Patent GB-594940-A (GB)
Title: Improvements in or relating to wireless receiving systems
IPC: G01S1/02
Filed: 1943-07-09
Abstract: 594,940. Radio direction-finding; automatic gain control. MOXON, L. A., and WRIGHT, C. S. July 9, 1943, No. 11198. [Class 40 (v)] In a radio receiving apparatus comprising several channels each embodying an individual continuously - acting automatic gain control system, the gains are equalized by locally-generated test signals of controllable amplitude which bring the a.g.c. systems fully into operation. The invention is applied to the maintainence of constant gain in the receivers of a twochannel " split-beam " D.F. system such 'as is used in a pulse radar system. The receiving aerials A, A<1>, Fig. 1, feed separate channels, comprising frequency changers M associated with a common local oscillator L.O. and followed by head amplifiers and I.F. amplifiers H.A. and I.F., detectors D and video amplifier V.A. The video signals are compared on a cathode-ray tube indicator C.R.O., the signals from the separate channels being distinguished by arranging them to be of opposite sign or by inserting a delay network D.L. in one channel. The detector stages D also produce automatic gain control voltages for application to the preceding I.F. amplifiers. The I.F. amplifiers are " gated " to pass only selected received signals and the test signals which are produced under the control of a pulse-generator P.G. during periods when the receivers are not required for reception. When the system is employed in a radar equipment, the pulse-generator P.G. may form part of that equipment. The " gating valve V, Fig. 2, is normally conductive and the I.F. amplifier V2 is biassed off by the voltage across the common cathode resistor R4, negative pulses applied to terminal T1, thus opening the I.F. amplifier and permitting the selected signals to pass to the detector D1. The video signals are fed via amplifier V3 and cathode follower V4, the input of which is provided with a D.C. restoring diode D2, to the output terminal T3 and to the A.G.C. rectifier D5 which is provided with a large negative delay bias. The A.G.C. voltage is fed to the D.C. amplifier V5, the output from which is obtained from the tapping on potential divider P1, the lower end of which is connected to a source of negative voltage considerably below earth potential. The output from terminal T4 which is fed to control the gain of the preceding I.F. stages is prevented from rising above earth potential by diode D4 and has A.C. components removed by condensers C4, C5. In the radar application the time constants of the A.G.C. circuits are made large compared with the received pulse durations so that the gains are controlled only by the test signals. The Provisional Specification also states that the A.G.C. rectifier may be arranged to be inoperative except for the duration of the test signal. The test signal generato T.S.G. comprises an electron-coupled oscillator, Fig. 3 (not shown), which is modulated by the pulse generator P.G. A harmonic is selected in the oscillator output and amplified at T.S.A. before being heterodyned to the radio frequency of the system in mixer T.M. by means of the local oscillator T.L.O. The test signal is then fed to the radiating element X located adjacent the receiving aerials A, A'. The gains of the receiving channels are controlled by variation of the strength of the test signal by gain control voltage fed to the test signal amplifying channel over lead M.G.C. In a modified system, described in the Provisional Specification, the outputs from the two receiving channels are opposed so that test signals give zero resultant output unless the gains in the two channels are unequal, Any resultant output is thus used to control the gain of the appropriate channel. With this method the gain of one channel is normally arranged to exceed that of the other and be reduced by the resultant control voltage.