Radio Broadcast (May 1929-Apr 1930)

300 500 1000 FREQUENCY 3000 5000 10,000 Fig. 5. curve and will give a fairly constant reflected load. In view of the fact that many types of loud speakers might be used, any of which might well vary with time or change of manufacturing conditions, it is probably best to use a resistive loading, preferably one of twice the a.c. tube resistance, for a one to one transformer. This will facilitate the comparison of various transformers against a standard. Two Kinds of Distortion WAVE-SHAPE distortion may be caused either in the tube, or the transformer. A curved tube characteristic or d.c. saturation in the transformer will produce even harmonics of the initial wave, and a.c. saturation in the transformer core will produce odd harmonics. Tubes are usually worked on as straight a portion of their curve as possible, or are connected in push-pull so as to eliminate the effect of the even harmonics. The only remedy for distortion introduced by transformer saturation is to run the transformer core at low flux densities, i.e., at values of d.c. plate current and a.c. signal voltage which will not cause the core to become saturated. Amplitude distortion may be divided into two classes. In the first, the output voltage of the amplifier is not directly proportional to the input voltage. At low frequencies low voltage inputs will not be amplified as much as higher ones due to the change of inductance of the transformer primary. The second class is that of frequency discrimination. Certain characteristics of the transformer and associated circuit cause it to pass some frequencies with less loss than others. In Fig. 9 are shown the actual circuit diagram, the equivalent diagram, and the vector diagram of the loaded transformer. The effective alternating current plate impedance of the tube must be added to the effective primary resistance of the transformer. As shown in the vector diagram, the leakage reactance causes a voltage drop in quadrature with the effective resistance in both the primary and secondary which causes voltage regulation. The greater the load, the greater the regulation from these causes. It is therefore desirable to have the effective tube, primary, and secondary resistances as low as lu 3 possible in order that the voltage output may be proportional to the voltage input. Fig. 5 shows a curve of output voltage against frequency for a resistive load. With a constant voltage input the curve drops off at both ends. The drop at the high-frequency end is due to the leakage reactance whose effect increases with frequency. The drop at the low end is due to the increasing magnetizing current causing magnetic saturation of the core, and increased IR drop in the primary circuit. The leakage reactance can be neglected where this effect comes in. The cone loud speaker load curve introduced on the same sheet shows the effect of an impedanca load which is low at low frequencies and high at the high ones. Apparent Losses FIG. 7 illustrates the effect on the apparent resistance and inductance of the primary circuit of a voltage transformer, of the addition of a secondary circuit. The added losses are very noticeable, the added capacity lowers the resonant frequency, and the loading lowers the effective low-frequency inductance. A coupled circuit effect produces an added TEST CIRCUIT 0 67.5 .25 V Ep^* — asv.Ep l.OV.Ep — s 30 50 100 500 FREQUENCY 1000 Fig. 6. 5000 FREQUENCY Fig. 7. jag in the curve about midway, and there is some parallel resonance effect at low frequencies. This curve illustrates the way a voltage transformer load looks to the tube which is feeding it. Therefore, the actual and equivalent circuits of the voltage transformer must include the distributed capacity of the secondary to enable a correct design to be worked out. In the equivalent circuit the leakage reactance is in series with the effective resistance and the distributed capacity of the secondary and the associated circuit. Of this associated capacity a large portion is formed by the effective tube capacity. At some frequency the circuit will be in series resonance and will give a high voltage drop across the capacity due to the resonant current. This produces a peak on the characteristic curve of frequency vs. ratio, as was to be noted in several of the figures. If the effective reactance and capacity can be made small enough the peak may be moved out of the normal range of transmitted modulation frequency. If the effective resistance is increased sufficiently the resonant current can be reduced to a value which will not produce a prominent peak. 3000 5000 10,000 Fig. 8. In Fig. 2 are shown curves for various improved designs of audio-frequency voltage step-up transformers. By pie winding, sufficiently low distributed capacity and leakage reactance are obtained to put the resonance frequency peak above the normal range of the radio-frequency band-pass of a receiving set. The curve shown for the transformer with a small window is an example of the tendency toward improvement in transformer design. This had a comparatively small core weight and a high primary inductance, but showed practically no tendency to saturate with normal d. c. applied. The small type transformer whose curve is shown, is interesting chiefly because of the method of manufacture. It is wound with a high space factor so that the turns in a layer bunch up during winding. This automatically short circuits enough turns to iron out the resonance peak. The cheapness of the core, winding and assembly enables this transformer to compete successfully with a much higher grade product. A carefully designed voltage transformer with a value of primary inductance to give a 3.0 per cent, drop at 60 cycles with a 10,000-ohm tube, that is, 110 henries, can be made with such a low leakage reactance that it will peak at above 7000 cycles. If the core laminations are so made that the eddy currents are large, they will increase the effective resistance sufficiently to flatten the resonance peak completely. This will not affect the gain over the lower frequencies, however. Such a transformer is fully sufficient for covering the range of frequencies from 30 to 5000 cycles which is considered as perfectly satisfactory for the reproduction of voice or music. In conclusion, the writer wishes to express his appreciation of the assistance in compiling this data rendered by the Engineering Department of the Pacent Electric Company. 3000 5000 10,000 Fig. 9. • may, 1929 page 35 #