Abstract:
A switched frequency multiplier receives pulses of pump energy as an input signal. The input signal is transferred to within a housing having a first cavity tuned to the frequency of the pump signal and a second cavity which is tuned to a harmonic of the input signal and is enclosed within the first cavity. An outlet port couples the second tuned cavity to a waveguide which includes a Q-switch that can be turned on and off. The interior of the housing has a planar grid of layers which includes a layer of nonlinear material and a frequency selective layer. The frequency selective layer is transparent to the input signal but reflective to the harmonic output signal thereby trapping energy in the second cavity. The multiplier operates by receiving a pump pulse and storing the energy while the Q-switch is closed. When the Q-switch is opened near the end of the pump pulse, the stored energy is suddenly released to produce a relatively high energy harmonic pulse having a shorter duration than the pump pulse.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention pertains in general to devices for multiplying the frequency of microwave signals and in particular to such devices, which have a plurality of different resonant cavities. 
     BACKGROUND OF THE INVENTION 
     Certain applications utilizing microwave signals require the production of a very high frequency signal in the microwave range. In general, the efficiency of producing a high frequency signal becomes less as the frequency of the signal increases. Therefore, the use of a frequency multiplier device with relatively high conversion efficiency can be a practical approach to the production of high frequency microwave signals. One type of microwave frequency multipliers is described in a white paper proposal entitled “High Power, High Efficiency, Monolithic Quasi-Optical Frequency Triplers Using Microwave Power Module Drivers” by N. C. Luhman, Jr. dated Feb. 27, 1996. The frequency multiplier described in this paper is an elongate waveguide device, which includes input and output filters and a multiplier array comprising diodes with antenna leads. A cavity multiplier, which does not utilize switching, is described in U.S. Pat. No. 5,731,752 entitled “Microwave Signal Frequency Multiplier.” 
     There exists a need for a switched, microwave frequency multiplier, which can store energy at a frequency which is a multiple of a source signal and selectively release this energy when required. 
     SUMMARY OF THE INVENTION 
     A selected embodiment of the present invention is a switched microwave frequency signal multiplier, which includes a pump signal cavity for receiving a pump signal via an input port. A nonlinear medium is positioned within the pump signal cavity for receiving the pump signal and producing therefrom a harmonic signal of the pump signal. A frequency selective barrier is positioned within the pump signal cavity for defining a harmonic signal cavity for storing therein energy at the harmonic signal frequency. The frequency selective barrier is substantially transparent to the pump signal and substantially reflective to the harmonic signal. An output port is coupled to the harmonic signal cavity. A two-state switch is positioned in series with the output port. When the switch is set to a first state it blocks the transfer of the stored energy from the harmonic signal cavity. When set to the second state, the switch releases the stored energy from the harmonic signal cavity through the output port to produce a microwave signal pulse. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a perspective, partially cut-away view of a Q-switched frequency multiplier in accordance with the present invention, 
     FIG. 2 is an schematic diagram of the multiplier shown in FIG. 1, 
     FIG. 3 is a chart illustrating the signal production of the multiplier shown in FIG. 1, 
     FIGS. 4A and 4B are waveform charts illustrating the pump pulse and resulting multiple frequency pulse with and without a Q-switch, and 
     FIG. 5 is a second embodiment of the invention having an intermediate energy storage stage. 
    
    
     DETAILED DESCRIPTION 
     A frequency multiplier  10  in accordance with the present invention is illustrated in FIG.  1 . The multiplier  10  receives an input signal ω 1 , which is also referred to as a pump signal. Within the multiplier  10 , the input signal ω 1  is multiplied to produce an output signal ω 3 , which may be, for example, three times the frequency of the input signal. The multiplier  10  includes a switch which causes a build-up of energy from the input signal and selectively releases this energy as the multiplied output signal ω 3 . 
     The multiplier  10  includes a lower housing  12  and a top plate  14  which is secured to the housing  12  by means of screws  16 ,  18 ,  20 , and  22 . The housing  12  and plate  14  are preferably made of brass, aluminum or a plated dielectric material. The input signal ω 3 , is provided to the multiplier  10  through an input waveguide  28 , which is secured to the top plate  14 . The input microwave signal ω 1  is transferred through an opening  30  in the plate  14  to the interior of the housing  12 . The multiplier  10  includes an output waveguide  32 , which is mounted on the plate  14  and receives microwave signal energy via an opening  34  in the plate  14 . The dimensions of each waveguide are principally determined by the frequency of the primary signal carried by the waveguide. The input and output waveguides may also be referred to as input and output ports. 
     A Q-switch  36  is mounted within the waveguide  32  and functions to either block or pass the output signal ω 3 . The Q-switch  36  receives a control signal via the line  37  to turn the switch  36  on and off Within the housing  12  there is provided a grid structure comprising a plurality of layers of materials. At the top of the grid there is provided a nonlinear layer  38 . Immediately below the nonlinear layer  38  there are provided temperature compensation dielectric layers  40  and  42 . Immediately below the layer  42  there is provided a frequency selective layer  44 . 
