Patent GB-546284-A (GB)
Title: Improvements in or relating to radio-echo distance measuring systems
IPC: G01S13/24
Filed: 1940-11-18
Abstract: 546,284. Radio receiving apparatus. ELECTRICAL RESEARCH PRODUCTS. Inc. Nov. 18, 1940, No. 16610. Convention date, Dec. 30, 1939. [Class 40 (v)] [Also in Group XXXVI] A frequency modulated wave is radiated and received after reflection, the reflected wave being combined in the receiver with an oscillation of mean frequency different from that of the transmitted wave but of cyclically varying frequency synchronous therewith, so as to give a wave of mean frequency equal to said frequency difference which is modulated in frequency at the same modulation frequency as the transmitted wave, the frequency deviation being proportional to the range of the reflecting surface. Frequency modulated oscillators 11, 12, Fig. 1, differ in mean frequency by an amount sufficient to produce a convenient intermediate frequency when their outputs are combined and detected. A lowfrequency oscillator 14 modulates both radiofrequency oscillators 11, 12, its frequency being variable to vary the range of the system. Variable phase and amplitude controlling networks 16, 17, 18, 19 may be provided for insuring exact correspondence between the frequency variations imparted to the oscillators 11, 12. Switches 20, 24 enable the output of the oscillator 12 to be connected directly to the detector 34 for zero adjustment. In operation the switches 20, 24 are in the position shown and the output of the oscillator 12 is connected to the transmitting antenna 30 which transmits a wave 29 towards the earth or other reflective surface. The reflected wave 31 is received on an antenna 32 and passed to the first detector 34 where it is combined with the output of the oscillator 12. The output from the detector 34 is passed through an intermediate frequency amplifier 36, amplitude limiter 38 and conversion circuit 40, where it is converted into an amplitude modulated signal whose modulation frequency is equal to the frequency of the oscillator 14, the amplitude of the modulation being proportional to the altitude or distance being measured. This signal is passed through a second detector 42, a filter 41 adjusted to pass the frequency generated by the oscillator 14, and a rectifier 43, to a D.C. meter 44 calibrated to read altitude or distance. In a modification a single high-frequency oscillator is employed, part of the frequency modulated signal being transmitted as the radiated wave while part is combined in a modulator with the output of a beating oscillator and passed through a filter to suppress one of the resulting side bands before being passed to the receiver. Alternatively the side bands may be separated by band pass filters, one side band oscillation being transmitted as the radiated wave while the other is passed directly to the receiver. Fig. 4 shows a circuit for obtaining adequate amplitude limitation of the intermediate frequency signals over a wide range of reflected wave intensities. A first detector valve 58 and intermediate frequency amplifier valve 74 are connected as shown to an amplitude limiter valve 94, which is operated at reduced plate and screen potentials so that it is readily overloaded by the intermediate frequency wave applied to its grid. The rectified voltage across a resistor 90 in the control grid circuit of the valve 94 is impressed upon the control grid circuits of the valves 58. 74 so as to reduce the variation of intermediate frequency voltage applied to the limiter valve 94. The amplitude of the intermediate frequency wave reaching the conversion circuit 40 is thus made substantially independent of the amplitude of the signals applied to the first detector.