Patent GB-724925-A (GB)
Title: Delay controlled pulse generator
IPC: G01S7/282
Filed: 1953-01-30
Abstract: 724,925. Radar; pulse-generating circuits. MORLE, C. W. (Philco Corporation). Jan. 30, 1953, No. 2727/53. Classes 40 (6) and 40 (7). An arrangement for producing a train of pulses comprises a pulse generator capable of being triggered by trigger impulses except during a first time interval of predetermined duration following the generation of a pulse, delay means delaying the pulse signals generated by a second time interval longer than the first, and connecting means applying the delayed signals to the pulse generator as trigger impulses. The embodiment described consists of a radar system capable of distinguishing between fixed and moving objects in which a pulse generator 1 generates pulses passed over line 3 to modulate transmitter 4. Echo signals are received by receiver and detector 7, rendered inoperative for the duration of the transmitted pulses over line 8, and mixed in unit 11 with pulses of larger amplitude direct from generator 1. The combined signal is used to modulate at 13 a carrier frequency suitable for passage through supersonic delay line 15 which introduces a delay equal to the required pulse spacing. The output of the delay line is amplified at 16 and detected at 22 to give a delayed signal equivalent to that fed from mixer 11. The delayed signal is fed to discriminator 28 which separates the delayed generator pulses and applies them over line 2 as trigger pulses back to the generator to initiate the next pulse after a time determined by the delay line. Signals from detector 7 are also applied to comparator 25 where they are mixed in opposition with echo signals received during the previous pulse period and delayed in line 15. The output of the comparator comprising signals corresponding to moving targets only is fed to an indicator over line 26. Short delay device 23a is introduced to compensate for the slight delay occurring in the triggering of generator 1 and ensures the echo signals are delayed exactly one pulse repetition rate period before application over line 23 to comparator 25. In a preferred form of generator 1 a multivibrator circuit including valve 30 is arranged to generate pulse signals with a period of recurrence greater than the delay introduced by line 15 and is triggered to generate pulses at the required rate. The triggering circuit includes means preventing the triggering of the multivibrator before a period. equal to half the desired, pulse period has elapsed in order to prevent a spurious impulse causing a doubling of the frequency of the output pulses. The output of the multivibrator is drawn from the anode of the left-hand section through isolating stage 47 and fed to units 4 and 11. Triggering impulses from detector 22 are applied to the grid of amplitude selector valve 40. This is biased with a time constant equal to several pulse periods to prevent signals smaller than the required triggering pulses (e.g. the echo pulses) to render the tube strongly conductive. It is also supplied with low anode and screen voltages to limit the output pulse amplitude and increase the sharpness of cut-off. Trigger pulses developed across anode resistor 41 are differentiated to produce a sharp negative trigger impulse which is applied to the grid of the left-hand section of the multivibrator. The potential across condenser 35 however, coupled with the operating conditions of valve 40, ensures that no triggering pulse fed in over line 27 can be effective until at least half the pulse period has elapsed. After such time triggering of the generator becomes progressively easier as the charge across the condenser leaks away. Reference has been directed by the Comptroller to Specification 578,690.