GB1007516A - Improvements in or relating to apparatus for producing coded signals - Google Patents

Abstract

1,007,516. Correlators. RAYTHEON CO. April 6,1962. No. 13427/62. Heading G1U. [Also in Divisions H3 and H5] Apparatus comprises means affording two signals having frequencies which vary with time and symmetrically with respect to each other, and means incorporating delay and phase-inverting devices to produce a signal from the two signals according to a quaternary code of a sequence of 2<SP>n</SP> symbols selected from four symbols a, b, a, #b where a, b, are the phase inversions of a,b. Further rules for constructing the quaternary code are given in the Specification. In the particular embodiment, which is a pulse radar wherein a time-expanded pulse consisting of sub-pulses according to that code is radiated and the echo signals are time-compressed, symbols a, b designate ascending and descending "staircases" of frequency Fig. 6a, b respectively and symbols #a, #b denote similar staircases but with carrier-phase inversion. The invention inhibits the range ambiguity arising from energy spaced in time from that corresponding to the correct echo range at the output of the time-compression device, and increases range accuracy by the use of a large transmitted signal time-bandwidth products. General operation. Figs. 1, 5a. In the pulse radar of Fig. 1 switches 15, 22, 40 and 42 are in the positions shown when transmission is to be effected and in their alternative positions for echo reception. A generator 14 produces a triggering pulse Fig. 5a consisting of a single cycle of a 60 Mc/s oscillation and a gating pulse generator 16 places the switches into their "transmit" positions. The triggering pulse causes a circuit 10, which incorporates pairs of coaxial resonators, to produce the ascending frequency staircase on line 18 and the descending frequency staircase on line 20. Network 11 forms the quaternary-coded signal which is applied after frequency translation in mixer 30 to power amplifier 32 and thence to the aerial via duplexer 34. In the receiving condition, the I.F. output from amplifier 38 is fed to circuit 10 one output of which is fed directly, and the other after inversion by circuit 24, to network 11 which acts by an effecitvely auto-correlation process to form a single output pulse of the same duration as that of a sub-pulse and of enhanced amplitude, Fig. 8 (not shown). In an example the individual sub-pulses are 0.25 microseconds long, the staircase waveform is 1.25 microseconds long, the transmitted waveform is 40 microseconds long, and the final output pulse of the receiver is 0.25 microseconds long. The frequency sweep need not be of staircase form; it may be linear. Production of staircase waveforms. Figs. 2, 3, 5, 6. The triggering pulse Fig. 5a causes successive ringing of pairs of coaxial resonators 61/62, 84/85, 95/96, 97/98 and 99/100 Figs. 3a and 5d.... n to give the ascending staircase pulse train Figs. 5p and 6a on line p Fig. 3b and the descending train Figs. 5a and 6b on line q; diode switches 50, 72, 93, 223 and 214 are closed in this mode and 0.25 microsecond delay lines 52, 78, 229, 222 and 207 ensure termination of ringing at the appropriate times and the successive actuation of resonator pairs. The ascending pulse train is applied to a phase inverter 151 Fig.2 and the descending train, to a delay line 150 in the first section, also incorporating a signal-splitting resistance network 154 and summing amplifiers 155, 156, of a multisection network 11 for forming the quaternary coding, see also Fig. 4 (not shown), from which there ensues on line 27 a pulse train of the form Fig. 6m; one section of the network is described with reference to Fig. 7 (not shown) and its outputs are illustrated by Figs.6h, 6i, wherein a negative sign indicates phase reversal. Network 11 may be composed from two tapped delay lines, Fig. 9 (not shown). The invention is stated to be applicable to communication systems. Reference has been directed by the Comptroller to Specifications 755,721 and 921,311.