Patent GB-582279-A (GB)
Title: Improvements in or relating to radio noise-reducing systems operating to eliminate noise that is displaced in time with respect to received pulse signals
IPC: G01S13/70
Filed: 1942-10-13
Abstract: 582,279. Radiolocation; valve circuits. SPERRY GYROSCOPE CO., Inc. Oct. 13, 1942, No. 14364. Convention date, March 12. [Class 40 (v)] [Also in Group XXXV] Relates to variable-phase pulse-receiving systems such as are used in reflected-pulse altimeters and radiolocation, and in pulse directionfinding systems. In order to reduce the effects of .atmospherics and stray reflections, the transmission of the received (reflected) pulse and of the atmospherics &c. is blocked except during predetermined periodic intervals of substantially the same duration as the pulses, means being provided for adjusting the phase of such intervals automatically, so that they agree with the phase of the reflected pulses. Such a blocking device is termed a " pulse gate." Fig. 1 shows the general arrangement of a distance measuring system. Oscillations of, say 2000 cycles per second, from oscillator 1 produce short pulses of the same frequency in the pulse generator 3 which modulate the short-wave transmitter 5 and are radiated at 7. The reflected waves are received at 9, amplified and detected at 11 and fed through the pulse gate 23 to the cathode-ray tube 13, or, if the switch 12 is in its lower position, direct to deflection plates 27 of the tube 13. The pulse gate is shown in block form in Fig. 4, current from the oscillator 1 passing through amplifier 37, variable phase-shifter 41, amplifier 45, clipper 49, square-wave-shaper 53, differentiator 57 and clipper 61, thereby producing pulses as shown at 63. These pulses pass over separate circuits, one containing a delay line 65, to mixers 73, 77, where they are combined with the received (reflected) pulses from 71, and thence to a comparator circuit 83 described below. The delay produced by line 65 is such that the delayed pulse 70 and the non- delayed pulse 76 are co-existent for a period equal to the duration of the received pulse. The mixers 73, 77 conduct only during the duration of the pulses 70, 76, so that atmospherics and stray reflections are excluded from the comparator circuit 83 and distance indicator 33. The output of the circuit 83 is used to control the phase shifter 41, so that the phase shift of the pulse 63 (in relation to the phase of the oscillator 1) is automatically adjusted so that the overlap of pulses 70, 76 coincides with the received pulse 72. In this condition, the system is in equilibrium and the meter 33 or tube 13 gives the distance to be measured. Fig. 5 shows the detail circuits corresponding to Fig. 4. Current from oscillator 101 passes through amplifier 37 to transformer 119 feeding the variable phase-shifter 41 comprising condenser 127 and the resistance of the valve in series, the output being taken from the midpoint of the transformer secondary and the junction of condenser 127 and the cathode. By varying the potential on lead 279, as described later, the resistance of the valve can be varied and consequently the phase-shift. (Compare Figs. 7 and 8, not shown). The phase-shifted currents pass through amplifier 45 to the clipper 49, the series grid resistance 165 serving to suppress the positive peaks and the bias on the control grid is such that the negative peaks are cut off; the output then passes to a square-wave-shaper 53 comprising a pair of diodes having their anodes connected together and through resistance 193 to battery. The squared pulse then passes through a differentiating circuit 203, 205, producing two short pulses, the negative one being suppressed by clipper valve 61. The anode of valve 61 is connected (1) through lead 225 and condenser 227 and amplifier 75 to the second grid of mixer 77, and (2) through delay circuit 265 and amplifier 69 to the second grid of mixer 73, the first grids of the mixers being connected to the circuit 247 in which the received (reflected) pulses are produced. The cathode circuits of the mixers are connected to a differential circuit comprising diodes 257, 260 and resistances 259, 273. As stated above, the mixers 73, 77 conduct only during the duration of the pulses 70, 76, and the arrangement is such that if the received (reflected) pulse is situated symmetrically between the two overlapping pulses 70, 76, both mixers conduct equally and the amplifiers 275, 277 also conduct equally, giving a zero indication on voltmeter 291. If, however, the received pulse is concurrent with pulse 76, but not with pulse 70, mixer 77 will conduct but not mixer 73, thereby disturbing the balance of the output circuits of amplifiers 275, 277 and producing a control voltage which is applied over lead 279 to the grid of the phase-shifter 41 in such a direction as to vary the phase of the pulses 70, 76 until they are symmetrical with reference to the received pulse. If the distance of the reflecting object varies, the position of the " phase gate " is automatically maintained. The distance may be indicated on voltmeter 33, on potentiometer 150, or on a cathode-ray tube connected to the output terminals 258 or 262. In order to initially adjust the " phase gate " manually to a position near the reflected pulse (after which it will be maintained automatically as described above), the cathode-ray tube, Fig. 9 (not shown), is controlled by the potential on a lead 293, Fig. 5, connected to the delay circuit 265.