Patent GB-1486138-A (GB)
Title: Function generator
IPC: G01S7/12
Filed: 1975-02-20
Abstract: 1486138 Radar RAYTHEON CO 20 Feb 1975 [25 Feb 1974] 7199/75 Heading H4D [Also in Division G4] A radar system comprises a rotating radar antenna 102 (used both for transmitting and receiving by means of a transmit/receive switch 106, a transmitter 114 and a receiver 108 and detector 110), one revolution of the antenna being divided into 4096 sectors and an ACP pulse (azimuth change pulse) being transmitted by a radar base 104 each time the antenna crosses a sector boundary; in addition an ARP pulse (azimuth reference pulse) is transmitted each time the antenna crosses North reference. Transmission of radar echo signals from the antenna is triggered by a pulse generator 116, the number of ACP pulses following an ARP pulse being counted by a counter 180 to provide a representation of the azimuth angle of the antenna. Counter 180 accesses a memory 182 holding stored second differences of the cosine and sine functions. The cosine and sine second differences are converted to cosine and sine first differences in respective accumulators 184, 188, these first differences being converted to cosine and sine function values in respective further accumulators 186, 190. Since the trigger pulses from pulse generator 116 may not be synchronous with the ACP pulses, means are provided (see later) at the outputs of accumulators 186, 190 for interpolating between the calculated values of cosine and sine at the sector boundaries. The received radar echo signals are displayed on a cathode-ray tube (CRT) 132 via a video amplifier 112 using a "vector stroke" technique, wherein the CRT beam is moved along a desired locus in a succession of equallength straight lines, each line having a length termed the stroke length. This is achieved by multiplying the stroke length by cosine and sine in respective multipliers 150, 160 to obtain the X and Y components of the stroke, deflection of the beam along the locus being obtained by means of clocked X and Y accumulators 152, 162, digital-to-analogue converters 154, 164, and deflection amplifiers 128, 130. Counter 180 is arranged to count forwards from 0 to 511 in even octants of azimuth angle and backwards from 511 to 0 in odd octants, the direction of counting of counter 180 being determined by a signal from an octant counter (204), Fig. 2 (not shown) coupled to the carry bit position of counter 180. Memory 180 is preferably an integrated circuit read-only memory divided into two portions (206), (208), portion (206) storing the cosine second differences from 0 to 45 degrees whereas portion (208) stores the sine second differences from 0 to 45 degrees. To obtain values of the cosine and sine functions in the remaining octants, viz. from 45 to 360 degrees, the connections between portions (206), (208) of the memory 182 and the accumulators 184, 188 are reversed (if necessary), a sign change also being incorporated (if necessary) by means of cosine and sine add/subtract circuits (230), (232) coupled to the accumulators and fed from the two higher-order bit positions of octant counter (204) (which are indicative of quadrants). The interpolation between sector boundaries referred to above is accomplished by right-shifting the outputs of accumulators 184, 188 by six bit positions (i.e. dividing by sixty-four), and then clocking accumulators 186, 190 sixty-four times between consecutive ACP pulses. If a trigger pulse is received between ACP pulses, the current contents of accumulators 186, 190 are then employed.