GB596651A - Improvements in or relating to radar systems - Google Patents

Abstract

596,651. Radiolocation; valve relay circuits. KEMPTON, A. E., WALLEY, C. A., and PARLEY, F. J. M. Dec. 6, 1944, No. 24451. [Class 40 (v)] In a pulse radar system for the detection of moving targets by the observation of Doppler frequencies, which are produced by the beating of the target echo signals with reflections from stationary objects or with reference oscillations from a local source, provision is made for the selection of echo signals corresponding to a desired range. The pulse selecting means or strobe may be set to select targets at a fixed range, or on a fixed perimeter if the aerial system is rotated or may be manually or automatically adjusted to scan in range. In the system of Fig. 1 the aerial 1, movable in elevation and azimuth is coupled through the T-R unit 3 to the transmitter 2 which operates at 3000 Mc/s. and at a p.r.f. of 1000 c.p.s. The receiver head amplifier and frequency changer 4 feeds two channels 5 and 6 each comprising a second frequency changer and I.F. amplifier followed by detector and output stages. The output from channel 5 is fed to the indicator units 7 and 8. Unit 7 provides a type A display of the full range, the time base circuit 10 being synchronised by a lock pulse from the transmitter over lead 11. Unit 8 provides a type A display of a selected 2000 yard interval of range, the high speed time base circuit 13 being triggered by the lock pulse through a variable delay circuit 14. The receiver unit 6 is normally biassed off in the LF. stages but 1 microsecond strobe pulses from a delayed pulse generator 15 are applied to open the channel to select echo signals of the desired range. The strobe pulse is also fed as a brightening marker pulse to the cathode-ray oscillographs of the indicator units. Automatic gain control in receiver 6 has a long time constant so that low Doppler frequencies are not smoothed out. When signals from the gated receiver include reflections from moving objects, beating occurs with reflections from fixed objects, or alternatively the receiver may have a reference oscillation injected bearing a constant phase relation to the transmitted radio frequency so that the Doppler beat frequency is produced. The output pulses are lengthened at 16 and are fed to a low-pass filter 17, cutting off at about 500 c.p.s. to eliminate the recurrence frequency. An audible indicator 23 or an oscilloscope may be operated by the Doppler frequency amplified at 18. An additional output circuit 19-22 comprising an automatic warning circuit operating in the presence of a sustained Doppler signal may give an audible warning or stop automatic range scanning at the range of the detected target. Device 22 may take the form of a display giving an indication of range or range and bearing of moving targets only. The delayed pulse generator, Fig. 2, comprises a phantastron circuit generating at the screen of V1 a positive rectangular pulse which commences coincidentally with the lock pulse from the transmitter which is fed to the suppressor grid of VI. The square wave, the duration of which is controlled by potential divider 30, is differentiated by circuit C3, R3 and fed to valve V4 where a positive pulse corresponding to the back edge of the square wave is produced in the anode circuit, and after amplification and reversal in valve V5, is fed through C4 as a brightening pulse to the indicator cathoderay tubes. The I.F. stages of receiver 6, which are normally biassed off by the voltage across potential divider R7, which is fed over connection 31, are made operative by the positive pulse appearing on the cathode of V5. The strobe pulse may be scanned automatically in range by a motor device for the potential divider 30 or a triangular saw-tooth waveform from a suitable generator may be applied to the anode of valve V1. In the Doppler frequency detection circuits, Fig. 3, the valve V10 with a long-time-constant cathode circuit R10, C10 lengthens the received pulses 32 to the form 33 which indicate echo pulses with Doppler frequency amplitude modulation. The lengthened pulses are fed by a cathode follower to the low pass filter 17 and the extracted Doppler frequency 34 is amplified by V9 and applied to a squaring valve V10. The resultant square topped wave, by charging condenser C12 through the coupling condenser C13 and resistance R13, reduces in steps the cathode potential of the gas discharge tube V12. When the cathode potential of V12, which is normally set by connection through R13 and diode V11 to the screen of V10, reaches such a value with respect to the grid bias derived from the potential divider R16 that V12 strikes, the discharge occupies about 1 millisec. and operates relay 35. The circuits of valves V10, 11, 12 are such that they provide discrimination in favour of the Doppler frequency against interfering pulses. The filter 17 may be a bandpass filter cutting off below the pulse recurrence frequency and above a frequency corresponding to a given radial velocity so that only targets with radial velocities exceeding the given value are detected.