Results 1 - 10 of 6373
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[en] The use of increased sensitivity and temporal resolution at mm-wavelengths has revealed a number of novel features associated with solar activity. Of particular interest are the ultra-fast time structures superimposed on impulsive bursts. They are distinguished by time scales and repetition rates much shorter than the impulsive time scales. (Auth.)
[en] A fast-hopping 3-band (mode 1) multi-band orthogonal frequency division multiplexing ultra-wideband frequency synthesizer is presented. This synthesizer uses two phase-locked loops for generating steady frequencies and one quadrature single-sideband mixer for frequency shifting and quadrature frequency generation. The generated carriers can hop among 3432 MHz, 3960 MHz, and 4488 MHz. Implemented in a 0.13 μm CMOS process, this fully integrated synthesizer consumes 27 mA current from a 1.2 V supply. Measurement shows that the out-of-band spurious tones are below -50 dBc, while the in-band spurious tones are below -34 dBc. The measured hopping time is below 2 ns. The core die area is 1.0 x 1.8 mm2.
[en] The development of a one gigawatt, high current relativistic klystron tube is underway for producing one microsecond long, 1.3 GHz microwave pulses at a 5 Hz pulse repetition frequency. This paper describes the theory, modeling, and experimental development of the microwave tube. The one microsecond pulse length is almost an order of magnitude beyond what has been achieved with a high current relativistic klystron. Achieving a peak power approaching 1 GW for 1 μs requires a stable electron beam on that time scale, and an optimized extraction efficiency in the output cavity. The microwave tube design was guided by theory and particle-in-cell code modeling that relate output cavity extraction efficiency to the amplitude of the beam harmonic current modulation and output cavity shunt impedance. Current experimental results are presented. The 1 MV, 10 kA, 1μs pulse length, 5 Hz pulse-repetition-rate pulsed power modulator used to power the relativistic klystron is also described. 4 figs., 6 refs
[en] The dielectric constant and the loss angle in the (111) direction of CdTe at room temperature between 65 and 75 GHz have been determined. The results are compared to those of other authors and discussed. Problems of sample preparation are dealt with. (G.Q.)
[en] We propose a tunable flat-top broadband defect mode in the terahertz (THz) frequency range by inserting multiple defects into a periodic waveguide with undulated tube walls. When introducing a defect into periodic structures, we can achieve a defect mode with a very sharp transmission peak in the forbidden band due to the local resonance effect. Here, in the proposed multi-defect structure, we find a broadband defect mode with a bandwidth of 38.3 GHz appearing in the transmission spectrum, which is over 21 times that of the traditional defect mode. The simulations reveal that the broadband defect mode is caused by the competition between Bragg scattering and local resonances in multiple defect mode coupling. The bandwidth and center frequency of this broadband defect mode can be manipulated by altering the number and size of defects, which could be applied in various THz systems as functional devices. (paper)
[en] In this paper, based on rotation phase-gradient principle, a single-layer, high-efficiency transmitting metasurface is designed and applied to high-gain antenna. In the case of circularly polarized incident wave, the PCR (polarization conversions ratio) of the metasurface element is greater than 90% in the band of 9.11–10.48 GHz. The transmitting wave emerges an anomalous refraction when left-handed circularly polarized wave are incident perpendicularly to the 1D phase-gradient metasurface, which is composed of cycle arrangement of 6 units with step value of 30°. The simulated anomalous refraction angle is 40.1°, coincided with the theoretical design value (40.6°). For further application, the 2D focused metasurface is designed to enhance the antenna performance while the left-handed circularly polarized antenna is placed at the focus. The simulated max gain is increased by 12 dB (182%) and the half-power beamwidth is reduced by 74.6°. The measured results are coincided with the simulations, which indicates the antenna has high directivity. The designed single-layer transmission metasurface has advantages of thin thickness (only 1.5 mm), high efficiency and light weight, and will have important application prospects in polarization conversion and beam control. (paper)
[en] In this paper we discuss the linearity response of the Planck-LFI receivers, with particular reference to signal compression measured on the 30 and 44 GHz channels. In the article we discuss the various sources of compression and present a model that accurately describes data measured during tests performed with individual radiomeric chains. After discussing test results we present the best parameter set representing the receiver response and discuss the impact of non linearity on in-flight calibration, which is shown to be negligible.