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Electrical Engineers - Part IIIA: Radiocommunication, Journal of the Institution of

Issue 13 • Date March-April 1947

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Displaying Results 1 - 21 of 21
  • Electro-acoustic transducers and intercommunication systems for aircraft

    Page(s): 441 - 451
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    The paper is a review of audio-frequency communication instruments and techniques used in military aircraft during the war years. It indicates also the purpose and trend of further developments in this field. A brief exposition is first given of the mechanism of speech communication in and from noisy locations, and of the requirements. thereby imposed on the design of transducers and audio circuits. The practical application of these considerations in aircraft equipment is discussed; details are given of instruments used in the Royal Air Force. Telephone distribution problems in aircraft intercommunication and radio installations are outlined, and a brief description is given of systems proposed for future Service and civil aircraft. View full abstract»

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  • Recent developments of aircraft communication aerials

    Page(s): 452 - 458
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    The paper describes the progress made in recent years in aircraft aerial design. The impedance characteristics of several typical kinds of aerial are stated. View full abstract»

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  • A panoramic receiver (3.5¿20 Mc/s)

    Page(s): 459 - 460
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (261 KB)  

    The receiver is designed to indicate the presence, frequency and relative magnitude of strong signals in the frequency band 3.5¿20 Mc/s, displaying all such signals simultaneously in ¿panoramic¿ form on a 12-in cathode-ray tube. To cover this large frequency range in the one instrument, a superheterodyne arrangement with the intermediate frequency higher than the signal frequencies is employed. The beating oscillator frequency is varied cyclically in sawtooth fashion by means of a motor-driven rotating capacitor. The output of the final detector is applied to the Y-deflection of the tube, and the X-deflection is derived from a sawtooth generator. The latter is synchronized with the frequency sweep by means of pulses, developed at a predetermined point in the cycle by applying the beating oscillator output to a tuned amplifier. View full abstract»

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  • Reference-crystal-controlled v.h.f. equipment

    Page(s): 461 - 474
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (2233 KB)  

    A brief allusion is made to the value and usage of v.h.f. equipment capable of crystal-controlled operation on a large number of channels, any one of which may be selected by the setting of one or more dials to the cipher or letter combination characterizing this channel. A survey is made of the methods of obtaining this facility from a small number of crystals built into the apparatus, one of which controls a harmonic generator whose frequency is related to the channel spacing. The authors have ventured to coin a generic term for equipment of the above somewhat cumbersome description, namely ¿asac¿, as representing ¿automatic selection of any channel.¿ The methods conceived by the authors are placed in relation to the field surveyed, and are elaborated with particular reference to the TR.1407 and some similar equipments. View full abstract»

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  • Power supplies for aircraft communication equipment

    Page(s): 475 - 483
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    The paper presents a brief historical review of the development of power supplies for aircraft communication equipment. A study is made of the aircraft general electrical system in so far as it affects radio power supply, particular attention being paid to the system voltage and the means employed for its regulation. A brief mention is made of the only experimental a.c. system which has yet been applied in this field. The various forms in which power is required by communication equipment are summarized. The paper gives a description of the assembly of the various components forming the power unit, giving by way of illustration some details of a typical power unit. A section of the paper is devoted to description of the voltage-conversion apparatus used. The broad principles and performance of the rotary transformer are described, with an outline of its design and application. The principal types of vibrator are described, with an outline of the design and performance of a particular type developed for Air Ministry service. The specialized requirements of transformers for vibrator convenors, and the application of vibrator convertors to the field of radio power supply are briefly discussed. The chief methods of providing control of voltage in radio power supplies are described. View full abstract»

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  • Radio interference from aircraft electrical equipment

    Page(s): 484 - 492
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    This paper deals with the radio-frequency disturbances caused by the normal operation of electrical equipment in aircraft. A brief account is given of the events leading to the adoption of the present system of suppression, reference is made to the special problems encountered, and attention is drawn to the more common sources and means of propagation of interference. Methods of reducing interference are described, and suppression at the source is indicated as being the most satisfactory method, emphasis being laid on the feasibility of designing radio-interference suppressors and suppression systems that will be effective over a wide frequency range with predictable behaviour. It is noted that some reduction of interference is possible by measures applied to receivers and aerials, and by careful design of equipment, and that these alone are rarely adequate. The design considerations for a radio-interference suppressor are stated and expanded for the particular application of a suppressor for aircraft use, and details are given of effective components and assemblies. Reference is made to test arrangements peculiar to the work. Experience has shown that, if adequate suppression is achieved continuously over the frequency range 150 kc/s¿150 Mc/s, no trouble will be experienced at higher frequencies, and that the system adopted obviates the necessity for interference tests in production. View full abstract»

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  • A method of obtaining linear frequency deviation in a wide-band frequency-modulation system

    Page(s): 497 - 502
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (601 KB)  

