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The work reported in this paper was carried out some years ago during the development of Morse's theory of reverberation, in an attempt to explain the difference in the observed values of absorption coefficients for sound waves which are normally incident on an absorbing surface in a room and those which are at grazing incidence. The theory has been developed further by works of Morse, Bolt, Ingard, Beranek and others. The work has thus some historical value; nevertheless, the results obtained are of practical value in case of cubical rooms with absorbing material concentrated on one wall-ceiling-and special placements of the microphone, as is the case with many broadcasting studios.

The method of measurement of reverberation time by tracing decay curves is tedious though accurate. Occasions frequently arise when the need for a quick determination of reverberation time vs. frequency curve is felt. An equipment constructed in the Research Department, All India Radio, which can plot the reverberation time vs. frequency curve of an enclosure under test, in a relatively short time, is described. The time taken for 40 db decay is measured and is taken as two-thirds the value of reverberation time, assuming an exponential decay of sound energy. The value of reverberation time thus obtained is indicated on a calibrated meter and also plotted on waxed paper as a straight line whose length is proportional to the value of reverberation time. A number of such lines is obtained automatically at close intervals, each line representing the value of reverberation time a frequency slightly higher than the frequency corresponding to the previous line. Since all these lines have the same zero a...

In order to determine the level of maximum interference at which a satisfactory broadcasting service is possible, a large number of listening tests were conducted. The study has been confined to the determination of protection required for a speech modulated broadcast signal against interference from speech and music and also against CW Morse. The experimental set-up is described in detail in the paper. More than 750 recordings were made and about 12,000 individual opinions collected. The analysis of the listener's opinion indicates that minimum protection of about 33 db is required for zero-beat operation and 52–54 db at frequency separation of 2–3 kc./s. Beyond 3 kc./s. the protection becomes gradually lower and after 5 kc./s. it drops down sharply. Other factors influencing the desired protection have been discussed.

This paper describes the principles of operation of high-speed electronic differential analysers called analogue computers, with particular reference to the analyser set up at the Indian Institute of Science. The potentialities of such an analyser in solving linear differential equations and also in dealing with physical systems in which several types of non-linearities occur are discussed. The paper then describes the solution of some typical problems with the aid of the computer installed at the Indian Institute of Science.

The paper relates to investigations made regarding the acoustics of the hall and the measures adopted to improve the same.The three main factors responsible for acoustic condition, namely (a) the reverberation time, (b) the distribution of sound pressure level and (c) the noise level, were studied. Measurements were taken when the hall was completely empty. The intelligibility was found to be poor, the main cause being the excessive reverberation. Based on these results, an estimate was made of the reverberation time that may be expected after furnishing it with the chairs and with audience present. Attempt was made to provide as much of the additional absorption necessary, as was possible with such objects as carpets, curtains, hangings, etc. Only the remaining amount of absorption was provided by treating with acoustical tiles, without affecting the architecture of the hall.Several sets of measurements were taken under different conditions of the hall during the execution of the suggestions and ...

Physical defects in the component parts of a system, such as saturation, dead-zone and back-lash, introduce complications in the behaviour of the system and make a rigorous mathematical analysis difficult. For a few types of problems, an approximate method based on the technique of equivalent linearization has been suggested by R. J. Kochenberger. This method has been successfully used by E. C. Johnson and others to obtain information on the stability, the degree of stability and the general behaviour of servo-systems, and in the design of compensation schemes. An analogue computer is, of course, the best means of studying such problems.This paper considers the specific case of a simple positioning servo-system and describes the study carried out on the electronic differential analyser recently set up at the Indian Institute of Science, to determine individually the effects of saturation that may be present in the electronic amplifier or the generator field of the servo and of hysteresis in the co...

