RF Losses Research Articles

Design considerations for adaptive active phased-array 'multifunction' radars

Adaptive active phased-array radars are seen as the vehicle to address the current requirements for a true 'multifunction' radar system. Their ability to adapt to the environment and schedule their tasks in real time allows them to operate with performance levels well above those that can be achieved from conventional radar designs in addressing the current and future threats. Their ability to make effective use of all the available RF power and to minimise RF losses also makes them a good candidate for future very long range radars. This paper addresses the design of such radars in terms of the system and its component parts and the operational requirements that drive the design. The paper also considers some futures uses outside the military field.

High temperature superconducting films on flexible substrates for cryoelectronics

High temperature superconducting (HTS) films on flexible, low loss, low thermal conductivity substrates offer unique advantages to transmit multichannel digital data with low signal attenuation for cryoelectronic applications. Polycrystalline, flexible yttria stabilized zirconia (YSZ) with a low thermal conductivity of 0.015 W/cm K, a low RF loss of 4×10 −4 (at 77 K, 5 GHz) and a relatively low dielectric constant of 28 is an excellent choice as a substrate if high quality HTS films can be developed with desirable RF properties. This paper presents recent data obtained on biaxially textured Y 1B 2C 3O 7− x (YBCO) films deposited on polycrystalline YSZ substrates with a biaxially-textured YSZ template. YSZ structural templates were developed by Ion beam-assisted pulsed laser deposition (PLD). YBCO films were deposited by PLD. The biaxially textured YBCO films exhibit in-plane (Phi-scans) FWHMs of ∼7°. The transition temperatures ( T c's) of the films are in the range 88–89 K with transition widths of 0.5 K. The critical current densities ( J c's) measured at 77 K, zero field, are in the range of 2×10 6 A/cm 2. The surface resistance of the films measured at 77 K, 10 GHz is 700 μΩ indicating a low microwave loss in these films. The high materials quality obtained in these HTS films on polycrystalline substrates can be used in fabricating a variety of components for cryoelectronic applications.

RF oscillation generated by an electromagnetic shock wave in coupled transmission lines with anomalous and normal dispersion

The generation of rf oscillation when an electromagnetic shock runs in synchronism with a backward wave in coupled transmission lines with normal and anomalous dispersions and ferrite-related nonlinearity is considered. The oscillation damping can be significantly reduced if a large portion of the rf energy flux is directed to the anomalous-dispersion line, where rf losses are relatively low.

Epitaxial growth of YBa 2Cu 3O x film on single-domain YBCO substrate for rf component development

In our previous experiments we have developed a high quality single-domain YBCO for rf component development. The material exhibits excellent physical properties including extremely sharp T c’s, well-controlled crystal orientation, and low surface resistance, R s. However, the surface resistance can be further reduced if the surface of the single domain can be modified. As the major rf losses may arise from the dissipation at the 211 particles, we have developed a new method to “cover” the 211 particles by depositing a YBCO thin film on the polished single-domain surface. In this method, the single-domain YBCO serves as a substrate that has a perfect lattice matching with the YBCO film. We report the experimental results on the film synthesis and discuss the film growth mechanism on the YBCO single-domain substrate.

Electric surface resistance R/sup E/(T, f, E/sup ⊥/) of Nb/Nb/sub 2/O/sub 5-y/-interfaces and Q-drop of superconducting Nb cavities

The RF losses, especially actual level and increase with RF fields limit most stringently the application of superconducting RF cavities. This is due to the needed cooling power to be supplied locally to the high field region and due to the nonlinearities causing harmonics and RF breakdown. The separation of RF residual losses R/sub res/(T,f) from the intrinsic losses R/sub BSC/(T,f) yields the quasi-exponential increases of the electric surface resistance with the electric field E/sup l/ perpendicular to the surface /spl delta/R/sup E/(E/sup /spl perp//)/spl prop/exp (-c/E/sup /spl perp//) and the power law increases of the magnetic surface impedances with the magnetic field H/sup /spl par// parallel to the surface /spl delta/R/sup H/(H/sup /spl par//)/spl prop/(H/sup /spl par//)/sup 2n/ (n=1, 2..). By the Nb/Nb/sub 2/O/sub 5-y/ interfaces at external and internal surfaces R/sup H//sub res/(T,f) and R/sup E//sub res/(f,E/sup /spl perp//) can be explained quantitatively. Especially, the drop of Q/sub 0/(E/sup /spl perp//)/spl prop/1/R/sup E//sub res/(E/sup /spl perp//) and its reduction by EP- and BCP-smoothening and by better interfaces by UHV anneal are well accounted for by interface tunnel exchange.

