Radio broadcast .. (1922-30)

An untuned amplifier used by the author in his experiments with cathode-ray tubes. MEASURING PERCENTAGE MODULATION Using the Cathode-Ray Tube for Determining Percen- tage Modulation. Desirable Characteristics of a Tube Used for This Purpose. Method of Procedure and Some Typical Examples of the Results Obtained by the Author. The cathode-ray tube has always been a most useful device in laboratory measure- ments on all kinds of pulsating or alternat- ing currents. Additional uses for the lube are always being found and in this artick Mr. von Ardenne (who has contributed several articles published in the Proceed- ings of the Institute of Radio Engineers) describes a method of using the cathode- ray lube in the measurement of percentage modulation. THE DETERMINATION of the percentage of modulation with the Braun tube may be made by two procedures: (1) the method of standing figures and (2) the direct taking of the modulation-time curve. The procedures included in the first group are the older and have been de- scribed numerous times in the last few years. A superficial explanation of the gen- eral method for the production of standing figures is as follows: the high-frequency to be demodulated is fed to a rectifier and the low frequency so obtained is coupled to one of the sets of plates of the Braun tube, while the high frequency is coupled to act on the cathode ray at right angles to the low frequency. If an oscillograph is made of a high-frequency transmitter in this way, with the modulating current having a constant frequency and ampli- tude, and if the high-frequency axis is made the horizontal one, then a figure is to be expected which has for its maximum and minimum widths the corresponding limiting values of the modulated high fre- quency. If the demodulated low frequency of the transmitter is put on the vertical plates of the tube without time lag, then a stationary trapezoid is obtained on the fluorescent screen. The parallel sides, a and 6, lie horizontal and the percentage modu- By BARON MANFRED VON ARDENNE Fig. 1 — The photographic result of experiments with standing figures. lation of the transmitter can be computed immediately from the expression — The advantage of the method lies in the possibility that tubes with very weak fluorescent spots can be used successfully when the photographic exposure is made long enough. On the other hand, a long photographic exposure is necessary, and in this fact lies a limitation of the use of the trapezoid method, if the transmitter is excited only with a constant amplitude of low-frequency power. If the amount of modulation varies, then the height of the trapezoid also changes so that a measure- ment of the parallel sides is no longer possible. If a broadcast transmitter is used, in which not only the amount but also the frequency of the modulation varies then the whole trapezoid changes. This will be true particularly at higher frequen- cies, since phase displacements take place in the coupling elements of the rectifier and amplifier which completely change the boundaries of the trapezoid. Further- more, the trapezoidal form is only obtained with very small high-frequency amplitudes, since with larger amplitudes at the input of the rectifier, the corners are rounded off. The Second Method The second method is free from these disadvantages. It consists in producing a time axis by means of a rotating mirror or a moving film. Naturally this method can be used for any sort of modulation curve, including those of broadcast transmitters, and at the same time it will allow observa- tion of peak values of excitation. How important such a control is in order to maintain a small amount of distortion in the detector of the receiving set is gen- erally understood. Since the envelope of the modulated high frequency is visible on the film or in the rotating mirror, the sinusoidal form can be seen beautifully when the excitation is from a low- frequency oscillator and the complex shape when it is due to speech. For photographing the modulation curve, it is very important to have a tube which gives a very bright fluorescent spot. The brilliancy of this spot determines the highest speed at which the film can be moved, and therefore the highest modula- tion frequency which can be recognized on the film. For direct observation of the modulation on a rotating mirror a bright spot of light is also very desirable, since this permits observations in daylight. In 334 RADIO BROADCAST FOR APRIL •