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DIGESTS OF PAPERSMODULATORS, FREQUENCY-CHANGERS AND DETECTORS USING RECTIFIERS WITHFREQUENCY-DEPENDENT CHARACTERISTICS

621.396.619.2:621.396.622 Paper No . 1139 RADIO SECTIOND. G. TUCKER, D.Sc, Associate Member

(DIGEST of a paper published in September, 1951, in Part III of the PROCEEDINGS.)The calculation of the performance of modulators andsimilar circuits using copper-oxide or selenium rectifiers iscomplicated by the fact that the resistance characteristic of therectifiers varies with frequency. The dependence on frequencyis very marked, since the simple equivalent circuit of the rectifierbased on resistance and capacitance in parallel shows a resistancecomponent at negative bias voltages which falls, in a typicalsmall copper-oxide rectifier, from perhaps 100 000 ohms ataudio frequencies to only about 100 ohms at 6 Mc/s. The fallof resistance with frequency is very much smaller at positivebias voltages, and the shunt capacitance also varies compara-tively little with frequency.

It is found that an approximately equivalent circuit of therectifier which allows at least some practical applications to becalculated is as shown in Fig. 1. Here a rectifier which has nofrequency dependence is shunted by a capacitance and also bya linear resistance which falls as the frequency rises. When thisdevice is used in modulator circuits with terminating impedances

which are finite only at a finite number of modulation-productfrequencies and are zero at all other such frequencies, then theequations developed for rectifiers without frequency dependence

JFreguencydependent

, Approx.' constant

Dr . Tucker is in the Royal Naval Scientific Service, and was formerly at the PostOffice Research Station.

^ VoltagedependentFig. 1.Equivalent circuit of a rectifier.

can be used, since the frequency-dependent linear resistance andthe capacitance can be absorbed into the terminating impedances.The paper includes some detailed examples and shows how theconversion loss of modulators using these rectifiers can bestabilized.

THE DEVELOPMENT OF THE ELECTRICAL SYSTEM ON THEBRISTOL BRABAZON I MARK I AIRCRAFT621.311.28:621.34:629.135 Paper No. 1131 UTILIZATION SECTIONM. J. J. CRONIN, Associate(DIGEST of a paper published in September, 1951, in Part I of the PROCEEDINGS.)

In the last seven to ten years there has been a great increasein the use of electricity in aircraft, and whilst this has been ageneral trend, it has been more evident in the large civil andmilitary post-war aeroplanes. Under the stimulus of the1939-45 War, great progress was made with the development ofaircraft electrical equipment, and a brief and general summaryof this is given in the paper. The main object of the paper is,however, to deal with the power supply.

Up to the end of the last war, our large bomber aircraft werefitted with four 1-5-kW or four 3-kW 28-volt d.c. generators,but it soon became obvious that the capacity of these machineswould be quite inadequate for the majority of the post-waraircraft, and 28-volt d.c. generators up to 10 kW were subse-quently developed. At the same time, in order to reduce cableweight, it became necessary to consider the adoption of 112-voltd.c. and 208-volt 3-phase 400-c/s a.c. systems, as both were beingconsidered as standard supplies for large American and Britishaircraft. A typical installation of the latter type is the one usedin the Brabazon I Mark 1 aircraft.

The electrical installation is, broadly speaking, a 208-volta.c./28-volt d.c. system, which uses the a.c. power for heatingloads, such as propeller anti-icing (76 kW) and other electro-thermal services, and the extra-low-voltage d.c. power for thelarge amount of electrical equipment, which has reached ahighly developed state and cannot be reasonably replaced byhigher-voltage equipment. Other special supplies for gyroscopicinstruments, auto-pilot, radio, radar, etc., are obtained from

Mr. Cronin is with the Bristol Aeroplane Co., Ltd.

115-volt 400/1 200-c/s inverters operating from the 28-volt d.c.busbars.

As it was estimated that approximately 150kW would berequired to serve the unusually large number of electricalservices, the choice of a medium-voltage system was natural,and in view of its inherent and prospective advantages, an a.c.system was chosen, using six 3-phase 30-kVA alternators as theprimary source of supply. Also, to avoid extra accessory drivesfor d.c. generators, and the added maintenance and brush wearproblem associated with these machines, six 3-phase 6-kW metalrectifiers were fitted to provide the extra-low-voltage d.c. supply.

Much of the development time on the Brabazon electricalinstallation was spent in overcoming the problem of operatingthe generators over the wide speed range of the engines. Inmost aircraft with smaller electrical power demands, it waspossible to disconnect the generators from line at the lowerspeeds and to permit the loads to fall on the batteries. In theBrabazon, however, a generated supply was necessary over a4 : 1 speed range (720-2 800 engine r.p.m.) and investigationswere made into the use of constant-speed drives, auxiliarygenerating plants and extra-fine-pitch stops on the propellers.Finally, as a result of these investigations, a wide-speed-rangeelectrical system was evolved.

The alternators used in this system are of optimum design,inasmuch as they are rated for their full output over a narrowspeed range (7 600-10 000 r.p.m.), but by accepting a reductionin the power output with reduction of speed, it is possible tooperate them over the 4 : 1 speed range as variable-voltage

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