Becker, Green and Pearson1 describe ‘thermistors’ or thermally sensitive resistors as “simple, small, rugged having long life and requiring little maintenance”. They further remark later in their paper, “It is surprising such versatility can result from a temperature dependent resistance characteristic alone. The ability to produce the sensitive element in a wide variety of shapes and sizes results in applications in diverse fields.” This paper attempts to show that the new ceramic composition developed at the National Physical Laboratory, Radio Components Development Section, has properties which are very similar to thermistors and it can be described in almost identical terms except that, with these, the change is that of a capacitive reactance with temperature instead of a change of resistance with temperature. In some applications a change of reactance can be more directly used for indication, control or measurement, if the operation has to be done from a remote station.

The present paper together with a subsequent paper discusses the design of I.F. amplifier and detector stages with stabilized bandpass characteristic suited for use in the community receiver. In the present paper a method of partial neutralization is described and it is shown that with surface barrier transistor it is possible to design a double tuned I.F. amplifier stage whose performance is close to that predicted from measurements carried out to determine transistor parameters at a single value of emitter current. This is particularly true as regards the variation in the gain and bandpass characteristics of a common emitter connected I.F. Amplifier stage.

This paper describes the effect on intelligibility in radio telephone channels when restricted speech frequency characteristics are employed. It is well known that advantages like improvement of intelligibility and reduction of adjacent channel interference are obtained when restricted frequency characteristics are used. The set-up to provide restricted speech characteristics and the procedure adopted to study their effect on intelligibility in radio telephone channels are described. It has been practically verified that a speech frequency band of 1000–2400 c./s. with specific cut-off characteristics provides good intelligibility in the case of radio telephone channels in general and aeronautical radio telephone channels in particular.

In this paper a new method of analysing active networks by means of admittance matrices is presented. The admittance matrices of vacuum tubes and transistors are derived and examples of the application are given. The concept of the indefinite admittance matrix as defined by Shekel is introduced. The indefinite admittance matrices are then derived for the vacuum tubes and transistors and it is shown how one can derive the admittance matrices of the various tube and transistor configurations from their respective indefinite admittance matrices.

An earlier paper discussed the design of an Ie-controlled I.F. amplifier stage employing partial neutralization. The inherent variation in the band-pass characteristics of such a transistor amplifier stage was demonstrated.In this paper, data are presented regarding a transistor I.F. amplifier stage employing partial neutralization and operating at low collector voltages. The gain of such a stage can be controlled by varying the collector voltages. A transistor collector detector stage can also provide, besides the demodulated audio frequency signal, the control voltage necessary to secure A.G.C, for the preceding high frequency stages.A modified circuit of such a detector stage is presented in this paper and this can be employed to drive simultaneously the Ie-controlled and the Vc-controlled transistor stages. The performance data for such a detector stage and a method of connecting in tandem an Ie- and a Vc-controlled stage are demonstrated. The resulting I.F. amplifier and detector stages have ...

The existing system of education in engineering colleges does not give good emphasis in the field of specialization, as a lot of time is spent in teaching general engineering subjects. Further the teaching is not sufficiently analytical and emphasis is not being given on basic sciences which is so essential for undertaking research and development work.This paper gives the general principles which should be borne in mind to frame the curriculum of Bachelor's degree in Telecommunication Engineering. However, these principles can also be followed for planning the education in any other field of engineering.

The technique of determining the amplification factor of a triode by bridge measurement on an electrolytic tank model of the tube is well known. This method does not appear to have been used for multi-grid tube structures hitherto, probably owing to the difficulty involved in a simultaneous adjustment of the various electrode potentials in the proportion required to produce the current cut-off condition on the tank. It is shown in this paper that this difficulty may be overcome by a technique of grounding the electrodes selectively. In a tetrode, for instance, the anode will be grounded in measuring the screen-grid to control-grid amplification factor, and the screen-grid will be grounded in measuring the anode to control-grid amplification factor. The authors have successfully used this method to study typical screen-grid tube and pentode configurations, and also to verify analytical expressions describing the ‘inselbildung’ effect encountered in beam power tubes. In the latter case, a probe is p...

With the advent of tunnel diodes a great amount of interest has been shown in its circuit applications. This paper will first briefly review the physics of the tunnelling mechanism and then discuss some of its applications in trigger circuits.

