| Popis: |
Considerable progress has been made in the development of broadcast receivers over the 10-year period 1929 to 1939, the chief features of which have been the almost complete supersession of the straight r.f. receiver by the superheterodyne and the increase in the number of accessory circuits. Design has been influenced to a great extent by the fact that the sales market consisted of a large majority of purchasers of first receivers. This has rather tended to over-emphasize the accessory circuit (which can be made a good selling point) at the expense of basic design and the potential user's needs. With market saturation approaching, replacements must become a larger and larger proportion of the total sales, and this should give the designer an opportunity of producing receivers calculated to fulfil the more exacting requirements of the user. This paper is intended to assist the designer in taking stock of the present position and is divided into two main sections, dealing with past and probable future developments. Part 1, compiled from an examination of over 1 000 circuit diagrams, is a review of receiver circuits which have stood the test of production. The receiver is analysed stage by stage in the general discussion preceding the detailed examination of typical circuits, the salient features of which are listed separately. Representative examples of the most popular types of present-day receivers and a short note on performance specification complete the section. Part 2 is concerned with some fundamental problems and the lines of possible development. Selectivity, fidelity, electrical interference and automatic accessory circuits for volume, frequency, selectivity and remote control are some aspects of receiver design to which attention is devoted. After a short discussion on performance specification deficiencies, the predominating influence of transmitter characteristic on the overall selectivity of the complete transmission chain is stressed. Part 2 concludes with a suggestion for improving broadcasting conditions by group allocation of wavelengths. |
