Patent GB-745347-A (GB)
Title: Improvements in radar receiver systems using a p.p.i. type indicator
Inventor: MATON REGINALD WESLEY
IPC: G01S7/14
Filed: 1952-02-06
Abstract: 745,347. Radar; cathode-ray tube displays. BRITISH THOMSON-HOUSTON CO., Ltd. Feb. 4, 1953 [Feb. 6, 1952], No. 3144/52. Class 40 (7). The plan position indicator of a pulse radar has the radial lines of the display generated by the application of scanning current waveforms to two pairs of fixed, orthozonally arranged deflection coils. Means are provided for offcentring the origin of the display comprising adjustable scanning current biasing devices for displacing the origin up to a predetermined position and adjustable scanning current delay means for off-centring beyond that position. The bias and delay controls are ganged together so that the transition from one to the other takes place automatically as the origin is displaced through the predetermined position. It is conveniently arranged that the biasing is effective to displace the origin over the range as far as the edge of the tube and that, for displacements beyond this, delay is introduced. In one embodiment a continuously rotating directive aerial system 2 is supplied with pulses from transmitter equipment 1 and received echo signals are fed to brighten up the display on a cathode ray tube screen by means not shown. Associated with the aerial drive mechanism is a resolver magslip 3 feeding A.C. voltages of amplitudes proportional to the sine and cosine of the instantaneous angular direction of the aerial to the respective tube scan control circuits of which only one (within the dashed outline) is shown in detail. Considering this one of the two similar circuits the A.C. signal received from unit 3 is demodulated at 4 and the envelope fed to scan wave-generating circuit 5 to control the amplitude of the generated waveform which is fed to one of the scanning deflection coil systems 10. A trigger pulse derived from the transmitter is fed through, shaping circuit 9 to initiate the scanning waveform. This pulse may be subjected to an adjustable delay in unit 6 before it is applied to generator 5. An adjustable bias can also be applied to the deflection coils by unit 7. If it is desired to off-centre the display origin to a position still remaining on the tube screen the ganged control is effective only to introduce bias voltages to bring the spot to rest at the new origin and the scans are not delayed. However, if it is desired to off-centre the origin from O to O 2  where O 2  is outside the area HJKL of the screen, the control is effective to introduce bias to bring the spot to rest at the point J and delay is introduced to correspond to X-direction off-centring a distance x-s and Y- direction off-centring a distance y-s. These delay times differ for each particular scan (for that reason unit 6 is fed from unit 4) and for the two directions are, for the scan NP, the time equivalents of the ranges Rx and Ry ; Rx being the range in the direction O 2 P of the intersection of the radius O 2 P with the line KJ and Ry the range in the same direction of the intersection of O 2 P and HJ. In general where # is the angle shown and K is the radar time for unit range. The y axis delay may be similarly obtained. Arrangements must, of course, be introduced to reverse the bias currents as required when the off-centred origin is in other quadrants. The delay times introduced are always positive. It may be arranged that when the scanning waveform has reached a predetermined amplitude in one or other of the pairs of deflection coils, corresponding to the beam reaching the edge of the tube, the scanning circuits are reset. This prevents current flowing in the scanning coils unnecessarily. In some circumstances the repetition rate of the transmitter makes it necessary to stop the scan short of the edge of the tube in order to allow time for the energy in the scanning coils to be dissipated before a new scan is started and resetting means are required to do this. The initiation of the deflection coil sweep current may be arranged to be dependent on the output of a coincidence circuit, such output trigger being given when two equal potentials are applied to the circuit. The first of these (for the X coils) is manually adjustable to K 2 X where K 2  is a constant and X the X axis off-centring of the origin. The second is a waveform such as is shown in Fig. 7. Each cycle being initiated by the transmitter trigger pulse, rising immediately to a value K 2 s (where s has the same value as in Fig. 5), and then continuing to rise at a rate such that K (x - s) a delay time ------ is reached for an offsin # centring of x.