Patent GB-226652-A (GB)
Title: Improvements in multiplex signalling systems
IPC: H04J1/12
Filed: 1923-11-12
Abstract: 226,652. Western Electric Co., Ltd., (Western Electric Co., Inc.). Nov. 12, 1923. Wired wireless systems.- In order to increase the number of carrier frequencies applied to a single circuit, these are so chosen that the second harmonics and the sum and difference frequencies, produced by the non-linear characteristics of the amplifying tubes employed, fall in the frequency spectrum substantially halfway between adjacent carrier frequencies. Preferably the selected carrier frequencies are the odd harmonics of a common fundamental frequency, say 125 cycles per second, derived from a distorting vacuum-tube generator. In addition means are provided for adjusting the initial phase-angles of each of the carriers in such a manner as to reduce the amplitudes of the various interfering or secondary frequencies to a minimum. The harmonic generator may be any known type of vacuum-tube distorter. As shown in Fig. 2, two vacuum tubes 10, 11 are connected in parallel across the input transformer 14 carrying the fundamental frequency from the source S. The even harmonics are balanced out across the output transformer 15, leaving the odd harmonics to enter the line 20, Fig. 1, where HG, represents the harmonic generator just described. Here the various harmonics are selecbed by fitter circuits, F<1>, F<2>, F<3>, and pass through phase-changers PC<1>, PC<2>, &c. to amplifiers A<1>, A<2>, &c. Signal frequencies are then impressed by means of keys T<1>, T<2>, &c. Band filters BF<1>, BF<2>, &c., of the Campbell type, shown in detail in Fig. 3, convey the modulated waves to the output line L. Each phase-changer PC', &c., comprises a circuit in which provision is made for adjusting the value of inductance in series and capacity in shunt, or vice versa, so as to secure any desired phase relation between voltage and amperage. The method of estimating the relative phase adjustments of the various carriers necessary to reduce to a minimum the resultant amplitude of the secondary or interfering frequencies is described in detail in the Specification, and involves a certain degree of trial and error. Fig. 7 shows a switching device used with the phase-changers PC<1>, &c. for connecting graduated amounts of inductance and capacity in series or in shunt, between the input lines 47, 48 and the output terminals 49, 50. The four switches 25, 26, 27, 28 are rotated bv a common shaft. In the position shown the current is in phase with the electromotive force as the conductor 48 is directly connected to conductor 50 through segment 35, switch arm 33, conductor 39, switch 26, switch arm 30, and segment 32, whilst conductor 47 is joined through lead 51, switch arm 29, and segment 31 to the conductor 49. If however the switch is rotated clockwise through 90‹. the whole of the inductance L<5> is inserted in series between the conductor 48 through segment. 37, switch arm 34, conductor 43, contact 7 of switch 27, switch arm 46, switch arm 30, and segment 32 to conductor 50. Simultaneously the large condenser C<5> is shunted across the line from junction 52. switch arm 42 (resting on contact 7), condenser C<5>, lead 39, and segment 38 to conductor 47. Under these conditions the current will lag the voltage by 90‹. As the switches are advanced step by step the values of inductance and capacity are progressively varied, a change from shunt to series taking place when the switch arms 45 and 41 contact with the second studs of switches 27 and 26 respectively. The switch 28 serves to reverse the outgoing connections so that the phase angle can be adjusted progressively through the full 360‹.