摘要:

A formula of Shimboet al. for the power spectrum of signals pulse-modulated in accordance with random sequences is specialised for Markov chains. In the analysis,ztransform techniques are used rather than explicit matrix methods. The resulting expressions may be used to find the spectra of a wide class of digital signals obtained by pulse modulating the amplitude, frequency, phase, or any combination of these, of a radio-frequency carrier. Attention is directed to phase modulation and, in particular, to ideal phase reversal and signals which practical systems might produce as approximations to it. In the first such signal, amplitude modulation is introduced in addition to the phase modulation, while in the other there is a nonzero switching time between the two phase levels. The power spectra of these signals are determined and compared with the phase reversal spectrum.

DOI:10.1049/piee.1970.0338

出版商:IEE    

年代:1970

数据来源: IET

摘要:

The paper calculates the output signal/quantisation-noise ratio, the dynamic range, and the effect of transmission errors for syllabically companded delta-sigma modulator systems, for different orders of companding. The conditions necessary to achieve a nearly constant signal/noise ratio at the decoder output over a wide range of input signal power are given.

DOI:10.1049/piee.1970.0339

出版商:IEE    

年代:1970

数据来源: IET

摘要:

During prolonged anticyclonic weather conditions, it is well known that abnormally high values of field strength sometimes occur during the night on overland transhorizon radio paths, usually returning to normal values after sunrise. The paper discusses simultaneous recordings of field strength and refractive index, made on two occasions when this occurred. On the first occasion, the return to normal values of field strength occurred within 2h, but, on the second, the change was much slower, taking some 6h. Use of a ray-tracing technique shows that the very high field strength on the first occasion was due to the return of the rays to ground by superrefraction in a thick elevated layer. On the second occasion, the layer was insufficiently thick to support refraction, and a partial reflection occurred. The magnitude of this reflection is computed from linear and cosinusoidal layer models, and the former leads to values of field strength agreeing, to within 5dB, with those measured.

DOI:10.1049/piee.1970.0340

出版商:IEE    

年代:1970

数据来源: IET

摘要:

Temperature-and thickness-dependence studies on silicon-carbide composites (fired matrices of silicon-carbide crystallites, clay and graphite) have produced a new insight into the electrical-conduction mechanisms in such devices. Using computer methods, an optimum empirical expression for the current/voltage characteristic has been obtained in the formI=C1exp (αT)V/L2+C2exp (βT)V3/L5+C3exp (δT)V6/L7whereTis the device temperature andLis the device thickness. The most probable explanation for this equation is that it represents three different electric-field modes of space-charge-limited current flow. The silicon-carbide composite can be thought of as a bulk polycrystalline insulator or semiconductor in which the crystallite-crystallite contact points act as ‘grain boundaries’, the clay holds the crystallites together and the graphite is an impedance moderator; The linear conduction term can be explained by assuming that the carrier drift velocity is independent of electric field at low fields (<1.5 × 103V/m). TheV3/L5term is due to the field-dominated double-injection process, while the conduction region represented by theV6/L7term is most probably the diffusion-dominated double-injection process.

DOI:10.1049/piee.1970.0341

出版商:IEE    

年代:1970

数据来源: IET

摘要:

A systematic method of generating different combinations of edges, takenmat a time, in complete graphs, and classification of these edge combinations based on their topological structure, are discussed. In the process, a simple algorithm is presented for determining whether or not two given sets of edges are topologically equivalent. The listing of edge-combination classes has a significant bearing on some problems of realisation. The procedures given are all simple and routine and are suitable for computerisation.

DOI:10.1049/piee.1970.0342

出版商:IEE    

年代:1970

数据来源: IET

摘要:

A system using self-synchronising digital codes to control several switching circuits through two wires is described. The system has a simple design and high reliability, and although the response is slow, it is thought that, on many power applications, the delay would be tolerable. A prototype 12-switch unit has been built and found to operate satisfactorily.

DOI:10.1049/piee.1970.0343

出版商:IEE    

年代:1970

数据来源: IET

摘要:

The design of very wideband amplifiers using transistor feedback pairs is discussed in detail, and typical experimental results are given. With source and load impedances of 50Ω, the product of gain and bandwidth is nearly 2000 MHz for a single pair, and over 10000 MHz for two pairs in cascade, subject to a maximum bandwidth of about 400 MHz. Noise factors range from 2 to 6dB. Source impedances other than 50Ω can be used.

DOI:10.1049/piee.1970.0344

出版商:IEE    

年代:1970

数据来源: IET

摘要:

Since the innovation of the practical tungsten-halogen lamp employing iodine as the regenerating agent, there has been continuous research and development in this field of lamp technology. Although there are still gaps in our knowledge, particularly regarding the kinetics of certain reactions, there is now a far better appreciation of the regenerative mechanism and the influence of materials, processing and design on the performance and reliability of lamps. Practical applications are expanding, and one of the most notable is motor-vehicle lamps, where scientific investigation into tungsten transportation using radiochemical tracers has led to the establishment of twin-filament headlamps. The use of bromine, hydrogen and hydrocarbon halides and mixed halogens, including chlorine, in addition to iodine, is quite common practice. The modern tungsten-halogen lamp is now a very reliable light source, in which the halogen additives, dimensions and gasfilling are carefully chosen to give optimum performance. At present, efficacy and life have reached a plateau with existing techniques and materials, but the prospect of alternative materials and techniques indicate that a further major advance could occur within the next few years, and it is not difficult to foresee tungsten-halogen lamps replacing most conventional incandescent lamps.

DOI:10.1049/piee.1970.0346

出版商:IEE    

年代:1970

数据来源: IET

摘要:

In conventional polyphase alternating-current machines, the number of parallel circuits per phase is always an integral submultiple of the pole number. This requirement can impose restrictions in relation both to the optimisation of design, and to the practical aspects of the manufacture of medium and large low-voltage machines. A general method is described in this paper, by which several alternative intermediate numbers of parallel circuits per phase and numbers of parallel circuits greater than the pole-number can be derived, for a wide range of combinations of armature slot number and pole number. The design and performance of a 4-pole machine with six parallel circuits per phase, and of a 6-pole machine with four parallel circuits per phase, are considered in detail, by way of example. The new windings are of the double-layer type. It is shown that, on the basis of identical coil pitches, the new type of winding is not inferior in rating or load performance to a standard winding; and that, in addition to the advantage of a free choice of the number of parallel paths, there is the further advantage of improved accelerating characteristics.

DOI:10.1049/piee.1970.0347

出版商:IEE    

年代:1970

数据来源: IET

摘要:

The paper discusses the process whereby some of the kinetic energy stored in a generator rotor is converted to magnetic energy when a generator is short circuited. This gives rise to oscillatory electrical torques at fundamental frequency which, hitherto, have not been recognised as an important factor in the initial angular movement of a generator rotor and have usually been ignored; this is shown to be a serious omission which, when corrected, will influence future studies of generator stability. The effect of these oscillatory torques is to cause a back swing of the rotor when a generator is faulted. This phenomenon has been noticed during several system tests, and the results of such tests are used in the paper to support the theory which is presented. The practical system conditions necessary for the existence of these torques is discussed. Since these torques are of fundamental frequency, the exact calculation of them in digital-computer programs is time consuming, and thus a simple approximate method for simulating their effect on rotor movement is presented.

DOI:10.1049/piee.1970.0348

出版商:IEE    

年代:1970

数据来源: IET