Abstract:
Burnout resistance ferroelectric material is used in the feedback path of a microwave oscillator as a frequency control element. The ferractor has rapid broadband frequency tuning capabilities limited only by the speed of an external DC source.

Description:
GOVERNMENTAL INTEREST 
   The invention described herein may be manufactured, used and licensed by or for the United States Government for governmental purposes. 

   BACKGROUND OF THE INVENTION 
   A need has existed for an extended period of time, in the armed services and industry, for an oscillator having the ability to generate an RF signal from a D.C. power source which has the capability of rapidly selecting different RF output frequencies. This need is particularly acute in applications by frequency agile sources used in electronic warfare and communication links. Commercial applications for the invention will include the use in transmitters and receivers using phased-locked loops to stabilize a desired frequency. Prior art devices did not provide a reliable method to control a wide range of frequencies without concern for “burning out” or the need for shielding of the ferroelectric frequency control material when used in pulsed systems with high power or in environments where the generating sources are unstable. Prior art devices using a varactor diode as a variable capacitance for voltage controlled oscillators also were unreliable in high power pulsed systems. The varactor diode was vulnerable under the aforementioned environment whenever the maximum current rating was exceeded. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a voltage tuned ferrroelectric resonant oscillator which has the capability of rapidly selecting different frequencies in high power pulsed systems. 
   An object of the present invention is to provide a reliable frequency tuned, “ferractor”, oscillator which can control a wide range of frequencies without concern for burn out. 
   Another object of the present invention is to remove the threat of burn out of a tunable devise under pulsed systems with high power. A further object of the present invention is to provide a tunable ferractor which removes the threat of burn out due to inherent properties of the ferroelectric material being polarity independent. 
   tunable ferractor which removes the threat of burn out due to inherent properties of the ferroelectric material being polarity independent. 
   For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following descriptions taken in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a ferroelectric controlled oscillator. 
       FIG. 2  is an isometric view of ferroelectric material positioned between two metal plates for use as a varactor replacement. 
       FIG. 3  is a partial cross sectional view of ferroelectric material in a microstrip structure for use as a tunable resonator. 
       FIG. 4  is an example layout of a frequency tuned ferractor oscillator with ferroelectric material used to change the capacitance of the resonator in a similar fashion to a semiconductor varactor. 
       FIG. 5  is an example layout of a frequency tuned ferractor oscillator with the ferractor material located in the resonator portion of the circuit. 
   

   Throughout the following description, like numerals are used to denote like parts of the drawings. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to  FIG. 1  the ferractor circuit comprises an amplification stage  10  which includes one or more amplifiers cascaded together. Additional amplification  25  may be attached to the output port  12  of the oscillator to achieve the required output power level. Input and output matching networks,  14  and  16  respectively, are connected to the amplifier ports. The signal from the output network  16  is then devided at the power split  18  between the feedback path  20  and the output path  22  of the device. This division can take the form of a coupler or a power devider as long as enough energy traverses the feedback loop  20 . The ferractor is tuned by placing the ferroelectric material, hereinafter described in the ferroelectric resonator  24  portion of the oscillator circuit. The resonator  24  can be any reactive resonant structure such as a half wave dipole or a microstrip ring resonator, with its dielectric being a ferroelectric material whose effective capacitance is a function of the tuning voltage. This change in capacitance will tune the resonant frequency. The corresponding change in frequency is proportional to the square root of the effective dielectric constant. 
   Referring now to  FIG. 2  ferroelectric material  26  is metal plated on both sides  28 ,  28 ′. A biasing voltage “V” is applied to these plates by conductors  30  and  30 ′  0 respectively, setting up an electric field within the ferroelectric material  26 . When used as a resonator, the ferroelectric&#39;s permittivity is controlled by an external D.C. source, which is connected in parallel with the capacitance of the microstrip trace  25 . The parameters of the microstrip shown in  FIGS. 2 and 3 , are unaffected by the addition of this field. The bottom plated metal  28 ′ of  FIG. 2  is in electrical contact with a ground plane  32 . The microstrip trace ground plane  32  is separated from the ferroelectric resonator  24 ′ by coupling gaps  34 ,  34 ′ and dielectric material  36 . The voltage addition changes the resonator&#39;s overall capacitance and in turn the resonant frequency. With a given D.C. voltage swing, the amount of capacitive shift can be altered by proper selection of the composition of the ferroelectric material. The ferractor oscillator can be made to tune over a wide bandwith, comparable with YIG—tuned oscillators, while at the same time, have the rapid tuning speed of a varactor-tuned oscillator. 
   Referring now to  FIGS. 4 and 5 , complete ferractor oscillators are shown with the ferroelectric material used in place of a conventional varactor.  FIG. 4  shows the ferroelectric material  26  used to change the capacitance of the resonator  38  in a similar  25  fashion to a semiconductor varactor.  FIG. 5  shows the ferroelectric material  38 ′ located in the resonator portion of the circuit.  FIGS. 4 and 5  show frequency tuned ferractor oscillator layouts having a ferroelectric voltage driver  40  operatively connected to the ferroelectric material  26  and  38 ′ in feedback loop  42 . 
   In operation the microwave signal goes through one or more amplification stages to boost the signal level. The first amplifier  44  and second amplifier  46  have input and output matching stubs  48 ,  48 ′ and  50 ,  50 ′ respectively, to minimize reflection at the ports of the amplifier. Once amplified, a portion of the signal power is coupled, or devided, into the feedback loop which once again amplifies the microwave signal, thereby reaching a steady state after a few iterations through the loop. The amount of energy fed back must cause the loop gain to be greater than unity. The result is a stable RF signal at RF output port  52  with a quality factor (Q) comparable to that of a varactor tuned oscillator. The development of a ferractor controlled oscillator is based on the intrinsic behavior of the ferroelectric material. The example ferractor layouts of  FIGS. 4 and 5  include gate bias amplifier input stub elements  54 ,  54 ′ which include gate resistors  56 ,  56 ′ electrically coupled to ground via metal conductors  58  and  58 ′ respectively. Each of the amplifiers  44  and  46  include drain bias stubs  60  and  60 ′ respectively which contain resistor and capacitor tank elements  62  and  62 ′ respectively. The aforementioned resistors and capacitances are connected to ground via metal conductors  58 ,  58 ′. 
   In an alternate embodiment the resonant devise, shown in  FIG. 5  may be constructed entirely of ferroelectric material. This embodiment creates a distributed effect in which the change in frequency is proportional to the change in the effective wavelength of the signal in the material. Presently, a limitation of this embodiment is that the ferroelectric material has a very high dielectric tangential loss. The device may be constructed using Ba×Sn1-×Ti03, however any ferroelectric material that exhibits a change in permittivity due to an applied field will work. By using alternate material compositions or changing frequency ranges, a broad range of device performances are obtainable. The size and thickness of the ferroelectric material can be customized for a particular application that requires a particular voltage range or region of frequencies. 
   While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.