Radio Broadcast (May 1928-Apr 1929)

An Engineer s Explanation REAL VERSUS APPARENT SELECTIVITY By KENNETH W. JARVIS Engineer, Crosley Radio Corporation 500 A RADIO set without selectivity is like a ship without a rudder, buffetted by every wind, and heeding the strongest wave and current. Some sets are selective — some are not; and broadcast transmitters and guileless amateurs are reviled alike. There is wailing and gnashing of teeth because of the radio engineers' inability to bring about the near impossible, and all because most of us do not quite understand what "selectivity" really means. Imagine the conglomerate mess of the radiated energy of all the broadcasting stations on earth, all of the commercial transmitters, all of the amateurs, and all of the natural and man-made static, swirling, fading, crossing and recrossing the bare wire of your antenna, each inducing a voltage therein. You sit down below, more or less patiently attempting to hear the beautiful strains of the "Serenade" originating a thousand miles away, all uninterrupted and unimpeded by the myriad of unwanted impulses in your antenna. The selectivity is that property of your receiver which makes for order out of this chaos. As we are interested in the usual broadcast reception, we need to consider only a comparatively small range of the frequencies in the whole spectrum, that is the band between 550 kc. and 1500 kc. Stations broadcasting in this range are known by their " carrier," i.e., the fundamental frequency of their radiation. Due to the frequencies needed on either side of the carrier for proper transmission, each carrier is spaced 10 kc. apart. Thus there is room for only 96 stations in the allotted broadcast-frequency range. In the new allocation plan, various stations either divide time or are located geographically so that they can use the same frequency without interference. For our purpose we can, therefore, consider that there are only 96 .stations, one on each 10-kc. band of the broadcast range. In explaining the mechanism of transmission, use is made of the "side-band theory" which says that in addition to the carrier frequencies, a broadcasting station radiates additional frequencies corresponding to the audio tones. If, as is approximately true, the frequency distribution and percentage modulation as based on these side bands is the same for all stations, we can neglect the side waves and consider only the relations of the various carrier frequencies. What is Selectivity? OBVIOUSLY, we are interested in selecting the energy of some one carrier frequency out of all the number present at the antenna. The strength of the signal (or carrier) at the antenna will influence the degree of selectivity. This strength is measured in microvolts ( million ths of a volt). The bigger the antenna, the more microvolts it will pick up. The field strength of the signal is, therefore, rated in microvolts per meter, and the field strength, multiplied by the effective height of the antenna in meters, gives the actual voltage induced in the antenna. Thus a ooooooooooooooooooooooooooo rsCO Ol O rn C\|CO ^ in ID N OD O — tC-^rQ <J iniO N O) Ol 0~.<\ifri mmtfiuJioioiDiDUJvoioioyJNNrsfsfNNsNsr^cOQOcoto FREQUENCY IN KILOCYCLES Fig. 1 — A powerful local broadcasting station often presents a peculiar selectivity problem. In the case illustrated above, it is impossible to receive stations between 650 kc. and 750 kc. without interference from the powerful local signal of 100 microvolts per meter acting on a four-meter antenna will produce an input voltage to the set of 400 millionths of a volt. Before making use of this relation of field strength, a brief view of the nature of selectivity is necessary. Fundamentally, selectivity means a greater response to one frequency than to any other. This leads to the idea of resonance. If a circuit or system is quite responsive to some one frequency, it is said to be resonant to that frequency. Forms of mechanical resonance are familiar to every This article by Mr. Jarvis, a member of the engineering staff of the Crosley Radio Corporation, attempts to interpret the real meaning of selectivity in the light of our present-day reception problems. How much selectivity is desirable, what compromises in set design must be made to attain optimum selectivity at reasonable cost, and how the problem is solved in the design of present-day sets — all this Mr. Jarvis covers. To dealers, attempting to answer customer-questions this analysis should be useful as well as to many others of our readers desiring to follow a general investigation of the subject. — The Editor. one. The piano and violin strings tuned to the same note are in resonance. If the piano note is sounded, the sound waves will travel through the air and start the violin string vibrating and emitting sound. The pendulum of a clock oscillates back and forth at a definite frequency. The pendulum system is resonant to that frequency. Sometimes a road has small ripples at regular intervals. If an automobile is driven at the proper speed over these little ripples, the car will begin to surge up and down with greatly increasing movement. If the car be driven faster, the springs take up the movement of the wheels and the chassis stays almost still. Driving slower allows the car slowly to follow up and down the ripples without any great movement. But at some particular speed, the weight of the car and the elasticity of the springs are "resonant" at the frequency of the ripples of the road surface and a terrific movement results. Obviously the bigger the ripples, the greater will be the car movement. This is equivalent to increasing the force applied. The nearer the car runs at the "resonant" speed, the greater will be the movement. If there are "snubbers" on the car or there is a lot of resistance to the motion, the amplitude will not be so great. Electrical circuits exhibit the same sort of resonance to electrical forces as the mechanical systems show. The condensers in the circuit correspond to the elasticity of the springs while the inductance corresponds to the mass of the car. Therefore it follows that a similar relation is obtained between the force and resistance in electrical circuits as in mechanical systems. Electrical Equivalent THIS may be illustrated in curve a of Fig. 2 where the height of the curve at any particular point represents the current flowing in the circuit at the frequency applied, the voltage remaining constant. It is obvious that this circuit has the property of "selectivity" and gives the greatest response at some particular frequency. Usually a single circuit does not have sufficient selectivity and two or more such resonant circuits are needed. The amplification might be adjusted to give the same maximum current in the last resonant circuit and then the system would have a curve like b, of Fig. 1. Having obtained some idea of why a receiver is selective, a little different viewpoint is now necessary. A receiver should be sufficiently selective just not to hear any interfering station. Obviously this depends on the strength and frequency of this would-be interfering station. Therefore, the usual selectivity curve is drawn as in Fig. 3. Here the height represents the field strength in microvolts per meter necessary to apply to the receiver an output just loud enough to be heard. The receiver is tuned to 1000 kc. and obviously it takes less voltage to hear an output at this frequency, only 2 microvolts. At 10 kilocycles off resonance, the input would have to be 3.5 microvolts, while at 20 kilo april, 1929 . page 399