Showing papers in "Journal of the Institution of Electrical Engineers - Part III: Radio and Communication Engineering in 1946"

Theory of communication. Part 1: The analysis of information

Abstract: Hitherto communication theory was based on two alternative methods of signal analysis. One is the description of the signal as a function of time; the other is Fourier analysis. Both are idealizations, as the first method operates with sharply defined instants of time, the second with infinite wave-trains of rigorously defined frequencies. But our everyday experiences?especially our auditory sensations?insist on a description in terms of both time and frequency. In the present paper this point of view is developed in quantitative language. Signals are represented in two dimensions, with time and frequency as co-ordinates. Such two-dimensional representations can be called ?information diagrams,? as areas in them are proportional to the number of independent data which they can convey. This is a consequence of the fact that the frequency of a signal which is not of infinite duration can be defined only with a certain inaccuracy, which is inversely proportional to the duration, and vice versa. This ?uncertainty relation? suggests a new method of description, intermediate between the two extremes of time analysis and spectral analysis. There are certain ?elementary signals? which occupy the smallest possible area in the information diagram. They are harmonic oscillations modulated by a ?probability pulse.? Each elementary signal can be considered as conveying exactly one datum, or one ?quantum of information.? Any signal can be expanded in terms of these by a process which includes time analysis and Fourier analysis as extreme cases. These new methods of analysis, which involve some of the mathematical apparatus of quantum theory, are illustrated by application to some problems of transmission theory, such as direct generation of single sidebands, signals transmitted in minimum time through limited frequency channels, frequency modulation and time-division multiplex telephony.

Theory of communication. Part 3: Frequency compression and expansion

Abstract: It is suggested that it may be possible to transmit speech and music in much narrower wavebands than was hitherto thought necessary, not by clipping the ends of the waveband, but by condensing the information. Two possibilities of more economical transmission are discussed. Both have in common that the original waveband is compressed in transmission and re-expanded to the original width in reception. In the first or "kinematical" method a temporary or permanent record is scanned by moving slits or their equivalents, which replace one another in continuous succession before a "window." Mathematical analysis is simplest if the transmission of the window is graded according to a probability function. A simple harmonic oscillation is reproduced as a group of spectral lines with frequencies which have an approximately constant ratio to the original frequency. The average departure from the law of proportional conversion is in inverse ratio to the time interval in which the record passes before the window. Experiments carried out with simple apparatus indicate that speech can be compressed into a frequency band of 800 or even 500 c/s without losing much of its intelligibility. There are various possibilities for utilizing frequency compression in telephony by means of the "kinematical" method. In a second method the compression and expansion are carried out electrically, without mechanical motion. This method consists essentially in using non-sinusoidal carriers, such as repeated probability pulses, and local oscillators producing waves of the same type. It is shown that one variety of the electrical method is mathematically equivalent to the kinematical method of frequency conversion.

The probability distributions of sinusoidal oscillations combined in random phase

Abstract: This paper discusses a fundamental problem involved in the theory of multi-channel transmission. In modern carrier telephone systems a number of channels pass through common line amplifiers. The loading of these amplifiers therefore fluctuates in accordance with the sum of the speech voltages applied to the individual speech channels in the carrier group, and the maximum loading depends upon whether there can be a simultaneous occurrence of peak values in all the speech channels. In general, this is an extremely rare occurrence. As an alternative to providing speech circuits, some of the channels may, however, be sub-divided to provide voice-frequency telegraph channels, the speech being then replaced by a number of single-frequency tones. When the telegraph channels are idle, these tones are continuous and the maximum loading of any common amplifier may be increased thereby. Some modern automatic signalling systems also use continuous tones or long periods of tone for the transmission of signals, so that possible overloading of the amplifiers must again be considered. It becomes important, therefore, to be able to determine the probabilities associated with the combination of sinusoidal signals from a number of different channels, and in particular the probabilities that the instantaneous value and the length of the resultant vector will exceed certain limits. The instantaneous value of the resulting signal is, of course, the sum of the values in all the combined channels. Under suitable conditions the peaks of the instantaneous value trace out an envelope for the resulting signal, and the probabilities associated with this envelope are, in general, the same as those associated with the resultant vector. The paper discusses theoretically the probabilities associated with the instantaneous value and the length of the resultant vector obtained by combining n cosine oscillations of equal amplitude and random phase rel

Resonance methods of dielectric measurement at centimetre wavelengths

Abstract: The paper deals with the theory and experimental development of resonator systems suitable for measurement of the permittivity and power factor of solid dielectric materials in the wavelength range below 50 cm (600 Mc/s). The relative suitability of three forms of resonator, namely a short-circuited length of coaxial transmission line operating in the principal mode, and hollow cylindrical cavity-resonators operating respectively in the E010 and H01n modes, is discussed. The theory governing the resonant behaviour of these systems when wholly and partially filled with ?lossy? dielectric is developed, leading to relations connecting the permittivity and power factor of the latter with the resonant wavelength and Q-factor respectively.

