Abstract:
A pulsed magnetron is caused to emit a series of RF energy pulses which are phase coherent with an injection signal by supplying the magnetron with a pedestal pulse during low-power signal injection. The pedestal being of a magnitude and duration such that the slow-rise-time pedestal portion holds the magnetron in the Hartree region during the signal injection.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention pertains to the field of electronics. In greater particularity, this invention pertains to microwave electronic circuitry. By way of further characterization, but without limitation thereto, this invention pertains to the modulation of magnetron oscillators. More specifically the invention will be described as it pertains to the generation of extremely short pulses of coherent microwave energy. 
   2. Description of the Prior Art 
   The development of high-resolution radar systems and other radar applications where high-resolution is desired, the use of short pulse duration transmitting modes have achieved popularity because of the reduction of minimum range coverage and high-target resolution. Great strides have been made in recent years in short-pulse operation by using pedestal pulse modulators. Examples of which are shown in U.S. Pat. No. 3,273,075 issued Sep. 13, 1996 to H. L. Kennedy for “Pulse Generator System Providing Pulse Superimposed On A Pedestal” and in U.S. Pat. No. 4,051,439, issued Sep. 27, 1997 to Reuben E. Nyswander for “Short Pulse Magnetron Transmitter”. 
   Another approach for radar signal processing employs coherent transmitted pulses which make possible much more target information than can be-obtained with a non-coherent system. For example, coherent pulses permit processing to obtain Doppler information indicative of target movement. Such arrangements have applications in synthetic aperture systems and moving target indicating systems. Heretofor, obtaining coherence in transmitting radar systems have ranges of coherence limited to approximately 10 Mz. Naturally, such limitations are reflected in limitations of application which could be vastly improved if coherence could be obtained over a wider frequency range. 
   SUMMARY OF THE INVENTION 
   The invention provides a method and system of radar pulse generation in which pulses of extremely short duration are generated with coherence over wide frequency ranges. These enhanced benefits of the system are obtained by using a signal injection of a coherent microwave signal together with a pedestal pulse generator. The magnitude and duration of a pedestal is chosen such that the resonant-cavity-oscillator or magnetron is held in the Hartree voltage region in which internal electrical oscillations occur but are of such magnitude that the output is negligible. In this fashion, the injected signal causes the magnetron to function in a coherent mode such that the short duration drive pulse causes full output therefrom. Thus, the primary object of the invention to obtain a short-duration, coherent burst of microwave energy is realized. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a voltage-current plot of a typical resonant cavity oscillator; 
       FIG. 2  is a block diagram illustrating the system of the invention; 
       FIG. 3  is a schematic diagram of the short pulse modulator illustrated in  FIG. 2 ; and 
       FIG. 4  is a waveform showing the pedestal and short pulse modulation signals. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , the operational characteristics of a typical magnetron are illustrated. Curve  11  starts from the origin of zero volts and current and rises rapidly in voltage until a level indicated at region  12  called the Hartree voltage region in which electron oscillation starts within the internal cavities of the resonant cavity oscillator. In this operational region, there is low magnetron current indicating a minimal output. If the drive voltage is increased beyond this point, the current flowing in the magnetron increases rapidly to reach a point  13  which is the optimum operating point for a given electrical operational level. When the voltage is removed, the current falls rapidly and the voltage returns to zero. A slight overshoot in the return voltage is indicative of the collapse of the electrical energy stored in the circuit components comprising the system. 
   Referring to  FIG. 2 , the system of the invention as illustrated. A crystal controlled oscillator  21  generates a frequency stable master frequency. This circuit is connected to a pulse recurring frequency generator which establishes the pulse recurring frequency, PRF, of the system. The output of the PRF generator  22  is coupled to a short pulse modulator  23 , to be described in greater detail herein. Modulator  23  generates a pedestal pulse which is applied to the magnetron  24 . 
   Crystal controlled oscillator  21  also outputs a frequency-stable signal to X-band reference generator  25  which may be considered a frequency multiplier which converts the crystal controlled oscillator output to a continuous-wave X-band signal. The output of X-band reference generator  25  is, in turn, connected via ferrite isolator  26  and ferrite circulator  27  to magnetron  24  where it serves as an injection frequency. 