     The dimensions and relative sizes shown in FIG. 1 are for illustrative purposes and do not necessarily represent actual dimensions or size relationships. Actual dimensions are primarily a function of the selected operating frequencies. 
     The interior of the housing  12  has a surface  50  which is reflective to the microwave energy present within the multiplier  10 . The region within the housing  12  between the lower surface of the plate  14  and the surface  50  comprises a pump signal cavity  52  which is tuned to the frequency of the input signal ω 1 . The region within the housing  12  between the lower surface of the plate  14  and the frequency selective layer  44  comprises a harmonic signal cavity  54  which is tuned to the frequency of the output signal ω 3  which is a harmonic of the input signal ω 1 . The signal ω 3  is preferably the third harmonic of the signal ω 1 , that is, ω 3  has three times the frequency of ω 1 . 
     The Q-switch  36  can be fabricated as any one of the following: 
     (A) PIN diode switch, as described in Application Notes for Bulk Window™ waveguide switch elements, pages 3-3 through 3-8, M/A Com Semiconductor Products Operation, Burlington, Mass. 01803. 
     (B) Micro-Electro-Mechanical Switches (MEMS) as described in U.S. Pat. No. 5,880,921 by Tham, et al. which issued Mar. 9, 1999 and is ent “Monolithically Integrated Switched Capacitor Bank Using Micro Electro Mechanical System (MEMS) Technology.” A further description of this type of switch is given in an article “DARPA Sows Seeds of a Telecom Revolution,” EE Times, Monday, Aug. 4, 1997, starting on page 1. 
     (C) Laser diode switched photoconductive materials/window as described in U.S. Pat. No. 5,796,314 to Tantawi, et al. entitled “Active High-Power RF Switch And Pulse Compression System.” 
     (D) Bulk avalanche semiconductor switch (BASS) as described in U.S. Pat. No. 4,782,222 to Ragle, et al. entitled “Bulk Avalanche Semiconductor Switch Using Partial Light Penetration And Inducing Field Compression.” 
     (E) A latching circulator such as Model OP320 sold by Channel Microwave Corporation, 480 Constitution Avenue, Camarillo, Calif. 93012. 
     The nonlinear material layer  38  is preferably a doped superlattice, which comprises a photonic band gap structure. Such structures are described in “Photonic Band-Gap Structures” by E. Yablonovitch in  Journal of the Optical Society of America Bulletin , Volume 10, No. 2, Febuary 1993, pp. 283-295. As described in the article, the dimensions of the material are a function of the operating frequency. An applicable photonic band gap structure is further described in “Applications of Photonic Band Gap Structures” by Henry O. Everitt in  Optics and Photonics News , November 1992, pp. 20-23. 
     The temperature compensation dielectric layers  40  and  42  are preferably dielectric layers chosen for their dielectric property behavior over frequency and thickness, so as to achieve a given level of volume fill to achieve temperature compensated device operation. This technique is described in HTS Microwave Cavity with Temperature Independent Frequencies-Mueller, et al. in IEEE Transactions on Applied Superconductivity, Vol. 5, No. 2, June 1995, pp. 1983-1986. 
     The temperature compensation dielectric layers may exist separately from, or form part or all of the frequency selective layer  44 . 
     The frequency selective layer is preferably fabricated as described in “Low-Loss Microwave Cavity Using Layered-Dielectric Materials” by C. J. Maggiore, et al. in  Appl. Phys. Lett. 64(11), Mar. 14, 1994, starting at p. 1451. The arrangement of dielectric materials described in this paper is a superlattice. 
     The multiplier  10  is shown in FIG. 1 with a rectangular configuration, which in a selected embodiment can have a length dimension of approximately one (1) inch and a thickness of approximately 0.25 inch. However, it can have other shapes, such as rectangular or round, with the size primarily depending upon the necessary sizes required for the tuned cavities  52  and  54 . 
     The embodiment shown in FIG. 1 uses microwave waveguides but can also utilize other microwave conductors such as microstrip and coaxial lines. 
     In operation, the switch  36  is placed in a first state to block the release of energy from the multiplier  10  while input energy in the form of signal ω 1  is received. This energy is transferred to the cavity  52 , but as the input signal passes through the nonlinear layer  38 , a harmonic signal having components, including a third harmonic, is produced, and this harmonic is at the frequency of the desired output signal ω 3 . The frequency selective layer  44  is transparent to the input signal ω 1 , the lower microwave frequency, but is reflective to the harmonic microwave signal ω 3 , which is at a substantially higher microwave frequency. Thus, the signal at the frequency of ω 3  is trapped within the smaller cavity  54 , but the input signal ω 1  is present within a larger cavity  52 . The lower frequency signal within the cavity  52  is converted into the higher frequency signal ω 3  and trapped within the cavity  54 . 
     When the Q-switch  36  is switched to its second state and becomes transparent to the signal ω 3 , the energy within the cavity  54  is transferred through the opening  34 , the switch  36 , and out through the waveguide  32  as signal ω 3 . In blocking the output signal by use of the switch  36 , energy from the input signal ω 1  can be stored as high frequency energy ω 3 , to produce a high-energy pulse upon opening the switch  36 . 
