Radio Broadcast (May 1928-Apr 1929)

342 RADIO BROADCAST OCTOBER, 1928 ® ® INAUDIBLE RESPONSE [ 580 600 610 KILOCYCLES FIG. I plate circuits of an ordinary amplifier tube invariably results in uncontrollable self-oscillation. When both grid and plate circuits are tuned a two stage r.f. amplifier has a total of five tuned circuits, including the grid circuit of the first tube. This increased number of tuned circuits would naturally provide a marked increase in selectivity. In fact the scheme looked so good on paper that an experimental receiver was constructed embodying these ideas. The circuit diagram of the experimental model was substantially the same as that of the finished receiver shown in Fig. 5. On test, this model performed in a truly remarkable manner, greatly exceeding expectations. The r. f. gain was very good — enough to bring in many distant stations, including one on the Pacific Coast. The selectivity was such that more than a dozen distant stations were received while the local stations were operating. This test was made last May using a 75-foot antenna located in midtown New York THE COUPLING TRANSFORMERS THE remarkable performance of this receiver can best be understood by consideration of the principles involved in its design. The interstage r.f. transformers are quite unique, in that they consist of two exactly similar coils. One constitutes the primary of the transformer and is connected in the plate circuit of the preceding tube, and the other coil acts as the secondary and is connected to the grid of the following tube. Each coil is tuned to resonance with the desired signal by means of a 0.00035mfd. variable condenser. Due to the rather unusual mounting arrangement the mutual inductance or coupling between primary and secondary is very small. However, this does not mean that the energy transfer from primary to secondary is inefficient. On the contrary, when two tuned circuits are coupled to each other, the maximum secondary voltage is obtained when the relation (2icf)2M2 = R.1R2 is satisfied, where f is the frequency to which both circuits are tuned, M is the mutual inductance in henrys, and Ri and R2 are the effective radio-frequency resistances of the primary and secondary, respectively. In the case of the coils used in the receiver under discussion the maximum secondary voltage is obtained with a coupling coefficient of the order of one per cent. The physical arrangement of the coils, as shown in Fig. 3, was chosen because it seemed the simplest way to secure such loose mechanical coupling while still keeping the coils close together, thus conserving space. Due to the inherent characteristics of loosely coupled tuned circuits each of these doubly tuned r.f. transformers really constitutes a band pass filter. This is quite clearly shown in Fig. 2. In this figure is shown the tuning characteristic of one of these double-tuned loosely coupled transformers. The dotted line represents the response curve of one tuned circuit alone, and the sclid line that of both circuits properly coupled. It will be noticed that the dotted curve of the single circuit is a typical resonance curve; very sharp on the top at exact resonance and sloping gradually toward zero as the frequency is increased or decreased. On the other hand the solid curve of the double circuit is quite broad and almost flat on the top over about seven kilocycles, but slopes more steeply on the sides and the response approaches zero much more rapidly above and below the resonant frequency. These curves in Fig. 2 are based on actual measurements of one of the new r.f. transformers used in the Hammarlund-Roberts "HiQ 29 While one of these doubletuned r.f. transformers provides an unusual degree of selectivity, the use of two such stages in cascade results in a vast improvement. As an illustration note that the response of an interfering signal at 20 kc. below resonance on the solid curve of Fig. 2 is but 9 per cent, or about yr of the response at the frequency for which the set is tuned. This is for one stage only. After going through the second stage, however, the intensity of this interfering signal will have been reduced to to per cent., or about rl^. At the same time the addition of the second stage does not materially affect the shape of the top of the re 0 ■\ Re '■of jxmse char ingle tuna i circuit Ri tw .■'it -ptmse char ry loos* tot uleristieoj aling 9 1 t / % / / \ \ \ \ 1 V y \ FIG. sponse curve. The top of the curve remains substantially the same as shown in Fig. 2; the sides become much steeper and the response approaches the zero line at a much more rapid rate. [These percentages, when reduced to losses in tu, give interesting figures. At 20 kc. off resonance, for example, the discrimination of a single resonant circuit — using Mr. Oram's percentages — is 14 tu, while the doubly tuned circuit gives an additional 8 tu or a total loss of 22 tu. When these signals are passed through an additional doubly tuned circuit this becomes 44 tu, and if the antenna stage has a discrimination of 10 tu at 20 kc. the total selectivity factor becomes 54 tu, which is the difference between signals from two stations at an equal distance from the receiver but differing in power by a ratio of 250,000. — The Editor] The width and flatness of the top of the solid curve shown in Fig. 2 has an important bearing on the quality of the received speech and music. This is due to the fact that broadcast stations do not transmit on a single frequency, but rather on a band Of frequencies. The width of the side bands varies somewhat, depending on the transmitter ad j u s t m en ts and also on the type of program being broadcast. They are, however, generally conceded to be about five kilocycles wide for high quality transmission. It is therefore apparent that the receiver should be capable of amplifying a band of frequencies with substantial uniformity if the program is to be received faithfully. Hence the desirability of the wide flat top on the overall response curve of a high-grade receiver. When the top of the response curve is sharp instead of flat all the frequencies in the band are not amplified equally. Consequently certain of these frequencies reach the detector much stronger than others with the result that even the most perfect a.f. amplifier and loud speaker will be unable to reproduce the program with its original quality. The two double-tuned r.f. transformers used in the Hammarlund-Roberts Hi-Q2o. necessitate the use of four variable condensers— one to tune each of the four coils. Since all four of the tuned circuits are identical these four variable condensers are rotated by a common shaft actuated by a new model drum dial having a smooth positive drive without backlash. The tuned input circuit to the grid of the first screen-grid tube, often referred to as the antenna coupler, is of the conventional type having a tapped primary making it adaptable to different length antennas. The variable condenser tuning this antenna coupler is on a separate shaft and has a separate drum dial, thus enabling this circuit to be tuned to exact reasonance with the received signal. 1000 1010 KILOCYCLES Both , 1 3" 'l'S about has a FIG. 3. AN R. F. COIL primary and secondary of this interesting transformer are in diameter and i\" long, wound with silk covered wire, 80 turns of No. 28 wire being used. The detector input coil tap at about the twentieth him from the grid end, as indicated in Fig. 5. other design features THE volume control is quite out of the ordinary and is made possible only by the characteristics of the screen-grid tubes. It consists of a