Radio Broadcast (May 1929-Apr 1930)

16 §12 FREQUENCY IN KILOCYLES Fig. 1. oscillator is set at 1160 plus 180 kc. or 1340 kc. The only other station that could produce the 180-kc. intermediate frequency in the first detector would be 1340 plus 180 or 1520 kc— and the broadcasting keyboard extends only to 1500 kc. "Here again is a virtue of the radiofrequency amplifier ahead of the intermediatefrequency amplifier. This first amplifier builds up the desired signal and discriminates against the unwanted, so that even if two stations offer equal signals at the antenna, one of them desired and one of them not, the undesired will be reduced — compared to the desired — by the amplification of the radiofrequency amplifier and when the two signals get to the first detector or mixing tube, the unwanted signal is already reduced so far it does not bother the listener. " So far the result is a tuned radio-frequency amplifier of such a design that all stations scattered over the radio keyboard can be received with equal facility; the quality of reproduction from these stations will depend only upon the stations — there is inappreciable loss of quality in the receiver; after amplification at the frequency of the stations, the signals are changed in frequency and amplified again. The first amplifier acts not only as a kind of filter letting in only the desired signals, but it gives some amplification too. "The intermediate-frequency (180-kc.) amplifier is of the type that has attracted considerable attention from radio editors. It is a band-pass amplifier which means simply that it is tuned so broadly on the top of its response curve and so steep on its sides that all desired audio tones are admitted and amplified, and others are rejected. This result is 16 14 12 lu 10 CO z O Q. CO o cc LU § 6 LU 4 i IHih i r c I \ => c ?c i => Z> 1_ 1 ■v > — 4 / 1 I! \\ I 1 A B| 1 A— 1 -r f 1 i -l1 # 1 I * f— 1 V ... 9 » * — V — % ✓ secured by tuning both primary and secondary of the intermediate-frequency transformers (the usual transformer has only the secondary tuned). The response characteristic is shown in Fig. 2a. The overall characteristic — which shows how the intermediate-frequency amplifier discriminates against unwanted signals is shown in Fig. 3. "And so the intermediate-frequency amplifier not only amplifies but selects as well. The Radiola 60 amplifies, selects, and detects twice. "The second detector is the increasingly 100 80 60 50 40 30 20 o cc 1.0 0.8 0.6 0.5 0.4 0.3 150 160 170 180 190 200 210 FREQUENCY IN KILOCYLES Fig. 3. 16 ] iC. 40 1 CCr 48 40 LU (3 £9 -J £E 32 o o £9 24 Q. < 9< 16 Q * AU 0 « V c t B 140 16Q 180 200 220 FREQUENCY IN KILOCYLES Fig. 2. 240 RADIO FREQUENCY MICROVOLTS (R.M.S.) AT FIRST R.F.GRID Fig. 4. popular 'power detector,' which means, so far as the layman is concerned, that it eliminates the distortion and noise that frequently occur in the first stage of a.f. amplification. It does this by eliminating the first stage of a.f. itself. Such elimination is possible by the great amount of amplification that has taken place in the preliminary amplifier and in the intermediate-frequency amplifier. "This power detector is adjusted so that it overloads at the same time the power tube does. When the detector overloads its output decreases as shown in Fig. 6. "Some of the Radiola 60 series (the 64 for example), have automatic volume controls. It is this device which makes the receiver suitable for reception in a transmittercluttered neighborhood, or out in the rural areas far from stations. Once the listener sets the volume-control dial to the maximum output he desires he cannot get a louder signal no matter how powerful the station is that he tunes to. In a local area this is of undoubted advantage. In the country the automatic volume control will tend to build up weak signals to the desired level. Of course, if the weak signal is surrounded by noise — static, 100 1000 5000 10,000 FREQUENCY IN CYCLES PER SECOND Fig. 5. arc lights, X-ray machines, etc. — the volume control cannot eliminate the noise and get the signal, and so the noise background comes up along with the station. But on a good night when the noise level is down all stations that deliver a certain minimum signal to the antenna will deliver a certain maximum output loud speaker signal, this output always at the control of the listener. The effectiveness of the control at the output levels A and B compared to no control, C, is shown in Fig. 4. "This receiver has eight tuned circuits. One's first reaction to such a statement is that there would be no high audio frequencies at all — but a look at Fig. 5 shows that such is not the case. The large primary winding of the first amplifier transformer, the bandpass effect in the intermediate or second amplifier, and the characteristics of the a.f. amplifier are such that undue suppression of the high audio notes does not take place. In other words, it is a high-quality receiver." If I were a salesman, I should use technical facts to back up my sales arguments. I should state that this particular receiver is selective because of the radio-frequency amplifier and because of the selecting effect of the second or 180-kc. amplifier. It is sensitive because amplification takes place at three different frequencies, first the frequency of the incoming signals, next at 180 kc, and finally at the audio or audible frequency. It has a power detector which eliminates some noise and some distortion. It is a high-quality receiver because its radio-frequency amplifier does not cut "sidebands," because its intermediatefrequency amplifier employs the band-pass idea, and because its a.f. system is good. In addition to all these advantages, the Radiola 64 has an automatic volume control which keeps down strong local signals, and boosts weak distant signals. 0 10 20 30 40 50 R.F. PEAK VOLTAGE ON DETECTOR GRID Fig. 6. • may. 1929 page 16 •