Patent GB-594798-A (GB)
Title: Improvements in or relating to selective circuit arrangements for pulse signalling systems
IPC: H04J3/00
Filed: 1945-04-09
Abstract: 594,798. Multiplex pulse signalling. BRIDGES, D. E. April 9, 1945, No. 8791. [Class 40 (v)] A circuit arrangement adapted to respond to a pulse signal having a desired pulse recurrence frequency, includes a gating valve fed with locally generated gating impulses of a recurrence frequency less than the desired frequency until a pulse signal is applied thereto, whereupon the gating recurrence frequency is automatically increased to and maintained at the desired frequency if the signal recurrence frequency is correct or subsequently reverts to the lower value if it is incorrect. Thus, the gating pulses, curve A, Fig. 1, have an invariable time interval between the trailing edge of one and the feading edge of the next, but the signal pulses, curve B, are arranged to terminate them so that they assume the form shown in curve C of minimum width and synchronous with the desired signal pulses. For signals of undesired but adjacent pulse recurrence frequency, the gating impulses will lock in for a short period only and will then be rejected. The gating pulses are derived from a valve forming one of a ring of three delay devices round which a pulse is continuously circulating, the delay introduced by the other two being invariable and pre-set in accordance with the desired signal recurrence frequency. The pentode gating valve V1, Figs. 3 and 4, is supplied with width-modulated signal input from terminal T1 and gating pulses are applied to its suppressor grid from the output of a delay device DEL3 comprising valves V2, V3, V4. Signals in the anode circuit of valve V1 are fed to the demodulator, comprising valves V11, V12, V13 and condenser C19, and to the suppressor grid of valve V2 of DEL3 to terminate the gating pulse. The pentode V2 and diodes V3, V4 are arranged in a circuit of the type described in Specification 582,758, triggered by a negative pulse applied to the anode of V2. The other delay circuits DEL1 (valves V7, V8) and DEL2 (valves V5, V6) are similar to DEL3 but are triggered by negative pulses applied to their control grids. The triggering pulses are derived from differentiating circuits DIF1, DIF2, DIF3, fed with the positive-going square wave output from the screen grid of their associated delay circuits. The waveforms of the voltages at various parts of the circuit are shown in Fig. 5. Curve A shows the negative triggering pulse applied to the control grid of V8 and also an ineffective positive pulse. The resultant square wave at the screen of V8, curve C, the duration of which is determined by the circuit constants is differentiated at DIF1 and the resulting negative peak, curve D, triggers valve V5. The resultant square wave at its screen, curve F, is differentiated at DIF2 and the resulting negative peak, curve G, triggers valve V2. At the screen of V2, the square wave gating pulse, curve K, has a maximum duration shown in dotted lines, but the signal reduces its duration and it is differentiated at DIF3 to provide the negative peak, curve L for triggering V8 to repeat the cycle. Curves L and A are the same. The remaining elements of Fig. 3 comprise a differentiating circuit DIF4 fed from V8 and producing a pulse applied through a cathode follower V9 to a gate valve supplyingpulses for retransmission. The output of the demodulating condenser C19 is fed through a cathode follower V14 to control the amplitude of the output of a tone generator V10 from a control valve V15. Details of these circuits are shown in Fig. 4. The delay device DEL1 comprises a pentode V8, to the control grid of which is applied a negative triggering pulse from the differentiating circuit C11, R28. The screen voltage rises to 210 volts, the increase of potential being fed through C13 to the suppressor grid so that when the trigger pulse ceases, anode current starts to flow. The consequent fall in anode voltage is fed back to the control grid through C10 to linearize the anode voltage fall during the delay period. Towards the end of this period an increasing current passes to the screen and the fall in voltage at the screen is fed to the suppressor grid until the anode is suddenly cut off, the screen voltage suddenly falls, the anode voltages rises exponentially with a time constant dependent upon C10, R34, followed. by the control grid potential until grid current flows. The anode voltage rises to that of the lower end of resistance R26 at which point diode V7 becomes conducting. The circuit is now restored ready for the next trigger pulse, but is self-running at a lower recurrence frequency due to the rise in suppressor grid voltage at a rate determined by C12, R36. The square wave at the screen is differentiated in circuit C9, R22 and the resultant applied to the control grid of valve V5 which forms the delay circuit DEL2 and operates in a similar way. The duration of the delay is determined by the valves of C10, which may be selectable from a number by switching, and R28 which is made up of carbon film and wire-wound resistances whose temperature coefficients are mutually compensating. The operation of the third delay circuit DEL3 comprising pentode V2 and diodes V3, V4 is similar to that of the other two, but the trigger pulse is applied, from the screen of V5 after differentiation in C17, R57, to the anode of V2 through diode V3. The valve V2 is also provided with an undecoupled cathode resistance R11. While the positive square gating wave exists at the screen and is applied to the suppressor grid of VI, a signal pulse from V1 applied. through C3 to the suppressor grid of V2 terminates the delay and restores the valve to its steady state after an interval of a little more than three microseconds determined by R6, C28, to allow signal pulses of three microseconds width to pass through V1. Pulse demodulation. In the demodulator circuit the charge on condenser C19, Fig. 4, is varied in accordance with the signal pulse width and a D.C. potential proportional to the steady voltage thus developed across the condenser controls the amplitude of a locally generated signal. The latter is varied between zero and maximum for pulse width variation between one and three microseconds and is zero for pulse widths less than one microsecond, In the absence of signals, diode V11 is non- conducting, its cathode being at +210 volts and its anode at + 140 volts. At the beginning of a signal pulse in valve V1 its anode potential falls at the rate of 70 volts per microsecond, due to condenser C29. After one microsecond diode V11 therefore becomes conducting and C19 is discharged. At the end of a threemicrosecond pulse, V11 is cut off due to the rise in potential of the anode of VI, which carries both electrodes of C19 to 140 volts, the condenser being still completely discharged. For a two-microsecond pulse the left-hand electrode of condenser C19 falls to 70 volts and rises to 140 volts at the end of the signal, the right-hand electrode rising then to 70 volts so that the condenser is only partially discharged. The condenser is maintained partially discharged until DEL3 is triggered by DEL2 when V13 which is normally cut off becomes conducting to charge C19, a positive pulse being applied to its, grid from the differentiating circuit C21, R59. The condenser C19 is thus maintained at a potential proportional to the signal pulse width in excess of one microsecond and except during the short charge and discharge period. This potential is applied to a cathode follower valve V14 whose output controls the suppressor grid potential of V15, to the control grid of which is applied a tone frequency from a cold-cathode relaxation oscillator V10. The amplitude-controlled tonefrequency output of V15 is fed to the utilization circuits through transformers TR1 or TR2. An abrupt change of pulse width can be used to operate a relay A in the anode circuit of V15.