2

We study the renormalized Coulomb interactions due to retardation effect in Na$_x$CoO$_2$. Although the Morel-Anderson's pseudo potential for $a_{1g}$ orbital $\mu^*_{a1g}$ is relatively large because the direct Coulomb repulsion $U$ is large, that for interband transition between $a_{1g}$ and $e_g'$ orbitals $\mu^*_{a1g,eg'}$ is very small since the renormalization factor for pair hopping $J$ is square of that for $U$. Therefore, the s-wave superconductivity due to valence-band Suhl-Kondo mechanism will survive against strong Coulomb interactions. The interband hopping of Cooper pairs due to shear phonons is essential to understand the superconductivity in Na$_x$CoO$_2$.Comment: 2pages, 2figures, Proceedings of ICM in Kyoto, 200

Phase coding of RF pulses in photonics-aided frequency-agile coherent radar systems

An innovative optical scheme to generate software-defined phase-modulated radio frequency (RF) pulses with carrier frequency agility from a mode-locked laser (MLL) is proposed. The technique exploits a direct digital synthesizer and a Mach-Zehnder modulator to apply an intermediate frequency modulation to the MLL's modes. The heterodyne detection of the optical signal allows the generation of amplitude- and phase-modulated RF carriers with very high phase stability, suitable for coherent radar applications. Further, a single MLL can be used to generate carriers simultaneously at different frequencies, enabling frequency hopping or multifunctional radars, with no need to increase the complexity of the transmitter. Results show chirped and Barker-coded pulses at around 10 or 40 GHz in a single setup, without any performance degradation while increasing the carrier frequency. The proposed technique allows the practical realization of compressed pulses for coherent radars over a wide carrier frequency range, allowing the development of software-defined radar systems with improved functionalities. © 1965-2012 IEEE

Photonic generation of phase-modulated RF signals for pulse compression techniques in coherent radars

A novel and flexible photonics-based scheme is proposed for generating phase-coded RF pulses suitable for coherent radar systems with pulse compression techniques. After selecting two modes from a mode-locked laser (MLL), the technique exploits an optical in-phase/quadrature modulator driven by a low-sample rate and low-noise direct digital synthesizer to modulate the phase of only one mode. The two laser modes are then heterodyned in a photodiode, and the RF pulse is properly filtered out. The scheme is experimentally validated implementing a 4-bit Barker code and a linear chirp on radar pulses with a carrier frequency of about 25 GHz, starting from an MLL at about 10 GHz. The measures of phase noise, amplitude- and phase-transients, and autocorrelation functions confirm the effectiveness of the scheme in producing compressed radar pulses without affecting the phase stability of the optically generated high-frequency carriers. An increase in the radar resolution from 150 to 37.5 m is calculated. The proposed scheme is capable of flexibly generating software-defined phase-modulated RF pulses with high stability, even at very high carrier frequency, using only a single commercial device with potentials for wideband modulation. It can therefore allow a new generation of high-resolution coherent radars with reduced complexity and cost. © 1983-2012 IEEE

10-GHz fully differential Sallen–Key lowpass biquad filters in 55nm SiGe BICMOS technology

Multi-GHz lowpass filters are key components for many RF applications and are required for the implementation of integrated high-speed analog-to-digital and digital-to-analog converters and optical communication systems. In the last two decades, integrated filters in the Multi-GHz range have been implemented using III-V or SiGe technologies. In all cases in which the size of passive components is a concern, inductorless designs are preferred. Furthermore, due to the recent development of high-speed and high-resolution data converters, highly linear multi-GHz filters are required more and more. Classical open loop topologies are not able to achieve high linearity, and closed loop filters are preferred in all applications where linearity is a key requirement. In this work, we present a fully differential BiCMOS implementation of the classical Sallen Key filter, which is able to operate up to about 10 GHz by exploiting both the bipolar and MOS transistors of a commercial 55-nm BiCMOS technology. The layout of the biquad filter has been implemented, and the results of post-layout simulations are reported. The biquad stage exhibits excellent SFDR (64 dB) and dynamic range (about 50 dB) due to the closed loop operation, and good power efficiency (0.94 pW/Hz/pole) with respect to comparable active inductorless lowpass filters reported in the literature. Moreover, unlike other filters, it exploits the different active devices offered by commercial SiGe BiCMOS technologies. Parametric and Monte Carlo simulations are also included to assess the robustness of the proposed biquad filter against PVT and mismatch variations

An improved reversed miller compensation technique for three-stage CMOS OTAs with double pole-zero cancellation and almost single-pole frequency response

