Patent GB-672863-A (GB)
Title: Improvements in and relating to radio ranging systems
IPC: G01S7/282
Filed: 1947-02-28
Abstract: 672,863. Radiolocation; valve super-regenerative circuits; automatic gain control. PHILCO CORPORATION. Feb. 28, 1947 [March 1, 1946], No. 5824/47. Classes 40 (v) and 40 (vii). [Also in Group XXXV] In a pulse radar the delay of a reference pulse relative to the transmitted pulse is progressively increased until it coincides with an echo pulse when the delay is abruptly reduced either by reducing the delay of the reference pulse or by delaying the transmitter trigger pulse; the progressive increase of the relative delay is then resumed and the process is repeated so that the reference pulse repeatedly sweeps out to an echo pulse and " bounces " back a short distance, the maximum delay of the reference pulse being measured to produce an indication of the range of the target producing the echo pulse; alternatively the reference pulse may search inwards starting with a maximum delay in which case the minimum delay provides the range information. The invention is described as applied to a system with a super-regenerative receiver operating in the linear mode, the reference pulses being used as the unquenching or sensitizing pulses. Fig. 1 shows a block schematic circuit of one embodiment of the invention the corresponding waveforms being illustrated in Fig. 3. The square-wave output D from the 2 kc/s. pulse recurrence frequency (P.R.F.) multivibrator 55 is differentiated at 56 and the positive pulses of the resultant waveform E are inverted and shaped to give negative pulses A which are further amplified and inverted and applied to modulator 57 to trigger transmitter 58 which is coupled to the aerial 60 through T-R switch 59, echo pulses B being applied via the T-R switch, frequency-changer 67 and single-stage I.F. (60 Mc/s) amplifier 68 to the super-regenerative receiver 66. The negative pulse A also triggers a flip-flop delay circuit 61 producing positive pulses F, the duration of which is cyclically varied in the absence of echo signals by a saw-tooth waveform from the sweep generator 69 of recurrence frequency 10 c/s and of amplitude such that the maximum duration of the pulses F corresponds to a range of 2,300 yards and the negative pulses C coincident with the trailing edges of waveform F, obtained by differentiating F and shaping the resultant waveform G, are applied to synchronize a quench oscillator 65 comprising a multivibrator having a natural frequency, e.g. 9.8 kc/s., slightly less than the fifth harmonic of the P.R.F. The output from oscillator 65 is differentiated so that each synchronizing pulse C produces a train of five narrower negative pulses which are applied as unquenching pulses to the super-regenerator 66 but the power output of the transmitter 58 is such that the receiver is insensitive to echoes produced by targets having a range greater than 16,000 yards corresponding approximately to a fifth of the equivalent range of the pulse recurrence period so that only the first unquench pulse coincident with a synchronizing pulse C can coincide with an echo and the waveform C can be taken as representing these unquench pulses. The purpose of the remaining four unquench pulses is to provide noise pulses for automatic regeneration control as described below with reference to Figs. and and 4A. When an unquench pulse 11, Fig. 3, C, coincides with an echo pulse 17, Fig. 3, B, the resultant amplified output pulse I from receiver 66 is applied to the sweep cut-off circuit 70 to reduce the amplitude of the sweep voltage output 71, Fig. 3, H, from 73 to 74 so that the next unquenching pulse 18 precedes the next echo pulse 20 and the sweep voltage increases along line 75 until an unquench pulse 19 again coincides with an echo pulse 21, the sweep voltage being again reduced at point 76 so that the peak amplitude of the sweep voltage is proportional to the range of the target producing the echo pulse and this is measured in the range indicator 79. If the target is approaching the radar, the delay of the echo pulse 20a succeeding the first coincidence condition 17, 11 will be reduced and the next coincidence 18, 20a will occur sooner, the sweep voltage being reduced from the full line 75, Fig. 3, H, to the dotted line 78. In a modification the quench oscillator 65 is omitted and the pulses C are applied directly as unquenching pulses to the super-regenerator. In a second embodiment of the invention, Fig. 2 (not shown), the quench oscillator is free-running with a recurrence frequency of 80 kc/s. and the frequency of the P.R.F. oscillator is automatically controlled to be approximately 10 c/s. above or below the fortieth subharmonic of the quench oscillator,' i.e. its recurrence frequency is equal to 2,000+ 10 c/s., so that the delay between a P.R.F. pulse and the succeeding unquenching pulse is progressively increased or decreased. In the former case the output from the super-regenerator obtained when an