MEMS microswitches for reconfigurable microwave circuitry

The performance is reported for a new microelectromechanical structure (MEMS) cantilever microswitch. We report on both dc- and capacitively-contacted microswitches. The dc-contacted microswitches have contact resistance of less than 1 /spl Omega/, and the RF loss of the switch up to 40 GHz in the closed position is 0.1-0.2 dB. Capacitively-contacted switches have an impedance ratio of 141:1 from the open to closed state and in the closed position have a series capacitance of 1.2 pF. The capacitively-contacted switches have been measured up to 40 GHz with S/sub 22/ less than -0.7 dB across the 5-40 GHz band.

Rf Field Dependence of Surface Resistance for a–b Plane-Textured YBa2Cu3O7–δ Films Deposited on Copper Substrate

Low rf loss at high rf field levels should be realized in order to apply high-Tc films to accelerator cavities. It is well known that c-axis perpendicular to the surface is essential to reduce Rs and its field dependence However, the effect of a–b plane texturing on Rs is not so clear because lack of experiments, especially for films deposited on metallic substrates. We developed a deposition technique that enables a–b plane texturing as well as c-axis orientation of YBCO films on a copper substrate. We prepared four samples with c-axis normal to the surface: two of them were a–b plane well textured films and the other two were a–b plane weakly textured films. The a–b plane well-textured films in low rf field exhibited a Rs higher than that of weakly textured films below 80 K. The rf field dependence measurements showed that the increase of Rs with increasing rf field for the a–b plane well-textured films was slower as compared with that of the weakly textured films.

Self-Assembled 3-Mercaptopropyltrimethoxysilane (MPS) ON Ba0.5Sr0.5TiO3 as an Adhesion Layer for Microwave Devices

ABSTRACTThe use of self-assembled monolayers in patterning, adhesion studies, corrosion protection, and electronic devices is a growing research field. Here we discuss the use of (3-mercaptopropyl)trimethoxysilane (MPS) as an adhesion layer in microwave components using metal-ferroelectric thin film-dielectric heterostructures. The MPS has been used as an adhesion layer between a ferroelectric thin film and gold (Au) in the development of tunable microwave components. The system studied was the metal/ferroelectric thin film/dielectric heterostructure with the MPS molecule as an adhesion layer between the metal and the ferroelectric thin film. Specifically, we have looked at the interface between the Au layer and the Ba0.5Sr0.5TiO3 (BSTO) ferroelectric thin film deposited on LaAlO3 (LAO). Typically, during metallization via e-beam evaporation, it is common to deposit a chrome or titanium adhesion layer (∼ 15 nm thick) between the Au and BSTO films. However, this underlayer may diffuse through the Au causing an increase of the RF losses of the device. Replacing the metal buffer layer interface with a self-assembled structure, such as MPS, can avoid the effect of inter- diffusion of chrome in the Au vapor deposit film. The surface modification of Ba0.5Sr0.5TiO3 (BSTO) thin film with MPS was studied by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. The effectiveness of MPS self-assembled monolayers as an adhesion layer was determined by analyzing the performance of Au/MPS/BSTO/LAO interdigital capacitors as a function of dc voltage and temperature at 1 MHz.

Outgassing of plasma facing antenna front for lower hybrid wave launcher

A 3.7 GHz mock-up antenna module using carbon fiber composite (CFC) was fabricated and tested for the development of a heat-resistive front of the lower hybrid current drive (LHCD) antenna. This module has four waveguides and a water cooling channel, the length is 206 mm. The CFC surface was coated with a thin titanium layer and was plated with copper in order to reduce RF losses, to bond rods and septum plates and to assemble them with cooling channel. The RF losses and the outgassing rates of this CFC module at high RF power were measured during long pulses. When the injected power varies between 30 and 100 kW, the RF losses measured by calorimetery, were found to be in the range of 1.0–1.2%. It is found that this experimental value is 2.5–3.0 times higher than the theoretical value of pure copper. Stationary operation of the CFC module with water cooling is performed at the RF power density of 45 MW m −2 during 1000 s. The outgassing rates from the CFC module are in the range of 0.93∼1.3×10 −6 Pam −3 s −1 m −2 at the module temperature of 120°C, it is low enough for an antenna material. No significant bonding defects occurred during the steady-state operation. It is assessed that a CFC module is an attractive candidate for a heat-resistive front of LHCD antenna.

Fluxon-induced losses in niobium thin-film cavities

New data obtained with niobium film cavities allow a refinement of the physical explanation of RF losses by trapped magnetic flux at small RF field amplitudes. A stationary model explains these data. As the diameter of the fluxons decreases, the RF bypasses them, and the RF losses are due to the voltage, to which the normal conducting electrons, located in the fluxon core, are subject to. This loss mechanism is similar in origin to that of the so-called BCS losses.

