The present invention relates generally to electronic circuit components, and more particularly to variable capacitors and variably tunable tank circuits.
Many high frequency electronic systems benefit from the use of tunable passive elements such as capacitors and resonators. However, the performance of these tunable elements is typically limited by linearity, intermodulation products, loss and power handling. For example, a varactor diode is commonly used to provide a variable capacitance, however, a varactor often suffers from a limited tuning range (20%), high loss, poor intermodulation performance, and limited power handling. In other circuits, ferroelectric devices are used as tuning elements in place of varactor diodes. In yet other instances, microelectromechanical variable capacitors are used as tuning elements. However, all of these techniques suffer from poor linearity, which is an especially relevant constraint under high RF signal power conditions.
As is known, resonators with a variable resonant frequency can be constructed by assembling discrete variable capacitor and inductor elements. However, these resonant circuits typically suffer from a poor quality factor (Q), resulting in diminished narrowband performance such as increased insertion loss in the case of a filter. It is desirable to construct a resonant cavity wherein the unloaded Q is very high, thus allowing the implementation of a low insertion-loss narrowband tunable filter, or a low-phase noise tunable oscillator. Generally speaking, the quality factor is limited by the Q of the discrete elements that comprise a circuit. Losses in either an inductor element or a capacitor element will have the effect of reducing the overall system Q. A circuit design which minimizes the losses associated with these reactive elements, and minimizes the interconnection and parasitic losses is very desirable.
Given the breadth of applications for tunable passive elements such as capacitors, inductors and resonantors, it would be desirable to overcome the aforesaid and other disadvantages, and to provide an electronic circuit component capable of providing a relatively wide tuning range and a relatively high Q, low intermodulation, high linearity and thermal stability.
A radio receiver is but one example of a wide variety of electronic devices that require the ability to tune to selected frequencies. Other examples include, but are not limited to, radio transmitters, power amplifiers, wireless telephones (voice and data), wireless modems, cable modems, radar systems, and scientific instrumentation, and all would make use of and be based upon the design and construction and operation disclosed in earlier U.S. Pat. No. 5,964,242 to Alexander H. Slocum, who is a co-applicant herein, and U.S. Pat. No. 6,914785 to Alexander H. Slocum et al, the contents of both of which are herein incorporated by reference.
Many electronic devices require the ability to selectively tune one or more circuits to receive or transmit a selected one of a variety of radio signals, each associated with a relatively narrow band of frequencies about a corresponding center frequency. For example, a conventional radio receiver is designed to manually or automatically tune to enable reception of a selected radio signal from among many radio signals. By selectively tuning the radio receiver, any selected one of the many of radio signals can be received, down-converted to an audio signal, and presented to a user for listening. As is known, the many radio signals span a relatively wide frequency range, while each individual radio signal spans a relatively narrow frequency range, each having a different center frequency.
While the conventional radio receiver has selective tuning to tune near selected ones of the many radio signals, i.e. with selective “coarse” tuning, it should also be appreciated that the conventional radio receiver also has selective “fine” tuning, to tune within a narrower frequency range. Such fine tuning can variably move a tuned center frequency, first selected by the coarse tuning, to more accurately select a particular center frequency.
As is known, fixed electrical components typically suffer from component value drift with time and temperature, which can result in drift of a tuned circuit. With the selectable tuning described above, tuning drift can be overcome, and a tuning circuit, regardless of component drift, can still tune to a desired center frequency.
Some characteristics that are important in determining the effectiveness of an electronic tuning circuit include a total frequency span over which the selective tuning can tune, i.e., a coarse tuning range, an accuracy of the tuning, i.e. a fine tuning range and accuracy, and a selectivity of the tuning. The selectivity will be understood to be characterized by a quality or Q factor (or more simply “Q”), associated with the relative amplitude of a resonant peak and hence the minimum filter bandwidth capabilities.
Conventional electronic circuits are known which can provide selective coarse tuning over a wide range of frequencies, but with only a relatively low Q. For example, a phase locked loop (PLL), having a programmable divider, can provide selective tuning in a relatively wide range of frequencies. Conventional electronic circuits are also known which can provide selective tuning over only a small range of frequencies, but with a high Q on the order of several hundred. For example, a varactor diode is known to provide a variable capacitance, which can be used in conjunction with a fixed inductor and other electronic components in a resonant tank circuit to provide selective fine tuning. To this end, there also exist other passive components used in tank circuits (e.g. crystals, surface acoustic wave (SAW) devices, and bulk acoustic mechanical resonators), which provide relatively high Q (on the order of a thousand), low noise, and high stability necessary for highly-selective, low-loss fine tuning at radio frequencies (RF) and intermediate frequencies (IF). While a high Q is obtained with tank circuits, if used in a radio receiver without coarse tuning circuitry, the tank circuit could not tune over the full AM and FM frequency bands. Therefore, it should be understood that with conventional circuits a tradeoff must typically be made between total tuning frequency range and Q.
