Patent GB-590701-A (GB)
Title: Radio locating and signalling systems
IPC: G01S13/78
Filed: 1944-11-02
Abstract: 590,701. Radiolocation; pulse modulation. STANDARD TELEPHONES & CABLES, Ltd. Nov. 2, 1944, No. 21519. Convention date, Nov. 6, 1943. [Class 40 (v)] A self-impulsing oscillator which mav be used to transmit or receive in a pulse communication system is arranged to transmit intelligence by variation in the pulsing rate while the reception of intelligence is detected by the triggering of the oscillator by received pulses of a differing rate. In the application of the invention to a radar system, Fig. 1, the ground station comprises a squegging oscillator 15 transmitting pulses 34a &c., Fig. 3, from aerial 14, the pulse recurrence frequency being determined by the time constant of condenser, 22 and resistor 23 and the grid bias applied over resistor 30 from the potential divider 32 which controls the critical triggering level 35 of the oscillator with respect to the grid voltage. The sweep voltage 33 from the grid circuit of the oscillator is clipped in the limiter 48 to produce the sweep voltage 50 which commences at the end of each transmitted pulse and is fed to plates 54, 55 of the oscilloscope 46 to produce the time base trace on which are displayed echo signals such as 12a from the aircraft 12 which are received by aerial 41 and receiver 40. Receiver 40 is normally blocked by a source of blocking voltage 54 but is unblocked following each transmitted pulse for a period corresponding at least to the time base sweep voltage by a voltage wave 58 produced by clipping and shaping the transmitter grid voltage 33 in circuit 56. Keying alteration of the pulse rate is produced by closure of key 60 which decreases the negative bias on the grid connection 24 and reduces the triggering level to 35a so that pulses 34d to 34h are produced at a higher repetition frequency. The slope of the oscillator grid voltage curve 33a, is not affected by the keying so that the sweep voltage fed to the oscilloscope still enables range to be determined to the same scale. The craft 12 to which. the communication is directed carries a transmitter receiver device 13, Fig. 2 (not shown), which comprises. an oscillator circuit substantially identical with that of Fig. 1. , The pulse recurrence of the device 13 is so adjusted by means of the variable grid condenser that the transmitter is synchronised with the unmodulated pulse recurrence of the ground transmitter or one of the pulse recurrence rates corresponding to keying. The locally produced pulses 64 and the received transmitted pulses are then simultaneous and no tone is heard in the telephones 38 which are connected via filter 37 to the averaging circuit 29, 29a in the anode circuit of the oscillator valve. When the transmitted pulses are keyed to 'the second rate indicated by pulses 34a-h, the received pulses 340d-h are not simultaneous with the normal pulsing periods 64 and those received pulses which cause the receiver oscillator grid voltage to exceed the triggering level 67 produce pulses 65 which are spaced by an interval such as t3 from the preceding pulse which differs from the unmodulated spacing t1. The receiver output pulses 69 are thus not uniformly spaced and contain a component corresponding to a beat frequency between the two pulsing rates which are themselves several hundred times greater than the keying rate. The beat frequency is passed by the filter to the headphones where it is audible as dots or dashes of tone corresponding to the 'keying. Signallings may also be carried out in similar manner from the transmitter-receiver equipment 13 in the craft to the ground station, the operation of the circuits being identical with the operation described, a beat frequency being produced in the oscillator circuit 15. Speech communication is obtained by connecting the circuit of microphone 81 and the bias source C2 in place of the keying circuit and bias source C1, by means of switch 31. The bias is preferably such as to produce a mean triggering level 85a, Fig. 4 (not shown) about midway between the triggerable limits of the receiving circuit. Voice modulation with given limits above and below the mean triggering level thus causes the transmitted pulses to be produced at rates which are representative of the modulating amplitude and which enable all pulses to trigger the receiving oscillator circuit; the receiving oscillator having a bias and triggering level corresponding to the pulse recurrence rate of the transmitter with the microphone switched out. The output pulses from the receiving oscillator are thus controlled in rate by the voice modulation applied at the transmitter and by virtue of the averaging circuit and filter in the output of the receiving oscillator, the modulation frequency is extracted as indicated in Fig. 5 (not shown). In a modified form of the transmitter receiver oscillator, Fig. 6 (not shown) the time constant of the grid circuit is changed for keying. This circuit cannot be employed in the arrangement of Fig. 1 as the speed of sweep fed to the oscilloscope would vary with keying.