A Theory of Valve and Circuit Noise

Abstract: The paper presents a fundamental theory of noise which the authors believe to be physically perspicuous and from which all the well-established formulae used in practice can be deduced by direct mathematical argument. Since these formulae almost all relate to mean-square noise in linear circuits, the theory will not be applied here to other problems, although its ideas are valuable in attacking more complicated problems.The subject is discussed under the following headings: (1) Introduction; (2) the basic postulate and the fundamental equations; (3) the “mean theorem” and the “mean-square theroem” (4) the calculation of mean-square noise; (5) thermal noise; (6) application to valve and circuit noise; (7) space-charge effects; and (8) transit-time effects.

Theory of communication. Part 2: The analysis of hearing

Abstract: The methods developed in Part 1 are applied to the analysis of hearing sensations, in particular to experiments by Shower and Biddulph, and by Burck, Kotowski and Lichte on the discrimination of frequency and time by the human ear. It is shown that experiments of widely different character lead to well-defined threshold "areas of discrimination" in the information diagram. At the best, in the interval 60-1 000 c/s the human ear can discriminate very nearly every second datum of information; i.e. the ear is almost as perfect as any instrument can be which is not responsive to phase. Over the whole auditory range the efficiency is much less than 50%, as the discrimination falls off sharply at higher frequencies. The threshold area of discrimination appears to be independent of the duration of the signals between about 20 and 250 millisec. This remarkably wide interval cannot be explained by any mechanism in the inner ear, but may be explained by a new hypothetical effect in nerve conduction, i.e. the mutual influence of adjacent nerve fibres.

Calculation of the electromagnetic field, frequency and circuit parameters of high-frequency resonator cavities

Abstract: A method for the calculation of the field components of free electromagnetic oscillations in metal cavities of any shape is described with reference to Klystron resonators. Whereas in a previous paper a method for computing solutions of Laplace's equations was given, the present paper is concerned with the wave equation ?? + (?2/c2)?=0. Again, the differential equation is replaced by a system of difference equations, which in the case of free vibrations are homogeneous and soluble only for certain values of the parameter ?2/c2, the proper values. A method for finding the lowest value of ?2/c2 without solving a determinantal equation is described. The boundary of Klystron resonators contains sharp corners, often feather edges, which present a special problem. In order to obtain the necessary accuracy for the circuit parameters, with a reasonable amount of computation work, it is essential to take account of the analytic behaviour of the fields near sharp corners. This is done in a manner which is particularly well suited to the relaxation method of solution of the equations, and a great deal of computation work is thereby saved. Once the field components and the resonant frequency are found, the beam impedance and the damping constant are easily determined.

Some Applications of Field Plotting

Abstract: By the process of field-plotting in the cross-section of a uniform transmission line, an orthogonal field pattern of curvilinear squares satisfying the boundary conditions is obtained. If Nv is a number of voltage steps and Nf the number of electrostatic flux lines in this field plot, the characteristic impedance is given by Z = 377Nv/Nf??/k where ? is the permeability, and K is the permittivity. Applications to coupling and screening are also considered. The use of models in an electrolytic tank for capacitance determination from field plots or direct measurement is discussed, and an axially symmetrical three-dimensional field is plotted. Mention is made of the relaxation process and rubber-sheet methods of field-plotting.

