Patent GB-698909-A (GB)
Title: System for composite cathode ray tube indication
IPC: G01S7/20
Filed: 1951-06-12
Abstract: 698,909. Radio-navigation, valve gating circuits. BENDIX AVIATION CORPORATION. June 12, 1951 [June 26, 1950], No. 13931/51. Classes 40 (vi) and 40 (vii). The indicator for a ground control approach radar system consists of a cathode-ray tube on which may be simultaneously made visible two presentations of sector form, one conveying elevation-range information and the other bearing-range information. Clipping means are used to prevent overlap of the two displays controlled by one of a pair of voltages each of which is a function of the scan position of the elevation and azimuth aerial systems respectively and including means generating a saw-tooth output voltage of an amplitude dependent on the amplitude of the control voltage, means to generate an energy pulse at the instant the saw-tooth exceeds a predetermined value, and the utilization of the energy pulse for blanking purposes. Fig. 3 shows the form of presentation on the screen 26 of a cathode-ray tube. The upper display with its origin at point 12 is a sector elevation range display showing glide path 14 and the lower display is an azimuth-range display with its origin at 18 and showing the approach path 20 and runway 21. The approach and glide path markings may be marked or projected on to the screen and the range marks marked or formed electronically. As shown, both displays are clipped along the parallel horizontal lines 13 and 27. In the system described a master switch unit directs alternate pulses to the elevation and azimuth antennae respectively and gates the receiver and display tube scanning circuits in synchronism so that video signals received only by the correct antenna system are displayed and only on the appropriate display. Range marks may be added to both displays. Besides this the switching unit also applies square waveform biasing voltages to the deflection coils of the display tube to alternate the origin of the display between 12 and 18 as required and renders the azimuth and elevation display clipping circuits effective in alternation. The circuit feeding azimuth and elevation scanning antenna position data alternately to the display sweep circuits comprises azimuth voltage input terminals 106 and 107 feeding respectively the grids of valves 121 and 124 and elevation voltage input terminals 113 and 114 feeding respectively the grids of valves 122 and 125. Valves 121 to 125 are cathode followers, 121 and 122 having a common cathode circuit coupled to output terminal 123 and 124 and 125 having a common cathode circuit coupled to output terminal 126. Opositely phased squarewaveforms J and K are applied to the grids of clamp valves 108, 109, and 117 and 118 to render the two pairs of valves alternately conductive and so alternately prevent the passage of azimuth and of elevation signals to the output terminals. The clipping circuit preventing the overlap of the two sector displays consists of two parts. In the first part azimuth angle voltage applied at 129, but rendered effective only during the azimuth display scans by a gate voltage applied at 156, is used to initiate the generation by relaxation oscillator 47 of a saw-tooth of an amplitude dependent of the input signal. Valve 134 conducts during the initial part of the saw-tooth and until a given amplitude is reached giving a substantially square-wave output 138 with a fixed rear edge and a front edge determined by the input. The pulse is differentiated and the front edge of the resulting waveform passed by gate valve 142 to trigger a flip-flop circuit controlling the application of blanking pulse to grid of the display tube. The second (elevation) part of the clipping circuit is of simpler form due to the clipping being along the scan line corresponding to the horizontal and consist of a flip-flop circuit 160 together with gate valves which trigger circuit 150 to produce clipping slightly delayed on the transmitted pulse for all scan angles below the horizontal. Specification 698,940 is referred to.