Patent GB-994757-A (GB)
Title: Improvements in or relating to a pulse radar transceiver device
IPC: G01S7/28
Filed: 1963-08-02
Abstract: 994,757. Pulse radar; automatic frequency control. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Aug. 2, 1963, No. 30760/63. Headings H3A and H4D. A pulse radar transceiver device of the type providing anti-jamming facilities by the carrier frequencies of consecutive pulses being different and the pulse repetition rate not being constant, comprises a transmitting tube having a tunnel circuit which is continuously adjusted to produce said variation of the carrier frequency, modulating means, which when excited causes the transmitting tube to transmit a radar pulse of a carrier frequency dependant on the instant of excitation, and a receiver having a mixer and a local oscillator so varied in frequency as to derive an I.F. signal of constant frequency from the echo signals irrespective of the frequency of the transmitted pulse, said local oscillator being controlled to have a frequency related to (and preferably equal to) the natural frequency of the transmitter tube for a time terminated by the excitation of the modulator and then to have its frequency maintained at the value of said termination time for the duration of the echo time interval, and thereafter returned to follow said natural frequency. The transceiver device of Fig. 4 embodies a frequency synchronizing arrangement whereby the frequency of the local oscillator 2, is made to follow the natural frequency of the tuned magnetron 14. In this arrangement energy from the oscillator 2 is reflected by the magnetron to an amplitude detector 4, with an amplitude depending on its frequency f 1  as shown in Fig. 2, there being a pronounced minimum in the amplitude of the reflected energy when its frequency is equal to f 0 , that of the magnetron. The frequency of the local oscillator signal is controlled by a signal comprising two parts combined in a driving unit 5. A first D.C. part which determines the centre frequency f 1  of the oscillator signal is supplied by an integrating circuit 11 via an amplifier 12, whilst a second A.C. part, which causes the oscillator frequency to vary between the limits f<SP>1</SP> and f<SP>11</SP>, is supplied by one oscillator 6. The reflected oscillator energy received at the amplitude detector 4 will thus be frequency modulated and amplitude modulated. The amplitude modulation is detected by the detector, and for the frequencies shown will have an amplitude represented by e, Fig. 2. The amplitude e will rapidly drop to zero as f 1  approaches f 0  and will increase as f 1  becomes greater than # 0 , the output of the amplitude detector undergoing a phase reversal at the frequency f 0 . The output of the detector is fed, via an amplifier 7, to a phase sensitive rectifier 8 which will give an output of magnitude dependant on the amplitude e and of a sign dependant on whether the detector output is in phase or antiphase with a reference signal fed from oscillator 6 via phase adjusting means 9 and amplifier 10. The output of the phase sensitive rectifier is as shown in Fig. 3. Any output from the phase sensitive rectifier will change the output of the integrator 11, which will in turn cause the centre frequency f 1  to tend to and track the frequency of the magnetron. The local oscillator 2 is then caused to track the magnetron until at time t 1 , Fig. 5, a trigger pulse is received at input terminal 25 which sets via trigger generator 18 and modulator 17 to cause the magnetron to transmit a radar pulse. The trigger pulse is also fed to a monostable flip-flop 20 (output shown at Fig. 5 (a)) which changes its state causing a pulse to be sent to shut gate 16 and shut off the control signal from the integrator circuit 11. The last passed signal from the integrator is however stored by capacitor C 1  the potential of which is shown by curve P 1 , Fig. 5(e), such that the frequency of the local oscillator (see curve fosc, Fig. 5(g)), is kept at the frequency of the transmitted radar pulse. The output from the local oscillator can then be used to mix with the echo pulse to produce the required IF signal of constant frequency. The trigger pulse is also fed to a bi-stable circuit 21 which changes its state (the output of the circuit being shown at Fig. 5(b)) to produce a signal which stops oscillator 6 and causes the potential of point P 2  (see dash and dotted line in Fig. 5(e)), to go strongly negative. At tim t 2  the flip-flop 20 returns to its stable state and reopens gate 16. The potential of point P 1  now falls to that of P 3  (see dashed line in Fig. 5(e)) causing the frequency of the oscillator 2 to correspondingly fall. The return of flip-flop 20 to its normal state also causes bi-stable circuit 22 (output shown at Fig. 5(c) to feed a voltage to an integrating circuit 23 which commences to feed a sweep voltage (see Fig. 5(d)) to point P 3 , via cathode follower 28, such that the potential of point P 3  and the frequency of the oscillator 2 start to rise. When the frequency of the oscillator 2 reaches the frequency of the magnetron 14 at time t 3  a null appears in the output of a differentiation circuit 30 connected to amplifier 7, which null is used to return bi-stable circuits 21 and 22 to their original states. Oscillator 6 is now restarted and the oscillator 2 again tracks the magnetron until the next trigger pulse is received at time t<SP>1</SP> 1 . The return of circuit 22 to its normal state causes the output of circuit 23 to decay and the potential of point P 3  to fall to its normal low voltage. To ensure that no time is lost in the recommencement of the frequency tracking due to the recharging of the capacitor in integrating circuit 11, the outputs of flip-flop 20 and the potential at point P 3  may be applied to the input of amplifier 12.-