The design procedure for receiving type tubes has all along been mainly based on a few empirical relations and the constructional experience of tubes of similar category and this involves a series of adjustments based on the measurements on sample production. In this paper, a systematic analytical method of arriving at the tube geometry and electrode dimensions to meet a set of given specifications is presented for the cases of low mu-power triode, medium mu, and high mu triodes. This design procedure makes use of the fact that corrections to the equivalent diode anode potential for contact p.d. and initial velocity of emission of electrons can be reduced to an equivalent internal correction voltage. The pertinent data useful in the design procedure have been presented graphically. These data based on Ollendorf's fourth approximation formula enable the evaluation of amplification factors of tubes which possess high screening fractions up to 0·75. The determination of amplification factors of tubes...

The errors in beam rider system are largely due to lags in the tracker and missile servos. To a certain extent it is possible to reduce these errors by increasing the stiffness of the servo system, but this would demand larger band-widths and hence increased response to noise. There is therefore an optimum bandwidth giving the minimum possible miss-distance. In his approach to the design problem, Whitfield3 assumes the target flying directly towards the radar set and accelerating perpendicular to the radius vector. This is a hypothetical trajectory, and the design approach has therefore to be modified to suit a particular tactical situation that the beam rider system is most likely to face during its operation. The present paper attempts to present this modified design philosophy.

The paper discusses an improvement over the RCA beading technique for the construction of electron guns for travelling wave tubes. The improved technique requires only a simple demountable jig for aligning, supporting and controlling the spacing of the electrodes to a close tolerance. It eliminates the undesirable heating of the entire jig along with the electrodes during the glass-beading, and presents deformation of the glass rod after softening. The cathode and heater mounting is also such that alterations are possible. A number of special electron-tubes built here have utilized this technique successfully.

The aim of this paper is to review the progress in the development of electronic wattmeters especially for the measurement of power at power and audio-frequencies. Efforts have been made to make this review up to date. The developments have been grouped according to the principles underlying their operation. In each group, the principle of operation is explained using the initial circuit and the subsequent improvements done in it are given in brief.

An attempt has been made to present the broad approach to the design of a transistorized feedback amplifier with the help of conventional techniques. The design procedure is equally applicable to other type of single loop feedback amplifiers with suitable modifications. It is hoped that the procedure outlined in the paper will find use in the practical design of feedback amplifiers.

This paper deals with the channelling equipment which is the basic 12-channel building block for large capacity carrier telephone systems. The design criteria for the various functional units, the synthesis of some of the elements and the use of transistors have been discussed. Equipment based on these considerations has been designed in the Telecommunication Research Centre of the Posts and Telegraphs Department and is in production in the Indian Telephone Industries. The details of construction adopted have also been dealt with.

Infrared guidance systems generally use a focussing device along with a chopper to get angular information of a target. This requires a critical design of the IR focussing system. In this paper a new technique for obtaining guidance information has been mentioned that uses four IR detectors and avoids focussing devices. It provides guidance signal that can be used with a missile having On-Off control which can turn the missile to keep the source of IR, the target, within a cone with its apex at the missile. As the missile approaches the target, it is thus ensured that the target is brought within the lethal range of the missile warhead.

Microwave filters form an important building block in the design of low noise parametric amplifiers, harmonic generators and various other microwave circuits. Conventional mode of transmission at microwave frequency is by waveguides and coaxial lines, which have certain disadvantages. The triplate line for transmission of microwave frequencies is a recent advance and offers a number of advantages, such as ease of manufacture and high order of reproducibility.The paper gives the design aspects of a constant K five-section low pass filter with m derived end sections, designed for specific application in the Land S-bands. The realization of the designed inductive and capacitive elements in triplate line is discussed. The experimental results on a triplate line filter in L-band and one in the S-band are presented with the corresponding theoretical designs.

After describing the advantages of magnetic amplifiers as a control element, this paper deals with the application of magnetic amplifiers to two types of regulated rectifier power plants, viz. a constant voltage power plant for automatic telephone exchanges and a constant current regulated power supply for power feeding over small tube coaxial cables.

