Radio today (Sept 1935-Dec 1936)

THE IMPORTANCE OF RECTIFICATION John Rider explains the function of many of radios complicated circuits in terms of rectification * DID you ever analyze the number of different functions performed in a radio receiver — which are referred to or spoken of by a number of different names — but which are founded upon one and the same basic principle? . . . The power of interpretation cultivated by recognizing the function as related to a basic principle does much towards facilitating comprehension of radio theory. . . . This is quite an important consideration, because it reduces the number of supposedly distinctly different elements of a modern radio receiver and makes it easier to understand the workings of these new innovations. Take as an example the process of rectification. Eectification, as we understand the term, involves one basic principle — namely, asymmetrical conductions. Expressed in a different manner, a rectifier as applied to radio and allied fields is a device whereby an alternating current is changed into unidirectional current, which naturally means that an alternating voltage is changed into a unidirectional voltage, varying between zero and maximum. . . . Or if the relation between the input and output circuits is such that input voltage and output current are involved, then the device as a result of its non-symmetrical conduction characteristic (asymmetrical conduction) will change alternating voltage into unidirectional current, varying between zero and maximum. . . . The variation of the output current, between zero and maximum, is in con .ELECTRON \EMITr£/1 formity with the variation from zero to maximum of the input alternating voltage. This is the basic principle of the rectifier and is applicable to all types of rectifiers, such as vacuum tube rectifiers, electrolytic rectifiers, gas rectifiers, oxide rectifiers, vibrating rectifiers, etc. ... At the present time, however, we shall concern ourselves solely with vacuum tube rectifiers. Power rectifiers The basic circuit for power rectifiers is shown in Figs. 1 and 2. Fig. 1 is that of a half wave rectifier and Fig. 2 shows a full wave rectifier. The electron emitter may be the cathode in tubes which employ a cathode, or is the filament in tubes which use a filament as the electron emitter. E in both cases is the load upon the rectifier system, across which the output voltage is developed. C is a condenser connected across the output of the rectifier system. Obviously, a power rectifier system which furnishes an output voltage, although unidirectional, but which varies between two wide limits, would not be satisfactory. Hence, since that condition exists in the rectifier tube and its output circuit, supplementary condensers and filter chokes are used, so that the output voltage supplied by the complete system has a steady value. It should, of course, be remembered that the process of arranging for a constant d-c. voltage output is something supplementary to the actual process of rectification and its ELECTRON EMITTER INPUT VOLTACE TolZl AAAAA (WITH NO FILTER) '''''' FIG.2 The half wave rectifier diagramed in Fig. 1, passes one-half of the cycle producing a fundamental ripple of 60 cycles. The full wave circuit of Fig. 2 rectifies both halves of the voltage and gives a fundamental ripple of 120 cycles to be filtered. The latter requires less smoothing. immediately related system, namely, the source of input a-c. and the rectifier. Detectors The detector tube or the detector action also comes under the main heading of rectification, for the process of detection likewise depends upon asymmetrical conduction. The detector tube rectifies the carrier and permits a greater flow of output current for one half of the radio frequency Detection as visualized by the cathoderay oscillograph. cycle than for the other half of the cycle. . . . As a matter of fact, we generally assume that in the conventional detector circuit there is no plate current flow during the lower or negative half of the modulated wave envelope. What really takes place during the usual process of detection is that the lower or negative half of the modulated wave envelope is eliminated and as a result of the elements in the load circuit (plate circuit) of the two or more element detector, the audio signal is extracted from the rectified output. This is shown in Figs. 3 and 4. Fig. 3 is an oscillogram of a modulated signal fed into a detector tube and Fig. 4 is the resultant audio output. Note that the wave shape of the audio output corresponds with the positive half of the wave envelope. The lower or negative half of the input modulated wave has been eliminated. 26 Radio Today