Radio Broadcast (Nov 1926-Apr 1927)

300 RADIO BROADCAST ADVERTISER All the Truth and nothing but the Truth! If your set gives you poor quality, it is telling lies about the sending station. If it fails to transmit those low base notes, it is concealing part of the truth. You want true reception. You are entitled to it. So is your family. There is a way to get the truth in radio: — FErrantI Ferranti Transformers can probably modernize that old set of yours or improve the reception of even a new one. Your dealer can help you install one or two. If you want to make the best of the power tube feeding the loud speaker, use Ferranti. If your dealer does not carry Ferranti, write us and we shall tell you where you can get one. HIGHSPOTS High • amplification ratio with flat 'Curve. Ferranti brings out the fundamental frequency of low tones — none are heard merely by inference from higher harmonics. Every transformer tested ten times — all short-circuit turns eliminated. Windings have high impedance. Built by an established manufacturing company with forty years' experience in the winding of coils of fine wire for electrical instruments and meters. Primary shunted with built-in condensers of correct capacity s Tested to iooo volts between primary .►and secondary and between primary and secondary and ground. For the best available transformer results-^-Ferranti Audio Frequency Transformer A.F. 3 — ratio 35 to 1 — $12. For a transformer far superior to the average, use Ferranti A.F. 4 — ratio l\ to 1—28.50. FERRANTI, Inc. 130 W. 42nd Street New York, N. Y. * No Better Transformer Is Available At Any Price No. 59 Radio Broadcast Laboratory Information Sheet What are Harmonics? January, 1927 THEIR ELECTRICAL CHARACTERISTICS PRACTICALLY none of the sounds that we hear can be said to be pure, in the sense that they contain only one frequency. Several different persons could all sing the same note and yet the different voices would be easily distinguishable. A violin and a flute might play the same note, but they would sound entirely different. The factor which causes this difference is the existence in practically all sounds of various harmonics, or overtones, about which something has been said in Laboratory Sheet No. 51. In the present Sheet, we will explain, from an electrical standpoint, what harmonics are. Acoustically, the difference between a fundamental note and, say, its fifth harmonic is that the pitch of the harmonic is five times as high as the pitch of the fundamental. Electrically, the difference is that, for every time that the fundamental note goes through one cycle, the fifth harmonic goes through 5 cycles. This relation between a fundamental and any of its harmonics always is true, i. e., while the fundamental passes through one cycle, a harmonic passes through a number of cycles, depending upon what harmonic it is; the second harmonic passes through two cycles, the third harmonic passes through three, the fourth through four, and so on. A cycle comprises one complete alternation of the wave and, therefore, to produce one cycle the wave must start at zero, rise to a positive maximum, decrease to zero, rise to a negative maximum and again decrease to zero. The sounds created by instruments are practically always very complicated and contain many harmonics. The violin, as an example, produces a very complex note containing a very prominent third harmonic, and many other harmonics as well, while some of the notes produced by a flute are perhaps the purest of any sounds that are generated by musical instruments. Many amplifying systems are not capable of amplifying the low notes but fortunately a considerable decrease in amplitude in these low frequencies is hardly noticeable to the ear. It is also generally true that the harmonics of these low notes will have the same effect on the ear as the fundamental note. Consequently, if an organ sounded a chord which contained a 30-cycle note and only the second harmonic, 60 cycles, of this note was heard, it would give the same effect to the ear as the fundamental note of 30 cycles. This characteristic, combined with the fact that these low notes are very seldom used, makes it hardly worthwhile to go to any great expense to set up apparatus capable of giving exact amplification of these low frequencies. Radio Broadcast Laboratory Information Sheet Filter Circuit Data January, 1927 CONDENSER VALUES SOME interesting data were given in the General Radio Experimenter of July, 1926, regarding the characteristics of filter circuits for use with B current-supply devices. In the diagram, the condenser marked Ci may be called a reservoir condenser as its function is to store up energy during the peak of the wave and feed it back into the circuit at the lower values of the wave. This condenser is especially valuable in keeping the voltage more nearly constant with varying loads. A small condenser at this point will cause the voltage to drop off very readily with increasing load. The condenser marked C2 is placed across the output, and is especially valuable in eliminating any ripple. The curves given on this sheet indicate the effect obtained with different values of condensers in the two positions. It should be noted that an increase in either one of these condensers improves the regulation (curve B and C both showing a decrease over A in voltage drop), due to increasing the size of the load. Curve C shows the best regulation where Ci is the larger. This indicates that an increase in Ci is more effective in improving the voltage regulation than C2. An oscillograph would show that an increase in either condenser would tend to eliminate the ripple, but that less ripple would be obtained by increasing C2 rather than Ci. Both experiment and theory seem to indicate that, with a certain total capacity in the current, the best regulation and the least ripple are obtained by making both of the condensers of the same value. 20 30 40 50. Is MILLIAMPERES The curves shown are for a single section filter using the Raytheon tube as a rectifier. A multi-section filter would, however, give the same type of curves. No. 61 Radio Broadcast Laboratory Information Sheet January, 1927 The Intermediate Frequency Amplifier CHOOSING THE BEST FREOUENCY '"PHE best operating frequencies for intermediate frequency amplifiers are 45, 55, 65, etc. rather than 40, 50, or 60. kc. At the present time, broadcasting stations are supposed to be separated by a frequency of 10 kilocycles. Consequently, it is quite possible for any two stations to be separated, by, say, 40 kilocycles. If two stations, one strong and the other weak, are separated by this amount, it may be quite difficult to completely separate them by means of a single tuned circuit such as a loop. Therefore, both of these frequencies will be present in the loop circuit, and will beat with each other to produce a frequency equal to the difference between their respective frequencies. That is, a station on £00 kilocycles would heterodyne a station on 460 kilocycles to produce a 40-kilocycle note. Should the intermediate-frequency amplifier happen to be tuned to this frequency, both stations will be heard in the output, even though the oscillator is removed from the circuit. If, on the other hand, the intermediate frequency amplifier is tuned to 45 kilocycles, only the heterodyne beat between the station wanted and the oscillator would be amplified. We have endeavored to show this idea in the diagram where a 40-kc. intermediate amplifier is used: "A" is the interfering station. "B" is the station desired, and "C" is the wave produced by the oscillator. C is tuned to 540 kilocycles and produces a 40-kc. beat note or heterodyne with the desired signal B which is fed to the intermediatefrequency amplifier. However, at the same time, we will suppose that there is a powerful local station operating on 460 kilocycles (indicated at A), and the interaction between A and B also produces a 40-kilocycle beat note. The result is, that the station broadcasting on 4C0 kilocycles will also be heard through the amplifier. When stations whose frequencies are multiples of 10 heterodyne, they naturally produce a beat note which also is a multiple of 10. By designing the intermediate amplifier for a frequency which is not divisible by 10, it will, therefore, exclude beat notes of two heterodyning stations if such are divisible by 10. Hence the desirability of a 45-, 55-, or 65-kc. intermediate amplifier. 40 Kc. Produced by A&.E 40 Kc. Produced by B&C 540 Kc. Examined and approved by R \mo Broadcast -A"