Patent GB-817486-A (GB)
Title: Improvements in and relating to direction finders
IPC: G01S3/786
Filed: 1956-02-28
Abstract: 817,486. Photo-electric control arrangements. TURCK, J. Feb. 28, 1956 [March 1, 1955], No. 6111/56. Class 40 (3). [Also in Groups XXXVIII and XL (a)] The invention relates to a photo-electric direction-finder capable of determining the misalignment between an incoming light beam and the directional axis of the apparatus and in particular to apparatus of the kind described in Specification 771,488 in which the incoming light beam passes through a modulator whose output will include a cyclic frequency modulation corresponding to any misalignment. In such a direction-finder the present invention proposes means sequentially scanning over a large field of view until the light beam is directed on to a relatively small modulator passing light to a photo-electric device. Output from this causes the large field scanning action to cease and replaces it by auto-follow means for maintaining the light beam directed through the modulator. In one embodiment incoming light is directed by adjustable mirrors 2 and 3 and brought to a focus in a plane containing a sheet 10 having a central circular aperture 11 immediately behind which is placed the modulating means 4. This comprises a flat plate having radial slots cut in it radiating from a central point (Fig. 2, not shown). The plate is mounted on rotatable eccentric supports which are driven at the same speed by motor 9 and cause the said central point to follow a circular path round the edge of, or just outside the edge of, circular aperture 11. A photo-cell 12 is mounted directly behind the modulator and its amplified output is fed both to detector c (Fig. 4) and through clipping circuit d to frequency discriminator f. The output from the discriminator is fed through a cathode follower stage to two phase discriminators h1, h2 fed also with signals  in phase quadrature derived from reference signal generator 13, 14, 15. This comprises a disc 15 driven by motor 9 and having a crescentshaped slot which modulates the light passing from source 14 to photo-cell 13. In operation generators m1 m2 supply signals through relay p to motors n1, n2 which oscillate the mirrors 2, 3 in two different planes and cause cell 12 to scan a large field of view. As soon as the light beam is received modulated energy is fed to detector c which rectifies the signal and feeds it, after amplification, to the operating coil of relay p. The generators m1 and m2 are thereby disconnected from the motors n1 and n2 and the scan stopped. Instead these motors are connected respectively to discriminators h1 and h2. If the light beam is brought to a focus at the centre of aperture 11 then the signal fed to discriminator e will be of a constant frequency and the discriminator will give zero output. If however the light beam is directed to a point away from the centre of the aperture 11 then the photo-cell output is cyclically modulated in frequency and the output of discriminator e will consist of a signal of rotation frequency whose phase is dependent on the angular position of arrival of the beam relative to the centre of the aperture. This signal is compared in phase discriminators h1 and h2 with the reference inputs and control signals are generated which drive motors n1 and n2 to cause the light beam to fall on and be maintained at the centre of the aperture 11. The outputs of the phase discriminators can be used for off-centre measurements or to control means for directing the axis of direction finder towards the light source. In a second embodiment the light is brought to a focus on the photo-cathode of a detector tube and the resultant electron beam is focused by an electron lens towards a target at the opposite end of the tube. The target itself is made up of alternate radially extending effective and non- effective sections and replaces the mechanical modulator of the previous embodiment. Two orthogonally placed pairs of deflection coils alter the direction of the beam as it passes along the tube. The first pair is fed by a reference signal oscillator (which also feeds the phase discriminators) and cause the electron beam to scan round the centre of the target in a circular orbit. The second pair are coupled initially to large scan generators m1, m2 and, after the electron beam has been moved on to the target, later by the output of the phase discriminators. The detector tube is described with reference to Figs. 6 and 7 and the circuit (which is substantially the same as in the previous embodiment except that multivibrators are used as switches) with reference to Figs. 8, 9 and 10 (Figs. 6, 7, 8, 9 and 10 all not shown).