This paper presents an improved reversed nested Miller compensation technique exploiting a single additional feed-forward stage to obtain double pole-zero cancellation and ideally single-pole behavior, in a three-stage Miller amplifier. The approach allows designing a three-stage operational transconductance amplifier (OTA) with one dominant pole and two (ideally) mutually cancelling pole-zero doublets. We demonstrate the robustness of the proposed cancellation technique, showing that it is not significantly influenced by process and temperature variations. The proposed design equations allow setting the unity-gain frequency of the amplifier and the complex poles' resonance frequency and quality factor. We introduce the notion of bandwidth efficiency to quantify the OTA performance with respect to a telescopic cascode OTA for given load capacitance and power consumption constraints and demonstrate analytically that the proposed approach allows a bandwidth efficiency that can ideally approach 100%. A CMOS implementation of the proposed compensation technique is provided, in which a current reuse scheme is used to reduce the total current consumption. The OTA has been designed using a 130-nm CMOS process by STMicroelectronics and achieves a DC gain larger than 120 dB, with almost single-pole frequency response. Monte Carlo simulations have been performed to show the robustness of the proposed approach to process, voltage, and temperature (PVT) variations and mismatches

Flexible multi-band OFDM receiver based on optical down-conversion for millimeter waveband wireless base stations

A novel and flexible photonics-based down-conversion scheme is proposed for wireless receivers in base stations. It allows simultaneous detection of multiple signals at carriers up to tens of GHz, enabling communications at millimeter waves. Experiments demonstrate the effective down-conversion of Wi-Fi signals at 2.4 and 39.8GHz with EVM<;-43dB

UWB FastlyTunable 0.550 GHz RF Transmitter based on Integrated Photonics

Currently, due to the 6G revolution, applications ranging from communication to sensing are experiencing an increasing and urgent need of software-defined ultra-wideband (UWB) and tunable radio frequency (RF) apparatuses with low size, weight, and power consumption (SWaP). Unfortunately, the coexistence of ultra-wideband and software-defined operation, tunability and low SWaP represents a big issue in the current RF technologies. Recently, photonic techniques have been demonstrated to support achieving the desired features when applied in RF UWB transmitters, introducing extremely wide operation and instantaneous bandwidth, tunable filtering, tunable photonics-based microwave mixing with very high port-to-port isolation, and intrinsic immunity to electromagnetic interferences. Moreover, the recent advances in photonics integration also allow to obtain very compact devices. In this article, to the best of our knowledge, the first example of a complete tunable software-defined RF transmitter with low footprint (i.e. on photonic chip) is presented exceeding the state-of-the-art for the extremely large tunability range of 0.5-50 GHz without any parallelization of narrower-band components and with fast tuning (< 200 s). This first implementation represents a breakthrough in microwave photonics

Photonics enabling coherent MIMO radar networks

The potential of coherent MIMO radar networks enabled by photonics is introduced. The first coherent dual-band 2 × 4 MIMO radar experiment is presented. Range/cross-range maps demonstrate the higher cross-range resolution due to the coherence and the enhanced performance introduced by dual-band operation

Diagnosing diabatic effects on the available energy of stratified flows in inertial and non-inertial frames

The concept of available energy in a stratified fluid is revisited from the point of view of non-canonical Hamiltonian systems. We show that the concept of available energy arises when we minimize the energy subject to the constraints associated with the existence of Lagrangian invariants. The non-canonical structure implies that there exists a class of dynamically equivalent Hamiltonians, related by a local (in phase space) gauge symmetry. A local diagnostic energy can be defined via the Hamiltonian density chosen imposing a specific gauge-fixing condition on the class of dynamically similar Hamiltonians. The gauge-fixing condition that we introduce selects a specific local diagnostic energy which is well suited to study the effect of diabatic processes on the evolution of the available energy. Non-inertial effects, which are notoriously elusive to capture within an energetic framework, are naturally included via conservation of potential vorticity. We apply the framework to stratified flows in inertial and non-inertial frames. For stratified Boussinesq flows, when the initial distribution of potential vorticity is even around the origin, our framework recovers the available potential energy introduced by Holliday & McIntyre (J. Fluid Mech., vol. 107, 1981, pp. 221-225), and as such, depends only on the mass distribution of the flow. In rotating flows, the isopycnals of the ground state are generally not flat, and the ground state may have kinetic energy. We finally demonstrate that flows in non-inertial frames characterized by a low Rossby number , the local diagnostic energy has, to lowest order in , a universal character

Widely distributed photonics-based dual-band MIMO radar for harbour surveillance

A new architecture for a widely distributed dual-band coherent multiple-input multiple-output (MIMO) radar system is illustrated, and its implementation and testing are reported. The system consists in a central unit where radar signals are coherently generated and detected, which serves multiple remote sensors connected over transparent WDM optical network. Every remote node operates coherently both in the S- and X-band, and is displaced over distances of several kilometers, allowing to monitor a scene under different angles of view. All the remote sensors share the same oscillator and digital signal processing unit, both located in the central office, allowing to perform centralized raw data fusion on the acquired signals. By virtue of the system coherence, the system takes advantage of the coherent MIMO processing strategy to offer a superior spatial resolution, which is even magnified by the dual-band approach