956 



Popular Science Monthly 



Preventing Interference of Signals 

 by Amplifying 



AN interesting [patent on a highly 

 selective arriingement for use at a 

 radio-telegraph recei\'ing station is No. 

 1,173.079, issued in 1916 to E. F. W. 

 Alexanderson. Realizing that the prin- 



Fig. 1. Inductively coupled circuits 

 in a series of magnifying relays 



ciple of selection bj' tuning to wave- 

 length may be applied several times in 

 the same group of instruments, the 

 inventor has arranged a .series of tuned 

 radio-frequency magnifying relays, as 

 shown in the diagrams. The circuits 

 look complicated, but are not very hard 

 to manipulate. Anyone who has two 

 or three audion bulbs, preferably of the 

 double-plate "amplifier" type, will be 

 able to try out a series of experiments 

 along the lines of this invention. 



Referring to Fig. i, it is seen that the 

 antenna i passes to earth through the 

 transformer primary 2 and series tuning 

 condenser 3. The secondary circuit is 

 tuned to the desired incoming wave- 

 length by means of condenser 8, and 

 connects via wires 5 and 6 to the grid 

 G and filament C of the first ani|)lifying 

 tube/. Battery 9 is in the grid circuit. 

 so as to adjust the relay to its best 

 magnifying condition, and battery B is 

 used to heat the filament (or cathode) C. 

 Wing circuit battery 1 1 has one terminal 

 attached to the filament and the other, 

 through the primary of transformer 12, 

 to the wing or anode .1. Tlie i)osili\e 

 side of II is connected to A; the proper 

 polarity of the other two batteries of 

 the first tube is to be found by trial. 



The second amplifying tube // is 

 connected in the same way. Condenser 

 15 serves to tune the secondary of 

 transformer 12; luiKeries 16 and 17 

 take the places of and il, respectively. 

 The plate circuit from 17 to .1 includes 

 the primary coil of the third transformer 



18, whose secondary is tuned sharply to 

 the incoming waves by means of con- 

 denser 19. Wires 25 and 26 run to the 

 grid and filament of the third vacuum 

 tube ///, which is arranged to rectify 

 and "detect" the desired signals instead 

 of merely amplifying them. It will be 

 noted that the grid circuit contains a 

 small series condenser 27, which is 

 shunted by a variable high resistance 28 

 and a polarizing battery 29; it is through 

 the co-operation of these three elements 

 that the third bulb is adjusted to rectify 

 the signal waves and so to produce pul- 

 sating response-currents in the telephone 

 recei\er 33 and condenser 34. 



It is not necessary to use inductively 

 coupled circuits as shown in Fig. i. If 

 the tubes are interlinked by suitably 

 designed and tuned auto-transformers, 

 as in Fig. 2, the same results will be ob- 

 tained. By comparing the two diagrams 

 the similarity of the various parts ma>- be 

 seen; the main difterence lies in the 

 substitution of single coils and condensers 

 such as 55 and 60 for the two-coil 

 couplers and capacities typified by 12 

 and 15. As indicated by the switch in 

 Fig. 2, the telephone condenser 62 (or 

 34) is not essential. 



The high degree of tuning anticipated 

 by the use of this entire arrangement is 

 gained by the successive selectivity of 

 a series of tuned circuits. If each tuned 

 circuit is adjusted to cause a response to 

 the desired signal ten times as loud as 

 to that which is causing interference, 

 and if the desired signal is amplified 

 five times in intensity by each relay 



m 



Fig. 2. 

 linked 



Here the tubes arc inter- 

 by tuned auto-transformers 



tube while the interfering signal is not 

 amplified, it is evident that the final 

 response will contain little of the un- 

 desired disturbances. In a normal 

 rei-eiwr, lia\ing the same sclcctiveness 

 for iiiiiiviilual steps, and not amplifying, 



