Patent GB-1176166-A (GB)
Title: Improvements relating to Radio Receivers for the Reception of Pulse Coded Signals
Inventor: COVILL DENNIS HENRY
IPC: G06J1/00
Filed: 1966-06-04
Abstract: 1,176,166. Pulse radar. E.M.I. ELECTRONICS CANADA Ltd. 13 Dec., 1966 [4 June, 1966], No. 24992/66. Heading H4D. In a radio receiver for reception of pulse coded signals a correlation network derived from a received sequence of pulses a correlation signal, the amplitude of which is a function of the correlation of the sequence with a particular code sequence, there is derived from the sequence a threshold signal of which the amplitude is a function of a criterion different from the correlation criterion, and a clipping circuit transmits the correlation signal to an output circuit only to the extent that it exceeds the threshold signal. The invention is applied to an ionospheric sounding system wherein the sounding signal consists of a sequence of pulses, e.g. thirty, having equal amplitudes but coded as positive or negative polarity, the frequency of the carrier wave for the pulses being changed in a pre-arranged way. In the receiver the pulse coded signal is applied to a pulse compression filter to enhance signal-to-noise ratio and facilitate selection of the best carrier frequency for transmission at a particular time. In one embodiment if each pulse is T (e.g. 100) microseconds long the receiver video output is fed to a delay line 2, Fig. 1, having consecutive taps 3 1  . . . spaced corresponding to delays of T. A " correct code " matrix 4 is such that when the desired code signal is positioned exactly within line 3 all the voltages from the taps add together in phase. A further matrix 5 termed a " universal code " matrix is designed to produce an output signal equal to the sum of the moduli of the signals at the n taps, and its output is applied after adjustable attenuation and subsequent inversion to be added in circuit 9 to the output of matrix 4. The resultant-signal is fed to a clipper 10 which removes any negative portion of its input. By selection of the attenuation most signals on either side of the compressed pulse are removed. The correct code matrix comprises resistors 201 . . . 20 k , Fig. 3, connected to those taps at which positive signals appear when the correct code is correctly positioned in the delay line and resistors 21 1  . . . 21 m  similarly but for negative signals; circuit 26 produces an output representing the sum of signals applied to resistors 20 minus the sum applied to resistors 21. In one embodiment the universal code matrix comprises two sets of resistors each set being connected to every tap on the delay line, one set is connected to a common point via diodes poled in one direction and the other set to another common point via diodes poled in the opposite direction, and the common points are connected to a transistor adding circuit which incorporates inversion, Fig. 4 (not shown). The inhibiting signal derived from the matrix 5 may be made dependent on the sum of pulses of extreme value from groups of delay line taps each group including three taps, Fig. 5 (not shown). In a modification, the inhibiting signal is dependent on the pulse of extreme value on any of the taps, Fig. 6 (not shown).