Patent GB-599087-A (GB)
Title: Improvements in or relating to radio direction-finding apparatus
IPC: G01S1/02
Filed: 1945-06-30
Abstract: 599,087. Radio direction-finding; valve circuits. SMITH, B. S., and GIRLING, F. E. J. June 30, 1945, No. 16707. [Class 40 (v)] Relates to switched-beam D.F. systems for indicating the direction of reception of a discontinuous signal, e.g. a pulse signal, and is more particularly applicable for use in a pulse interrogator-responder system in which the bearings of responders at different ranges are displayed simultaneously on a cathode-ray tube indicator. According to the invention, the connection to the indicator is changed from one directional system to the other during receipt of the signal the change over being effected automatically a predetermined time after the occurrence of the signal. In one embodiment of the invention the two signals obtained successively from the two aerial systems are applied to the indicator through a common amplifier and the first signal is subject to A.G.C. to produce a constant output the gain being then maintained constant at the same level during reception of the second signal so that the output then obtained is proportional to the ratio of the two signals ; when such an output is applied to a deflection-type indicator, equal and opposite departures from the equi-signal condition would produce unsymmetrical deviations from the equi-signal deflection and this is rectified by deriving a deflection proportional to the logarithm of the second output. The two overlapping directional systems may be provided by two separate aerial systems or by a single aerial system the directivity of which is varied in synchronism with the signal switching. The system in Fig. 2 indicates whether or not the direction of the received pulse is within a narrow sector bounded by the equisignal lines OA, OB, Fig. 1C, produced by the aerials 1 and 2 the directive characteristics of which have respectively a maximum and a minimum along the central path OC. The aerial 1 is connected to a normally operative amplifier A1 and aerial 2 is connected to a normally inoperative amplifier A2 the outputs from the amplifiers being connected to a common receiver A3 and thence through a limiter and delay line D1 to a pulse generator L which applies delayed negative and positive pulses to amplifiers A1 and A2 to reverse their operative states. Thus the first component of the pulse is received by system 1 and the second by system 2 and the two components are compared by applying the output signal, Fig. 3C, from terminal 3 to a valve with a shortcircuited delay line connected across the anode load, Fig. 5 (not shown), so that the inverted undelayed output, Fig. 6A, is added to the delayed output, Fig. 6B, to give a composite waveform, Fig. 6c, the amplitude of the portion 12 being equal to the difference of the components. The composite waveform may be applied to a cathode-ray tube fed with a brightness pulse during the portion 12 so that only this portion is displayed. The sector AOB may be reduced by reducing the gain of amplifier A1 below that of A2. In Fig. 7 a single equi-signal axis is defined by the overlapping lobes of aerials 17 and 18, Fig. 8 (not shown), and the amplifiers A1 and A2 are switched during reception of a pulse by the switching pulses 30 and 31, Fig. 9, so that the first component of the input 21 to the receiver A3 is due to the signal 19 received by the aerial 17 and the second component is due to the signal 20 received by the aerial 18. The switching pulses 30 and 31 are obtained by producing a negative pulse 27 of the same duration as the received RF pulse but slightly delayed with respect thereto, inverting it at RSI to give a positive pulse 28 which is mixed at M2 with a delayed pulse 29 to give the positive and negative switching pulses 30 and 31 having durations equal to the coincident parts of pulses 28 and 29. During the first component of the received pulse, if the output 22 from the receiver A3 exceeds a predetermined level it produces at G a negative A.G.C. voltage 24 which is fed through the closed switch S1 and the line 25 to the gain control point of the receiver so that the output 22 is brought to a fixed level 26 independent of the received signal strength. The switching pulse 31 is also applied to render the A.G.C. circuit G inoperative and open switch S1 so that the gain control voltage built up across the condenser C remains constant during the second component of the pulse and the receiver output is then proportional to the amplitude ratio of the two components of the pulse. The A.G.C. line is normally earthed through the switch S2 which is opened by the pulse 27. In order to prevent operation of the switching by a signal less than the A.G.C. level, the negative gain control, voltage 25 is mixed at with a negative output 23 from the receiver A3 so that an output pulse 27 is only produced when both input voltages are present. The receiver output is applied through a logarithmic ampli - fier LA to the X plates of a cathode-ray tube 35 and a time-base synchronised with the interrogator pulses is applied to the Y plates, the tube being brightened only after the switching when the horizontal deflection of the spot indicates the amount by which the target is displaced from the axis, Fig. 11 (not shown). The brightening pulse 33 is obtained by delaying the pulse 29 at DL4 by an amount d to give a pulse 32 which is mixed with pulse 28 at M3. The circuit diagram corresponding to the block diagrams of Fig. 7 is shown in Fig. 10 in which V1 is the A.G.C. valve which is cut off at the changeover by applying the negative switching pulse 31b to its suppressor grid. The switch S1 is formed by the diode D3 in parallel with the normally conductive triode V3 which pass the control voltage 24 until the negative switching pulse 31c cuts off V3. The coincidence valves V5 and V6 together with the phase inverter V4 constitute the mixer M1, one output 27b being applied to cut off the diode D4 (switch S2) and another output 27a being applied to V7 (RSI) to give a negative pulse 28a and a delayed pulse 29. These two pulses are applied to the suppressor and control grids of a coincidence valve V9 to produce in the anode circuit a negative pulse 31 having a duration equal to the coincident portions of pulses 28 and 29 and this pulse is fed to one phase splitter V10 feeding another phase splitter V11 so that low impedance push-pull switching pulses 30 and 31a are produced together with other switching pulses 31b and c. The coincidence valve V8 functions in a similar manner to V9 to produce the brightening pulse 33.