Patent GB-1197235-A (GB)
Title: Improvements in Correlation Devices of the Optical Type
IPC: G06E3/00
Filed: 1967-06-29
Abstract: 1,197,235. Pulse radar. COMPAGNIE FRANCAISE THOMSON HOUSTONHOTCHKISS BRANDT. 29 June, 1967 [6 July, 1966], No. 30017/67. Heading H4D. [Also in Division G1 To facilitate determination of the variation of the cross correlation function of a test and a predetermined reference signal for different relative time delays, light emitted by a first emitter is modified by transmitting it through a screen whose transmission coefficient is spatially varied to represent one or more reference signals each related to the predetermined reference and, if more than one, delayed relative to it by different amounts. The screen is moved relative to the first light emitter. The transmitted light is detected by photoreceivers to provide first output signals. The first light emitter may be modulated by the test signal so that the first output signals vary according to the instantaneous products of test and reference signals, or a further light emitter may be modulated by the test signal to provide illumination for further photoreceivers connected in circuits to which the first output signals are applied and which provide second output signals corresponding to the instantaneous products. The first or second output signals respectively are integrated to provide values of the required correlation function at different relative delays. In a system for analysing received radar signals Y(t), similar reference patterns may be conveniently arranged serially in a track 201 around the circumference of a rotating drum. The spacing between similar patterns and the drum velocity are such as to correspond with the frequency at which the radar signal is transmitted and received. The transmitted waveform may in fact be derived from a further track 27 on the drum. A light beam from a gallium arsenide laser diode 11 modulated by the received signal Y(t) is spread over an angle corresponding to the maximum measurable delay by a fibre optic system 41, 51, and a mosaic of photodetectors 602 is placed behind the track of the reference pattern. The relative positions of an individual photodetector and the reference pattern at a given instant correspond to a given delay, different for each photodetector. The output from each photodetector is separately integrated at 701, and the largest integrated output identifies a photodetector where the changing reference pattern caused by movement of the drum synchronizes or registers with the output from the light modulator. The position of this detector determines the delay in the received radar signal. A number of different reference patterns respectively arranged in different tracks 201, 202, 20n are in fact used, each representing the transmitted waveform modulated by different noise or interference. The particular band producing the largest correlation output indicates which reference waveform corresponds most nearly to the received signal. The separate integrating circuits for each detector in the blocks 701 &c. are switched round under the control of a system 700 operated from a programme 22 recorded on the drum 20. In the correlation system described above, a modulated light beam which is angularly spread to impinge on an array of photodetectors is used. Different arrangements within the scope of the invention may be used. In one (Fig. 9) the reference pattern is repeated in horizontal tracks arranged one below the other, each being differently displaced horizontally and a vertically collimated beam is used. Detectors in a vertical line in the path of the beam are placed one behind each reference waveform. The detector producing the largest integrated output as the reference waveform array moves across the beam indicates the relative delay in the signal used to modulate the light beam. In another system (Fig. 10) the reference pattern tracks are arranged serially but the frequency at which each successive reference occurs as the drum or other support is moved is different to the repetition frequency of the signal modulating the light beam. Thus each successive reference represents a different delay. Only one detector is necessary in this system and the time at which it produces maximum correlation output indicates the relative time delay of the received signal. In an alternative system (Fig. 14), an unmodulated light beam is used to illuminate a set of reference pattern tracks. The voltages produced by corresponding photodetectors represent reference signals S(t-to) and are applied to energize corresponding members of an array of photo-resistant receiver cells, to which a modulated light beam, representing the input signal Y(t) is applied. The current in each cell will be proportional to a corresponding product Y(t) S(t-to) and a voltage representing this is derived from a load across the cell, and integrated. The largest correlation function so obtained identifies a photodetector whose position represents the relative delay in the input signal.