Patent GB-790778-A (GB)
Title: Automatic frequency control systems
IPC: H03L7/14
Filed: 1955-02-17
Abstract: 790,778. Radio navigation. SPERRY CORPORATION. Feb. 17, 1955 [March 25, 1954], No. 4800/55. Class 40 (7). Relates to a Loran receiver of the type described in Specification 761,524 and U.S.A. Specification 2,651,033 for measuring the time interval between two trains of signal pulses of equal repetition rate transmitted respectively from spaced A and B stations in which a first locally generated pulse train A<SP>1</SP> derived from an oscillator of higher recurrence frequency is maintained coincident with the received A signal pulses and a second locally generated pulse train B<SP>1</SP> derived from said oscillator by means of a calibrated variable delay device is maintained coincident with the received B signal pulses. The system is set up initially by manual adjustment of the frequency of the oscillator (i.e. " drift control ") to align the A and A<SP>1</SP> pulses and then by manual adjustment of the delay device to align the B and B<SP>1</SP> pulses. According to the present invention the alignment of the signal pulse train of greater amplitude with its corresponding train of local pulses is then effected automatically by generating first and second control voltages proportional respectively to any subsequent displacement between A and A<SP>1</SP> pulses and any displacement between the Band B<SP>1</SP> pulses by means of a single discriminator and selecting the first or second control voltage to control the frequency of the oscillator according to whether the A pulses are of greater amplitude than the B pulses or vice-versa. The alignment of the weaker signal pulse train with its corresponding train of local pulses is then maintained by manual adjustment of the delay device. As described in Specification 761,524, positive video A and B pulses, Fig. 3U, are applied to a gating circuit 350 (" gain synchronism ") controlled sequentially by the positive A<SP>1</SP> and B<SP>1</SP> pulses 145, 146, Fig. 3T, and the sampled A and B pulse outputs, Fig. 3X, are applied respectively to storage capacitors 352, 353 by a relay 130, 131 controlled by a square-wave D derived from the timing circuit 25 to give D.C. voltages Y and Z of amplitude proportional to the A and B pulses respectively so that at relay contact 130 there appears a square-wave voltage BB whose amplitude and polarity relative to that of waveform D depends on the relative amplitude of the A and B pulses; the waveform BB is applied through circuit 382 to balance the amplitudes of the A and B pulses in the receiver 12 and the voltages Y and Z are applied to an A.G.C. circuit 356 to provide a gain-control voltage of amplitude proportional to the weaker of the A and B pulses. In accordance with the present invention positive video A and B pulses, Fig. 3U, from a phase inverter 425 are differentiated at 424 and the resultant signals, Fig. 3V, applied to a gating circuit 426 controlled sequentially by positive A<SP>1</SP> and B<SP>1</SP> pulses 145, 146, Fig. 3T, from circuit 422, the sampled A and B outputs being applied respectively to storage circuits 428- 430, 431-433 by a relay 121, 122 controlled by the square wave D to give the first and second D.C. control voltages, the appropriate one of said control voltages being selected by a relay 434 and applied through a cathode follower 452 to a reactance tube 48 which controls the frequency of the oscillator in the timing unit 25. The operating coil of relay 434 is in the anode circuit of a valve 438 whose grid is controlled by a negative voltage produced by rectifying at 442 the resultant waveform obtained by adding waveforms D and BB such that when the received A and B pulses are of equal magnitude or when the A pulses are of greater magnitude than the B pulses, the anode current of valve 438 is insufficient to actuate relay 434 and the first control voltage from the storage circuit 428-430 is applied to the reactance tube 48 but when the B pulses are of greater magnitude than the A pulses, relay 434 is actuated and the second control voltage from storage circuit 431-433 is applied to the reactance tube 48. Potentiometer 456 in the cathode circuit of valve 452 serves as the manual frequency control during the initial alignment. The initial alignment of the local A<SP>1</SP> and B<SP>1</SP> pulses with the signal A and B pulses is effected by conventional pulse matching on a cathoderay tube having selectable slow, medium and fast time-base sweeps as described in more detail in U.S.A. Specification 2,651,033, the automatic frequency-control operates as described in Specification 668,777, and the variable delay between the A<SP>1</SP> and B<SP>1</SP> pulses is controlled as described in Specification 644,579.