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The determination of the amplitude-response characteristic is an important means of checking a network design. However, this measurement is usually a time-consuming procedure and at best does not yield a continuous curve. A device which would produce a continuous curve corresponding to the amplitude-response characteristic would be extremely useful in network design and development. It is shown that the system response to a frequency-modulated signal can be made to approximate the amplitude-response characteristic if the frequency is varied slowly enough so that the "quasi-stationary" conditions exist. The physical realization of this slowly varying frequency requires an oscillator with an extremely large frequency range, controllable by one circuit parameter. The greatest difficulties involved in the design of this oscillator were the development of a simple and stable subtractor and the synthesis of the frequency-determining networks. A mathematical analysis was made to determine the characteristics of the network necessary to produce a logarithmic relation between the oscillator frequency and the control position. The audio-frequency sweep generator was constructed using networks designed to approximate the required characteristics and when tested proved to have a satisfactory output waveform. Any improvement in the oscillator performance would require a better approximation to the specified network characteristics.
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A program of research has been undertaken to examine the interaction between vortex shedding and the galloping type oscillation which square cylinders are subject to when immersed in a water stream. It is possible that the fluctuating force from the vortex shedding could quench the galloping oscillation if it acts as a forced oscillation (independent of cylinder motion). An experiment was designed where a square cylinder with one degree of freedom could oscillate transversely to the water flow. The amplitude and frequency of the cylinder oscillation were measured. By using a hot film anemometer spectra of the fluctuating velocity in the wake were taken to determine what frequencies vortex shedding occurred at. The results show that for velocities greater than the resonant velocity the galloping oscillation is dominant and the cylinder motion controls the frequencies of the wake. For velocities less than the resonant velocity no galloping occurs and the vortex shedding seems to control any cylinder motion which occurs. To explain this type of response a mathematical model has been constructed. The model is a set of two coupled self excited oscillators} one with the characteristics of the galloping oscillation and the other with the characteristics of the fluctuating lift force from the vortex shedding. Using the model some aspects of the observed interaction are explained.
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An investigation into various methods of calculation of the high frequency performance parameter f[formula omitted] for microwave heterojunction bipolar transistors is presented. Two high frequency representations of the device are developed: equivalent circuits consisting entirely of lumped circuit elements, and a two-port network based on drift-diffusion equations. Proper account is taken in these representations of the phase delay associated with carrier transit time through the base and base-collector space charge region. Also included are the charging time effects due to the various parasitic circuit elements associated with actual devices. A single-sided isolated structure is used in simulations and it is found that both representations yield remarkably similar characteristics for the behavior of unilateral gain U with frequency. For devices in which the dominant factors limiting high frequency performance are the parasitic resistances and capacitances, it is found that U rolls off at 6 dB/octave through the region where U = 1 and the values predicted for f[formula omitted] via these two methods, as well as via the widely-used analytical expression involving f[formula omitted] and (R[formula omitted]C[formula omitted])[formula omitted], are in agreement. However, when the periods of the oscillations are on the order of the carrier transit times, and the device parasitics are sufficiently low so as to not limit performance, resonance effects occur in U in the region where U = 1 and the prediction of f[formula omitted] obtained via the two equivalent circuit approaches deviates markedly from the predictions of the analytical expression.
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In the development of a pulse position modulation analogue computer the requirement arose to control accurately the frequency of a magnetic drum. A control system was designed which compares the drum frequency to a 1000 cps tuning fork oscillator. Any deviation in phase produces a corresponding change in torque on the drum drive motor which then corrects the phase error. As part of the over-all project a precision power supply was developed. The requirements were met by modifying existing designs.