The measurement of the activity of quartz oscillator crystals

Abstract: Consideration is given to the various electrical quantities associated with the equivalent network of a quartz crystal which might be used to assess the "activity" that the crystal will exhibit in an oscillator circuit. It is concluded that the most generally serviceable criterion is afforded by measuring the anti-resonant impedance of the parallel combination of mounted crystal and a standardized value of capacitance typical of that encountered in current oscillator designs. This is especially true for the high-frequency thickness-shear-mode plates which at present constitute the bulk of the output of the industry, since these plates are practically always used in circuits in which this anti-resonant impedance determines directly the amplitude of oscillation. The relations between the various possible criteria of activity are investigated. The following pieces of equipment are described:- 1. A flexible laboratory instrument capable of measuring the series-resonant or anti-resonant impedance of any crystal response in the frequency range 50 kc/s-20 Mc/s. 2. A simpler laboratory instrument capable of measuring the anti-resonant impedance of any response in a restricted frequency band around selected spot frequencies. 3. An instrument designed for factory or other routine testing, in which the parallel dynamic resistance of the dominant crystal response is measured by a substitution method in an oscillator circuit. This instrument is suitable for frequencies above 3 Mc/s. 4. An instrument similar to No. 3 but slightly more complex, for the frequency range 80 kc/s-3 Mc/s.

Radiation from large circular loops

Abstract: In this paper is calculated the radiation resistance of a circular loop of any radius and also its polar diagram of electric field at a large distance. It is shown that, according to the radius, the field can be zero in the equatorial plane or at any angle of elevation. It is shown that the ?high-angle? radiation can be sensibly removed by using two concentric and coplanar loops having suitably chosen radii: but with this disposition the current must be supplied to both loops and it is impracticable to induce one current from the other. The ?high-angle? radiation can also be much reduced by the use of two similar coaxial large loops in parallel planes, and this offers a disposition which may be useful in practice. The power gain of a stack of N similar small loops is shown to be equal numerically to the gain of an N-member in-line array of electric doublets; and it can be shown that this is true whatever the radius of the loop, but one or more side lobes can be sensibly obliterated by suitable choice of this radius. Having regard to the difficulties of ?feeding,? inherent in the construction of all aerial systems, it would seem that a pair of parallel large loops is one of the simplest systems for obtaining a horizontal electric field concentrated mainly in a zone of about ± 30° from the equator. The choice between two such loops and a stack of small loops depends very much on the wavelength to be used.

A standard of frequency and its applications

Abstract: The paper discusses the importance of frequency standardization to the telecommunication engineer, and outlines the work which the Post Office has carried out in this field. As published information on the subject is scanty, the historical development of frequency standards is reviewed, a bibliography is included and the factors affecting the design of a standard are briefly analysed. The design, performance and application of the Post Office standard are considered in some detail. In particular, the absolute calibration of the group of crystal-controlled oscillators comprising the standard is discussed, and details are given of equipment designed for the precise intercomparison of the frequencies of the individual oscillators. A specialized form of frequency standard, known as a ?quartz clock,? which is now being used by astronomers in addition to the older type of pendulum clock, is briefly described. The methods available for the comparison, both national and international, of frequency standards are analysed, and a resume of international comparisons reveals the advances achieved in frequency standardization during the past two decades. At present it is considered that the absolute value of the Post Office standard of frequency is known to ±1×10?8. Finally, the development work now being carried out to improve the stability and reliability of the standard is outlined.

The super-regenerative detector: an analytical and experimental investigation

Abstract: On the basis of an exponential build-up of oscillations in its grid circuit, the super-regenerative phenomenon is analysed and a general expression for the output at modulation frequency is obtained. This expression is examined term by term and reveals the mechanism of (a) high amplification, (b) self-a.v.c. action, (c) excessive distortion of the modulation frequency at modulation depths in excess of 50%. All these properties are quite characteristic of the super-regenerative detector in practical operation.Histograms illustrate the distortion effect in terms of modulation depth. Various oscillograms verify the mathematical results, particularly in regard to the law of build-up of oscillations as a function of the amplitude of the applied signal.The dependence of radio-frequency selectivity and modulation-frequency gain upon the quenching frequency is shown in the analysis and verified by measurements. It is shown that an optimum quenching frequency exists for maximum gain at the modulation frequency. If this optimum frequency is slightly exceeded, the noise background may be removed without severe reduction of gain.The ability of the super-regenerative detector to discriminate agamst impulsive radio-frequency interference (e.g. motor-vehicle igmtion interference) is analysed briefly. The reception of frequency-modulated signals is also discussed briefly.

