Patent GB-603896-A (GB)
Title: Improvements relating to railway communication apparatus
IPC: B61L15/00
Filed: 1945-11-03
Abstract: 603,896. Railway communicating apparatus. WESTINGHOUSE BRAKE & SIGNAL CO., Ltd. Nov. 3, 1945, No. 29242. Convention date, Dec. 12, 1944. [Class 105] For communicating between spaced railway vehicles or between such vehicles and wayside posts, the rails and the existing wayside line wires are used as a channel for carrier current which is frequency-modulated by a microphone and transmitted from or received on that vehicle which is fitted with a loop circuit or a longitudinal capacity wire for impressing the frequency-modulated carrier current on the channel ; the communication is thus canalized along the railway. As shown in Fig. 1, a brake van CO is provided with a loop circuit LC tuned to the carrier frequency by a condenser C1 and comprising a conductor extending the length of the van and connected to the wheel bogies 4, 5 which are preferably insulated at IN from the body of the van. The loop circuit gives a coupling with the rails and line wires LW and, when transmitting, is fed from a primary coil SC energized by a transmitting apparatus TA brought into action at the front contacts 19, 17 of a relay DR1 energized by a push-button PB. Preferably, the coil SC comprises separate coils over each rail and connected additively. Normally, a receiving apparatus RA is conditioned for receiving by the back contacts 48, 72 of such push-button relay DR1, the signals being collected from the rail and line wire channel and applied to the receiving apparatus RA by a receiver coil RC on the roof of the van. Power is supplied from a motor generator 7-8, the output terminals B300, N300 of which are connected to the armatures of push-button relay DR1. The transmitting apparatus TA comprises a microphone M1 applied to the control grid 10 of a pentode valve VI, the anode current 'of which acting through a capacity C2 modifies the reactance in a Hartley oscillatory circuit of carrier-current frequency comprising inductance 23, capacities 24, 25 and resistance 26, such Hartley circuit being connected at an intermediate point of the inductance 23 to the cathode 28 of a pentode valve V2 and at the two ends of the inductance 23 to the first two grids of such valve V2, the anode output of which therefore is frequency-modulated in accordance with the microphone signal. This frequency-modulated current from anode of valve V2 is amplified by pentode valves V3, V4 and fed to the primary coil SC, whereby through the loop circuit LC on the van a frequency-modulated carrier current is applied to the channel provided by the rails and line wires LW. A cold-cathode gas tube V8 serves as a voltage regulator for the anode of valve V1. The receiving apparatus comprises an amplifier AM1 for the signal received from the rail and line wire channel LW through the receiver coil RC on the top of the van and applied to the amplifier through a transformer T3. The output of the amplifier AM1 is applied through a transformer T4 to a pentode valve V5, the anode voltage of which is made low by the inclusion of resistances 51, 52 thereby to limit the amplification by such valves so that approximately uniform amplitude is obtained in the anode output of valve V5. The output of valve V5 is applied to a duo-diode valve V6 through a coupling transformer T5 with each winding 53, 56 respectively tuned to carrier frequency by a capacity C7, C8 and resistance 78, 65. The mid-point of winding 56 is connected to the mid-point of a resistance 62, 63 joining the two cathodes of the duo-diode valve 16 and accordingly a voltage arises across the resistances 62, 63 which varies in accordance with the frequency modulation of the signal. This voltage variation is amplified by a triode valve V7 and pentode valve V8 to supply a loud-speaker LS. Fig. 4 shows modifications of the transmitting and receiving apparatus. ]n the modified transmitting apparatus, a frequency - modulated low - frequency carrier current is obtained by a heterodyne beat arrangement. Thus, microphone current from a microphone M1 amplified by an amplifier AM2 is applied to the control grid 150 of a valve V13, the anode current of which is applied through a capacity 152 to vary the reactance in an oscillatory circuit of a valve V14 comprising inductance 154, capacity 155 and resistance 156 giving a frequency of 50 kilocycles per second. The voltage in the oscillatory circuit of valve 14 is thus of 50 kilocycles per second frequency and frequency-modulated by the voice and is applied to a screen grid 157 of a valve V15, the control grid of which is connected to an oscillatory circuit, comprising inductance 159 and capacity 160, of such frequency that the anode current of valve V15 resulting from the heterodyne beat is of a frequency of 7.5 kilocycles per second and is frequency-modulated at voice frequency. This current amplified by an amplifier AM3 is fed to the loop circuit LC and so applied to the rail and line wire communication channel. In the modified receiving apparatus of Fig. 4, the pick up of noise by the receiving coil is prevented from affecting the loud-speaker ; this is effected by a squelching circuit arrangement. Current in the receiving coil RC1 is amplified at AM1 and applied through a transformer T9 passing the necessary frequency band to the control grid 101 of a valve V9, the anode voltage of which is sufficiently low by the anode lead being tapped from a point B between the two resistances 110, 109 forming a voltage divider across the power supply. The valve V9 accordingly gives a constant amplitude of the frequency-modulated signal which frequency-modulated constant-amplitude current so obtained is applied through a lowpass filter LPF to the first grid 111 of a duotriode valve V10. The first-anode current of this portion of valve V10 is fed to a filter RPF and the output point D thereof is applied to the second grid of the valve V10 and the second-anode current is applied through a transformer T10 and a voice-frequency network to the first grid 120 of a duo-triode valve V11. The second grid of this valve V11 is connected to point A at the transformer T9 following the initial amplifier AM1. Picked-up noise will give a small negative voltage at point A so that the second-anode current of valve V11 will still flow and thereby cause a voltage drop in resistances R8, R9 to bias the first grid 120 of valve V11 to prevent first-anode current flow. A proper signal will, however, cause a greater voltage drop at point A and this will prevent any second-anode current flow in valve V11; accordingly, the first-grid bias at resistances R8, R9 is reduced and the frequencymodulated signal, which becomes amplitudemodulated after the filter LPF and has the carrier wave suppressed in the network BPF (transformer T10, inductance 112, capacities 113, 114 and transformer T11) is amplified by the first portion of the duo-triode valve V11 and further amplified by the valve V12 to actuate the loud-speaker LS.