Patent GB-944098-A (GB)
Title: Pulse echo type detecting system
IPC: G01S13/24
Filed: 1960-03-24
Abstract: 944,098. Pulse radar. HOLLANDSE SIGNAALAPPARATEN N.V. March 24, 1960 [March 25, 1959], No. 10482/60. Heading H4D. Relates to a pulse-echo locating system in which the echo time corresponding to the maximum range of the system is less than the pulse recurrence period, the time interval between such an echo time and the transmission of the next pulse being defined as the rest interval. According to the present invention if interfering signals are received, a control signal is derived from such signals received during the rest intervals which causes a cyclic variation in the carrier frequency and/or the direction of linear polarisation of the transmitted signal until the receipt of interfering signals ceases. If the means for selecting signals received during the rest intervals is responsive only to pulse signals or to amplitude or frequency modulated signals having a modulation frequency greater than the pulse recurrence frequency an additional control signal is derived from any received CW signal which is unmodulated or modulated at a frequency less than the pulse recurrence frequency. The invention is described as applied to a pulse radar but it may be applied to other types of pulse-echo systems, e.g. those employing sound waves. As shown in Fig. 1 the radar comprises a pulse generator 101, transmitter 103, T/R switch 105, mixer 123, receiver 125 and a local oscillator 132 whose frequency is automatically maintained at a fixed difference from the transmitted frequency by means of a conventional control circuit comprising an auxiliary mixer 121, frequency discriminator 129 and servomotor 131. The receiver 125 is blocked during pulse transmission by a circuit 128 controlled by the pulse generator 101 and the frequency of the transmitter 103 is controlled by a motor 116. Any pulse type output signals from the receiver 125 occurring during the rest intervals are selected by a gate circuit 111 controlled by a mono-stable trigger circuit 109 triggered by the pulse generator 101 and the control signal output from the gate circuit 111 is applied through an amplifier 113 and reversing circuit 115 to actuate the tuning control motor 116. Any received unmodulated CW signals or CW signals modulated at a low frequency will produce at the output of the receiver 125 a D.C. or low-frequency signal which is passed to the amplifier 113 by a low-pass filter 134 having a cut-off frequency less than the pulse recurrence frequency. Circuit details of the components 109, 111, 113, 115 and 134 are shown in Fig. 2. The gate circuit 111 comprises a pentode 214 whose anode is connected to one plate of a capacitor 219, the negative video output from the receiver 125 being applied to the other plate of the capacitor through an inverter 216 and a pulse stretching cathode follower 217. During the rest intervals the pentode 214 is rendered conductive by a positive pulse from the trigger circuit 109 and any positive pulse from the cathode-follower 217 will produce a negative charge on the capacitor 219 which is applied as a negative control signal to valve 224 of the amplifier 113 via a rectifier 222. Outside the rest intervals the pentode 214 is cut off and any positive pulses from the cathode follower 217 cannot pass the rectifier 222. Any positive D.C. output signal from the inverter 216 due to CW signals is applied as a positive control signal to valve 225 of the amplifier 113 via the low-pass filter 134. The valves 224, 225 have a common cathode resistor 226 and are normally biased so that valve 224 is conductive and valve 225 is cut-off. A polarised relay 227 is connected between the anodes of the valves 224, 225 such that when a control signal is applied to either of the valves the relay 227 operates to close a contact 228 thereby energizing the two-phase tuning motor 229. The direction of rotation of the motor 229 is controlled by a contact 230 of a relay 231 which in conjunction with limit switches 233, 234 and a holding contact 232 of the relay 231 causes the tuning of the transmitter 103 to vary between predetermined limits as long as a control signal is being produced. In a modification (not shown) the positions of the capacitor 219 and the pentode 214 are interchanged so that the capacitor 219 is then positively charged by all types of interference signals received during the rest interval; in this case the output from the valve 216 via the lowpass filter 134 is omitted. If the direction of polarisation is varied instead of the carrier frequency, a linearly polarised aerial element, e.g. a dipole, Fig. 3 (not shown), or waveguide form, is rotated by the motor 229 but in this case it is not necessary to reverse the motor rotation. Alternatively separate motors are provided for varying the frequency and polarisation cyclically and sequentially, the two motors being controlled by a relay system similar to that shown in Fig. 2 so that the carrier frequency is first varied through the complete transmitter tuning range and the polarisation is then rotated through 180 degrees.