Radio Broadcast (May 1929-Apr 1930)

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Illl illlllllllllllllll How to Use Technical Facts in Selling IF I WERE A SALESMAN By AN ENGINEER JF I WERE a salesman and a prospective customer wanted to know why the Radiola 60 was "the one receiver that is built for all broadcasting conditions" — to quote from an advertisement in the New York Evening Post — I should use the following sales argument. The points are somewhat technical because all matters relating to what is inside the cabinet of a radio receiver concern technical matters, just as talk about the vermiform appendix, the dorsal fin, the intake manifold, or wing struts concern technical things, and naturally must be couched in technical jargon. "There is little use in telling you as a customer that you need a selective set if you have listened-in at all, or have read or heard the tirades against the present overcrowded condition of the radio channels. There is no use to remind you that the multiplicity of stations, many of them on very high power, the general increase in high-quality programs more hours per day, and the more general use of high-quality loud speakers mean simply that the receiver must be designed for conditions that were undreamed of four years ago. What you want to know is how the Radiola 60 meets these problems. "The Radiola 60 is a combination tuned radio-frequency receiver and a super-heterodyne. These two receiver systems are fundamentally different. The majority of receivers now made are tuned radio-frequency sets only. Some receivers are super-heterodynes only. This receiver is a combination of both of them. The object is first to gain selectivity, second to gain voltage amplification, and third to secure these essential receiver characteristics without destroying the fidelity with which the receiver will translate radio signals into audible sounds. "The tuned radio-frequency receiver first picks up the desired signal from an antenna, amplifies it, separates the audio modulations from the inaudible radio or carrier wave, and then amplifies these audible tones. Now here, in this complicated process which may involve six or more tubes and a corresponding number of electrical circuits, distortion must not occur. "Radio stations are now put on channels which are 10 kilocycles apart, just as keys on a piano are stationed along the keyboard a certain number of tones apart. When one wants to listen to one station, he does not want to listen to several others, but at the same time he wants to get all the desired station is transmitting — all tones from the highest to the lowest. He must select a certain radio signal but not select it so well that he loses part of the desired signal in the process. "The usual tuned radio-frequency amplifier is inherently more sharply tuned at low frequencies — at the left-hand end of the usual radio keyboard — and broader at the other end. Thus, it is more difficult to separate two stations that occupy adjacent channels at the high frequencies. At the same time the amplification at the high frequencies is higher than at the low end of the keyboard. And these two difficulties work together to make the problem of the radio-frequency-amplifierdesign engineer anything but pleasant. The ideal condition would be to make the amplification and selectivity the same all along the radio keyboard. "The usual radio-frequency amplifier is composed of a tube and a transformer which The Radiola 60 receiver in a tabletype cabinet. connects this tube to the following tube. This transformer has a few turns on the primary and many t urns on the secondary. The primary tends to resonate at some frequency higher than any to which the secondary is ever tuned. This tendency to resonate at some high frequency makes the receiver have greater amplification and less selectivity on the high frequencies. "The Radiola 60 uses large primary coils instead of small ones, so that the primary tends to resonate at a lower frequency than any to which the receiver will be tuned. This tends to bring up the amplification at the lower radio frequencies, and to prevent such selectivity at these frequencies that part of the audio tones are cut off. "The direct result of making the primary of the transformer large instead of small is, first, an increase in amplification at low radio frequencies, second, prevention of 'sideband cutting' at low radio frequencies, and third, the amplification over the whole keyboard of radio channels is even. Fig. 1 shows what this amplification is. Believing the importance of using technical facts in selling radio receivers was being overlooked by the majority of salesmen, the Editors asked an engineer to write the article which appears here. The data for it were taken from a paper presented before the Institute of Radio Engineers, March, 1929, by G. L. Beers and W. L. Carlson, which are the result of the development work on the Radiola 60 series. Although the facts used here apply only to this particular receiver, similar presentations of sales points could be prepared on any other receiver — and in our opinion ought to get many salesmen out of tight places when the prospective customer demands facts, instead of glib phrases about excellent tone quality, extreme selectivity, and perfect "DX." — The Editor. "Such is the radio-frequency amplifier of the Radiola 60. It is followed by a detector, just as in any receiver, but into this detector is introduced another frequency coming from a tube acting as a miniature transmitter, the oscillator tube. The frequency at which this tube oscillates is varied automatically at the same time that the radio-frequency amplifier is tuned to the desired signal and the frequency it introduces into the detector always differs from the incoming signals by 180 kc. The modulations which are separated from the radio wave in this detector are impressed on this 180-kc. signal and modulate it. Thereupon amplification takes place again at 180 kc. instead of the frequency to which the receiver is tuned. "So the Radiola 60 in addition to amplification at broadcast frequencies amplifies again at 180 kc. Finally these 180 kc. signals are fed into a second detector which separates the audio frequencies. "The 180-kc. frequency to which the second or intermediate-frequency amplifier is tuned was chosen for the following reasons. If this frequency were low, it would amplify audio tones and any noises appearing in the preceding tubes; that is, microphonic bongs, tube hiss, etc., would be passed through and amplified in the intermediate-frequency amplifier. If the frequency were made too high, trouble from oscillation, and lack of amplification would occur. The 180 kc. is a compromise frequency. "Let us look into this mixing of frequencies in the detector tube. Suppose the intermediate frequency is 50 kc. If we are receiving a 1000kc. station we can set the oscillator at either 1050 kc. or 950 kc. and still have the desired 50-kc. intermediate frequency modulated by the audio tones. For this reason we can receive a 1000-kc. wave at two points on the oscillator dial. Again let us suppose we have the oscillator timed to 1000 kc. and that two stations equally powerful are transmitting on 950 and 1050 kc. Roth of these signals enter our first detector and with the 1000-kc. oscillator frequency produce a 50-kc. wave modulated with the audible tones of both stations. The result is hash; both stations are spoiled. "If the intermediate frequency is 180 kc. such trouble cannot possibly take place at frequencies lower than 1140 kc. Suppose the • may, 1929 page 15 0