Microwave losses induced by vortex propagation in highly epitaxial YBa 2Cu 3O 7−δ films

The rf and microwave losses of devices based on YBa 2Cu 3O 7−δ superconducting films are dominated by extrinsic mechanisms that can be controlled by a careful device fabrication and design. Often the granular nature of the films shows up in the measurements of the residual surface resistance. We have measured the surface impedance of a large number of highly epitaxial, dc sputtered YBa 2Cu 3O 7−δ films patterned by argon ion milling as meanderline microstrip resonator (T c>90K and ΔT c<1K). The plot of R s vs. H rf shows that the excess surface resistance is determined by Josephson vortices propagating in the grain boundaries, in good agreement with the theoretical prediction of Halbritter. Further, the temperature and frequency dependence of Z s is consistent with this hypothesis.

The effect of solder paste residues on RF signal integrity

Wireless devices such as pagers and cellular phones are becoming common consumer items. These products require low loss RF signal propagation which is affected by material choices and processing conditions. This paper examines the effect of a series of no-clean solder pastes on signal integrity using an RF test circuit which sends a broadband signal through a gallium arsenide antenna switch and measures its transmission using a network analyzer. The test circuit also measures signal leakage. This paper reports on two different test vehicles, one that uses a 900 MHz antenna switch, and the other that uses a 2.0 GHz antenna switch. The transmission and leakage readings were taken daily for 16–21 days while the test vehicles were under accelerated aging conditions of 85°C and 85% RH. Average values for the readings for each solder paste were plotted to provide comparison among the pastes. The comparison data clearly distinguish solder pastes that provide consistency throughout the test period from those which do not.

Theory of long term gain growth in traveling wave tubes

The properties of the attenuator materials used in traveling wave tubes (TWT) are a major factor in determining the gain of the tube. Carbon is a commonly used attenuator material which is typically deposited on the dielectric rods used to support the helix slow wave structure in TWTs. The deposited carbon layers are susceptible to ion bombardment induced damage during operation and outgassing of the tube, which increases the electrical resistivity of the layer and reduces the rf loss in the tube. The total gain in the tube grows during this process until the bombardment ceases or the layer damage saturates. A simple theory is developed that relates this gain change to the gas evolved in the tube that results in the particle bombardment of the layers during operation. The model accurately describes the observed time dependent behavior of the gain, and the time constant for outgassing the tube is found for several tubes ranging from S to Ka band. The time constant increases significantly for smaller, high frequency tubes with a low conductance and poor pumping speeds. The model shows that stabilization of the gain occurs after several time constants have passed, which may take hundreds to thousands of hours.

Radiofrequency magnetoresistance of Fe/Cr superlattices

The rf magnetoresistance of Fe/Cr superlattices is studied for two orientations of the current: parallel and across the superlattice layers. A mutually single-valued correspondence is established between the relative magnetoresistance measured at dc current and the change in the transmission coefficient of electromagnetic waves in the magnetic field. When rf currents flow across the layers, the relative change in the signal amplitude is proportional to twice the change in the electrical resistance of the superlattice and is of opposite sign. It is shown that the rf losses are determined by the surface resistance which is proportional to the superlattice thickness and inversely proportional to its conductivity. An equation is derived for the rf electric field distribution in the superlattice. It is established that when the thickness of the superlattice is small compared with the skin layer depth, field and current components which penetrate through the entire superlattice exist.

Very high-current-density Nb/AlN/Nb tunnel junctions for low-noise submillimeter mixers

We have fabricated and tested submillimeter-wave superconductor–insulator–superconductor (SIS) mixers using very high-current-density Nb/AlN/Nb tunnel junctions (Jc≈30 kA cm−2). The junctions have low-resistance-area products (RNA≈5.6 Ω μm2), good subgap-to-normal resistance ratios Rsg/RN≈10, and good run-to-run reproducibility. From Fourier transform spectrometer measurements, we infer that ωRNC=1 at 270 GHz. This is a factor of 2.5 improvement over what is generally available with Nb/AlOx/Nb junctions suitable for low-noise mixers. The AlN-barrier junctions are indeed capable of low-noise operation: we measure an uncorrected double-sideband receiver noise temperature of TRX=110 K at 533 GHz for an unoptimized device. In addition to providing wider bandwidth operation at lower frequencies, the AlN-barrier junctions will considerably improve the performance of THz SIS mixers by reducing rf loss in the tuning circuits.

