Patent GB-1480498-A (GB)
Title: Pulse radar apparatus
IPC: G01S13/24
Filed: 1974-09-04
Abstract: 1480498 Pulse radar HOLLANDSE SIGNAALAPPABATEN BV 4 Sept 1974 [21 Sept 1973] 38713/74 Heading H4D A pulse radar apparatus is provided with a continuously variable transmitting device and with a control circuit to fine tune a tunable local oscillator at a momentary frequency f l  to a frequency f lo , which shows a difference f 0 , fixed in sign and magnitude, with the momentary transmitter frequency f z , where f o =f z -f lo , said control circuit comprising an error voltage generator, which derives an error voltage from the signals supplied by the transmitting device and the local oscillator, said error voltage being proportional to the frequency difference |#f| - |f o |, where #f equals the difference between the momentary transmitter frequency f z  and the momentary local oscillator frequency f l , wherein said control circuit is provided with a frequency control circuit to obtain the tuning voltages for the local oscillator, said frequency control circuit being supplied with a setting voltage proportional to the momentary rate of change of the transmitter frequency [df/dt), as well as with said error voltage, and wherein the pulse radar apparatus is further provided with an angular position indicator, coupled to the shaft of the servomotor required for the transmitting device, and also with a conditional circuit which, with the aid of said angular position indicator, delivers a switching signal during a period when the transmitting device is operative in a defined frequency range comprising a momentary transmitter frequency (f l +f o ), the switching signal enabling said frequency control circuit to adjust itself to the then prevailing setting voltage during a first time duration T 1  at the end of each pulse repetition time and to the then prevailing error voltage during a second time duration T 2  at the beginning of each pulse repetition time. As described, the transmitting device is a dither-tuned magnetron 2, Fig. 1, which is pulsed at either a constant or a variable rate by a H.V. modulator 3. The magnetron delivers power at the momentary frequency f z  to the radar aerial 6 via a waveguide 4 and a duplexer 5. Return signals are delivered from the duplexer 5 to a pre-amplifier 7 which provides one input to a mixer 8, from which said signals are obtained at intermediate frequency. Another input to the mixer (8) is provided by a local oscillator 1, which is electronically tuned by a signal from a frequency control circuit 11. The local oscillator 1 also provides one input to a mixer 14 of an error voltage generator 10, said mixer 14 receiving another input via a directional coupler 13 from the waveguide 4. Output from the mixer 14 at the frequency #f is coupled through an amplifier 15 to a frequency discriminator 16 having two filter combinations, of which the output voltages are detected separately and with opposite polarities. The filter combinations are so tuned that there is a change of polarity at the frequency f 0  (Fig. 2, not shown). An error voltage from the discriminator 16 is supplied to the frequency control circuit 11, and two other output signals are supplied to a conditional circuit 17. In the resonator cavity of the magnetron 2 a plunger is driven up and down by means of a rotating servo-motor 12 to produce a periodic variation of f z  from a maximum value, f zmax , to a minimum value, f zmin , (Fig. 3, not shown). An angular position indicator 18 comprises a resolver 21 coupled to the shaft of the motor 12 and connected to an oscillator 20. A detector 22 compares the phases of the outputs from the resolver and the oscillator and produces a voltage which is applied via an amplifier 23 to a differentiator 24. The output voltage of the dif ferentiator, which is proportional to df/dt, is applied to comparators 25, 26, and may be applied via a switch 31 to the frequency control circuit 11. The comparator 25 produces a positive output when the output voltage of the differentiator 24 is above a fixed positive value, indicating that the magnetron is tuned to a frequency in a higher range, and the comparator 26 similarly produces a positive output when the magnetron is tuned to a frequency in a lower range. The signals from the comparators 25, 26 are supplied to a logical circuit 19, comprised in the conditional circuit 17. The tuning voltage for the local oscillator I is provided by an integrator 30 in the frequency control circuit 11. For coarse tuning, the integrator is fed by a multiplier 38 via a time switch 36, which is closed by a monostable element 37, controlled by a pulse #Sp, for a short time T 1  before the next transmitted pulse #S m . The multiplier 38 has one input from the switch 31 and another from a sawtooth generator 39, which is reset by each #S m  pulse. In position "a" of the switch 31 the differentiator 24 supplies the multiplier 38. Alternatively, in position "b" of the switch 31, the multiplier input is derived from the tuning voltage output of the integrator 30, after processing in a differentiating network 32. This comprises first and second differentiators 33, 34 which are connected in series, and which each provide an input to a summer 35. For fine tuning, the integrator 30 is fed by the frequency discriminator 16 via a time switch 40, which is closed by a monostable element 41, controlled by the pulse #S m , for a short time T 2  after said pulse. The integrator 30 is controlled by the conditional circuit 17, being released when an output is given by an AND gate 29. The logical circuit 19 provides one input for the gate 29, and another is provided by a pulse stretcher 28 which is fed by a comparator circuit 27, receiving the aforesaid two inputs from the frequency discriminator 16. The conditional circuit 17, which also comprises a reset circuit 42, ensures that the local oscillator 1 is tuned only if #f is within a required range (indicated by "L" in Fig. 2) and the maximum frequency value f zmax  has been reached. A second embodiment comprises a generator the amplitude of the voltage of which is directly proportional to f 1 , the momentary value of the local oscillator frequency. There is also provided a differerence circuit which derives a difference voltage from the output voltages of said generator and of the angular position indicator 18. The conditional circuit 17 comprises a comparator, receiving said difference signal, and delivering a switching signal as soon as the difference voltage is below a fixed threshold value, such that the transmitting device is active in a frequency range f l +f o .