     An electrical schematic circuit  60  is shown in FIG. 2 for illustrating the operation of the multiplier  10  shown in FIG.  1 . Elements that are common to FIG.  1  and FIG. 2 carry the same reference numeral. A source  62  indicates a generator for the input signal ω 1 . A resistor  64  represents the loss of the source and the loss in the input waveguide. An impedance  66  represents a matching section between the input signal source and the first filter cavity  52 . 
     A variable capacitor  68  represents the nonlinear layer  38 . A resistor  70  represents losses in the nonlinear layer  38 . An impedance  72  represents a matching section between the output cavity  54  and the Q-switch  36 . Resistor  74  represents the desired load of output waveguide  32 . The cavities  52  and  54  shown in FIG. 2 correspond to the similarly numbered cavities shown in FIG.  1 . The switch  36  shown in FIG. 2 corresponds to the Q-switch  36  shown in FIG.  1 . 
     The operation of the multiplier  10  as represented by the schematic circuit  60  is as follows. The pump source signal ω 1  is introduced to filter cavity  52 , which encompasses nonlinear layer  38 . Cavity  54  is nested within filter cavity  52  and also encompasses nonlinear layer  38 . Due to the action of the input pump source signal ω 1  on nonlinear layer  38 , harmonic signals are generated that resonate in filter cavity  54 . Switch  36 , while in a closed condition, provides a highly reflective condition to the signal ω 3  in cavity  54 . Due to the high Q value of cavity  54 , the signal ω 3  builds up increasing energy during the pump pulse of signal ω 1 . After a suitable energy level has built up in cavity  54 , a signal is sent to switch  36  to place it in an on condition, which causes the energy of cavity  54  to be suddenly released. The Q of cavity  54  is much reduced with the switch  36  in the on position. The stored energy is able to rapidly exit the Q cavity  54 , which enables a well-defined short pulse to be transmitted into output load  74 . This is shown in FIG.  4 A. 
     FIG. 3 is an illustration of the transfer function provided by the varying dielectric value, shown by curve  76 , of the nonlinear layer  38  which is acted on by input signal ω 1  ( 78 ), and generates an increasing amplitude energy signal ω 3  ( 80 ). Signal  80  is a harmonic of signal  78 . 
     FIGS. 4A and 4B show input and output pulses of the signals ω 1  and ω 3 . FIG. 4A shows input pump pulse  82  of signal ω 1  and output pulse  84  of signal ω 3  for the multiplier  10  shown in FIG. 1, which includes the Q-switch  36 . The Q-switch  36  is closed when the pulse  82  begins. The Q-switch  36  is opened to start the generation of the pulse  84 . The switch  36  is closed at the end of the pump pulse or at a later time, but in any case before the next pump pulse begins. 
     FIG. 4B illustrates what an output pulse ω 3  ( 86 ) would be in response to input pump pulse ω 1  ( 82 ) should the multiplier  10  not include the Q-switch  36 . In such a case, the input and output pulses are essentially the same duration, but the peak power of the output pulse is much less. 
     As shown in FIG. 4A, the output signal pulse is produced near the end of the pump pulse at a much larger energy level than would have been available had no Q switch operation been performed to store energy. This is illustrated by a flat top output pulse  86 , which represents the output pulse without Q switching, which pulse lasts as long as the input pulse, and a higher level output pulse  84 , which exists only near the end of the pump pulse  82 . 
     FIG. 5 illustrates a multiplier  90  in accordance with a further aspect of the present invention. The elements of multiplier  90  that are the same as those for the multiplier  10  shown in FIG. 1 have corresponding reference numerals. An additional cavity  55  is added to FIG. 5 representing an idler cavity. This enables the multiplier to provide additional frequency outputs by adding frequency terms for the pump harmonics. The idler cavity  55  can be nested within the input pump cavity as shown in FIG. 5, enclosing the nonlinear layer, as well as the desired output cavity  54 . 
     The multiplier  90  includes a second group of layers  96 ,  92 , and  94  which are offset by a gap below the layers  38 ,  40 ,  42 , and  44 . The layers  96  and  92  are temperature compensation dielectric layers corresponding to the previously described dielectric layers  40  and  42 . The layer  94  is a frequency selective layer that corresponds to frequency selective layer  44 . However, layer  94  has a different cut-off frequency. The frequency selective layer  94  is transparent to the input signal ω 1 , but is reflective for an intermediate frequency ω 2 , which is at a greater frequency than ω 1 , but a lesser frequency than ω 3 . For example, the signal ω 3  can be three times the frequency of signal ω 1 , and signal ω 2  can be twice the frequency of signal ω 1.    
     U.S. Pat. No. 5,731,752 entitled “Microwave Signal Frequency Multiplier” which issued Mar. 24, 1998 is incorporated by reference herein. The structures shown in this patent may be utilized as a part of the present invention. 
     Although several embodiments of the invention have been illustrated in the accompanying drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention.