GB656294A - Improvements in and relating to direction finding indicating, control and like systems employing radiant wave energy - Google Patents

Abstract

656,294. Frequency measurements. GUANELLA, G. Dec. 31, 1947, No. 35288. Convention date, May 12, 1940. [Class 37] [Also in Groups XXXVIII and XL (c)] In a radio direction-finding system in which the direction a relative to the receiver of a transmitter radiating a frequency modulated carrier is determined by heterodyning in a detector the signals received at two spaced aerials to produce a beat frequency proportional to cos a (see Group XL (c)), means are described for producing a quantity which depends both in magnitude and sign on cos a, i.e. the magnitude of the quantity is proportional to the beat frequency and the sign of the quantity depends upon which of the two signals received has the greater instantaneous frequency. The cyclic variation of the transmitted frequency may be sinusoidal or linear, e.g. triangular or saw-tooth, the modulating frequency being constant. As shown in Fig. 4, the signal received by one aerial R12 is applied directly to detectors D11 and D12 and the signal received at the other aerial R11 is applied directly to detector D11 and through a 90 degree phase shifter NO to detector D12, so that the beat frequency outputs from D11 and D12 are in phase quadrature and these are applied to the field windings of a two-phase induction motor M10 driving a D.C. generator G12, the output from which has an amplitude proportional to the beat frequency and the polarity of the output being dependent on which of the signals received at R11 and R12 has the greater instantaneous frequency. In another embodiment, Fig. 5, the signal received at R11 is applied directly and through delay line d3 to detectors D1 and D3, respectively, and the signal received at R12 is applied directly and through delay line d1 to detectors D3 and D1 so that the beat frequency outputs f1 and f3 from detectors D1 and D3 are proportional to (t 0 +b/c cos a) and (t 0 - b/c cos a), where b is the spacing of the aerials R11 and R12, c is the velocity of electromagnetic waves and t 0 is the delay time of lines d1 and d3, which is arranged to be greater than b/c. The beat frequencies f 1 and f3 are applied to circuits HI and H3, Fig. 3 (not shown), of the type described in Specification 655,575, each of which produces an output D.C. voltage of amplitude proportional to the input frequency and the two outputs are applied to the two X plates of a cathode-ray tube J so that the magnitude and sign of the resultant deflection is proportional to the magnitude and sign of cos a. When the transmitter frequency is varied sinusoidally, the beat frequencies fluctuate at the modulation frequency and a trace is produced on the cathoderay tube having an amplitude proportional to cos a. A perpendicular pair of aerials R21, R22 are coupled to the Y plates of the cathode-ray tube through a similar circuit producing a deflection proportional in magnitude and sign to sin. a so that the angular position of the resultant spot or trace indicates the value of a.