Patent GB-994657-A (GB)
Abstract: 994,657. Radar systems. COMPAGNIE FRANCAISE THOMSON-HOUSTON. Jan. 14, 1964 [Jan. 16, 1963], No. 1617/64. Heading H4D. In a pulse radar system, wherein the transmitting means includes a radar transmitter keyable by the application of any one of a plurality of keying pulse trains, having different repetition frequencies, to a keying input thereof, so as to transmit radar pulses at a corresponding repetition frequency, said frequencies being so selected that pulses in the respective trains intermittently coincide, and wherein the pulse repetition frequency of the transmitted signal is commanded to change when an echo pulse approaches a subsequent transmitted pulse, the changeover of the keying pulse train at the input of the transmitter is delayed from the command until the next occurrence of a coincidence between pulses in the keying pulse trains. The delay in carrying out the command is said to obviate the possibility of losing a fast-moving target in changing from repetition frequency to another at random. In the embodiment of Fig. 2 a master oscillator MO feeds a signal So to a pulse former 2, and to two keying pulse generators GE 1  and GE 2 , which produces sine waves Sm 1 , Sr 1  and Sm 2 , Sr 2    at frequencies of #o/m 1 , #o/m 1 n 1  and #o/m 2  # o /m 2 n 2    respectively where fo is the frequency of the signal So from the master oscillator, and pulse trains E 1 , Lr 1  and E 2 , Lr 2 , at repetition frequencies of #o/m 1 n 1  and #o/m 2 n 2 . A logic circuit   L, under the control of ranging means to be described, determines which of the keying pulse trains E 1  or E 2  is to be fed to the transmitter T, and changes the said trains over in response to the said command. The waves Sm 1 , Sr 1 , Sm 2  and Sr 2 , together with the oscillator output So are fed to respective phase shifters 16 1 , 17 1 , 16 2 , 17 2  and 15, and thence two follow-up pulse generators GP 1  and GP 2 , and a pulse former 2<SP>1</SP>. The phase shifters are controlled from gear-box B by a motor M such that the generators GP 1  and GP 2  produce followup pulse trains P 1  and P 2 , having the same respective repetition frequencies of trains E 1  and E 2 , but variable in phase relation thereto. Logic circuit L<SP>1</SP> determines which of the follow-up pulse trains to use in correspondence with circuit L, and the chosen pulse train is fed to a comparator C, where it is time compared with an echo pulse from receiver R. A signal representing the time difference is fed to motor M, which adjusts the phase shifters such that the difference is reduced to zero, whence the angle of rotation of the shaft adjusting phase shifter 15 can be calibrated to read the distance of the target, with the other phase shifter providing coarse but unambiguous distance readings. A cam, coupled to the motor M determines when the echo pulse is approaching the next transmitted pulse and produces signals whereby the changeover of the keying and follow-up pulse trains is commanded. The two keying pulse generators are shown in Fig. 3, and comprise frequency dividers 3 1  or 3 2  dividing the signal So down by a factor of m 1  or m 2  to produce the signals Sm 1  and Sm 2 . These signals are converted to pulse trains Km 1  and Km 2  (see Fig. 7), in pulse formers 4 1  and 4 2  and further frequency divided in dividers 5 1  and 5 2  to produce signals Sr 1  and Sr 2 . These signals are converted into pulse trains Kr 1  and Kr 2  which are passed to AND gates 71 and 72 together with the pulse trains Km 1  and Km 2 , to produce pulse trains Lr 1  and Lr 2  at the gate outputs. These pulse trains are then passed to further AND gates 81 and 8 2  together with the pulse train Io, to produce the keying pulse trains E1 and E2. More or less than two frequency dividing steps may be used. The follow up pulse generators, Fig. 5 (not shown) are identical to the keying pulse generators, except that the frequency dividers 3 1 , 3 2 , 5 1  and 5 2  are not provided, the signals Sm 1 , Sm 2 , Sr 1  and Sr 2  being provided by the keying pulse generators. The logic circuit L is shown in Fig. 4 and comprises a cam γ<SP>1</SP> coupled to the shaft varying phase shifter 17 1 , and a cam γ2 coupled to the shaft varying phase shifter 17 2 . Those phase shifters control signals having the same frequency and phase as the follow-up pulses and so have control shaft positions indicative of the targets range and time position of the echo pulse. The cams γ<SP>1</SP> and y<SP>2</SP> can therefore be arranged to close a switch and generate a signal F 1  or F 2  when the echo-pulse approaches the next transmitted pulse. The signal F 1  or F 2  is passed to a bistable device 10, which passes a signal to AND gate 11 or 11<SP>1</SP> depending on which cam last produced a signal. The other inputs to the AND gates are fed from a further AND gate 9, which gives a pulse whenever the pulses in pulse trains Lr and Lr 2  coincide. These coincidence pulses Lc after passing through gate 11 or 11<SP>1</SP> trigger a second bi-stable device 12, which delivers a signal at a corresponding output to open AND gate 13 or 13<SP>1</SP> and pass the keying pulses E 1  or E 2  to the transmitter via OR circuit 14. Assume for example that output H1 of bi-stable device 10 is active and that the coincidence pulses Lc are passing through gate 11 to keep the output J 1  of the bi-stable device 12 active and so allow the keying pulse E 1  to pass to the transmitter. A pulse F 1  will now switch over device 10, activating output H 2  and opening gate 11<SP>1</SP>. However, it is not until the occurrence of the next coincidence pulse Lc that a pulse is fed from gate 11<SP>1</SP> to switch over device 12 and thus charge the keying pulse train being fed to the transmitter from E 1  to E 2 . The logic device L<SP>1</SP> is the same as device L, except that the bi-stable device 10 is omitted, and the gates 11 and 11<SP>1</SP> are switched by the outputs J 1  and J 2  of bi-stable device 12. The switch over of the follow-up pulse train is delayed slightly from the switch over of the keying pulse train so that echo pulses arriving from the previous keying pulse train, may be correctly followed.