Patent GB-624413-A (GB)
Title: Improvements relating to electric signalling systems
IPC: H04J3/10
Filed: 1945-07-18
Abstract: 624,413. Pulse modulation. SIEMENS BROS. & CO., Ltd., and PARKS, G. H. July 18, 1945, No. 18420. [Class 40 (v)] Relates to circuits for producing a train of pulses which are duration or phase modulated to represent the amplitudes of the modulating signal at equal intervals of time, and circuits for demodulating such pulse trains to produce, as a stage in the demodulation process a train of pulses of fixed duration and occurring at equal intervals of time which are amplitude modulated so that their amplitudes correspond to the duration or phase of the input pulses. Modulating circuits.-In one embodiment, a modulating signal a, Fig. 2, is first converted into amplitude modulated pulses b. The modulating signal is applied through terminal TA, Fig. 1, to the control grid of a pentode VV, the pulses c and d being applied to the circuit through terminals TC, TD, respectively, and the output pulses b being derived from the anode terminal TB of valve VV. When a positive pulse is applied from terminal TD to the suppressor grid of VV, condenser Q in its anode circuit is unchanged, but a constant anode current then flows for the duration of the pulse, charging Q to a potential depending on the signal amplitude at the control grid of VV. The suppressor grid is prevented from rising above cathode potential by diode VZ. At the end of the pulse, Q remains charged until the next positive pulse is applied to terminal TC. This pulse renders diode VX conducting to discharge condenser Q, which is shunted by diode VW to prevent it from being charged to the opposite polarity. At the end of the pulse the anode of diode VX returns to the potential to which it is held by diode VY and potential divider RS, RU connected across the H.T. supply. The resulting negative pulses b at the output terminal TB have amplitudes depending on the amplitudes of the modulating signal at equal intervals of time. In a modification of this circuit, Fig. 3 (not shown), the modulating signal and pulses d are both applied to the control grid of the pentode which is provided with negative feed-back. The negative amplitude modulated pulses a, Fig. 5, are converted into duration modulated pulses d by applying them through terminal TA, Fig. 4, to the control grid of valve VU. Negative pulses b which commence at the termination of pulses a, and of duration greater than the gaps between pulses a, are applied through terminal TB, and diode VX, to the junction TC of charging resistance RR and condenser Q. The point TC is connected to the control grid of valve VV, valves VU and VV, having a common cathode resistance RE. Duration modulated pulses d are taken from the anode terminal TD of valve VV. When a negative pulse b is applied to point TC, condenser Q is charged, but the pulse immediately discharges it to a minimum potential determined by the anode potential of diode VY, the condenser being connected to its cathode. The condenser remains in this condition for the duration of the pulse b, the anode current of VV being cut off and the output terminal TD being at the potential of the H.T. source, curve d. Thereafter, condenser Q charges linearly through RR until the next b pulse, curve c. The potential at TD remains constant until current begins to flow in valve VV, this time depending on the current flowing in valve VU and resistance RE which depends upon the a pulse amplitude applied to the control grid of VU. When anode and screen current commences to flow in VV, the circuit suddenly reverts to the condition of full current in VV and anode current cut-off in VU, due to the coupling between the screen grid of VV and the control grid of VU. The potential of terminal TD thus falls abruptly, curve d, to form the leading edge of the duration modulated pulse d, whose trailing edge is determined by the onset of pulse b. Phase modulated pulses may be produced by the circuit of Fig. 4 by coupling the suppressor grid of valve VV to the screen grid through a condenser so that a transitron connection is produced and the output pulses terminate shortly and a fixed time after their commencement. Demodulating circuits.-In one circuit suitable for demodulating positive duration modulated pulses a, Fig. 7, they are applied to terminal TA, Fig. 6, positive pulses e being applied to terminal TE and amplitude modulated output pulses c being produced at point TC. For the duration of the modulated input pulse condenser Q is charged linearly through resistance RR so that point TB reaches a potential proportional to the pulse duration curve b, the right-hand terminal TC of condenser Q being clamped by diode VY and potential divider RQ, RS. When the pulse terminates the point TB falls abruptly to the datum value H.T. negative determined by clamping diodes VS, VU. This causes an equal fall of potential of point TC, the diodes VW, VX, VY being non-conducting and forms the leading edge of an amplitude modulated pulse c. The pulse is terminated by the arrival of pulse e which renders diodes VW, VY conducting to discharge condenser Q. The circuit comprising diode VZ, condenser QQ and resistance RN converts pulse train c to the waveform d, which is applied to amplifier VV feeding output terminals TO through a filter FF which selects the modulating signal. In one modified circuit the condenser Q is charged through a constantcurrent pentode valve circuit, Fig. 9 (not shown).