Radio Broadcast (May 1928-Apr 1929)

MAY, 1928 AS THE BROADCASTER SEES IT 43 The volume indicator described above has an input impedance of about 12,000 ohms and it must be used across a 500 or 600-ohm circuit if its calibration is to hold. It is so connected in the set-up for a line frequency run shown in Fig. 1. The output of the oscillator is of 500 ohms impedance. This feeds a 10-TU artificial line which presents an impedance of 500 ohms in each direction. The usual repeating coil is inserted ahead of the line. The line is assumed to have an impedance of about 500 ohms also. At the other end of the line there is an equalizer (See article on "Types of Equalizers," Radio Broadcast, June, 1926) followed by a twostage amplifier, with an output impedance of 500 ohms. This amplifier must be terminated with a resistance of this magnitude, therefore, before the level readings of a volume indicator bridged across it will be valid. Since the equalizer is at the far end of the line, the latter will not present a strictly constant impedance at the transmitting end, and this would affect the output of the oscillator if instrument were connected directly to the line. The artificial line acts as a buffer, in that it provides a more constant impedance for the oscillator to feed into; in some cases the artificial line network also permits measurement at more convenient levels without excessive input to the telephone line. The procedure for a frequency run is obvious from this point on. The oscillator is set at various frequencies, the outgoing level checked with the volume indicator across it, and similar readings taken at the receiving end. A curve of received level against frequency may thus be secured for a given setting of the equalizer. If the equalizer is omitted, and the transmitted level remains constant, such a curve will show the line attenuation characteristic, which is a curve descending with frequency. The object of the equalizer being to correct this loss of the higher frequencies, a number of frequency runs may be taken, until a horizontal curve of received level is secured. The line is then equalized. Communication between the two terminals may be maintained over the line by telephone or telegraph in the intervals between readings, or over a separate pair. Of course before an attempt is made to take a frequency characteristic of a line, or to set the equalizer for a flat characteristic, the usual d.c. wire chief's tests are made for defects like open circuits or grounds. Nothing in the way of audiofrequency testing can be accomplished until such faults have been eliminated. HOW NOT TO DO IT CTG. 3 shows a method of taking line fre1 quency runs which is illegitimate. I have seen it used, and so mention it here with the caution that results so secured will usually be misleading. The oscillator, with the volume indicator bridged across it, is connected across a 500-ohm resistance and the level is read. The output of the oscillator is then switched to a line, the equalizer being at the other end. In this way a frequency run is made and the line is thought to be equalized. Actually, as the impedance of the line varies with the frequency, the output of the oscillator will also vary with frequency and the result of the experiment will merely be to show how the oscillator behaves with a variable impedance connected across its terminals. Sometimes it is convenient to send out tone on a line using the regular broadcast amplifier set-up. For example, in chain operation it is a sound precaution to transmit tones at a number of important frequencies before a program. The network stations take level readings at the various frequencies transmitted, which may be 500 ohms Equalizer FIG. 3 100, 1000, and 5000 cycles, and telegraph them back as a check on the condition of the lines. Any irregularity will show up in these readings and necessary changes in routing of circuits, adjustment of terminal apparatus, etc., may be made before the program begins. Fig. 4, from the input of the three-stage amplifier, is the usual setup for broadcasting. The input to the first amplifier would normally be a microphone. For the microphone there has been substituted the audio oscillator, a repeating coil, and a variable attenuation network, which can be adjusted to any loss up to 30 TU. By means of this pad the level of the outgoing tone may be made the same as that normally used during broadcast'ing — usually around zero level (12 milliwatts on peaks, or about 5 milliamperes into a 500-ohm circuit). In the December, 1924, issue of the Proceedings of the Institute of Radio Engineers, Mr. Julius Weinberger showed a means of taking the audiofrequency characteristic of the modulation system of a radio station. The diagram is reproduced, with some slight modifications, in Fig. 5 herewith. The audio oscillator in this case amplitude compared with the transmission of the mean speech frequency (1000 cycles), or in TU, the horizontal axis representing frequency. The thermo-galvanometers in such a set-up as that shown in Fig. 5 must necessarily have the right full-scale reading for the circuits under measurement. The required capacity can readily be calculated, since the output of the oscillator, the amplification of the audio system, and therefore the alternating voltage developed by the modulators, will all be approximately known. Where there is any doubt a large instrument is first used, until one of the right sensitiveness and current-carrying capacity is found. As the output measurements are made across the full plate voltage the engineer who works on this end must take the usual precautions against accidental contact with the high-tension portions of the equipment. Radio Folk You Should Know 4. E. B. Pillsbury IF THERE is a communication man in the 'United States it is Edward Butler Pillsbury, the Vice President and General Manager, as well as a Director, of the Radio Real Estate Corporation of America, the holding company for the realty properties of the R. C. A. Mr. Pillsbury has spent his entire career in telegraphy, starting as a messenger, working ten years as a Western Union operator, followed by many years in the service of the Postal TelegraphCable Company, first as Chief Operator in Bos Audio Oscillator Variable Pad Repeating Coil 3 Stage Audio Amplifier 10 Tu Pad 2 Stage Audio Amplifier 10 Tu Pad Repeating Coil FIG. 4 feeds into a 500-ohm resistance, matching its normal output impedance. A thermo-galvanometer in series measures the a.c. emitted by the oscillator. A portion of the voltage across the 500-ohm resistor is fed into the line amplifier of the station with enough added resistance on either side to maintain the 500-ohm impedance. The tone passes through the entire audio-frequency system and the level is measured at the output of the modulators. A fixed condenser of 5-mfd. capacity blocks the direct plate voltage and allows only the audio component to affect the measurement circuits. The lower terminal of the condenser is connected to ground through a resistance of the order of 20,000 ohms, which is so high that the characteristics of the transmitter will be unaffected by the addition of the measuring circuit. A relatively small portion of the audio voltage across the resistor is tapped off for the thermo-galvanometer. A radiofrequency trap is usually required to keep r.f. out of the galvanometer circuit. The current readings of the input and output galvanometers will now give the transmission characteristics of the modulation system at any frequency within the compass of the oscillator. The curve may be drawn with ordinates of percentage of ton, then advancing to the grades of local Manager in that city, Assistant Superintendent, District Superintendent for New England, and finally General Superintendent of the Eastern Division of the company, with jurisdiction over the lines and offices in thirteen states from his headquarters in New York. This position Mr. Pillsbury held for six years, until he resigned to take up radio work as General Superintendent of the Transoceanic Division of the Marconi Wireless Telegraph Company of America, and later for the Radio Corporation. In 1922 he was elected to his present office. While serving as an operator Mr. Pillsbury was renowned as an expert Morse man. He was among the first chief operators to adopt the practice of using the Wheatstone bridge method of locating faults on telegraph lines. Anyone else who has seen all he has of the communication business and of life would be writing his memoirs. But when Mr. Pillsbury was asked to supply information for this biographical sketch he replied plaintively. " I regret to say that no interesting anecdotes or experiences have come my way." Press agents should thank God that the country is not crowded with Edward Butler Pillsburys. 150 ohms Audio Oscillator 500.. jj; ohms \ 200 ohms Audio Amplifier Modulator FIG. 5