A description is given of the electronic circuits used for the excitation of the caesium resonance and the measurement of the resonant frequency, attention being drawn to the precautions which have to be taken in order to obtain an accuracy of ± 1 part in 1010. The exciting source is derived by the multiplication of the frequency 5.0069 Mc/s of a quartz oscillator. The oscillator is then set to give the resonant frequency exactly. The measurements are made at the high frequency of approximately 9 Gc/s so that an accuracy of 1 c/s is obtained in one second using a counter in the final stage.An alternative and rather simpler system is also described, and some comments are made on the problem of frequency synthesis.
An instrument is described for the very precise comparison of 100 kc/s frequency standards. A sensitive phase-comparator is combined with a trigger circuit to produce a timing pulse at the instant the two 100 kc/s signals are in phase opposition. The time interval between successive pulses is determined by a counter-type chronometer. For a 10 sec beat period a timing accuracy of 1 microsec is obtained. This corresponds to an accuracy of frequency comparison of 1 in 1013.
Reflections from the surfaces of dispersive materials used in broadband aerial systems are highly frequency-dependent. A technique for matching such materials is described, and results are included of a successful application to the input surface of a dispersive prism.
The direction of a radiated beam can be altered by passing it through a prism. If the prism material is dispersive, the final direction is frequency sensitive and beam scanning can therefore be achieved by frequency modulation. Two types of dispersive artificial dielectric are considered for use in such a prism, one consisting of an array of rods, and the other of an array of sheets containing a pattern of resonant slots. Measured values of the electrical constants of both arrays over a wide range of wavelengths are included, and the properties of the beam-scanning prism are described in detail.
The problem of synthesizing apparatus that will automatically simulate man's ability to recognize and to learn to recognize patterns is discussed and it is concluded that analogue circuits, rather than the digital switching circuits that have been employed in the past, provide the simpler solution. A new circuit unit that possesses many of the essential functional characteristics exhibited by nerve cells in the brain is derived from earlier work on the electrical simulation of nervous-system functional activity and forms the basic element of the circuits.The new analogue apparatus consists of a number of distinct functional circuits arranged in a definite sequence, through which signals derived from the patterns to be recognized pass simultaneously on their way to the final output terminals. Classification information may be built into the apparatus initially if it is available, but if not, it can be stored automatically in a special unit during a setting-up procedure in which samples of the pattern types that the apparatus will be required to recognize are presented, together with identification signals. Low-resolution automatic pattern-recognition apparatus is described, and examples illustrate the setting-up procedure and subsequent performance of the apparatus.
When a semiconducting crystal is mounted in the path of an electromagnetic wave, a Hall e.m.f. is set up in the material along the direction of propagation with a value proportional to the power transmitted. The device operates most satisfactorily when the wave impedance is small, so that for a given power the electric-field component is reduced to a minimum. The use of a resonant cavity with the crystal erected in it at a point of strong magnetic field enables such favourable conditions to be established, and this arrangement forms the basis of the wattmeter described. The instrument is capable of measuring power at 4 Gc/s with an error of ±3% from 30 mW to about 20 watts, and with the unique feature that this performance can be achieved at any standing-wave ratio between unity and 0.1 while absorbing only about 3.4% of the power measured.Some experiments on semiconductors mounted directly in a waveguide are also described, showing the possibility in this case of using several crystals in cascade.
It is demonstrated that a composite material of a solid dielectric with embedded conductors can give a wave impedance equivalent to that of free space, and thus eliminate interface reflections. A particular material, the rodded dielectric, is analysed, and design procedures are given which are exemplified by numerical results. An experimental verification of the design theory when the electric-field vector is wholly parallel to the axes of the rods is described, and the results show good agreement with the theory. Finally, methods of manufacture, and the tolerances required, are discussed.
