Pulse Doppler Proximity Fuze

This invention relates generally to high frequency oscillating and detecting circuitry, and more particularly to a self-pulsing oscillator and peak detector circuit having noise-free and insensitivity operating characteristics superior to those obtained in present day circuitry of the class to which the instant invention pertains.

Although self-pulsing, or so-called "squeeging", oscillator-detector circuits have been heretofore devised, the majority of the componential oscillator circuits were found to develop relatively large noise signals which resulted in inherent erratic variations of the repetition rate of the impulses generated by the self-quenching oscillators. Additionally, the majority of the present day componential detector circuits have not operated entirely satisfactorily because of their design as averaging superregenerative detectors, which detectors are unduly sensitive to inherent and spurious noise signals, as well as the erratic repetition rate of pulses generated by the squeeging oscillator.

Oscillator-detector systems of the type related to the instant invention are utilized in transceiver systems, particularly those systems which relate to the detection of moving objects or targets, such for example as a pulse-doppler proximity fuze. Although the prior art oscillator-detector circuits have, in the past, been employed in proximity fuzes, their performance leaves much to be desired in such applications. In addition to the limitations of the present day squeeging oscillator-detector circuits hereinbefore described, these present day circuits have also been found to be unduly sensitive to environmental disturbances, such for example as rain drops, water surface waves, cloud formation and the like. Moreover, in many instances the inherently generated noise level has been found to exceed the energy level of the reflected target signal thereby resulting in complete operational failure of the fuzes. Adjunctive circuitry necessary to overcome the hereinbefore described operational limitations and problems have been found to be too complex and bulky for ordnance fuzing applications.

Accordingly, a principal object of the instant invention is to provide a new and improved high frequency oscillator-detector circuit.

Another object of the present invention is the provision of a new and improved self-pulsing oscillator and peak detecting circuit.

A further object of the instant invention is the provision of a novel double-tube squeeging oscillator and peak signal detector system responsive to variations in the peak amplitudes of successively generated radio frequency modulations commonly known as a pulse or impulse.

A still further object of the present invention is to provide a new and improved electromagnetic energy impulse generating and detecting system having operational characteristics substantially insensitive to inherent and spurious noise signals.

Still another object of the instant invention is the provision of a new and improved pulse doppler proximity fuze being less susceptible to environmental disturbances and enemy counter-measure tactics than present day proximity fuzes.

Another still further object of the present invention is to provide a pulse doppler fuze having superior target range cut-off operational features.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein:

FIG. 1 is a schematic wiring diagram of the oscillator-detector circuit according to the present invention;

FIGS. 2 a and 2b are graphical illustrations of certain waveform characteristics of the circuit of FIG. 1, as hereinafter to be described; and

FIG. 3 is a view illustrating the use of the novel circuit of FIG. 1 in a pulse doppler proximity fuze system.

Referring now to the drawing wherein like reference characters indicate like parts throughout the several views and more particularly to FIG. 1 wherein the oscillator-detector circuit of the instant invention is indicated generally by the reference numeral 11. As shown thereon, the circuit 11 includes a pair of electron tubes 12 and 13, such for example as triodes, parallel connected across common plate and filament operating potential sources 14 and 15, respectively. The triode 12 is connected in a conventional high frequency oscillator circuit 16 which utilizes the inter-electrode capacitances of tube 12 and a portion 17 of antenna coupling coil 18 as the oscillator frequency determinative resonant network. The tank coil 17 is electrically coupled across the grid 19 and plate 21 of tube 12 by means of the grid biasing network 22 comprising resistor 23 and capacitor 24, and the high frequency by-pass capacitor 25. In like manner, tube 13 is connected in a high-frequency oscillatory circuit 26 which employs inductance coil 27, and the tube interelectrode capacitances as a frequency resonant network and the parallel combination of resistor 28 and capacitor 29 as the grid-leak or bias network 31. Coil 27 is inductively coupled to the tank coil 17 of oscillator circuit 16. The R. F. circuit from grid 32 to plate 33 is completed through the resonant and grid-leak networks by means of the high-frequency by-pass capacitor 34. For the purpose of isolating any R. F. potentials appearing on filaments 35 and 36, high frequency choke coils 37 are interposed between the filaments and the A supply 15.

