Patent GB-732910-A (GB)
Title: Improvements in or relating to radio range-measuring systems
IPC: G01S13/70
Filed: 1952-01-25
Abstract: 732,910. Pulse radar; valve circuits. HILLIER, A. (Sperry Corporation). Jan. 25, 1952, No. 2173/52. Classes 40 (6) and 40 (7). Relates to automatic-ranging pulse radar systems of the type in which a strobe pulse searches cyclically between predetermined range limits until it encounters an echo pulse and then remains automatically locked thereto thereby tracking the echo. The invention is described as applied to a system in which the strobe pulse comprises contiguous early and late gate pulses which are applied together with the echo signals to a discriminator which produces a control signal when an echo pulse is coincident with the gate pulses, the control signal being applied via a " memory circuit to terminate the search and control the timing of the strobe so that the gate pulses are maintained symmetrical with respect to the echo pulse. A wide gate pulse of fixed duration and timing is applied to the discriminator to prevent operation thereof by the transmitted pulse and is also applied to the search control means together with a narrow gate pulse (formed by combining the early and late gate pulses) to make the maximum limit of the search correspond to the trailing edge of the wide gate pulse. The receiver is also blocked by a long pulse (100 Ás. duration) initiated by the trailing edge of the narrow gate pulse to prevent strong reflected signals from objects beyond the object being tracked from affecting the operation of the system. According to the present invention the time constant of the signal input circuit of the discriminator is such that it is not actuated by noise signals. As shown in Fig. 1, the video output 12, Fig. 2, from receiver 46 is applied via a cathode follower 47 and amplifier 48 to a mixer 49 where it is combined in opposition with a blanking pulse 26, Fig. 3, from a flip-flop 50, the resultant output 28 being limited in cathode follower 51 (output 29A) and applied together with the early and late gate pulses 9 and 10 to the discriminator 52. The early gate pulse 9 is generated by a blocking oscillator 43, the late gate pulse 10 is produced by applying the early gate pulse to a delay line 45 and a narrow gate pulse 11 produced by combining the early and late gate pulses at 44 is applied to trigger the flip-flop 50. The blocking oscillator 43 is triggered by applying a pulse 1 synchronized with the pulse transmission to a wave shaper 36, Fig. 5 (not shown), whose output 2 is converted to a positive sawtooth 3 at 37 which is applied to a clipping circuit 38 together with a D.C. range voltage from amplifier 56 so that the delay of the output waveform 5 from the clipper is proportional to the amplitude of the range voltage and the output 5 is differentiated at 39 and applied via pulse transformer 40, inverter 41 and amplifier 42 to trigger the blocking oscillator 43. In the absence of received echo pulses, the timing of the strobe is cyclically varied by applying to amplifier 56 a negative sawtooth 17, Fig. 4, from a relaxation oscillator 58 which results in a positive sawtooth range voltage 18. As shown in Fig. 6, the relaxation oscillator 58 comprises a thyratron 75 and the amplifier 56 comprises pentode 102 and cathode follower 112. The cathode circuit of V75 comprises a resistor 84 in series with a parallel network of capacitors 97, 99 and 116, capacitor 99 having the largest effective capacitance (see below) and the waveform 17 is produced by charging the capacitors from a negative voltage source 87 via resistor 85 and the normally closed contact of relay 62. The outer limit of the strobe sweep is determined by the timing of the trailing edge of the wide gate pulse 24, Fig. 3, which is applied from buffer stage 54, Fig. 1, to the cathode of V75 via terminal 88 and the differentiating network C89, R84, the narrow gate 11 being applied from circuit 44, Fig. 1, to the grid of V75 via terminal 76 and an integrating circuit 63, Fig. 1, comprising R77 and distributed capacitance 79, Fig. 6, the circuit constants of V75 being such that the thyratron fires only when the integrated narrow gate (waveform 30, Fig. 3) coincides with the negative pulse (waveform 31A) derived from the trailing edge of the wide gate. The waveform 17 is applied to the grid of V102 whose anode output is applied to cathode follower 112. The cathode load of V112 comprises capacitor 116 in parallel with resistor 117 so that the cathode follower output is fed back to the cathode circuit of V75 thereby amplifying the effect of capacitor 99. When the amplitude of waveform 17 reaches the cut-off point of V102, there is no amplification of the capacitance of C99 and the slope of waveform 17 becomes quite steep; the slope of the initial portion is also steep since the grid resistor 100 tends to maintain the grid voltage of V102 substantially constant when V75 is conductive. When a received echo is coincident with the narrow gate pulse an output 19, Fig. 2, from discriminator 52 is applied via inverter 59, memory circuit 60 and amplifier 61 to energize relay 62 thereby opening its contacts and stopping the charging of capacitors 97, 99, 116, Fig. 6. Two outputs 13, 14, Fig. 2, from discriminator 52 are also applied to an integrator 55 whose output is applied to the input of amplifier 56 (input terminal 96, Fig. 6), the system being such that the resultant voltage applied to amplifier 56 automatically maintains the early and late gate pulses 9 and 10 symmetrically disposed with respect to the echo pulse. The memory circuit 60 ensures that should the echo fade, searching does not commence until the expiration of a predetermined memory period, during which the gate pulses 9 and 10 drift slowly. As shown in Fig. 7, the discriminator 52 comprises two pentodes 120, 121. The video signal is applied at terminal 131 in parallel to the control grids, the fixed wide gate pulse is applied at terminal 135 in parallel to the screen grids and the early and late gate pulses 9 and 10 are applied to the suppressor grids of V120 and V121 respectively. The values of the R.C. components in the control grid circuits of the pentodes are such that they are self-biased by random noise and are rendered conductive only by signals of amplitude greater than the average noise level. The separate pulse outputs 13, 14 for integrator 55 appear at terminals 152, 153 and the combined negative pulse output 19, Fig. 2, appearing at the junction of R154, R155 connected in series between the anodes of V120, V121 is inverted by V159 (59, Fig. 1, output 20, Fig. 3) and applied to the grid of triode 166 which is normally biased to cut-off by the negative voltage source 81. The anode of V166 is coupled via terminal 175 and amplifier 61, Fig. 1, to relay 62 and is connected by switch 176 to capacitor 180 or 181 which are in series with R178 and 179 respectively between earth and H.T. +. Assuming switch arm 176 to be making contact 182, when V166 is rendered conductive by the positive pulse 20, capacitor 180 is discharged, thereby operating relay 62, but if the echo fades, capacitor 180 will be gradually charged positively through R178 and relay 62 will be deenergized after a memory period depending on the time constant of C180, R178. If it is desired to change from track to search, switch 177 is actuated to connect terminal 175 to the positively charged capacitor 181.