Patent GB-955527-A (GB)
Title: An echo distance measuring system
IPC: G01S13/18
Filed: 1962-04-19
Abstract: 955,527. Radar; transistor comparator circuits. BENDIX CORPORATION. April 19 1962 [April 24, 1961], No. 15318/62. Headings H3T and H4D. In a pulse echo distance measuring system, e.g. an altimeter, wherein the receiver which is normally off is switched on for a short period in each measuring cycle, the timing of the switching-on period of the receiver within the measuring cycle is varied in dependency on the echo signal energy received during a preceding " on " period, such that the said period tracks the echo signal reception instant and the distance of the object can be determined from the time lapse between the instant of signal transmission and the switching on of the receiver. General low altitude operation. Referring to Figs. 1a and 1b and to the waveforms of Figs. 5a a 3 kc/s. generator 30 feeds waveform (1) through a switch 32, generator 33 and modulator 21 to produce a transmitter modulating pulse (4) for triggering a 4.3 Gc/s. transmitter cavity oscillator 20. The modulating pulse (4) is also clipped in a clipper 100 and inverted and chopped in 101, the two resulting outputs being combined to produce a microwave switch control pulse (6) comprising a narrow " on " pulse on a wide " off " base. At low altitudes the transmitter pulse (5) passes to the microwave switch 25 to produce a transmitted pulse (7) at aerial 26 having a large peak due to the " on " part of the control pulse, the width of the said " on " part of the control pulse being varied by a programme gate 102 to compensate for loss of echo signal strength with increased height. The 3 kc/s. generator 30 also controls a fast ramp generator 52 giving an output waveform (9) which is compared with a slow ramp waveform (8) from a free-running generator 50, in voltage comparator 55. A pulse is produced by the comparator every time the two waveforms cross and is fed to a grid pulser 46 to switch on the receiver cavity oscillator 40, the switching on periods of the receiver thus occurring a little later each time, enabling the receiver to sweep through the reception period. An echo pulse received at 41 coinciding completely or partly with the on period causes the receiver to oscillate and produce a waveform which is detected by detector 43 and amplified by amplifier 44. The amplitude of the amplified signal depending on the degree of the above coincidence, is then detected by detector 90 to produce a corresponding D.C. bias for controlling the slope of the slow ramp waveform and thus altering the sequence of pulses from the voltage comparator and the on periods of the receiver. This alteration continues until the slow ramp waveform is horizontal and the receiver on periods are locked on to the received echo signals. If the echo producing object changes its distance, the receiver will again commence to sweep the reception period. The distance of the object thus becomes proportional to the time interval between the transmission of the pulse and the corresponding " on " period of the receiver. This time interval is measured in time comparator 13 by the " on " pulse (6) produced by unblanking gate 101 starting the generation of a two-part altitude ramp at 65 which is then stopped by the corresponding receiver enabling pulse from pulser 46. A detector 66 then measures the peak voltage reached by the altitude ramp and passes this information to altitude indicating means 62. Auxiliary low altitude operations. (a) Automatic receiver triggering level.-The transmitted pulse (7) contains a pedestal caused by stray energy by-passing the switch 25. Means are provided at low altitude to adjust the triggering level of the receiver cavity oscillator 40 so that when it is switched on, the said pedestal energy does not cause it to oscillate. Such means comprise a monostable multivibrator 56 triggered by pulses from the 3 kc/s. generator 30 and generating pulses (2) of such a length that when they are applied to the gate 101, alternate " on " pulses are stopped and only the stray pedestal energy is radiated. The pulses from multivibrator 56 simultaneously alternately activate gain control 45 such that when the echo of the pedestal energy is received and causes the receiver to oscillate, the grid bias on valve 42 is adjusted to just stop such oscillation. The receiver oscillator triggering level is thus set on alternate cycles. (b) Low altitude receiver blanking.-At low altitudes the transmitted pulse may overlap the echo pulse and cause erroneous readings. This is overcome by ensuring that the receiver oscillator is never on during the transmission period. Pulses from grid pulser 46 and from unblanking gate 76 are fed to AND gate and upon overlapping therein cause a large negative pulse to be applied to the grid of valve 42 overriding the pulses received direct from the grid pulser 46 by wire 150. (c) Transmitted pulse width programming.