    This report describes a method of obtaining linear frequency deviation in a wide-band frequency-modulation system. The primary cause of harmonic distortion in a frequency-modulation system is the non-linearity of the slope of the reactance valve. A method is described of linearizing this slope, and thereby obtaining very good linearity of the frequency modulation. The harmonic distortion is of the order of 60 db below the fundamental. This report also describes a method of checking high-fidelity frequency modulation. View full abstract»

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  • Pulse communication on lines

    Page(s): 503 - 510
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    A dispersive medium, i.e. one in which the waveform of an individual pulse changes its type at successive points during propagation, is not normally appropriate for pulse communication, since greater efficiency can be achieved through it in other ways. However, conditions can arise in which it becomes desirable to employ such a link, and it is necessary to discover the restrictions imposed by the medium on multichannel pulse communication, and the steps that can be taken to minimize these limitations. The paper briefly describes a theoretical and experimental investigation by the authors into the particular problems of pulse communication over a 50-mile open-wire line of the standard Army type. The major problem, of adjacent-channel interference caused by the progressive deterioration of the individual propagated pulse waveforms to a point where they appreciably overlap, is adequately solved by adapting and extending a technique employed for wide pulses in line telegraphy to the case of narrow pulses. This is the device of reducing the interfering effect of each trailing pulse response by transmitting, instead of a simple rectangular pulse, a controllable curbed pulse, i.e. a flexible combination of contiguous pulses of opposite sign. By this means satisfactory crosstalk results are achieved for multichannel pulse operation in one direction. For two-way operation the dominant problem now becomes the near-end interference arising from the irregular sequence of pulse reflections from various inhomogeneities in the line. Unless the line is sufficiently smooth, the crosstalk so occasioned may become intolerable. View full abstract»

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  • A new method of wide-band modulation of pulses

    Page(s): 511 - 516
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (632 KB)  

    In a pulse-modulation system it may be required to convey a wide band of modulation-frequency components with a high degree of fidelity, as when the modulating signal consists of a multichannel carrier wave having its intelligence channels separated on a frequency basis. In this case the occurrence in the pulse system of any appreciable non-linear distortion introducing intermodulation amongst the pulse modulating tones cannot be tolerated, since it leads to mutual interference between the sub-modulation signals. The particular problem investigated theoretically and experimentally by the authors was the design of a modulator and a demodulator for a single-channel pulse system to operate as a link in a multichannel carrier-telephony system (0.3¿32 kc/s) having low overall noise and crosstalk requirements. The method employed, of scanning the modulating signal ¿periodically¿ instead of ¿synchronously¿, enables a low recurrence frequency to be used (theoretically twice the highest pulse modulating frequency, but in this case 80 kc/s to permit ease of filtering). There is a consequent gain of signal/noise time excursions, although each pulse position is limited to one recurrence period. View full abstract»

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  • The development of the Wireless Set No. 10: an early application of pulse-length modulation

    Page(s): 517 - 527
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    The experimental development of the system is described briefly, being a historical record of the design of an interlaced pulse-length modulated, multi-channel telephone circuit, and its application to a centimetre-wave radio link. The multiplexing equipment, radio sender and receiver, and the complete station, are described. A more detailed account of the electrical and mechanical difficulties encountered in the production is given and, where relevant, a technical description of the various units is included. Where a variety of possible solutions to lesser known techniques were considered, reasons are given for the final decision. The radio-frequency technique was developed concurrently with the engineering of the equipment and, where applicable to the particular problem, is described in detail. A brief description of the complete station includes overall electrical characteristics, its limitations and the method of testing to ensure a uniformity of performance. A general description of the equipment in its final form has been given elsewhere.1 View full abstract»

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  • A common-wave duplex pulse-communication system

    Page(s): 528 - 532
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (697 KB)  

    The paper details the principles of a duplex pulse-communication system using only one carrier frequency and suited for air-to-air and air-to-ground radiotelephony. The duplex facility is achieved by suppression of the receivers at each end of a communication link. Pulsephase modulation is employed and the pulse-recurrence frequencies of the transmissions in the two directions are unsynchronized. By using a memory pulse-phase discriminator, the introduction of noise of any considerable level is avoided. View full abstract»

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  • Noise problems in pulse communication

    Page(s): 533 - 545
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    Formulae for random noise in the audio-frequency output in pulse-communication receivers are given for pulse-length-, pulse-phase- and pulse-frequency-modulation systems in which the received pulses are sliced; they are also given for pulse-amplitude modulation. Noise resulting from random distribution of the i.f. phase at the onset of pulses is also considered. Optimum slicing level giving minimum output noise is evaluated. Signal/noise ratio and threshold formulae are obtained from these data for the pulse-modulation systems and are compared with those for conventional amplitude- and frequency-modulation systems having the same mean or peak power. Final conclusions as to the merits of all the above systems are drawn from graphs of output signal/noise ratio, in which the performance below threshold is also taken into account. View full abstract»

