Abstract:
A high power slide tuning capacitor ( 200 ) integrated into a circuit board adapted to function at RF and microwave frequencies. The capacitor ( 200 ) is comprised of a dielectric substrate ( 26 ) with a cavity ( 28 ) cut into a side of the substrate ( 26 ); a ground plane ( 24 ) mounted on a bottom surface of the dielectric substrate ( 26 ); a microstrip patch ( 22 ) mounted on a top surface of the dielectric substrate ( 26 ) and positioned above the cavity ( 28 ); and a movable dielectric stub ( 30 ) which engages the cavity ( 28 ) such that a variable length of the stub ( 30 ) is positioned beneath the microstrip patch ( 22 ). The microstrip patch ( 22 ) can be coupled to a desired circuit trace ( 20 ), effectively forming a shunt capacitor to ground.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to electronic circuits and systems. More specifically, the present invention relates to variable capacitors for use in high power radio frequency applications.  
           [0003]    2. Description of the Related Art  
           [0004]    High power radio frequency (RF) transistors are ever increasing in power. In the last several years, the power of RF transistors in the cell phone band has doubled approximately every two years. Amplifier circuits often use several of these transistors together to generate thousands or even millions of watts of power. However, the power handling capability of other lumped element components that comprise a high power transistor amplifier (e.g. capacitors, resistors, inductors, etc.) has not increased nearly as rapidly. Therefore, the design of high power RF amplifiers using the newest high power transistors is difficult due to the lack of high power lumped element components. In particular, variable RF tuning capacitors that can handle the required power levels are not available.  
           [0005]    In a typical high power RF microstrip amplifier, a 50 ohm input transmission line transitions into transmission lines of much lower impedances (the wider transmission lines). At the transition between different width lines, a shunt capacitor to ground is needed. The low impedance line is then connected to the input (e.g. gate) of the transistors. A similar embodiment is used at the output (e.g. drain) of the transistors.  
           [0006]    These types of matching networks are used to transform 50 ohms into the smaller input/output impedance (˜1 ohm) of the transistor. By making the shunt capacitors variable, the matching networks can be tuned. This tuning is necessary to account for differences in transistor performance, circuit board characteristics, and circuit etching tolerances as well as variations in any other lumped element components that my be in the amplifier circuit.  
           [0007]    Lumped element variable capacitors are available which can handle up to 200 watts of power. However, if the power output is pushed just a small percentage beyond this, the capacitors will fail. Higher power variable capacitors are available, but they are not suitable at RF frequencies (they self resonate below 900 MHz).  
           [0008]    Hence, a need exists in the art for an improved capacitor which can handle the voltage and current levels applied in high power, high frequency applications.  
         SUMMARY OF THE INVENTION  
         [0009]    The need in the art is addressed by the variable capacitor of the present invention. In a most general implementation, the inventive capacitor includes a dielectric substrate with a cavity disposed therein. A first conductive surface is mounted on a first side of said cavity and a second conductive surface is mounted on a second side of the cavity. The second conductive surface is mounted so that it is at least partially diametrically opposed to the first conductive surface. In accordance with the present teachings, a dielectric stub is provided which is adapted for reciprocal motion within the cavity.  
           [0010]    In a specific implementation, the first conductive surface is a ground plane and the second conductive surface is a microstrip patch. In the illustrative embodiment, the cavity is rectangular with four rectangular side walls, a rectangular top and a rectangular bottom. In the illustrative embodiment, the patch and stub are also rectangular. An electro-mechanical actuator reciprocates the dielectric stub into and out of the cavity to provide a desired amount of capacitance in response to a control system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a diagram of a conventional high power RF microstrip amplifier.  
         [0012]    [0012]FIG. 2 is a diagram showing a slide tuner capacitor designed in accordance with the teachings of the present invention.  
         [0013]    [0013]FIG. 3 a diagram showing the critical dimensions of a slide tuner capacitor designed in accordance with the teachings of the present invention.  
         [0014]    [0014]FIG. 4 is a diagram of a slide tuner capacitor with robotic tuning designed in accordance with the teachings of the present invention. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0015]    Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention.  
         [0016]    While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.  
         [0017]    [0017]FIG. 1 is a diagram of a typical embodiment of a conventional high power RF microstrip amplifier  100 . A 50 ohm input transmission line  10  transitions into transmission lines  12  of much lower impedances (the wider transmission lines). At the transition between different width lines, a shunt capacitor (C 1 , C 2 ) to ground is placed. The low impedance line  12  is then connected to the input (e.g. gate) of the transistors (M 1 , M 2 ). A similar embodiment is used at the output (e.g. drain) of the transistors. The output of the transistors (M 1 , M 2 ) is connected to low impedance transmission lines  14  which transition to a 50 ohm output transmission line  16 . At the transition between different width lines, a shunt capacitor (C 3 , C 4 ) to ground is placed.  
         [0018]    The capacitors (C 1 , C 2 , C 3 , C 4 ) are typically commonly available lumped element capacitors. These capacitors, however, cannot handle the power levels available from high power RF transistors. The present invention is a high power slide tuning capacitor that is integrated into the circuit board itself which can function at RF and microwave frequencies.  