Radio measurements in the decimetre and centimetre wavebands

Abstract: The paper deals with the technique of radio measurements evolved as a necessary aid to the development of decimetre- and centimetre-wave valves, circuits and equipment, and surveys some of the methods applicable in this wide field. It commences with a review of the circuit theory of coaxial and wave-guide transmission lines, followed by a short section on high-frequency oscillators.Succeeding sections deal with the measurement of the fundamental quantities of frequency, power, impedance and voltage. Heterodyne methods of frequency measurement, and coaxial and resonant cavity wavemeters, are described. The design of bolometer and calorimeter circuits is discussed for powers between 1 microwatt and several hundred watts. Measurements of reflection coefficient and Q-factors are included in the section on impedance.The concluding sections deal with derived measurements: receiver sensitivity, aerials and field strength. A standard of receiver sensitivity is established, based on thermal noise voltage, and the design of suitable signal generators and sensitivity measuring equipment is discussed. The section on aerials comprises measurements of polar diagram, gain and impedance. Finally, equipment for the absolute measurement of field strength at centimetre wavelengths is outlined.

The theory of the non-linear bridge circuit as applied to voltage stabilizers

Abstract: The paper deals with the theory and design of the non-linear Wheat-stone bridge circuits used in voltage stabilizers. It is restricted to the use of the bridge as a source of voltage proportional to the change in the supply voltage and to non-linear elements obeying the law V = kIn, where n and k are constants. Four circuits are considered, and formulae are deduced for the general case of a resistive load across the output terminals. The effect of changes in the various parameters is discussed. Formulae are also given for the load required for maximum power output from the bridge for a given change in input voltage. The paper concludes with a review of the advantages and disadvantages of the various circuits.

The characteristics of lamps as applied to the non-linear bridge, used as the indicator in voltage stabilizers

Abstract: The paper is concerned with the use of lamps in a non-linear bridge, operating as the “indicator” of a voltage stabilizer. The characteristics of various types of lamp are considered with regard to their suitability for this purpose, e.g. the effect of ambient temperature and vibration. Experimental and mathematical results are given for the response time of various lamps when used in this circuit. Methods of overcoming this delay by means of suitably designed capacitance-resistance networks are given, together with experimental results. The paper is restricted to the use of a d.c. supply for the bridge.

A method of transmitting sound on the vision carrier of a television system

Abstract: The paper describes a television system in which sound pulses having a constant height and variable width are inserted in the line synchronizing periods. It is claimed that this method of transmission leads to a simplified receiver and that the programme quality is better in the presence of severe interference. Other advantages are that the frequency band-width for transmission is reduced; the method of receiving sound ensures automatic volume control; the sound pulses provide a fixed reference level for automatic volume control on the vision channel; mutual interference between vision and sound often present on two-channel reception is avoided; reduced transmission band-width simplifies the design of the receiving antenna; mutual coupling between the vision and sound antennae at the transmitter is avoided; and the installation and maintenance costs of the sound transmitter are saved. The frequency range of the system operated in conjunction with the pre-war British transmission would be limited to 5 kc/s.

Fluctuations of electric current

Abstract: Random fluctuations in the amplitude of an electric current were originally discovered in two different circumstances: (a) the fluctuation in the current through a thermionic valve, which is known as "shot noise," and (b) the voltage fluctuation at the terminals of a resistor, which is known as "Johnson noise." The theory of these two types of fluctuation is presented in outline form; and it is shown how these two aspects of the fundamentally discrete nature of the flow of electric current, when it is a stream of electrons, can be unified into a single theory of current fluctuations due to random motions of the electrons. The theory is applied also to the controversial case of a space-charge-limited thermionic valve. Subsidiary factors, such as partition of current in a screen-grid valve and the behaviour of secondary-emission amplifiers, are of great technical importance, and methods of estimating the magnitude of their effects are described. The equivalent temperature of the radiation resistance of an aerial is also considered.

Tendencies in the design of the communication type of receiver

Abstract: The term ?communication receiver? is sufficiently vague to demand some measure of definition. The first Section of the paper therefore discusses briefly the historical background and the features and applications of the type of receiver which, to the best of the authors' knowledge, are generally accepted as being implied by this term. The second Section covers the mechanical considerations which apply to (and to some extent limit and define) the complete receiver. The third Section, in which it is hoped that the main interest of the paper will be found, discusses the electrical features of this type of receiver. From a brief review of overall circuit considerations, it passes to a discussion of specific characteristics, dealing in some detail with the extent to which theoretical limits of signal/noise ratio are achieved in representative designs. In addition to treating the factors limiting such fundamental characteristics as signal/noise ratio, selectivity, etc., this Section gives some attention to such ancillary features as electrical band-spread methods, listening-through facilities, etc. In the discussion of both the electrical and the mechanical features of these receivers, the object has been to give a critical survey of the nature of the design problems and the solutions adopted in typical receivers, rather than to cover detailed design information in connection with any particular feature. The paper concludes with Sections devoted respectively to current practice in supply-circuit arrangements and possible future tendencies.