This paper describes a device for simultaneous monitoring of audio tapes that are duplicated at high speed. The device has proved to be very useful as it has resulted in the saving of considerable time as well as man-power.

This paper describes some preliminary results in a study of a twin-channel frequency-modulated radar which is capable of measuring both the range and bearing of a target from a stationary aerial system consisting only of one transmitter and two receiving elements. The basic operation of the system is described along with some experimental results for range measurement. Also the proposed extension of the work to examine the bearing resolution of the system is outlined.

The general class of network functions which can be synthesized by passive elements is very much limited by the fact that we have only three basic components available, +R, +L and +C. Once active elements such as the vacuum tube, the transistor and the tunnel diode are allowed as components in the network, a much wider class of functions can be synthesized.The paper briefly presents the techniques now available to synthesize network functions using active elements in addition to +R, +L and +C. Typical active networks such as the negative impedance converter are discussed and synthesis procedures are presented wherein these networks are used freely as basic building blocks.

The coded system was initially evolved to devise a suitable apparatus for railway signalling, the receiving equipment in the engine has to pick up the inductive energy from the track. A.C. voltages were initially superimposed on d.c. track circuits and later on the development of a.c. coded track circuits was made.The present paper deals with various types of coding transmitters and their corresponding receivers with accessories; schematic explanation is being afforded by the line diagrams.The chief advantage of coded track circuit over the conventional d.c. track circuits presently employed in Indian Railways is that the application of coded track circuits does away with the requirement of the line wires for the control of the multiple aspects and the lengths of the track circuits are increased without affecting the efficiency of performance.

This paper describes the design and development of a semiconductor diode parametric amplifier at L-band. It is a single-port device operating in the quasi-degenerate mode. Results are given for a single-tuned amplifier, with two different diodes. Also described are the effects on the bandwidth by the inclusion of one additional resonator in the signal circuit. The parametric amplifier has a gain bandwidth product which is as good as for other amplifiers in L-band, reported in the literature.

In many applications of digital techniques it is required to convert an analogue signal into a digital form. For this purpose there are many types of analogue-to-digital converters in use. But not much work has been done on these utilizing the principle of frequency modulation. This paper describes two types of analogue-to-digital converters employing the principles of frequency-modulated radar. The advantages of this system along with a brief mention of further work are also included.

The paper deals with a simple transistor circuit whose input impedance is equivalent to that of an inductance L in series with a resistance R, both L and R being frequency dependent. B can be adjusted to have a positive, zero or a negative value so that the circuit can net as a high Q inductor, or in conjunction with a suitable capacitor, as a tuned oscillator. The circuit may be readily integrated into a single semiconductor block and, as such, it will be useful in the microminiature electronics field where the realization of a high Q inductor has long been a problem.

This paper treats a transmission-line model of the insulated-gate field-effect transistor (also known as the metal-oxide-semiconductor or MOS transistor) in the pre-pinchoff mode in order to determine the small-signal response of the device. The results are presented in terms of the canonical set of four complex admittance parameters which are dependent on frequency, the bias conditions, and known constants of the device. It develops that the transmission line involved is a non-uniform one composed of non-linear elements leading to differential equations of some complexity. These are solved by each of two methods (one an infinite-series technique, the other an analogue simulation) and the results compared. Finally, the behaviour of the device connected as a simple amplifier in the common-source configuration is investigated.

General purpose analogue computers are now being extensively used. These are, however, fairly expensive to buy. For teaching principles of computing and use of computer to solve problems, it is possible to build an analogue computer at considerable less cost. This paper gives the design of such a computer having 20 operational amplifier which can be conveniently built in a laboratory.

The paper describes a stable multi-decade transistor frequency divider for use in Time and Frequency Standardization and others. Coincidence technique for binary to decimal conversion has been suitably adapted for yielding the highly stable operation of the instrument.

This paper gives an account of the studies carried out on a filter type of up-converter Parametric Diode Amplifier operating on the sum-mode at a signal frequency of 755 Mc/s. and pump frequency of 9805 Mc/s. The diode used was a Kita diode type GSB-1A with a cut-off frequency of 60 kMc/s. At the output of the amplifier, a bandpass filter has been provided to select the signal at the output frequency and a band rejection filter to eliminate the pump frequency. The paper gives a brief account of the amplifier designed and also of the measurements carried out.