echo pulse coincides with an unquenching pulse is applied to decrease momentarily the frequency of the P.R.F. oscillator thereby increasing the duration of the current cycle of the oscillator and consequently delaying the commencement of the subsequent cycle so that the delay between the next P.R.F. pulse and the succeeding unquenching pulse is abruptly decreased, the delay being then progressively increased in the following P.R.F. cycles until a coincidence occurs again. The maximum delay between a P.R.F. pulse and its succeeding unquenching pulse is thus a measure of the range of the target producing the echo. In this embodiment the power output of the transmitter is such that the receiver is insensitive to echoes produced by targets having a range greater than 2,000 yards corresponding to approximately <SP>1</SP>/ 40  of the equivalent range of the pulse recurrence period so that only the first unquenching pulse following a P.R.F. pulse can coincide with an echo pulse, the remaining thirty-nine unquenching pulses occurring before the next P.R.F. pulse being used to provide " noise", pulses for automatic regeneration control as described. below with respect to Figs. 7 and 7A. The circuit diagram illustrating the first embodiment is shown in Figs. 4 and 4A, in which the square-wave output D from the P.R.F. multivibrator 96, 97 is differentiated at 99, 100 (waveform E), inverted and amplified at 136 (waveform A), further inverted and amplified at 101 and applied through cathode follower 102 to trigger the thyratron modulator 104 which produces a pulse waveform by means of the resonant choke 105 which is shaped by the artificial line 106 and applied via lead 108 to the pulse transformer 107 to trigger the magnetron 109 coupled to the aerial 111. Echo pulses received by the aerial are applied via T-R switch 112, crystal mixer 178 and LF. buffer stage 180 to the tuned circuit 54, 184 which is coupled to the tank circuit 182 of the super-regenerator, i.e. the left-hand side of double triode 181. The frequency of the local oscillator 177 is controlled automatically by the circuit comprising valves 297-302 (see Group XXXV). The negative pulses A from valve 136 are also applied to trigger the delay flip-flop 115, 116, and in the absence of received echo signals the duration of the positive pulse output F from valve 116 is cyclically varied by a 10 c/s. sawtooth waveform applied to the grid of valve 115 from the thyratron sweep generator 124. Waveform F is differentiated at 117, 118 and the negative pulses of the resultant waveform E coincident with the trailing edge of F are selected and amplified by valves 119, 120 and applied as negative synchronizing pulses, waveform C, to the quench oscillator which comprises a multivibrator 121, 122 having a natural frequency, e.g. 9.8 Mc/s., slightly less than the fifth harmonic of the P.R.F. so that each synchronizing pulse initiates four successive quench oscillator cycles each having a recurrence period corresponding to the natural frequency of the oscillator, the fifth cycle being cut off before completion by the next synchronizing pulse, Figs. 5, A-C (not shown). In order to increase the permissable frequency variation of the quench oscillator, the circuit constants may be adjusted so that the voltage on the grid of valve 121 during the positive half cycles decays substantially linearly instead of exponentially so that when the positive half cycle is terminated prematurely by a negative synchronizing pulse, the succeeding negative half cycle is correspondingly reduced, Figs. 5, D-G (not shown). The super-regenerator is normally quenched by a damping circuit comprising a normally conductive double diode 186 and an inductance 185 which is coupled to the tank circuit 182 and the square-wave output from the quench oscillator is differentiated at 140, 141, the negative peaks of the resultant waveform being applied via lead 123 to block the double-diode and unquench the super-regenerator as described more fully in Specification 672,862. When an echo pulse coincides with an unquenching pulse, the oscillations in the super-regenerator tank circuit build up to a large amplitude and the resultant wave pulse 188 is rectified by the diodes 190 to give a positive D.C. pulse which is amplified at 193, 194 and applied over lead 128 to trigger the sweep cut-off thyratron 129 which partially discharges the sweep condenser 125 thereby reducing the amplitude of the delay voltage applied to the flip-flop 115, 116. The delay voltage is also applied through an integrating circuit 132, 133 and cathode follower 134 to the range indicator 135 which indicates the maximum peak value of the delay voltage. When the system has " locked " on to an echo, the succeeding variations of the delay voltage are relatively small and in order to test for the presence of more distant echoes, the " out " switch 130 is closed to render the sweep cut-off valve 129 inoperative, thus allowing the delay voltage to increase towards its maximum