Patent GB-712308-A (GB)
Title: Improvements in or relating to frequency modulated wave apparatus
IPC: G01S7/28
Filed: 1952-07-21
Abstract: 712,308. Frequency-modulated radar. COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. July 21, 1952 [Aug. 2, 1951], No. 18409/52. Class 40 (7). [Also in Group XXXVI] In order to determine the curve of instantaneous variation with time of frequency modulated oscillations, the voltage is applied to a resonator of adjustable resonance frequency associated with means for producing a voltage impulse each time the frequency passes through one of two values f 1 , f 2  such that f 0 -f 1 =f 2 -f 0 =0.29f 0 /Q, where f 0  is the frequency of resonance of the the resonator and Q its Q-value, these impulses being applied to one deflector system of a cathode ray oscilloscope. As shown in Fig. 1, an ultra-high frequency generator 2 is frequencymodulated by a modulator 1 which also synchronizes a saw-tooth time base circuit 7. The field of the wave of the generator 2 is coupled to a cavity 3 of high Q whose resonance frequency f 0  is adjustable. The output current of the cavity 3 is passed through a crystal detector 8 followed by a differentiating circuit 4, the output of which has two peaks at frequencies f 1 , f 2  equally spaced on either side of f 0  and related by the equation f 0 -f 1 =f 2 -f 0 =0.29f 0 /Q. A circuit 5 suppresses the central part of the output curve and reduces it to a pair of impulses I1, I2, which are applied to the vertical deflection terminals V of a cathode ray, oscilloscope 6 whose horizontal deflection terminals H are fed from the time base 7. The mid-point t 0  between the times t 1 , t 2  at which the. impulses I1, I2 occur is the time at which the modulation frequency passes through the natural frequency f 0  of the cavity 3. The tuning of the cavity is then changed so that its natural frequency becomes f 0 , the impulses 11, 12 then appearing in new positions I1, I2 at times t 1 , t 2 , the central point to giving the time of passage of the modulation cycle through the resonance value f 0 . By taking a series of these values the desired curve may be built up point by point. The Q of the cavity 3 may be measured by noting the frequency f 0  and the positions of the impulses I1, I2 and adjusting the resonance frequency to a value f, or f, such that the centre between the new impulses I1, I2 coincides with the position previously occupied by one of the impulses I1, I2. The Q-value is then Q=0.29 f 0 /fr-f 1  or Q=0.29 f 0 /f 2 -f 0 . Fig. 4 shows a radar installation in which an impulse I3 causes the appearance of a spot on a panoramic screen. The elements 1, 2, 3, 4, 5, 7, 8 are the same as in Fig. 1. The generator 2 feeds a rotating aerial 10 through a transmissionreception switch 9, and the frequency-modulated wave reflected by a target and received by the aerial 10 passes again through the switch 9 and, together with the wave then being produced by the generator 2, is fed to a receiver 11 which supplies the impulses 13. The impulses 11, 12 of opposite polarities supplied by the unit 5 are fed through two oppositely connected detectors 15 from which the impulses 12 are passed through a valve 16 which shifts their phase by 180 degrees, without amplification. These impulses, together with the unchanged impulses I1 are fed to identical amplifier valves 18, the outputs from which are combined in a circuit 12 and superimposed on the impulses I3 from the receiver 11. The impulses I1, I2, I3 of the same sign delivered by the circuit 12 are passed to the control electrode of the cathode ray tube 13, which operates as a panoramic indicator. Electromagnetic deflection coils I4 are fed with saw-tooth current from the time base 7 and rotated around the tube 13 in synchronism with the rotation of the aerial 10. The impulse I3, which occurs when an echo signal occurs, causes the appearance on the screen of a spot T whose position indicates the direction of the target. The imnulses 11, 12 cause two concentric circles the radii of which are measures of the times t 1 , t 2 . The resonant frequency of the cavity 3 is then adiusted to a value f, such that the circles of radii t 1 , t 2  are symmetrically on opposite sides of the spot T, i e. the time t 3  of the impulse I3 is the mean of the times t 1 , t 2 .