Patent GB-1268896-A (GB)
Title: Electrical detecting means
IPC: G01S13/56
Filed: 1970-04-20
Abstract: 1,268,896. Radar. MINNESOTA MINING & MFG. CO. 20 April, 1970 [21 April, 1969], No. 18779/70. Addition to 1,219,202. Heading H4D.  A pulsed radar of the type described in Specification 1,219,202 with reference to Figs. 6, 7A, 7B therein, exhibits a change in oscillator frequency and pulse frequency when the environment (i.e., the medium through which the pulses pass) changes. To overcome this the present radar comprises: a feedback oscillator including a transistor 30, Fig. 1, with an internal, tuned, feedback loop comprising capacitor 42 and transmitter inductor 44; a transmitting antenna 32 for radiating the oscillatory output; a receiving antenna 33 for receiving reflections of the transmitted wave and for superimposing a signal representative of the phase and amplitude of the reflected wave upon the signal in the internal feedback loop to form a composite feedback signal; a bias C/R network 46, 48, 50 which switches on the oscillator to give the pulsed output at a repetition rate which is a function of the phase and amplitude of a received signal and which is directly proportional to the amplitude of a charging voltage; an output circuit 36 which produces a pulsed signal 224, Fig. 2, having a repetition rate corresponding to the oscillator pulse repetition rate; and a negative feedback voltage regulator 14 responsive to the pulsed signal on line 20 to produce the charging voltage on line 16 at an amplitude inversely proportional to the repetition rate of the pulses signal, and adjusting the charging voltage when a change in the oscillator pulse repetition rate is detected caused by a change in the phase and amplitude of the received signal which lasts greater than a predetermined time and is thus indicative of a change in the environment, the adjustment being such as to restore the oscillator pulse repetition rate to a "standard environment" rate. The only substantial detected changes in the oscillator pulse repetition rate will thus be short and due to moving targets. The regulator 14 comprises a monostable circuit 56 comprising two balanced transistor pairs 129 and a pulse width determining adjustable C/R circuit 57, 59, 61, and producing a train of pulses 226, Fig. 2, of determined width. The pulse train is fed to an integrator 60 which has a time constant such that it integrates-out (i.e. removes the influence of), rapid changes of pulse repetition rate, e.g. due to a moving target, and any increase in the output of the integrator which provides the charging voltage on line 16, must be due to a slow change in environment. In the elaborated embodiment of Fig. 3, an alarm and indicator circuit 26 is used and an additional integrator stage 64A, 64B is used in the regulator 14 having a much smaller integration constant than integrator 60 so that very sudden changes in the pulse repetition rate, e.g. from birds, do not actuate the alarm. The output of the integrator stage 64B is taken via emitter follower 58, as a non-inverted output, to the alarm circuit. The latter comprises a deviation indicator 66 which gives an alarm output when the signal on line 22 departs by more than a predetermined amount from a value indicating no-moving targets in a standard environment. The alarm output on line 80 is fed to a decaying memory circuit 68. A capacitor 96 is charged up by successive short pulses enabling transistor 84, until transistor 86 conducts and alarm relay coil 88 is energized. The alarm state is stopped by a positive signal on the emitter of transistor 84 produced after 1 min. 15 sec. by a programmed unijunction transistor 70. A capacitor 106 at the gate and input terminals of the transistor is charged up to trigger the transistor after said period. To stop an alarm condition being indicated when the system is switched on, a balanced transistor pair 76 conducts and holds line 80 down to ground due to the non-conduction of transistor pair 116 of flip-flop circuit 72. After 1 min. 15 sec. unijunction transistor 152 conducts and its output switches the left-hand transistor of pair 116 into conduction. Circuit 72 now flips, and line 80 is able to transmit normal signals.