Patent GB-582632-A (GB)
Title: Improvements in or relating to electrical signalling systems
IPC: H04B14/02
Filed: 1943-12-23
Abstract: 582,632. Duplex wireless signalling; pulse modulation. CANDELAND, H., and HASS, Z. K. Dec. 23, 1943, No. 21567. [Class 40 (v)] For converting phasemodulated signal pulses into width - modulated pulses, the signal pulses are applied to unblock one electrode of a valve and local pulses of fixed phase to unblock another electrode so that at a third electrode pulses occur during their overlap. Synchronization of the locally generated pulses depends on the mean degree of overlap. A complete duplex communication system is described in which either station may act as the Master station. The repetition frequency of the system is controlled by a gate oscillator 20, Fig. 1, which produces a sawtooth wave I, Fig. 2, from which phase - modulated pulses are derived in the modulator 21 and square gating pulses II used in the demodulator 32, which is also fed with signal pulses V from the receiving aerial system 1, 2 detector 30 and video-frequency amplifier 31. The demodulator output synchronizes the gate oscillator 20, operates the calling and warning device 36 and feeds width-modulated pulses VIII to low pass filter 33 and output stage 34. In the transmitter, speech current from source 22, band-pass filter 23 and amplifier 24, pass to modulator 21, the output pulses from which are shaped in 25 to produce curve IV and fed to a magnetron oscillator 26 via modulator 27, being radiated by aerial system 28, 29. Aerial system. The receiving dipole 1, Fig. 3, is at the focus of a parabolic mirror 2 and a non-parasitic dipole reflector 9 is arranged in front of dipole 1. This reflector is non- resonant at the operating frequency and its length and spacing are not critical. .Receiver. The dipole 1 is coupled by a coaxial line 3 to a resonator 4 with tuning plunger 5 and crystal detector 6 attached to the inner conductor. The crystal mounting 7 provides a R.F. bye-pass capacity to the outer conductor. The output from terminal 8 is fed to the input terminal 11, Fig. 4, of the videofrequency amplifier, the first three stages V1, V2, V3, of which are conventional resistance-capacity coupled amplifiers feeding negative-going pulses to the fourth stage V4. To avoid swamping of the pulses from the distant transmitter by those from the local transmitser, which are interlaced therebetween, the input circuit of valve V4 acts as a D.C. restorer, the grid components C1, R1 having a suitable time constant and the latter being returned to a positive point on the potential divider R3, R4. The positive pulses applied to the last stage V5 are clipped by anode current cut-off and grid current to remove noise voltages and limit pulse amplitudes. The gate and saw-tooth oscillator V9, Fig. 5, operates as a Miller oscillator, the screen and suppressor grids being coupled by condenser C2 and the anode and control grid by condenser C3. The saw-tooth wave at the anode is passed to amplifier V11 and through condenser C4 to the transmitter. The corresponding square negative pulses II, Fig. 2, are amplified by valve V10 and fed through condenser C7 to the demodulator V6, V7. If the station is operating as a Master station the frequency at which the oscillator V9 operates is controlled by the positive grid bias from potentiometer R11 through contact M of switch S1 and a voltmeter connected to terminal 12 provides an indication of frequency. When the station is a Slave station, switch S1 provides through contact S a synchronizing voltage from the demodulator V7. Incoming phase-modulated pulse signals from the video amplifier are applied to the grid of valve V6 which, with valve V7, form a triggered flip-flop circuit producing lengthened positive pulses VI, Fig. 2, at the grid of V7. The amplified gate pulses from valve V10 are applied to the suppressor grid of V7 a D.C. restorer circuit R17, V14 being provided to maintain the grid negative between pulses. For the duration of the overlap of the gate pulses and lengthened signal pulses, V7 is conducting and the resultant width-modulated pulses at the screen grid are passed through condenser C8 to a low-pass filter 33 removing the recurrence frequency and feeding speech components to the output stage 34. In a modified circuit, Fig. 11 (not shown), a separate triode coincidence valve is used the lengthened signal pulses being applied to the control grid and the gate pulses to