Radio Broadcast (May 1928-Apr 1929)

Record Details:

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AS THE BROADCASTER SEES IT Design and Operation of Broadcasting Stations ig. Frequency Runs THE various elements of the circuits used in broadcasting exhibit effects which depend, among other factors, on the frequency of the potentials applied to them. A line, for example, tends to attenuate voice currents of high frequency more than currents of lower frequency, because of the shunting effect of the distributed capacity, which varies with the frequency. More specifically, we may say that every piece of apparatus has a definite transmission characteristic with frequency, which it is necessary to know if organizations of apparatus are to be brought about for given objects, for example, impartial or "flat" reproduction of sounds of different pitches. Such a curve of amplitude against frequency is secured by means of a frequency run. In broadcasting the most common frequency runs are made within the audio band, say between 50 and 10,000 cycles per second, and typical circuit 10 Tu Artificial Line Audio Oscillator — vww 500 ohms tvvwvv Volume Indicator elements which require this sort of investigation are telephone lines and the audio circuits of transmitters. Representative methods of making such tests will be briefly described in this article. Fig. 1 is a diagram showing how a frequency run may be made on a wire line, using an audio oscillator at the transmitting end and vacuumtube voltmeters for the indicating instruments. The audio oscillator in all such work must fulfil several requirements. It must cover the frequency range over which the circuit is to be equalized. For ordinary line work, by present standards, this would be from 100 to 5000 cycles, hence the oscillator of Fig. 1 will have to more than cover this band — a 50to6ooo-cycle oscillator would be suitable. The output must be substantially free from harmonics. Obviously since the instrument is to be used in determining frequency characteristics one must be able to secure oscillations of any frequency in the range without the admixture of other frequencies. If, for example, the behavior of the line is to be studied at 200 cycles, the harmonics (400, 600, 800 . . . cycles) must be suppressed. Usually a harmonic amplitude of 5-per cent, is allowable, but the proportion must not be greater. The power output of the oscillator should be reasonably constant over the range of frequency, and it is not difficult to design an oscillator which will meet this requirement within 5 per cent, output voltage variation over a 50 to 10,000-cycle band. The oscillator may be one of several types. One form consists of audio tuned circuits, generally employing fixed condensers and obtaining the frequency variation by means of taps on an iron core coil of suitable inductance. The inductance and capacitance together tune to the audio frequency directly. Another type of audio oscillator utilizes the heterodyne principle. Two radiofrequency oscillators have their outputs combined, rectified, and, if necessary, amplified at audio frequency. Generally one of the component oscillators has its frequency fixed; the other radio frequency is varied, and the beats may be made to cover the whole audible range. Precautions must be taken to avoid too much fre lume licator quency drift, owing to varying voltages, and there is also a tendency for the two radio oscillators to pull into synchronism at the lower beat frequencies. Some information on the construction of audio beat oscillators for laboratory testing is contained in several 1927 papers in the Proceedings of the Institute of Radio Engineers (Wolff and Ringel: "Loud Speaker Testing Methods," May, 1927; Dickey: "Notes on the Testing of Audio-Frequency Amplifiers," August, 1927; Diamond and Webb: "Testing of Audio-Frequency Transformers," September, 1927). In general, broadcasters who lack labora FIG. 2 + 42 tory training in measurements will do better if they buy such instruments as audio oscillators. Such apparatus is sold by the General Radio Company, Graybar Electric Company, and other concerns. Oscillators covering a range of from 10 to 50,000 cycles, or higher, with definitely known output characteristics, are obtainable. One form covers from 1 5 to 9000 cycles, continuously variable through a single control; the price is a little more than $200. Returning now to Fig. 1, we note that the receiving instrument is a "volume indicator" of the vacuum-tube type. The circuits of a typical form are shown in Fig. 2. The action will not be taken up in great detail, as a previous article in this series ("Volume Indicators," Radio Broadcast, May, 1927) dealt with the general theory. In the form shown the negative grid bias is adjusted until the d.c. galvanometer in the plate circuit of the tube reads 5 scale divisions out of a total of 60 full-scale. Then the tap on the secondary of the input transformer is set to give peak readings, with modulation, of, say, 30 scale divisions. The level of the circuit across which the instrument is bridged may then be read on a scale attached to the transformer tap switch. High levels, obviously, correspond to settings in which only a small portion of the total transformer voltage is utilized, whereas when the telephonic level is low, more of the winding must be included by means of the tap switch in order to get the requisite galvanometer swing. Obviously the readings of such an instrument are the resultant of many factors, such as the wave form of the alternating currents under measurement, the ballistic characteristics of the galvanometer, the size of the galvanometer shunt, the smoothing characteristics of the inductancecapacitance filter in the plate circuit, the type of vacuum tube employed, and other details, but it is possible to design such level indicators to read in telephonic transmission units with sufficient accuracy for the usual purposes of broadcast transmission or measurement. The prototype is the Western Electric 518-B type, which, in its lowest range, from minus 10 to plus 10 TU, is constructed as shown in Fig. 2, but extends the range of measurable levels to as high as plus 40 TU by the addition of a potentiometer arrangement across the secondary of the input transformer. Obviously a level indicator must always be a bridging instrument, a circuit element, that is, with a relatively high input impedance, intended for connection across circuits of low impedance without drawing enough energy from the lowimpedance circuit to affect conditions therein. Line FIG. I