Patent GB-721108-A (GB)
Title: Automatic control systems for synchronising an electric oscillator to a train of pulses
IPC: H03L7/06
Filed: 1953-05-15
Abstract: 721,108. Automatic frequency and phasecontrol systems. SPERRY CORPORATION. May 15, 1953 [May 24, 1952], No. 13646/53. Class 38 (4). [Also in Group XL (c)] Relates to synchronizing systems for automatically controlling the frequency and phase of an oscillator by sampling the amplitude of received repetitive signal pulses by pulses derived from the oscillator, and comprises means for preventing synchronization being brought about by any interfering pulses that occur shortly after the signal pulses, such for example as pulses received by a sky-wave path. The system is particularly applicable to radio navigational apparatus of the Loran type. The received radio signals, Fig. 1, are fed to an amplifier 4 and detector 5 and the detected pulses are fed to two sampling gates 6, 8, the outputs of which pass to the anodes of rectifiers 10, 11. The rectifier cathodes are connected to each other and to the control grid of a reactance tube 13 associated with the controlled oscillator 1. Sampling pulses derived from this oscillator are fed on the one hand through a fixed-delay line 7 to the gate 6, and on the other hand to the gate 8 through a circuit 9 giving a delay which is repetitively varied over a range from near zero to the fixed delay time of the line 7. The gates 6, 8 furnish an output voltage which is a measure of the amplitude of a received pulse at the moment of sampling, and the arrangement of the rectifiers 10, 11 ensures that the reactance tube 13 is under the control of that gate of the pair 6, 8 which is furnishing the larger positive output voltage. Figs. 2A ... 2F show the sequence of operations leading to the desired synchronized condition of Fig. 2F, it being assumed that the fixed delay of the line 7 and the maximum delay of the circuit 9 are 1100 microseconds, and that the interfering pulses 21 are sky-wave pulses following the desired ground-wave pulses 20 after an interval less than this delay-time. In Fig. 2A, the gate 6 is in control and has temporarily locked the oscillator on the sky pulse by means of the sampling pulse 6<SP>1</SP> which intersects the sky pulse at the synchronizing level 22. This is an undesired locking, and is upset as soon as the increasing delay of the sampling-pulse 8<SP>1</SP>, after the oscillator pulse 2<SP>1</SP>, has brought the pulse 8<SP>1</SP> to a sampling level on the ground pulse 20 equal to that of the pulse 6<SP>1</SP> on the sky pulse 21 as shown in Fig. 2B. As the delay of the pulse 8<SP>1</SP> continues to increase, it tends to increase the frequency of the oscillator 1, thereby causing the oscillator pulses 2<SP>1</SP>, 6<SP>1</SP> to occur earlier whilst leaving the pulse 8<SP>1</SP> at the synchronizing level 22 on the ground pulse. In Fig. 2D the sampling pulse 6<SP>1</SP> has reached the right-hand slope of the ground pulse and is taking control. This however is an unstable position and merely increases the frequency of the oscillator to carry the pulse 6<SP>1</SP> over to the left-hand slope of the pulse 20, where it assumes permanent control of synchronism, Fig. 2E, whilst the pulse 8<SP>1</SP> is abruptly returned to its position of minimum delay, Fig. 2F. The sampling gates 6, 8 may be of the four-diode type disclosed in Proceedings of Institute of Radio Engineers, January, 1943, page 12, and the delay circuits 7, 9 may comprise delay lines, or multivibrator or phantastron circuits. The output pulses 2 from the oscillator 1 may synchronize deflecting circuits of a C.R. tube in a Loran receiver, e.g. of the completely automatic type described in Specification 661,954, [Group XL (c)].