By appropriate selection of the magnitudes of resistor 23 and capacitor 24, oscillator circuit 16 operates as a self pulsing, or squeeging, oscillator, the initial oscillation of which is initiated by the presence of random, or so-called "seed", noise. Upon the circuit bursting into oscillation, capacitor 24 will be charged negatively at a predetermined rate until sufficient grid bias is developed thereacross to cut-off tube 12 and terminate the oscillations. The grid bias potential will then decay exponentially at a rate proportional to the time constant of resistor 23 and capacitor 24 until it reaches a preselected reduced bias level whereupon the circuit 16 is again induced into oscillation and the hereinbefore described operation repeated. Inasmuch as the amplitude of the oscillations build up faster than the grid-bias potential, a relatively large impulse of oscillations is produced by the squeeging oscillator 16 before the grid-bias increases sufficiently to cut off tube 12. The effect of this intermittent operation of the oscillator circuit results in the development of a high frequency impulse across the tank coil 17, and a negative sawtooth output voltage waveform across grid biasing network 22 as more clearly illustrated in FIGS. 2a and 2b by waveforms 38 and 39 respectively. Although the tube and circuit capacities and inductance of coil 17 establish the carrier frequency of impulse signal 38, the impulse characteristics, such for example as the pulse shape or envelope, and the impulse repetition rate are controlled by the time constant characteristic network 22.

The circuit 26 associated with electron tube 13 is basically oscillatory like the intermittant oscillator circuit 16 and is tuned to oscillate at the same carrier frequency. However, as shown by waveform 41 of FIG. 2b the grid 32 of tube 13 is normally more heavily biased than the grid 19 of tube 21 thereby effecting a shorter duty cycle of circuit 26 than exists for circuit 16. Additionally, the discharge time constant characteristic of biasing network 31 is substantially larger than that of biasing network 22, thereby resulting in a reduced amplitude sawtooth signal appearing across network 31 with a proportionate reduction in the noise level. Interposed between the biasing network 31 and an output signal terminal 42 is a suitable carrier frequency filter consisting of resistor 43 and capacitor 44.

In response to the intermittent oscillation of circuit 16, the bias on grid 32 of tube 13 is reduced by the transformation of a positive potential signal, resulting from the decreasing bias on grid 19 through inductively coupled coils 17 and 27. Upon the grid bias potential of tube 13 reaching a preselected lower level corresponding to the peak portion 45 of impulse 38, circuit 26 is driven into oscillation. In view of the foregoing operational description of the oscillator-detector circuit 11, it will be apparent to those skilled in the art that the amplitude 46 of the output sawtooth signal 41 of circuit 26 will only be affected by a variation induced in the amplitude of pulse 38 occurring during the peak portion 45 thereof, such as will result from loading of the oscillator circuit 16 by antenna coil 18. Effectively therefore, the oscillatory circuit 26 operates as a peak detector.

Referring now to FIG. 3 of the drawing, the use of the novel oscillator-detector 11 in a transceiver system such for example as a pulse doppler ordnance fuze for which use the novel circuit is especially suited, is illustrated. As shown thereon, the squeeging oscillator 16 is coupled to a suitable electromagnetic energy radiator, or antenna 47 which will radiate into the surrounding spatial region the intermittent high frequency impulses 38 generated by the oscillator 16. The presence of a suitable target in the surrounding spatial area will result in a reflection of a portion of the radiated impulse which will be intercepted by the antenna 47. The reflected signal will effect a variation in the antenna radiation resistance, which in turn will result in a loading of oscillator circuit 16. If the reflected signal is received while the original impulse 38 is still being generated, the loading of the oscillator circuit will effect a variation in the amplitude of the pulse being developed. Any amplitude variation occurring over the peak portion 45, during which period circuit 26 is oscillating, will be transmitted as a potential variation through coils 17 and 27 to grid 32 of tube 13. It will be understood that the duration of the generated impulse 38 is preselected so that only electromagnetic energy reflected from a target within lethal range will effect the generated impulse during the peak portion 45 thereof. This amplitude variation will in turn proportionally vary the amplitude 46 of sawtooth signal 41 of peak detector 26 in a manner which results in the development of a sinusoidally varying audio frequency, often referred to as the "doppler frequency", having an envelope 48. As the distance between the proximity fuze and the reflecting target shortens, the frequency of envelope 48 will approach the predetermined band pass frequency of a conventional tuned audio amplifier 49 whereupon the amplifier will develop an output signal having an amplitude suitable for actuating a conventional thyratron firing circuit 51. Actuation of firing circuit 51 results in the ignition of an electroresponsive primer 52 whereupon the fuze will be detonated within lethal proximity of the target.

From the foregoing comprehensive description of a pulse doppler proximity fuze utilizing the novel squeeging oscillator peak detector 11 of the instant invention, a more sensitive proximity fuze has been developed inasmuch as an amplitude variation of a small magnitude is more readily detectable in a sawtooth waveform having the amplitude 46 than in a sawtooth waveform having the relatively large amplitude 53. Additionally, by reason of the relatively short duty cycle of the regenerative peak detector 26, a pulse doppler proximity fuze has been developed which will exhibit an absolute range cut-off feature. That is, the nonresponsive characteristic of the peak detector except at the peak of the transmitted impulse provides valuable protection from enemy repeater jamming as well as insensitivity to reflected signals from targets beyond the effective lethal range of the fuze.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practical otherwise than as specifically described.