The paper describes a transistor digital display system using ordinary filament lamps. The lamp driving unit affords sufficient flexibility in the selection of the transistors used in it and the current rating of the filament lamps. The particular form of the AND matrix used prevents loading of the binaries of the counter.

There are two important problems that arise in the design of bi-directional thyristors, particularly for high-current applications. These are the problem of (1) gate isolation and (2) commutation. The former arises in bi-directionat thyristors with two gates, one to fire in the 1st quadrant and the other to fire in the 3rd quadrant of the device's characteristics. The normal requirement is that the 1st quadrant gate should not fire in the 3rd quadrant and vice versa. Successful gate isolation consists of meeting this requirement. The problem of commutation arises when a bi-directional thyristor which has been carrying current in one quadrant is switched rapidly into the other quadrant, where it is expected to present a blocking characteristic. Under some circumstances stored charge effects can cause the device to turn on in the other quadrant (i.e. fail to commutate), In this paper analyses are made of both these problems, some practical results are given, and the implication in actual device des...

The field-effect transistor (FET) has a very high input resistance compared with the conventional junction transistor. Unfortunately, however, the traiiHconductance, gm, is relatively low in the FET, and its cut-off frequency,fct, also is low because of the large input and output capacitances. The FET designed to have high gm has inevitably a large input capacitance. These disadvantages are, however, easily compensated by connecting the FETs so that their input and output capacitances form the shunt elements of constant-4 type circuits or m-derived type circuits with the insertion of inductances between each FET. This method makes it possible to increase the degree of freedom in designing the FET and is suited to integrated circuits by the physical simplicity of the structure. In this paper the design theory is explained first, and this is then found to agree closely with the experimental results. The design gives the ultimate high-frequency limit and the number of FETs to realize the required vo...

The paper deals with the design and construction of a precision temperature controller for use with a three-zone silicon diffusion furnace, to operate in the range 900–1400°C. Thermocouples are used as sensing elements. The controller utilizes adjustments of proportion bandwidth, rate action and reset action for precise control of the temperature. The power is controlled through silicon controlled rectifiers, phase controlled by the unijunction transistor oscillators.

The paper presents the design and development of very wide band log-periodic antenna structure for the reception of electromagnetic energy over the frequency band 500–3000 Mc/s. Printed circuit technique was utilized for fabricating the structure. The antenna has a standard type of feeding system. The VSWR over the entire desired band was found to be within 2·2. The gain varied from 7 to 10 db. over an isotropic radiator. The impedance and radiation characteristics achieved are presented.

This paper describes a charge control approach to the analysis of the transient response of an RTL inverter. The procedure adopted in predicting the propagation delay of RTL inverters consists of a preliminary worst case d.c. design of the circuit, followed by the evaluation of the individual components of the propagation delay time, viz. the storage, fall, delay and rise times. In the calculation of switching times, the driving waveform which is actually an exponential time function is approximated by a ramp time function.The present investigation has yielded formulae which enable the prediction of propagation delay time as a function of the fan-in and fan-out and the value of the transfer resistance. This analysis enables the circuit designer to choose such values of the circuit resistances which will yield minimum propagation delay under the worst transient conditions without violating the d.c. stability criteria. Experimental verification of the predicted times has also been carried out.

This paper describes a transistorized version of the completely automatic PRF doubler employing hard tubes which has been previously described [J. Inst. Telecom. Engrs, 9 (8) (Nov. 1963), 551–60].

Emitter coupled differential amplifier with constant current feed is often used as the input stage in high gain direct coupled amplifiers. Although, the problem of zero drift can be partially solved by using matched pair transistors in such amplifier stages, but there is a limit to which drift compensation is achieved in such stages. The paper presents two methods by which further reduction in drift is possible in transistorized differential amplifiers. In both methods temperature sensitive devices are used for compensation of drift resulting from the change in ambient temperature. The circuits were analysed and the condition for minimum possible drift was found out mathematically. Finally the experimental results and the limitations of these circuits were discussed.

