Patent GB-548518-A (GB)
Title: Carrier wave production and control for multiplex telegraph systems
IPC: H04L5/06
Filed: 1941-09-05
Abstract: 548,518. Multiplex systems. STANDARD TELEPHONES & CABLES, Ltd. Sept. 5, 1941, No. 11370. Convention date, Sept. 10, 1940. [Class 40 (iv)] [Also in Group XL]. A carrier-wave supply system comprises a source of complex waves consisting of a plurality of harmonics of a given base frequency wave and a plurality of individual valve oscillators tuned to a different harmonic of the said base frequency and adapted to be controlled and synchronized by the said complex waves. Fig. 1 shows a multi-channel carrier-wave telegraph system in which the basic frequency is supplied by an oscillator S such as that described in Specification 510,379, [Group XL], the output from which is applied through an amplifier A and filter TC to a producer of odd harmonics such as that described in Specification 477,875, [Group XXXV], feeding a load resistance 12 across which the channel oscillator generates a different odd harmonic of the basic frequency and the outputs are fed through corresponding phase shifters PS, key circuits K and band filters BF to a transformer T coupled to a multi-channel transmission line including line amplifiers LA. Thus the same complex odd harmonic control wave from the harmonic producer HP is applied to all of the channel oscillators and pulls each oscillator into a fixed phase relationship with the basic wave. This enables the phases of the separate channels to be set in a random fashion either by means of the phase shifters PS or by reversing the connections of selected channels, and so to prevent the formation of peaks which would overload the line amplifiers LA. Two forms of channel oscillator are shown in Fig. 2. Each includes a valve 11 having an anode-cathode circuit including a blocking condenser 16 and a tuned circuit 17, 18 coupled to an inductance 14 connected in the grid circuit in series with a resistance 13 and blocking condenser 15 through which the mixed harmonics are fed to the grid. In this way the oscillators are locked in step with the appropriate harmonic and the output is taken from an adjustable transformer 19. The oscillators differ in that in the upper one the grid leak 20 is connected between grid and cathode while in the lower one it is in shunt to the elements 13, 14, 15. This results in a greater control voltage being applied to the lower oscillator. This is necessary in those of the oscillators O1 ... O16 in which it is found desirable to use a large value of the ratio of C to L in the tuned circuit for considerations of frequency stability. Fig. 5 shows an assembly of two systems as in Fig. 1 which in case of failure, may obtain carrier from an alternative source. A source of mixed harmonics S1 supplies through switch 30 a first bank B1 of oscillators 01, 02 &c. which supply the individual key circuits through switches 35. A second source S2 supplies through switch 37 a bank B1 of oscillators identical with B2 and supplying the key circuits through switches such as 38. SS is a spare source of mixed harmonics. On failure of either source Sl or S2 the corresponding switch 30, 37 is thrown to switch in the spare source SS. On failure of one of the oscillators 01, 02 &c. in any bank, the appropriate switches 35 and 38 are thrown to supply the correct harmonic from the corresponding oscillator in the other bank. The sources S1 &c. are provided with alarm circuits 48a including a rectifier bridge 40 which holds open the relay of an alarm signal AS except on failure of the source. Similar alarm circuits 48 are provided for the oscillator circuits. A more complex arrangement for enabling substitution to be made without interrupting the service is also described (Fig. 6 not shown).