Design and fabrication of coplanar YBCO structures on lithium niobate substrates

YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) with low RF losses has been successfully deposited onto lithium niobate (LNO) to improve the performance of electrooptic Mach-Zender modulators. Epitaxial, c-axis oriented superconducting YBCO thin films have been grown on X-cut LNO single crystals with a yttria-stabilized zirconia (YSZ) buffer layer by RF magnetron sputtering. This buffer layer is needed to obtain good superconducting properties of the YBCO grown. Numerical tools have been developed to analyze CPW structures based on YBCO/YSZ/LNO trilayers, and they indicate that YSZ thickness has to be kept to the minimum necessary for good YBCO growth. With this restriction, the RF losses of YBCO/YSZ/LNO samples have been measured. The results from these measurements are used to quantify the performance enhancement in a Mach-Zender modulator using YBCO electrodes.

Gain stability of traveling wave tubes

The long-term gain stability of traveling wave tubes (TWT's) with helix slow-wave structures is examined. A major variable in the gain of TWT's is the stability of the attenuator material that is placed in the tube to damp oscillations and improve input-to-output isolation. Thin carbon layers are often used for this purpose in TWT's and are deposited onto the helix support rods by several different techniques that produce a variability in the material structure and properties. The carbon layers are also susceptible to physical damage due to the environment in the tube during conditioning and long-term operation. Modification of the electrical conductivity of the layer by energetic particle bombardment and chemical erosion decreases the net RF loss in the tube and causes the gain to increase with time. The presence of impurity gases and rapid conditioning procedures produce gain increases due to the lattice damage of the attenuator material of up to 10 dB in a TWT in the first several hundred hours of operation. Properly designed attenuator loss-patterns and minimization of the gas evolution in the TWT causes these effects to saturate and the gain to stabilize quickly. Techniques to ensure long-term stability of these layers are discussed.

Competition between Josephson and electromagnetic interactions in single crystals

We have studied the low magnetic field, high-temperature region of the H-T phase diagram of (Bi2212) single crystals by measuring magnetic field induced rf dissipation in the mixed state. Normally the rf power loss P(H) versus H in a superconductor increases with the increase in the field with a minimum at H = 0. However, in Bi2212 the rf loss initially decreases with increasing field, shows a minimum at a certain field and increases continuously beyond . We understand the unusual minimum at as resulting from a competition between (i) the increasing attractive electromagnetic interaction between the pancake vortices (that constitute the vortex line) in adjacent bilayers with the increasing field and (ii) the decreasing Josephson coupling between the CuO bilayers with the increase in the field. The line defines a new boundary in the H-T phase diagram at which the tilt modulus of the 2D vortex stack in Bi2212 is at a maximum. It is found to obey the behaviour with the exponent n = 3/2 reminiscent of the predicted behaviour of the melting line.

Performance of quasi-optical SIS mixer with NbN/AlN/NbN tunnel junctions and NbN tuning circuit at 760 GHz

Abstract An all-NbN quasi-optical superconductor–insulator–superconductor (SIS) mixer for the terahertz band has been designed and tested. This mixer consists of a MgO hyperhemispherical lens with anti-reflection cap, an NbN twin-slot antenna, and NbN tuning circuits. The size of the NbN/AlN/NbN junction was about 0.5 μ m in diameter, and the current density was about 30 kA/cm 2 . The junctions showed good d.c. I – V characteristics, with a high gap voltage of about 5.4 mV and a small sub-gap leakage current. The double side band (DSB) receiver noise temperature, measured by the standard Y-factor method, was about 2700 K at 761 GHz. This value is much higher than the theoretical sensitivity based on the experimental I – V curve calculated using Tucker’s quantum theory of mixing. The difference between experimental and theoretical mixer performance may result from large RF losses in the tuning circuit of NbN fabricated on SiO.

Characteristics of trenched coplanar waveguide for high-resistivity Si MMIC applications

A novel low RF loss trenched coplanar waveguide (CPW) transmission-line structure fabricated using evaporated aluminum tracks on a high-resistivity (10-k/spl Omega/ cm) silicon (HRS) substrate is reported. By assuming that Schottky contact boundaries exist at the metal silicon substrate interface in the CPW line, the finite-element analysis method is used to determine the simulated behavior of the structure. The distributed capacitance, leakage conduction current, and dynamic shunt conductance for the line are shown to be a function of dc bias applied to the line, and also to reduce as a function of trench depth in the normal bias regime. Experimental results show: (1) the reduction of RF losses in comparison with conventional aluminum conductor CPW line structures may be as much as 0.5 dB/cm at 30 GHz; (2) by proper positive dc biasing of a CPW line on a p-type HRS substrate, a further reduction (0.2 dB/cm) in RF loss at 30 GHz can be achieved; (3) predicted trends in line leakage current, capacitance, and relative characteristics impedance are experimentally verified. The proposed waveguide structure may be utilized in a special fabrication process designed for RF/microwave applications.