The design and use of radio-frequency open-wire transmission lines and switchgear for broadcasting systems

Abstract: The paper describes the various types of transmission lines in use at transmitting stations of the British Broadcasting Corporation, their impedance, attenuation and power-carrying characteristics, together with results of various tests, general details of construction, and methods of matching the load to the transmission line. The major part of the work described was carried out before the end of 1943. The paper is written primarily from the point of view of the engineer engaged on design and practical work in the field. Data are included enabling the most suitable transmission line for any practical purpose to be designed. The range of frequencies considered is from 0.2 to 25 Mc/s; of this range those up to 2 Mc/s are considered as medium waves and those from 2 Mc/s to 25 Mc/s as short waves.

New methods for locating cable faults, particularly on high-frequency cables

Abstract: Normal fault-locating procedures become impractical in certain conditions, either for purely technical reasons or on account of the loss of service time involved. This paper commences by summarizing a theoretical investigation of the possibilities of applying pulse and frequency-modulation methods to this problem on wide-band coaxial telephone cables. A practical fault-locator employing d.c. pulses is then described. The problem is first contrasted with that of radar, and the factors controlling the choice of the transmitted waveform and those limiting the accuracy of location attainable are then discussed. The fundamental requirements of a frequency-modulation system are examined in some detail, and it is concluded that, although an f.m. instrument would be attractive in certain circumstances, the practical advantage lies with the pulse type of fault-locator owing to the clarity and reliability of its indications when more than one fault is present. The d.c.-pulse instrument described has been in use for some time, and faults on coaxial cables have been located within 1% of their true distances at ranges up to 10 miles.

The calculation of intermodulation products by means of a difference table

Abstract: Espley? has described a convenient method for the numerical calculation of the harmonic output of a circuit containing a non-linear element, such as a valve, the characteristic of which is given for equally-spaced values of the input voltage. A difference table was formed from these data; the terms of the central line of this table were then multiplied by the values of certain functions dependent on the type of the harmonic wanted and the amplitude of the input, and finally the products were added. The present paper shows that a similar procedure can be applied to the determination of intermodulation products of two input frequencies. The factors which now appear as multipliers of the difference terms can be obtained from a multiplication table of such Espley functions. The procedure is summarized in Section 5. The derivation of the result makes use of the operator properties of Sheppard's symbols ? and ?.

A method of increasing the range of v.h.f. communication systems by multi-carrier amplitude modulation

Abstract: The initial development of a method of extending the range or improving the coverage of v.h.f. communication systems of the type used for police services is described. The method is based on the simultaneous amplitude modulation of a number of carriers closely spaced in frequency. The frequency spacing between the carriers is so chosen that they lie within the band-width of the v.h.f. receiver, without producing audible interaction components of importance. Two-carrier schemes employing separate transmitters at the same site have been found to give improved coverage, while two- and three-carrier schemes using separate sites have been found to give greatly increased range. An unsuccessful attempt to achieve the same object with frequency-modulated (f.m.) transmitters using the same nominal carrier frequency is described. The difficulties of employing frequency modulation with common modulation are discussed and considered to be fundamental. Single-station v.h.f. schemes upon which the development of the multi-carrier scheme is based are briefly described.

Tuning devices for broadcast radio receivers

Abstract: The wide choice of entertainment provided by the broadcast receiver is perhaps its most important feature, and the development of tuning devices to simplify the exercise of that choice has played an outstanding part in radio receiver design. Both station selection and ready identification of a programme found by random searching are involved. The paper opens with a brief historical review of tuning-device evolution from the introduction of single-knob tuning to the immediate pre-war designs. Listening tests are used to establish the degree of mistuning to cause observable deterioration of quality, and to suggest a target design tolerance for tuning errors. A consideration of frequency stability on the broadcast bands leads to an analysis of design limitations and establishes pre-set tuning on medium and long waves and band-spread tuning on short waves as important problems. The design of pre-set tuning devices is reviewed, together with their power drive and remote control, and the development of band-spread tuning is described, together with the associated receiver circuits. Appendices give the measured frequency response and harmonic-content characteristics of the receiver used in the listening tests, together with a mathematical analysis of these characteristics as derived from the intermediate-frequency response.