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Coupling of the flexural, longitudinal, and torsional vibration modes of a column subjected to periodic axial loading was analytically and experimentally investigated. The initial crookedness of the column and longitudinal inertia give rise to coupled flexural-longitudinal vibrations. Further, the Weber effect and longitudinal inertia result in coupling between longitudinal and torsional oscillations. To assess the validity of the theory, an experimental apparatus was set up to axially excite a column using a vibration control generator and an electromagnetic shaker. The experimental results were in good agreement with the theoretical predictions. Coupled longitudinal vibrations exhibiting a frequency ratio of 1:2 were observed. Coupled flexural oscillations were also observed, though a frequency ratio was not established. Further, the experimental results suggest that coupled vibrations other than those theoretically expected were present. In particular, a longitudinal coupled vibration with a frequency ratio of 1:3 was observed, and a corresponding coupled flexural oscillation was present. A coupled torsional mode was experimentally observed when the applied frequency was twice the fundamental torsional frequency. A second coupled torsional mode appeared when the excitation frequency was three times the fundamental torsional frequency. The phase relationship between the coupled vibrations was observed. The resonant coupled vibrations were found to be significant at certain frequencies.
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Following a review of the theory of terahertz generation via optical rectification in nonlinear crystals, a method for enhancing the nonlinear conversion efficiency of this process is proposed. A nonlinear crystal is placed at the intracavity focus of a passive optical resonator, which is seeded by an ytterbium-doped fibre laser. Models of this arrangement indicate that an enhancement of the optical field of several orders of magnitude is possible. As the difference frequency radiation produced through the optical rectification process results from a mixing of the spectral components of the optical field, one expects a corresponding increase in the terahertz field. We present a design of optical resonator that compensates for the large group velocity dispersion of the nonlinear crystal. Our experimental results indicate that below bandgap absorption in the crystal severely limits the resulting enhancement of the optical field, and hence the terahertz field one would expect from this nonlinear process. A scanning-delay terahertz time-domain spectrometer has been constructed, using a gallium phosphide guiding structure to increase the interaction length of the optical and terahertz fields, thereby increasing the terahertz power produced. Our experiment demonstrates 20 dB signal to noise ratio over the spectral range of 0.5-1 THz. We propose a method for increasing the spectral resolution, whilst simultaneously reducing the required data acquisition time of such a terahertz spectrometer, through the use of two femtosecond optical frequency combs. One of these fields drives the nonlinear optical rectification process, whilst the second serves as a sampling local oscillator field to probe the terahertz field via electro-optic sampling in a second nonlinear crystal. By precisely controlling the relative pulse repetition rates of the two oscillators, we show that the full spectral content of the terahertz field can be acquired at rf frequencies, and without the slow mechanical delay lines associated with conventional terahertz time-domain spectroscopy. Finally, we present experimental efforts towards the demonstration of this technique, and show that, to be effective, steps must be taken to increase the strength of the expected rf signal over that of the measurement noise floor.
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A series of Q-spoiled neodymium glass oscillators has been constructed to produce radiation which, when frequency doubled to 5300 A, can be used in a optical scattering experiment on a plasma. Though unexpectedly the required light power was never obtained, an exhaustive theoretical analysis has revealed that this failure was due to superfluorescence depopulation of the inversion. When the problem is corrected by means of a samarium doped glass cladding for the neodymium glass rods, the power necessary to observe the electron satellite feature can easily be obtained.
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Performance of a set of torus-shaped nutation dampers, suitable for arresting relatively low frequency oscillations, is studied experimentally using a simple test facility. More important parameters affecting the damper performance are established and their influence assessed for a variety of configurations with the aim to arrive at more promising geometries. Results suggest damping characteristics to be particularly sensitive to physical properties of the liquid used, its height in the torus, damper geometry, and dynamical parameters representing amplitude and frequency. Among the configurations studied, dampers with perforated inside tubes, baffles, horizontal layers and floating rectangular pieces of wood in flow showed most favorable performance in terms of energy dissipated per unit volume. Such nutation dampers are likely to be suitable in tackling a variety of vibrational problems of industrial aerodynamics, earthquake engineering and off-shore structures.