In recent years, there has been a tendency to introduce new navigational aids which utilize rotating aerial arrays of large electrical dimensions in order to obtain highly accurate navigational information; one such example is the well-known system called Tacan. In general, the rotation of large arrays introduces severe mechanical problems which fix a lower limit of about 300 Mc/s to the operating radio frequencies of these aids. For many purposes it is desirable to operate in the 100 Mc/s region, and the paper describes a method by which this object may be achieved. Briefly, it consists of rotating an aerial array of small mechanical dimensions and allowing it to be coupled electrically to a static array of very large dimensions which acts as the final radiator of electromagnetic energy. The efficiency of coupling and the form of the static array are such as to produce a final radiation which makes the rotatable array appear to have the large dimensions of the static array. In effect, the method is similar to the familiar `goniometer¿ technique. The method is described with reference to its application in a new and more accurate form of Vor system (v.h.f. omni-range) known as Vorac. The paper also describes how the new method itself helps to improve the vertical elevational performance of a conventional Vor system.
Recognizing the evanescent character of the surface-wave field distribution over the equi-phase planes and the important part played by the inclination of these planes with the normal to the interface when power is transferred across it, calculations are made for radiation arising when a wave of this kind circulates around a highly reactive supporting surface of cylindrical form. It is concluded that, when the surface has a finite loss, there will be a particular radius of curvature for which the surface wave progresses for a limited distance without attenuation.
The graphical method of measuring a reflection coefficient through a junction, originated by Deschamps, is used to determine the transmission efficiency of a surface wave at a discontinuity. The radiations excited by the surface wave at a discontinuity in surface reactance, and at an edge of a metallic strip above a reactive surface, are investigated. It is shown that the discontinuities cause the radiation of an appreciable fraction of the incident power. A matching step at the discontinuity in surface reactance is found to have no effect on the excitation of radiation. Further, it is found that, compared with the radiation excited at the edge of a metallic strip, the radiation excited at a discontinuity in surface reactance is confined to a narrower angle above the surface.
The paper gives a practical account of parallel-plate, or strip, transmission lines, which are becoming of increasing importance in microwave applications. The different types of strip line are considered and their basic characteristics summarized. Transducers to conventional systems and various circuit-elements are described, while the application of strip-line techniques to the construction of microwave components and filters is examined. Manufacture by photo-etching is outlined.
The paper describes a new method for the automatic recognition of patterns. The method may be applied to any form of spatial pattern, but in the present instance, patterns consisting of line figures are considered. The pattern is presented to a flying-spot scanner connected to a digital computer. The shape of the pattern is analysed and a statement is prepared describing the basic features of the pattern. The pattern is then recognized by comparing this statement with a number of others already stored in the computer which relate to named patterns. Patterns are recognized independently of the angle at which they are presented to the scanner, and may be of any size provided that limits imposed by the resolution of the scanner are not exceeded. The average time to recognize a character is 60 seconds with the system programmed on a medium-speed computer. Special-purpose equipment built to perform certain of the stages of the process, together with the use of higher-speed computers now envisaged, will reduce this time by at least a thousandfold.If a new pattern is presented to the machine it will indicate its inability to recognize the pattern, but by giving the machine the name of the pattern, it may become one of the standard patterns which it can subsequently recognize. All the patterns recognized by the machine are hand-drawn and consist of such symbols as the capital letters of the alphabet and numerals, although the system is in no way limited to any special set of characters. Using exactly the same method but with an increase in the degree of complexity, it will be possible for machines to read handwriting.Special allowances are made for imperfections in the patterns, including breaks and general ill-definition. Where there is some confusion and an unknown pattern resembles two or more of the standard patterns, the relative degrees of similarity of the unknown to each of these standard patterns is printed out by the machine.
Testing methods, such as `articulation', `immediate appreciation', etc., at present available for providing a direct measure of the performance or effectiveness of a link for speech communication are not individually or even collectively applicable over any wide ranges of levels of performance or types of link. Many practical links fall outside the range over which these methods are reliable, and there is need for a general method that will both provide a safeguard against unrealistic results and serve, if possible, to extend the usefulness of existing techniques. The paper examines the mechanism of conversation over a speech link and deduces a sequence of criteria each applicable over an appropriate range of performance. A general assessment method follows from this, and the corresponding experimental techniques (which depend on the use of ordinary untrained subjects) and some applications are discussed. Some existing assessment methods can be fitted into the framework provided by the general method and can then be used with increased confidence. Assessments as described in the paper are of little value for indicating the merit a link has for the user. Their most valuable use is to facilitate (as part of the process known as rating) realistic comparisons of links with Standard Speech Links whose evaluation can be studied in actual use.
