Patent GB-603284-A (GB)
Title: Improvements in radar devices
IPC: G01S1/02
Filed: 1944-08-16
Abstract: 603,284. Radiolocation thermionic valve circuits. WESTERN ELECTRIC CO., Inc, Aug. 16, 1944, No. 15671. Convention date, Aug. 19, 1943. [Classes, 40 (vi) and 40 (vii)] In a pulse echo ranging system with a cathode-ray tube display, the range is determined by shifting the trace by a directly-calibrated shift potentiometer until the echo signal is brought into alignment with a fixed index mark. The general principles are explained with respect to Fig. 1 (not shown), in which the time-base voltage is applied to one X plate of a cathoderay tube and the voltage from the calibrated potentiometer is applied to the other X plate, but in the preferred embodiment the time-base voltage is applied in push-pull to the X plates through a direct-coupled paraphase amplifier and the shift is produced by varying the earthed tapping point on a calibrated range potentiometer connected across the floating H.T. supply for the time-base circuit. The timebase is generated by charging a condenser through a resistance and means are described for obtaining a linear calibration with an exponential charging voltage and also for expanding the sweep. In Fig. 2, a negative synchronizing pulse from the pulsing circuit 44 triggers the flip-flop circuit comprising the two sections 61 and 62 of the double triode 60 with a direct connection between the anode of 61 and the grid of 62. Triode 61 is unbiassed and normally conductive and triode 62 is cut off by the cathode bias derived from the potentiometer. 70, 114. The trigger pulse reverses these conditions for a period determined by the time constant of condenser 78 and resistance 69, producing a negative square wave at the output of triode 62 which is taken from the junction of the load resistances 75 and 76, resistance 75 being shunted by the condenser 77. The time-base condenser 22 is held discharged by the normally conductive valve 31 whose grid leak 83 is returned to H.T. + and the negative square wave from the flip-flop cuts the valve off allowing the condenser to charge through resistance 21. The sweep voltage commences slightly after the trigger pulse from the pulsing circuit 44 due to the delay in the multivibrator and this may be reduced by applying an additional negative trigger pulse from the pulsing circuit to the grid of the time-base valve 31. The initial voltage across condenser 22 is adjusted to 12 volts by means of the rheostats 203 and 204 in the anode and screen circuits respectively. The floating H.T. supply 17 for the time-base circuit is shunted by the resistance 19 and potentiometer 18, the variable tap 23 of which is earthed. The time-base output is directly coupled to the paraphase valve 48 of the direct coupled paraphase pushpull pair 48, 49 which are provided with cathode feedback to improve their linearity and which are supplied by the H.T. source 200, the negative end of which is earthed. The outputs are directly coupled to the X plates 12 and 13 of the cathode-ray tube 11 and the potential divider 88, 89 is such that the outputs are balanced and linear for an input voltage swing on the grid of valve 48 of plus or minus 8 volts about an initial value of 12 volts at which point the anodes of valves 48 and 49 are at the same A.C. potential as illustrated in Fig. 3 (not shown). Thus, when the range potentiometer is set at zero, i.e. negative line of the time-base circuit earthed, the initial position of the cathode-ray spot is in the centre of the screen and the trace sweeps from the centre to the right as illustrated in Fig. 4, which shows the ground pulse 90 aligned with the index 16 and followed by echoes 91 and 92. When the tapping on the range potentiometer is moved up, the negative line of the time-base circuit is made negative with respect to earth and the initial D.C. input to valve 48 is correspondingly reduced causing the trace to shift to the left, and if the shift is continued until an echo 91 is aligned with the index 16, Fig. 5, the range may be read off the potentiometer scale 24. The index may be marked on the inside of the cathode-ray tube screen to avoid parallax or may be provided by periodically shorting the X plates by a relay 140 operated by a 60 c/s. source 141. If the increase in voltage across the time-base condenser is a small fraction of the line voltage,' the sweep is approximately linear and a linear scale 24 may be used. Under these conditions, temperature compensation is obtained by using a time-base condenser 22 with a temperature coefficient equal to and of opposite sign from the temperature coefficient of the range potentiometer 18. If it is desired to observe longer ranges, the range potentiometer is used for the first portion, e.g. up to 20,000 yards, and then the range of more distant echoes read off a scale on the screen. A linear potentiometer scale may be used for longer ranges by using an exponentially varying potentiometer or by adding a compensating resistance 109 between the slide and the top of the linear potentiometer. For more accurate range determinations, the time-base is expanded about the balance point of 12 volts input to valve 48 by closing switches 100 and 101. Switch 100 connects a rheostat 102 in shunt across the cathode feedback resistances 103, 104, thereby increasing the amplification and switch 101 connects the junction of the potential divider comprising resistance 107 and resistance 106 in series with rheostat 105 to the grid of valve 49 to increase the attenuation of the input from valve 48 and to maintain the initial D.C. input at 12 volts. If the expanded time-base is adjusted by the rheostat 105 after setting up the normal trace, the range potentiometer calibration holds for both time-bases ; a scale covering the expanded portion of the time-base may be provided on the screen. A negative square wave from the flip-flop is applied to the cathode-ray tube through a blanking amplifier 131 to blackout the flyback and a conventional common aerial system with T.R. box is also shown. Suitable values for all circuit components are given.