         [0019]    [0019]FIG. 2 is a diagram showing a slide tuner capacitor  200  designed in accordance with the teachings of the present invention. A distributed capacitance  22  is etched into the microstrip circuit, shunting a desired circuit trace  20 . On the underside of the circuit board  26 , a cavity  28  is milled beneath the distributed capacitor  22 . A heat sink or ground plane  24  is then installed under the circuit board  26 . The cavity  28  forms a channel in the circuit board  26  under the distributed capacitance  22 . A rectangular piece of dielectric material  30 , slightly smaller than the channel  28 , is then inserted into the channel  28 . By changing the depth of insertion of this dielectric slide tuner  30 , the value of capacitance of the distributed shunt capacitor  22  is changed.  
         [0020]    [0020]FIG. 3 a diagram showing illustrative dimensions of a slide tuner capacitor designed in accordance with the teachings of the present invention. In this figure, the length of the distributed element capacitor  22  is L and the amount of insertion of the dielectric slide tuner  30  is d (the length of the slide tuner  30  which lies under the distributed capacitor  22 ).  
         [0021]    For frequencies around 1000 MHz, in the frequency range between those designated for lumped element and distributed element, this invention can be modeled either as a parallel plate capacitor or as a transmission line stub.  
         [0022]    In the parallel plate model, the ground plane  24  is one plate and the distributed capacitance  22  is the other plate. The region between the two plates is partially filled with dielectric (length d), and partially filled with air (length L−d). If this is modeled using Maxwell&#39;s equation, it is found that there is capacitance at this port, and changing the length of dielectric that is between the two plates effectively changes the capacitance.  
         [0023]    In the transmission line model, the distributed capacitance  22  is a transmission line which is open circuited at one end. Electromagnetic waves travel in the region between the distributed capacitance  22  and the ground plane  24 . This region is partially filled with dielectric (length d), and partially filled with air (length L−d). Waves traveling in the dielectric will have a shorter electrical wavelength than those in air. Thus, if a larger length d of the dielectric stub  30  is inserted beneath the distributed capacitance  22 , a larger capacitance is found at that port.  
         [0024]    Using mathematical software to solve for these two models results in the same solution. The shunt capacitance C can be shown to be:  
       C   =       1       2      π                 F                               Z   c1          Z   c2       +       Z   c2   2                     tan        (       β   1        d     )                       tan        [       β   2          (     L   -   d     )       ]                 Z   c2          tan        (       β   1        d     )         +       Z   c1                     tan        [       β   2          (     L   -   d     )       ]                                     
 
         [0025]    where F is the frequency, Z c1  and β 1  are the characteristic impedance and wave number, respectively, of the distributed capacitor line for the portion of the line with the slide tuner underneath, and Z c2  and β 2  are the characteristic impedance and wave number, respectively, of the distributed capacitor line for the portion of the line without the slide tuner underneath.  
         [0026]    By changing the value of d, the capacitance C can be varied over a wide range. Slide tuners of this nature have been successfully built and tested that vary in range from a couple of picofarads up to 20 or 30 picofarads.  
         [0027]    This invention can be practiced using standard materials and techniques for fabricating printed circuit boards. The dielectric slide tuner  30  can be fabricated using the same or different material as the dielectric substrate of the circuit board. Using a slide tuner with a higher dielectric coefficient than that of the circuit board will result in a more pronounced capacitance effect. This allows the use of a smaller patch of conductor for the distributed capacitor  22 ; however, if the patch becomes too small, the distance over which the dielectric tuner  30  can be slid becomes too small, making the tuned point difficult if not impossible to achieve.  
         [0028]    One method for producing the channel  28  follows. Those skilled in the art will appreciate that any method for producing the channel can be used without departing from the scope of the present teachings. After a circuit has been etched onto a circuit board, the board has copper (or another conductor) on both sides of the board: one side has been etched and the other is the ground plane. The channel  28  is carefully cut out from the side with the ground plane, cutting away some of the copper of the ground plane. A heat sink (a plate made of aluminum or some other conductor) is then attached to the ground plane. This essentially replaces the original ground plane so that the portion which was cut out is no longer a problem.  
         [0029]    The equation given above for the capacitance C assumes that all of the dielectric substrate between the distributed capacitor  22  and the ground plane  24  is removed. In practice, attempting to remove all of the dielectric would likely result in destroying the copper etching which is very thin. An effective compromise is to remove about 70-80% of the dielectric, leaving the rest as a structural support for the copper etching.  
         [0030]    An additional advantage of the slide tuner is that it can be easily adapted for robotic tuning. Conventional tuning capacitors require an operator to manually turn a screw next to the board. With the higher power amplifiers currently being designed, this has become unsafe.  
         [0031]    [0031]FIG. 4 is a diagram of a slide tuner capacitor  200  with robotic tuning designed in accordance with the teachings of the present invention. In this setup, the dielectric slide tuner  30  is connected to an electro-mechanical actuator  32 . A computer or controller  34  drives the actuator  32  to automatically slide the dielectric  30  to provide a desired amount of capacitance in response to user and/or system inputs. Those skilled in the art will appreciate that this type of arrangement can be used in an automated test/tune environment, where the amplifier performance can be automatically optimized via the slide tuner positions.  
         [0032]    Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof.  
         [0033]    It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention.  
         [0034]    Accordingly,