Radio broadcast .. (1922-30)

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nearly the same with all three curves, being slightly greater with inductive coupling at the higher frequency. The side- band variation, however, is very differ- ent; at 600 kc. it is 15 per cent., and is in the form of a high-note accentuation. At 1200 kc., with inductive coupling it is only 6 per cent., but with capacity cou- pling there is a high-note loss of 30 per cent. This difference in the amount of sideband variation at different frequen- cies is not the disadvantage it may at first appear to be; as will be shown later, when considering the applications of the filter to a tuned r.f. amplifier, it is rather an advantage than otherwise. Coupling Tubes The band-pass filter may be used with very similar results as a means of coupling two tubes in a r.f. amplifier. It is usually KC OFF RESONANCE Fig. 4 unnecessary to do this, however, as it is but rarely that more than one filter is needed in a receiver; and if only one filter is used, it should always be included in the antenna circuit, owing to the greater freedom from "cross-talk" which can then be obtained. The formula necessary for calculating the amplification given by a tube with a filter in the plate circuit is given below, where the symbols have the same meanings as before— (7) A = amplification Rp= tube plate resistance u, = amplification factor At the frequency at which coL = 1/wc this reduces to (8) since X 2 /ox3 in the right-hand bracket of the denominator is usually negligible in comparison with the other terms. When the tube has a plate resistance greater than about 500,000 ohms, it is sufficiently accurate for most purposes to calculate the shape of the tuning curve from equation (1), and the amplification at resonance from equation (8). The re- sults will show the selectivity and side- band variation as being slightly higher than is actually the case, but the error is usually inappreciable. Of course, if the tube has a low plate resistance the correct formula (7) must be used, and this will always give correct results. Plate-Circuit Filter The remarks made earlier in this ar- ticle on the respective merits of inductive and capacitative couplings apply with equal force to the plate-circuit filter. The only difference in the results is due to the damping exerted by the tube's plate re- sistance on the primary circuit of the filter. If this resistance is high, as it usually is with good modern screen-grid tubes, the difference is very small. With modern screen-grid tubes, how- ever, sufficient r.f. amplification for most purposes can be obtained with two stages, with the usual methods of coupling the tubes. This amplification is sufficient to permit the use of one filter, but it is not usually great enough if two filters are used; since the loss of amplification in each filter must be at least 50 per cent. The carves of Fig. 2, A and B, show the am- plification and selectivity of a particular, r.f. amplifier at frequencies of 600 kc. and 1200 kc., respectively. The amplifier, the skeleton circuit of which is given in Fig. 3., consisted of two Mazda AC/SG screen-grid tubes coupled by the tuned plate circuit, with coils having an induc- tance of 240 mh. and a r.f. resistance at 600 kc. of 10 ohms, and at 1200 kc. of 20 ohms. The tuned plate circuit was adopted because, owing to the high plate resis- tance of these tubes, no greater amplifi- cation could be attained by the use of a transformer nor could the selectivity be increased to any great extent. In addition, the tuned plate circuit allows of very simple switching arrangements being used to change from one wave range to another. This is a very important point in de- signing receivers for use in England, for every set must be able to receive not only on the normal waveband of 200-550 me- ters, but on the 1000-2000-meter wave- band also. The characteristics of English a.c. screen-grid tubes are given in Table II. Amplifier Characteristics It will be seen that the amplification given by this amplifier is very high; at 1200 kc. it is 200 per stage, and this with perfect stability. Now it is obvious that if this amplifier were preceded by a normal single tuned antenna circuit, it would re- sult in poor selectivity at the higher fre- quencies, for each of the three tuned cir- cuits would give greater selectivity at the low frequencies. In addition, the sideband loss would be far too high, for with only two circuits it is 60 per cent, at 600 kc. The results, when the amplifier is pre- ceded by a filter, however, can be seen from Fig. 4, in which curve A is for a fre- quency of 600 kc., and for either capacity or inductive coupling in the filter; curve B shows the results with capacity cou- pling at 1200 kc., and curve c for an in- ductively coupled filter at the same fre- quency. It is included so that a compari- son can be made between inductive and capacitative coupling. The values of the coupling components are the same as those mentioned earlier. The input volt- age for these curves is such that the out- K C OFF RESONANCE Fig. 5 Fig. 6 put voltages at resonance are the same for all three; in this way the differences in selectivity and sideband variation may be seen readily. The figures for amplifica- tion are given in Table I, but it must be remembered that these figures show the amplification of the voltage induced in series with the primary circuit of the filter, not the voltage in the antenna. Selectivity Variation The superiority of capacity coupling is even more obvious in these curves than it is in those for the filter alone, for the poor selectivity of inductive coupling at the higher frequencies is accentuated by the same defect in the amplifier. With ca- pacity coupling the selectivity varies in a 5-1 ratio between 1200 kc. and 600 kc., but with inductive coupling this ratio is 14-1. Not only this, but an advantage is gained in fidelity also. The high-note loss at 600 kc. is 54 per cent, with both methods of coupling; at 1200 kc. it is only 25 per cent, with inductive coupling, and 47 per cent, with capacity coupling. It is decidedly an advantage that the loss should remain constant over the whole tuning range of the receiver, for it makes it possible to compensate it by increased amplification of the upper audible fre- quencies in the a.f. amplifier. Although, with the loss of 50 per cent, given by this particular arrangement, it is rather doubtful whether any great increase in fidelity would result from such compen- sation, as the ear is very tolerant of a high-note loss. It is probable that a loss of this order is unnoticeable by the aver- age ear. The curves of Fig. 6 show the results to be expected from the same amplifier, but with a capacitatively coupled filter substi- tuted for one of the tuned plate circuits. (Continued on page ,149) 328 RADIO BROADCAST FOR APRIL •