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The TIGER-3 radar is being developed as an “all digital” radar with 20 integrated digital transceivers, each connected to a separate antenna. Using phased array antenna techniques, radiated power is steered towards a desired direction based on the relative phases within the array elements. This paper proposes an accurate phase measurement method to calibrate the phases of the radio output signals using Field Programmable Gate Array (FPGA) technology. The method sequentially measures the phase offset between the RF signal generated by each transceiver and a reference signal operated at the same frequency. Accordingly, the transceiver adjusts its phase in order to align to the reference phase. This results in accurately aligned phases of the RF output signals and with the further addition of appropriate phase offsets, digital beamforming (DBF) can be performed steering the beam in a desired direction. The proposed method is implemented on a Virtex-5 VFX70T device. Experimental results show that the calibration accuracy is of 0.153 degrees with 14 MHz operating frequency.

Effects of growth conditions and buffer structures on crystal quality of 1.9-eV In

This paper presents a novel antenna impedance matching system used in the latest Australian SuperDARN class HF radar, at Buckland Park, South Australia. Earlier radar designs used an off-the-shelf log-periodic wideband antenna that is significantly easier to match over the SuperDARN frequency band, but expensive to buy and mount, and had limited capability for azimuthal beamforming. The newer TTFD antenna, used in many recent SuperDARN radars, offers improvement in these areas, but is in essence a narrow band antenna. It is capable of wideband operation at the cost of being difficult to match, frequency dependant, high-impedance and complex load. Previous TTFD matching transformers utilising toroids have been measured and evaluated for their suitability for the Buckland Park radar. A new system based on an LC matching network circuit has been devised to replace them. The design approach and results of the new matching circuit are detailed.

The precise definition and physical interpretation of a tunneling time is a fundamental problem in quantum mechanics. The lack of a well‐defined time operator in quantum theory precludes the calculation of time in terms of an expectation value. Consequently, model calculations and simulation studies have been proposed and used to determine the time for a particle to traverse a quantum‐mechanical barrier. The results offer both qualitatively and quantitatively, disparate predictions. One of the more commonly used approaches is the phase method, which we have applied to several one‐dimensional model potentials to show that the tunneling time is characterized by the ratio of the typical decay length in the barrier to the incident velocity, τ∼(1/κ)/v0. We also show that the phase and the spin precession methods are equivalent when the magnetic field is applied throughout the space containing the particle wave function and the tunneling barrier. In principle, the spin precession method can be regarded as an operational definition of the tunneling time. It also has been shown that a tunneling time can be defined operationally for a time‐dependent barrier, for which the model calculations predict a tunneling time given by the mean barrier width divided by the magnitude of the imaginary velocity of the particle in the barrier, τ∼d/‖v‖. This pseudokinematic result has been inferred by magnetic field dependent tunneling through heterostructures and computer simulation of wave packet tunneling through one‐dimensional square barriers. Other tunneling time theories predict one or the other of the above mentioned characteristic times; several theories predict both. We interpret these different results for τ to mean that a tunneling time is not uniquely defined as a function of particle energy and the barrier shape unless the process in which the tunneling is involved is also specified. That is, the tunneling time is a meaningful and unique concept only for particular tunneling processes or definite operational procedures.

High luminance (brightness) thin-film phosphor materials have potential use in a variety of applications including heads-up, helmet-mounted, and electroluminescent displays, as well as in emerging flat-panel displays based on field emitter technology. Phosphor materials in thin film form offer several advantages over conventional powder phosphor screens. Since the film is nearly fully dense and in intimate contact with the underlying substrate, thin film phosphors transfer heat to the face plate much quicker than conventional, more porous, powder phosphor materials. This allows thin film phosphor screens to be driven at higher power levels, and therefore produce higher luminance, assuming the efficiency of the powder and film are the same. Fully dense phosphor films have smaller surface area, and will outgas less than conventional powder phosphor materials. Thin film phosphors have smaller grain sizes than conventional powder phosphor materials which will provide for smaller spot size, and thus, higher resolution. Furthermore, in applications such as field-emitter displays, powder phosphor particles can be physically dislodged from the screen due to the high electric fields produced by the large potential difference between the anode screen and gate electrode (or cathode), or dislodged by arcing which may occur if a powder particle protrudes significantly above the screen surface. Dislodged particles or damage produced by arcing could degrade display performance. Dense, thin-film phosphor materials which are well adhered to transparent substrates will provide much smoother surface morphologies, and should be able to withstand significantly higher electric field strength without arcing or screen degradation due to the dislocation of particles.

This paper elaborates on the idea, recently advanced by the authors, of using the scanning tunneling microscope (STM) in new experiments on laser frequency synthesis and laser rectification at optical frequencies. We first introduce the subject of harmonic generation and mixing of laser beams by the related tunneling device called ‘‘point contact diode.’’ Then, we present new quasistatic current–voltage characteristics of several STM junctions, emphasizing their nonlinear and rectifying aspects. The physical origin of the observed asymmetrical I–V curves is discussed. Finally, we describe the proposed rectification experiment which should result in an operationally meaningful definition of a tunneling time for electrons crossing an STM junction and we present some very preliminary results.

The adsorbate-induced reconstruction of the room-temperature Ni(110)-H surface has been resolved by scanning tunneling microscopy. The structure, known as ``streaky'' (1\ifmmode\times\else\texttimes\fi{}2) from LEED observations, is found to be primarily a (5\ifmmode\times\else\texttimes\fi{}2) Ni reconstruction with small domain size along the 〈001〉. Top-layer Ni atoms are paired long the 〈001〉 by a 0.5-A\r{} lateral displacement to form tetramers and every fifth 〈001〉 row is missing. The barrier height, measured simultaneously with a constant-current topograph, is found to be higher along the 〈001〉 missing rows.

The reconstruction of Au and Ni surfaces has been studied by scanning tunneling microscopy (STM). The correlation between the tunneling tip geometry, observed by field ion microscopy, and the resultant STM image leads to an operational definition of STM resolution. New structural models for Ni(110)–H and Ni(110)–Au surfaces are proposed.

Recent measurements of dc current-voltage characteristics of scanning-tunneling-microscope (STN) junctions have confirmed their expected high rectification property. We propose to exploit this property (i) to study rectification at infrared and optical frequencies and (ii) to arrive at a procedural definition of an electron tunneling time. We envisage focusing a laser beam of linearly polarized light onto a STM junction and detecting the dc bias induced across the junction by the alternating, asymmetrical tunnel current. The asymmetry may be of geometrical, material, and/or thermal origin. The dc rectified bias should vanish at sufficiently high laser frequency for fixed tip-to-surface distance or when withdrawing the tip away from the surface for fixed laser frequency. The average electron tunneling time is then one half of the inverse cutoff frequency observed in such a laser-rectification experiment. Similar experiments have already been conducted successfully with metal-whisker diodes and have resulted in the recent redefinition of the meter. The new opportunity offered by STM is the controllability of the junction and its microscopic size, entailing high responsivity. Additional possible laser-STM studies are proposed.