- This is operative between 30 and 500 ft. to compensate for the increasing energy losses with height and is brought about by the slow ramp signal from 50 defining the height instantaneously swept and controlling programme gate 102 which in turn controls the width of the on pulse generated by 101. General high altitude operation. The switching of the system from Low to High Altitude operation is automatically brought about by the signal from the slow ramp generator 50, the said signal increasing in slope at a point corresponding to 500 ft. so as to sweep the measuring cycle in larger steps at high altitude. The said signal energizes relay 70 at the said 500 ft. point to operate switches 23/24 so that the microwave switch 35 is by-passed and all the signals from the transmitter cavity 20 is radiated by the aerial 26. In addition an enabling signal generated by relay 70 at low altitudes to over-ride the alternate disabling pulses applied to switch 32 from multivibrator 56, ceases so that the transmitter cavity oscillates and a signal is transmitted on alternate cycles only. The receiver oscillator triggering level can then be automatically adjusted or alternate cycles to a very low level for use with the weak echo pulse energizes. Detailed description of circuits. (a) Voltage comparator 55 (Fig. 1a).-This comprises a transistor 140 having its collector positively over-biased into the avalanche mode. The fast ramp voltage is applied to the base of the transistor and the slow ramp voltage is applied to its emitter. Equality of the two voltages causes the transistor to switch to a second avalanche operating point producing a negative-going voltage spike across a capacitive load in the collector circuit, for application to the grid pulser 46. (b) Modulator 21 (Fig. 3).-This comprises an artificial line arrangement 124 charged up from source 120 and connected to a load comprising the primary of transformer 125. The artificial line is shunted by a series of four layer diodes which have a high impedance at normal voltages, but which break down to a low impedance on application of an additional voltage over line 17, causing the rapid discharge of the artificial line through the transformer winding, producing the required rectangular pulse, choke 134 preventing the flow of current through the diodes. (c) Time comparator 13 (Fig. 2).-As broadly described above this comprises a start gate 60 and a stop gate 61 respectively starting and stopping the generation of a rising voltage by altitude ramp 65. A peak detector 66, Fig. 1b, detects the maximum voltage generated by the altitude ramp and converts this to an altitude reading. The start gate 60 comprises a monostable multivibrator producing a positive pulse on line 68 on the application of a negative spike on line 62 from the unblanking gate 101 (Fig. 1a). The altitude ramp comprises a capacitor 165 chargeable through resistance network 167. The capacitor is prevented from charging up until the positive pulse is applied from starting gate 60, by an applied negative bias. When the voltage at point 173 becomes equal to that produced by stabilized source 170, diode 172 conducts and adds capacitors 174 and 175 to capacitor 165, to decrease the slope of the altitude ramp for high-altitude purposes. The capacitor 165 is stopped from charging by a large negative discharge pulse produced by stop-gate 61, from a transistor 180 operating in the avalanche mode and triggered in a similar manner to transistor 140 (Fig. 1a) by a voltage on line 63 from grid pulser 46 (Fig. 1a). A wider negative discharge pulse is also applied to capacitor 165 from a monostable multivibrator circuit comprising transistors 181 and 182. (d) Slow ramp generator, Fig. 4.-This is in the form of a relaxation oscillator comprising a capacitance multiplication circuit 110 in series with a resistance network 113 forming a charging circuit which is connected to the base of transistor 114 to provide a slow rising voltage at its emitter. The emitter voltage is applied to a resistance diode network and a tapping is taken on line 53 to the voltage comparator 55. The diode network is used to by-pass resistor 120 when the emitter voltage reaches a selected value, so as to increase the slope of the ramp voltage applied over line 53 to provide larger sampling increments of altitude at high altitudes. The capacitor 112 is shunted by a fourlayer diode network such that when the emitter voltage of the transistor rises above a certain value the diode breaks down and the capacitor is discharged.