the anode, the output and synchronizing control voltages both being taken from the cathode circuit. Synchronization. The synchronizing voltage for the control grid of the gate oscillator V9 is taken from the junction of the cathode resistors R14, R15 of valve V7 through contact S of switch S1 when the latter is in the Slave position. Frequencies above about 400 c.p.s. are bye-passed by condensers C9, C10 and a D.C. voltage is thus developed across R15 which depends upon the mean degree of overlap of the gate and lengthened singal pulses. The total voltage across R15 is also variable by the coarse and fine variable resistances R12, R13 which connect the H.T. line to R15. If synchronism is lost, in order to prevent the control voltage from falling to too low a value the valve V8 is provided, connected between the H.T. and control bias line, its grid being connected to a potentiometer R18 across the H.T. supply. The cathode and hence the control bias are thus prevented from falling below the grid potential. Calling 'and warning circuits. Calling is initiated by producing a change in the synchronizing bias sufficient to operate an alarm bell - which is provided also to provide a warning when synchronism is lost due to other causes. At a Master calling station the change of synchronizing bias at the called station is produced by short-circuiting terminals 13 to vary the frequency of the gate oscillator V9. The limiter valve V8 at the called station is rendered inoperative by switch S2. At a Slave calling station the change of synchronizing bias at the called station is produced by shifting the phase of the transmitted pulse by short-circuiting terminals 14, Fig. 6, the circuit of which is described later. The positive synchronizing voltage is applied to valves V12, V13 which are biased negatively by potentiometers R23, R24 respectively so that V12 is normally conducting and V13 non-conducting. The contacts of relay L1 in the anode circuit of V12 are normally held open and render inoperative the calling bell circuit. When synchronism is upset by the reduction of overlap between gate and signal pulses below a predetermined value relay L1 releases to close the bell circuit. The reverse effect closes contacts L2 in the anode circuit of V13 to short-circuit the winding of relay L1 and energize the bell circuit. Relay L2 may be omitted and the anode of V13 connected to the screen grid of V12 to produce the same result. Transmitter. At the transmitter the sawtooth wave I, Fig. 7, from-the gate oscillator is applied through condenser C11, Fig. 6, to the grid of valve V15 which is normally negatively biased by potentiometer R25 so that the valve is conducting only for levels of the saw-tooth input above the valve X, Fig. 7 (I), its output consisting of rectangular pulses II. The level x is modulated by the speech signals from source 22, amplifier V16 and transformer T1 in the cathode circuit of V15. The position of the leading and trailing edges of the rectangular output pulses from V15 are thus modulated and the pulses are fed to valve V 17 having in its anode circuit a resonant circuit L3, C13, R31 which is shock excited by the sharp rise and subsequent sharp fall in anode potential. The resulting heavily damped waves IV, Fig. 7, are fed to the grid of valve V18 which is biased so that it is conducting only for the duration of the positive pulse p which rises above the level Y. V18 is in series with the magnetron oscillator V19 which thus generates oscillations for the period of the pulse p. In a modification of the pulse-phase modulator the saw-tooth potentials from the gate oscillator are fed to a valve V24, Fig. 13, the steady bias of which is provided by the potentiometer R43 supplied by oppositely directed peak rectifier circuits V25, C20 and V26, C21 fed from the saw-tooth input. This arrangement enables the bias to be set to any point on the saw-tooth wave for initial phasing purposes. General. The Specification develops the relationship necessary between the interstation distance and the mean phase of the pulses transmitted from the two communicating stations to ensure the correct interlacing of transmitted and received pulses. The system may be adapted to transmit telegraph or teleprinter signals in addition to the speech channel by causing them to shift the recurrence frequency or phase of the pulses and observing the signals at the receiver as changes in the control bias obtained from the demodulator circuits.