Synthesis techniques are presented in this paper which establish the sufficiency of a differential amplifier embedded in an unbalanced grounded RC structure to realize any driving-point function, and simultaneously certain pairs of short-circuit admittance parameters of a two-port network. The freedom existing in the choice of two short-circuit driving-point admittances of the passive sub-networks with a given relation between them is used to alter the level of the associated transfer admittances without additional elements being used for the purpose. The expressions obtained for the various parameters to be realized arc somewhat simpler than those of Sipress (1961), thus reducing the number of passive sub-network functions to be determined and also the computational work involved.

This paper presents a way to investigate the transient performance of a varactor doubler. The rise and decay times are computed. The analysis of the transient shows the cases where overshoot exists and where the voltage across the diode is larger than the breakdown voltage.

The design of a linear feedback control system normally implies a compromise choice of loop gain between the conflicting requirements of (a) high gain for good steady-state accuracy and bandwidth, and (b) low gain for satisfactory stability and damping. The optimum gain would be that which leads to a system with the best possible step-response as indicated by the minimization of a suitably chosen index of optimization. The paper presents the results of an attempt to develop a frequency-domain approach to the direct determination of optimum gain for linear systems. A figure of merit for linear systems in the form: gm. sin φ m.v−1m is suggested, where gm, φm, Vm are respectively the gain-margin, the phase-margin and the frequency-margin (ωcp/ωcg) for the system. The value of loop gain which maximizes this figure of merit is also found to correspond to the minimization of chosen optimization indices of step-response for several typical systems.The above figure merit is identified as the imaginary par...

Within the last few decades medical electronics has rapidly developed and become an important part of medical research, clinical medicine and surgery. The present paper surveys the developments in medical electronics. Taking the measurement of physiological parameter aspect the design criteria for electronic circuits to meet biological requirements have been described with particular reference to the electromagnetic blood flowmeter. The necessity of correlation between biological factors and adaptation of conventional electronic techniques has been stressed. The possibility of construction of these equipment in India in terms of the components available has also been discussed.

While generating very low frequency sinusoidal voltage, using operational amplifiers, one experiences difficulties in stabilizing the amplitude and every endeavour has been made to get a satisfactory solution. The paper describes one method by which stable oscillation can be obtained to a frequency as low as 0·01 c/s. The oscillator consists of two integrators and one sign changer interconnected in the form of a closed loop so as to solve a second-order differential equation. The amplitude is stabilized with the help of an extra stabilizing loop which consists of a squaring network. A brief analysis of the oscillator has been carried out and the precautions that have to be taken for improving the oscillator performance are discussed

The main requirement of any pulse transmission network is small rise time (10-90%) with little or no overshoot for step inputs. In this paper an attempt is made to find the polo locations of third, fourth, fifth and sixth-order ail-pole transfer functions to give the least rise time. In the third-order case, the optimum pole locations, subject to the step response being monotonic. are found, while in the higher-order cases the pole locations along the Butterworth angles giving the least rise time are obtained.

The present paper deals with the design of one type of balanced mixers that are currently used in modern radar receivers. The use of a cross-bar configuration ensures compactness in size of the mixer and thus enables it to be employed in radars where space is at premium. The experiments carried out on the Laboratory model indicated that the mixer, when used with an IF amplifier of 3·5 db. noise figure, gives an overall noise figure of 11 db. and an isolation better than 20 db. between the signal and local oscillator arms over a range of 1 Gc/s. at X-band.

With the advent of Silicon-controlled rectifier, there has been a tremendous interest in the solid static inverter technology. This paper first discusses the various principles of inverter circuits and describes at length the widely used Capacitor-Commuted parallel inverter circuit. It then discusses the various methods of inverter voltage control and of converting the square wave output into sinusoidal. The paper concludes with a detailed description of a 4 kVA. static inverter designed at the Telecommunication Research Centre for use in ‘No break’ power plants for carrier communication equipment, coaxial and microwave systems.