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  • High-power pulse communication at centimetre wavelengths

    Page(s): 546 - 550
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    The paper describes several applications of microwave radar technique to pulse telephony for a single channel. Two methods of modulation were used, one of which has the advantage of reducing interference by a gating system with automatic synchronization. Pulsed magnetrons and klystrons were used, giving an output peak power of ¿ kW and higher. Some measurements were made of the signal/noise ratio, and reasonable agreement was obtained between theory and measurement. Suggestions are made for improved discrimination between channels. View full abstract»

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  • Random noise characteristics of a pulse-length-modulated system of communication

    Page(s): 551 - 555
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (645 KB)  

    The paper deals with a theoretical analysis of the random noise characteristics of a pulse-length-modulated system of communication. The result is demonstrated by two different methods, one of which is rigorous and based on the frequency spectrum of a rectangular pulse, while the other is an approximation based upon calculation of the average value of the pulse waveform, plus noise, integrated over a period of time that is short compared with a cycle of the a.f. cut off frequency. View full abstract»

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  • The spectrum of modulated pulses

    Page(s): 556 - 564
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    Some methods proposed for evaluating the spectra of modulated pulse trains are discussed. The basic pulse-frequency, -phase, -length and -amplitude modulation systems are defined and the spectrum of a train of rectangular pulses sinusoidally modulated in any one of these ways is derived. Modulation by more than one tone is also considered. It is shown that in none of these methods is there any harmonic distortion or audio crosstalk. Anharmonic distortion arises from sidebands of harmonics of the pulse repetition frequency, and graphs are given to illustrate its magnitude. The formulae are extended to cover non-rectangular pulses, and it is shown that the distortions to be expected are practically the same as for rectangular pulses. The reproduction of transients is briefly examined. It seems that they suffer little distortion in form but that their timing is modified by the pulse modulation, the maximum time-shift being half the pulse repetition period. View full abstract»

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  • Some theoretical and practical considerations of pulse modulation

    Page(s): 565 - 572
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    This paper gives a summary of theoretical and practical studies on the properties of pulse-phase modulation, developed mainly in 1943. The properties of pulse-phase modulation are studied by means of Fourier transformations. Although some approximations are introduced, the calculations lead to the following definite conclusions: (1) Pulse-phase modulation introduces no amplitude distortion except at sub-multiples of the recurrent frequency. (2) The harmonic distortion, if any, is negligible and this method of modulation can be used for high-quality broadcasting. (3) Pulse-phase modulation is subject to a special type of distortion called ?cross-distortion,? produced by side bands of the recurrent frequency appearing in the signal bandwidth. Curves of the approximate amount of this type of distortion are given, and it is shown that, in practical multi-channel systems, this distortion is negligible, provided that the recurrent pulse frequency is at least double the highest signal frequency to be transmitted, and preferably equal to, or greater than, three times this frequency. This study is followed by considerations on the signal/noise ratio in pulse-phase modulation. Pulse-phase modulation is compared with amplitude modulation and a formula, giving the improvement in the signal/noise ratio due to pulse-phase modulation, is established by very simple considerations. It is shown that this ratio improves as the frequency bandwidth used in pulse-phase modulation. It is shown how an improvement of 3 db in signal/noise ratio can be obtained by suppressing the noise on the synchronizing pulse, and a practical circuit developed and applied in 1943 by the author is described. Finally, a typical example of pulse technique is given. In practical circuits the modulator and demodulator pulses are not perfectly shaped, because of the departure from linearity due to finite time-constants. This introduces harmonic distortion. It is shown how this distortion can be practically elimi- nated by designing circuits so that the time constant is equal at modulation and demodulation. View full abstract»

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  • A 60-cm multi-channel system employing pulse-phase modulation

    Page(s): 573 - 583
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    A description is given of the principles of operation of a pulse-modulated radio-telephone system intended for ground point-to-point use and providing up to twelve simultaneous channels of two-way telephone communication. Each channel is capable of accommodating up to four channels of teleprinter signals as an alternative to speech. The operating frequency band is from 450 to 500 Mc/s, and directional aerials having a beam width of approximately ±15° are used. All the pulses are of equal amplitude and duration, the intelligence from each channel being separately conveyed by successive pulses whose positions in time relative to a set of marker signals are varied in accordance with the voltage from the associated channel signal. The marker signal consists of a regularly recurrent group of three equally-spaced pulses each identical with the channel pulses. The transmitter power of approximately 200 watts peak is sufficient to permit working over line-of-sight paths up to 100 miles long. The necessary arrangements, including ringing facilities, are provided for connection to two- or four-wire landlines, and a self-contained two-channel-unit construction economizes in equipment when the number of channels required is less than the maximum. View full abstract»

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Aims & Scope

The Journal of the Institution of Electrical Engineers - Part IIIA: Radiocommunication was published by the IET in 1947.

Full Aims & Scope