Patent GB-499114-A (GB)
Title: Improvements in or relating to alternating current telegraphy systems
IPC: H04L27/04
Filed: 1938-05-13
Abstract: 499,114. Code telegraphy. SIEMENS & HALSKE AKT.-GES. May 13, 1938, No. 14319. Convention date, May 13, 1937. [Class 40 (iii)] Relates to alternating current telegraphy systems of the kind in which A.C., of approximately constant amplitude, is supplied to a transmission line through a four-pole network comprising series and shunt rectifiers, and wherein there are provided means for biassing the rectifiers and a switch for producing the signals by so changing the respective biasses on the series and shunt rectifiers that the attenuation of the network may be varied from a high value A<1> to a low value A<2>. Fig. 1 shows a known system of this type, in which D.C. source 15 supplies the bias, and the variations of bias are effected by operating a switch 16. In the position shown, the series rectifiers 10, 11 present a low impedance to the alternating current, while the shunt rectifiers 12, 13 present a high impedance; with the switch in the other position, the opposite condition exists. According to the invention, the network is so designed that when the bias applied to the rectifiers is zero (as when no signals are to be sent), the attenuation of the network varies so little from either A<1> or A<2> that the receiving means in the system assumes a definite position unaltered by small variations in the amplitude of the received A.C., such as might arise from transmission fluctuations in the communication line, which might otherwise produce spurious signals. The receiving means may comprise a relay having an armature which is attracted when the amplitude of the signal current is high, and retracted when the amplitude is low, or vice versa. Fig. 3 shows one arrangement according to the invention in which, as in Fig. 1, 1 is the source of alternating e.m.f. of constant amplitude, a, b and c, d are the input and output terminals respectively of the four-pole network, 2 is the communication line, and 3 is the receiver. The biassing and switching means are denoted by 18. In the arrangement shown in Fig. 3, each of the series rectifiers 10, 11 of Fig. 1 is replaced by four rectifiers connected in parallel. With this arrangement, when no bias is applied to the rectifiers, the impedance presented by the series rectifiers 10, 11 is only a quarter of the impedance presented by the shunt rectifiers 12, 13. Thus, the attenuation of the network is so small that the amplitude of the transmitted current is sufficiently high (i.e. so far removed from the mean value of signal currents) that the receiver remains in a definite position, unaltered by small variations in the amplitude of the transmitted current. In an alternative arrangement, Fig. 4 (not shown), each of the shunt rectifiers of Fig. 1 is replaced by three rectifiers connected in series. In further modifications, for the purpose of the invention either the shunt rectifiers or the series rectifiers are connected to tappings on the secondary 8 of the input transformer. Fig. 5 shows the first of these alternatives, and also shows the biassing means connected to the shunt rectifiers through a resistor 20 and to the series rectifiers through resistors 21, 22, via the half windings (separated by a condenser 23) of the output transformer. Further arrangements are described, Figs. 7 and 8 (not shown), in which the series and shunt rectifiers are connected respectively to separate secondary windings of the input transformer 5, these windings having different transformation ratios with respect to the primary winding. The shunt rectifiers (as shown) or the series rectifiers consist of four rectifiers arranged in a bridge.