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The underlying mechanism of formation of spontaneous oscillations in tissue oxygen tension and their physiological significance remain unknown. The working hypothesis of this study were that the oxygen fluctuations are based on arteriolar vasomotion and that the frequency of oscillations in tissue oxygen tension is similar to spontaneous vasomotion of isolated pressurized arterioles. In addition we investigated whether oxygen fluctuations are altered under pathological conditions such as in malignant tissue. In this study, pO2 oscillations in the brain and skeletal muscles of rats were measured with an Eppendorf Oxygen electrode. Fluctuations in pO2 and changes in local temperature were coherent. These fluctuations did not correlate with heart rate, respiration rate or electroencephalogram (EEG). Surgical sectioning of the sciatic nerve and intravenus (i.v.) injection of the ganglion blocker, mecamylamine, did not alter pO2 oscillations in skeletal muscle. The histology of blood vessels in the measurement areas was assessed by light microscopy. Arterioles from muscle and brain surrounding the electrode track, were dissected, cannulated and pressurized in a myograph. Spontaneous oscillations were observed at normal physiological intravascular pressure. The frequency of spontaneous oscillations in vitro matched the frequency of pO2 fluctuations in vivo. Pharmacological studies of these oscillations show that local administration of phenylephrine (PE), an α-adrenoreceptor agonist to skeletal muscle, in vivo, increased the amplitude and frequency of pO2 oscillations but had only minor effects on oscillations in brain. Prostaglandin (PGFα1) increased the amplitude of these oscillations in the brain but had only a moderate effect in skeletal muscles. Similar results were observed when phenylephrine and U46619, a thromboxane analogue, were administered to isolated pressurized arterioles. L-type calcium channel antagonist, nifedipine was shown to decrease the frequency and amplitude of these oscillations both in vivo and in vitro. In mice implanted with squamous cell carcinoma (SCCVII) tumours, pO2 fluctuations (1-3 c/min) were observed in subcutaneous control and peripheral tumour tissue (2 c/min). These fluctuations were absent in most central zones close to necrosis areas. Injection of nicotinamide increased pO2 significantly (p<0.01) and also reinitiated pO2 fluctuations in 5 out of 10 hypoxic tumour areas. In conclusion, oscillations of oxygen tension recorded in tissue are similar to spontaneous vasomotion of isolated pressurized arterioles surrounding the electrode. These fluctuations are altered in hypoxic areas of malignant tissue.
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The absorption line profiles of Spica (α Virginis, HD116658, B1.5IV, m=0.97) show features, at about the 1% level, moving from the blue wavelengths towards the red wavelengths. A series of spectra were taken, at the 1.22 m telescope at the Dominion Astrophysical Observatory on three nights in April, 1982 and two nights in April, 1984, to study these moving features. As Spica is a member of a binary system, the effect of the secondary had to be removed from the observations. This was done by subtracting a template spectrum which had been scaled, broadened and shifted to match the secondary, from each observation. The required shifts were determined using the orbital elements on blended nights and using the Fahlman-Glaspy small-shifts technique on unblended nights. An average of all the spectra was then subtracted from each observation. The resulting series of residuals clearly show the motion of the features seen in the line profiles. The acceleration of the features was estimated to be between 0.0055 and 0.0068 kms⁻². Assuming the features are due to nonradial oscillations, this acceleration corresponds to waves moving slowly, about 5 to 20 kms⁻¹, in a prograde direction. The angular frequency of the oscillations, after accounting for the effects of rotation, would be about 3.4X10⁻⁵ rads⁻¹. A computer model that produces line profiles, under the assumption of a single nonradial oscillation, was used to produce profiles for comparison with observations. Using an [formula omitted]=8 and m=-8 mode, an intrinsic frequency of 3.4X10⁻⁵ rads⁻¹ and a stellar rotation rate of 190 kms⁻¹, the model produced profiles similar to those observed. The change in the model profiles with time was also similar to that observed.
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