Patent GB-1014756-A (GB)
Title: Improvements in or relating to receivers for use in aircraft landing systems
IPC: H04L12/54
Filed: 1962-09-21
Abstract: 1,014,756. Automatic gain control; clipping circuits; pulse modulation circuits; radio navigation. CUDER-HAMMER Inc. Sept. 21, 1962 [Oct. 13, 1961], No. 36038/62. Headings H3Q, H3W, H4D and H4L. In a radio navigation system utilizing a scanning beam pulse-coded in accordance with the beam angle from a predetermined reference angle, a receiver for decoding the received pulses incorporates a variable-threshold gate the threshold level of which is varied so as to -maintain the number of pulses passing therethrough substantially constant. The embodiment relates to an elevation-scanning beam system as described in Specification 992,543 in which the pulse coding is pulse spacing, the carrier frequency is 16 Gc/s. and beam width some � degree; the video pulses from the receiver demodulator are fed to a circuit 41, Fig. 5, which generates successive sweep voltage of amplitudes proportional to pulse spacing and applies them to a comparator 42 also fed with pulses from a gate 43 and an angle memory signal from a unit 32. The error signals from the tracking comparator are integrated in a unit 45 and thence correct the stored signal. The pulses from line 30, Fig. 6a or Fig. 7a, are also fed to a coincidence gate 46 controlled by a circuit incorporating a thresholdbiased gate 47 which passes only pulses exceeding its threshold 58-59, Fig. 6a, or 58<SP>1</SP>- 59<SP>1</SP>, Fig. 7a. A circuit 48 produces pulses of substantially constant amplitude 61, Fig. 6b, 61<SP>1</SP>, Fig. 7b; an integrator 49 produces a signal varying with the number of pulses per beam reception and this signal controls the threshold of gate 47 to maintain the number of pulses passing therethrough constant. Upper and lower limits of threshold bias are provided. The output on line 43 is thus as shown in, e.g., Fig. 6c or Fig. 7c, and may also be utilized to develop a voltage to automatically control the gain of the receiver pre-detection circuits. The upper limit set to the threshold bias inhibit loss of pulses or beam transmissions at low angles; and the lower limit thereof eliminates the possibility of measurement on side-lobe reception at high angles. In a system employing several beacons, which are then coded for identification by pulse pairs, Fig. 3b (not shown), it is preferred to pass only the anglecoding pulses to the threshold control circuit. Detailed circuit arrangement, Fig. 8.-The angle-coded pulses are fed to a cathode-coupled triode pair 71, 72 and threshold control is obtained by impressing a D.C. control voltage to the grid of triode 72; the amplification in the triodes is sufficiently high to ensure that pulses exceeding the threshold level are at a constant amplitude on line 75. These pulses are fed to a cathode follower 82; a potentiometer 83 permits adjustment of the lower limit of threshold control. In the absence of pulses diode 85 conducts and diode 87 is non-conducting; positive pulses on line 75 cause diode 85 to be cut off. Diode 87 then conducts and a capacitor 79 charges linearly. When the number of pulses on line 84 decreases sufficiently capacitor 79 discharges and more pulses are allowed, by the corresponding change of bias at the grid of valve 72, to appear on line 75. The threshold level is thus controlled to give a constant number of pulses on line 75. A potentiometer 92 permits adjustment of the upper limit of threshold control. The constant-amplitude pulses on line 77 are applied via a diode 93, and the input pulses on line 30 are applied via a diode 97, to a cathode follower 95; the diodes form a coincidence circuit. The output pulses, on line 43, vary in amplitude in accordance with beam shape in the central portion thereof, Figs. 6c, 7c.