Patent GB-856104-A (GB)
Title: Electric circuit for a radar system
Inventor: RAGNI VICTOR F
IPC: G01S13/44
Filed: 1957-06-27
Abstract: 856,104. Radar. SPERRY RAND CORPORATION. June 27 1957, No. 20403/57. Class 40(7). An electric circuit comprises a frequency modulator; a discriminator connected to the output of the frequency modulator; a multiplier arranged to modulate the output of the frequency modulator in accordance with a multiplying signal applied to its input; a first periodically energized gating circuit connected to apply in degenerative fashion a portion of the output of the discriminator representing the instantaneous frequency of the output of the frequency modulator to the input of the frequency modulator during the periods when the first gating circuit is energized; and a first means to ensure that the amplitude of the multiplying signal is at a predetermined value at the expiry of each of the periods. The circuit may be used in the radar height finding system shown in Fig. 1. Equal pulses are transmitted simultaneously from two aerials 18, 19 having overlapping vertical lobes diagrams, as shown. The outputs of two receivers 20, 21, each coupled to one of the aerials, are subtracted in angle computer unit 25 to give voltages representative of the sines of the angles of elevation of targets detected. The output of unit 25 controls a frequency modulator 27 the output of which is passed through a limiter 29, which removes any incidental amplitude modulation, to a unit 30 where the signal is amplitude modulated by a saw-tooth wave representative of range, and this is fed to a discriminator 31, one output of which is fed to the vertical deflection plates of a cathode-ray tube 55. A normal scanning deflection is applied to the horizontal deflecting plates of the tube and the grid of the tube is modulated by the sum of the outputs of the two receivers 20, 21. A point in the discriminator circuit responsive to the instantaneous carrier level is connected to one plate of a condenser 37, the other plate of which is connected to a constant reference voltage. In charging from an initially low potential condenser 37 generates a sawtooth wave which is amplified by unit 38 and fed through unit 40 to the amplitude modulator 30. Fig. 2 shows the waveforms at various parts of the system; B and C shows the outputs of the two receivers when there are three echoes from targets of low, medium and high elevation; D shows the output from the angle computer 25, the pulse 79 corresponding to zero elevation and being produced at the end of a cycle in response to a pulse from AFC restorer gate 46. The sawtooth produced by unit 40 is shown at F and is flat for the first 120 microseconds, corresponding to the clutter region, and thereafter rises linearly so that its height is proportional to time from zero. E shows the output of limiter 29 and G the input to the discriminator, whose output is shown at H. J shows the AFC recovery pulse produced in unit 46 1730 microseconds after the start of the cycle. This pulse is fed to unit 25, when it produces pulse 79, and to unit 48 where it opens a gate and completes a negative feedback path from the discriminator to the modulator 27 so that the latter is correctly set at the beginning of each cycle; the pulse also disables receiver 20, 21. L shows the range recovery pulse produced by unit 45 at the end of the AFC recovery pulse and lasting to the end of the cycle and for 120 microseconds into the next cycle. This pulse is fed to a gate 42 and thus connects condenser 37 to a constant reference potential, and to a gate 43 connecting the input of unit 40 to a constant reference potential so ensuring that the initial flat region in F is at the correct height.