   Ferrite isolator  26  and circulator  27  are conventional state-of-the-art devices which are used to establish a directional coupling in the direction indicated by the arrow. As is conventional in their applications, Ferrite circulator  27  has a load resistance  28  connected thereto which in a radar system would be a radar antenna and, ferrite isolator  26  has a load resistance  29  connected thereto. It will be understood by those familiar with the operation of these devices that energy from X-band reference generator  25  is coupled through these units to magnetron  24 , whereas energy reflected back to the unit is circulated in the direction indicated by the arrow and is dissipated in the associated load resistor  29 . 
   The pedestal or low rise time long pulse component of short pulse modulator  23  serves to elevate magnetron  24  to the Hartree voltage region. During this time interval, the injected signal of X-band reference generator  25  via ferrite isolator  26  and ferrite circulator  27  causes oscillation in magnetron  24  to commence but at such a reduced level that no output is evidenced therefrom. With the application of the short duration pulse carried on the pedestal pulse, magnetron  24  is driven to full output such that each pulse is coherent with the injected signal and, hence, with each other pulse. It will be obvious, that such a system permits much greater latitude in radar circuitry than heretofore possible. In laboratory tests, using a type MA249B magnetron, pulse coherence over a 200 Mz range has been obtained. This wide range permits X-band reference generator  25  to generate a local oscillator signal which is useful in processing the received pulses although the magnetron output may be at a frequency separated by the intermediate frequency therefrom. 
   Referring to  FIG. 3 , short pulse modulator  23  is illustrated in greater detail. An energy storage means such as capacitor  31  is charged to a reference level approximately twice E PS-1  due to D.C. resonance charging via a transformer  32  connected thereto. Capacitor  31  may be discharged by an associated switch device such as gas tube  33 , through transformer  32 , to produce a voltage pulse applied to magnetron  24 . Of course, the waveform, including rise time and duration, will be controlled by conventional LC circuitry and, for example, may be regulated by an inductor  34  connected in series therewith. In a similiar fashion, the short pulse portion of the pedestal pulse may be generated by an energy storage device such as capacitor  35  charging to a reference voltage approximately twice E PS-2  via a second transformer  36 . Similarily, capacitor  35  may be discharged via a gas tube  37  to produce a short pulse which is transformer coupled to magnetron  24 . Gas tube  37  may be triggered by the discharge of a capacitor  38  which is similiarly charged to the voltage value on capacitor  31  and discharged through switch  33 . By choosing the value of components in the discharge path of capacitor  38 , a predetermined delay for the operation of switch  37  may be achieved. The long pulse and short pulse circuits are connected to magnetron  24  via an RF choke  41  and capacitor  40 , respectively such that interaction therebetween may be minimized. The resulting waveform, appearing at point E A , is illustrated in the accompaning FIG.  4 . 
   Referring to  FIG. 4 , waveform E A  is indicated by a gently rising voltage at point  42  having a short-duration pulse  43  imposed thereon. As previously noted, the rise time and duration of waveform E A  may be controlled by selection of component values in the discharge path of capacitor  31 . 
   As will be familiar to those versed in the art, the selection of these values will depend upon the particular magnetron type employed. Similarly, the selection of the values of E PS-1  and E PS-2  which, ultimately, determine the magnitude of waveform E A  such that the pedestal portion indicated at  42  holds the magnetron in Hartree region a sufficient length of time that coherent oscillations may be reliability started. Likewise, the value of E PS-2  is similiarly chosen such that the narrow pulse  43  causes magnetron  24  to achieve full output potential illustrated at point  13 ,  FIG. 1 , of the particular magnetron chosen. The time duration of 1 microsecond illustrated in  FIG. 4  is representative of values obtained with the system. The duration of short duration pulse  43  in such systems have reliably achieved pulse widths of less than 10 nanoseconds. 
   In the construction and operation of the system, conventional engineering practice for the microwave arts is, of course employed. Within the limits of good design practice, substitutions of component values and types may be practiced with the expected engineering tradeoffs. For example, the invention is illustrated using thermonic switches, however, within the availability of suitable hardware, their solid state equivalents may be employed. 
   The foregoing description taken together with the appended claims constitute a disclosure such as to enable a person skilled in the microwave and electronics arts, having the benefit of the teachings contained therein, to make and use the invention. Further, this structure and a method herein described is seen to constitute a meritorious advance in those arts unobvious to such a person not having the benefit of those teachings.