In order to achieve both a wide range of tuning and a high Q, many conventional electronic circuits incorporate both coarse tuning circuits, which conventionally have a wide tuning range but low Q, and fine tuning circuits, which conventionally have a low tuning range but a high Q. It will, however, be understood that the coarse tuning circuits and fine tuning circuits in combination represent a relatively complex and expensive electronic structure.
To replace the circuits described above, researchers have sought to develop micro electromechanical systems (MEMS) to provide on-chip voltage-tunable capacitors, low-loss inductors, and on-chip mechanical resonators. MEMS capacitors with a tuning range of approximately 6:1 at radio frequencies (RF) are known, but their robustness and Q have not met requirements. In addition, very low-loss inductors have yet to be demonstrated by other research groups.
It would, therefore, be desirable to overcome the aforesaid and other disadvantages, and to provide an electronic circuit component capable of providing a relatively wide tuning range and a relatively high Q.
The present invention provides a tunable capacitor and/or a tunable tank circuit capable of tuning at relatively high signal frequencies, over a relatively wide range of frequencies, and with a relatively high Q factor, fabricated using electroforming, ceramic printed circuit board, and joining technology.
In accordance with the present invention, a circuit component has a first structure provided from an elastically deformable material. The circuit component also has a second structure with a surface proximate a surface of the first structure. The first and the second structures are coupled with a support structure which also acts as an elastic constraint to the first structure. The first structure can be elastically deformed, causing a portion of the surface of the first structure to move relative to the surface of the second structure, varying a gap. In one particular embodiment, the gap can range from microns to nanometers in size and is controllable with nanometer resolution. In one particular embodiment, the surface of the first structure and the surface of the second structure which are in proximity, each have a first conductive region, forming a first capacitor, the capacitance of which varies in proportion to the movement of the first structure relative to the second structure. In another embodiment, the surface of the first structure and the surface of the second structure which are in proximity, each also have at least one other conductive region, forming an inductor in parallel with the capacitor, and therefore, forming a tank circuit. In yet another embodiment, the circuit component includes a piezoelectric disk laminated or otherwise attached to the elastically deformable region of the first structure to form a piezoelectric bimorph actuator. In yet another embodiment, a flexible circuit element comprised of insulating and conducting layers may be disposed on the upper surface of the resonator or on the lower surface of the piezoelectric disc to electrically insulate the piezoelectric actuator from the elastically deformable metal structure, and to provide an electrical contact to the bottom surface of the piezoelectric disc.
To simplify the manufacturing process and reduce manufacturing costs, an inventive fabrication process for the production of the variable electrical circuit components of the present invention, incorporating metal electroforming techniques known for use in other applications was developed. The first elastically deformable structures of the inventive variable electrical circuit components may advantageously be fabricated by electroplating one or more thin layers of conductive material onto a mandrel having a complementary shape, polishing the surface of the electroplated layer until it exhibits a fine surface finish, dicing the electroplated layer into individual components and then releasing the electroplated layer from the mandrel using standard techniques, resulting in thin free-standing metal structures. This first metal structure may then be joined to a second structure having a conductive circuit topography patterned onto its surface. The first and second structures may be joined by means of an intervening conductive adhesive, or by direct joining techniques such as ultrasonic welding or thermocompression bonding. In one particular embodiment, a piezoelectric ceramic may be laminated onto the top surface of the first elastically deformable structure, providing a means of deforming the first structure in response to an applied electric field, and thus electronically controlling the capacitor gap. In another embodiment, the piezoelectric ceramic may be incorporated into the electroforming mandrel and is intimately joined to the first elastically deformable structure without intervening adhesives. This provides a significant advantage in reducing mechanical hysteresis associated with the deformation of the adhesive layer, and assembly complexity. By creating multiple such features on a larger mandrel, many such devices may be made in a single batch process.
With this particular arrangement of the present invention, a MEMS capacitor having a selectably variable capacitance value is provided. The capacitor can be provided as part of a variable tank circuit having a relatively wide tuning range and a relatively high Q.
In another arrangement, a stripline circuit pattern may be disposed upon the second substrate wafer forming the second structure of the variable electrical circuit component of the present invention, such that a variable input coupling capacitor, a variable tank capacitor and a variable ouput coupling capacitor may be formed between the second substrate and the top deformable conductive region of the first structure. In such an arrangement, the input and output capacitors have the effect of transforming the resonator impedance to the impedance of the input and output striplines respectively. Adjusting the size of the coupling capacitors allows the designer to adjust the electrical bandwidth of the resonator. In another embodiment, a circuit pattern may be disposed upon the second substrate wafer such that a fixed inductive input coupling structure and a fixed inductive output coupling structure are formed. Thus, either magnetic or capacitive coupling circuits can be formed to couple electromagnetic energy into and out of the variable tunable element.
With this particular arrangement, the method provides a variable capacitor and/or a variable tank circuit having a relatively wide tuning range and a relatively high Q.
The foregoing features of the invention, as well as the invention itself may be more fully understood from the following detailed description of the drawings, in which:
Before describing the circuit components of the present invention, mention is made as to the format of some of the figures. Those figures shown and described as cross-sectional figures are drawn without some hidden lines representing features behind the section region. Those lines behind the section region, if drawn, would add unnecessary complexity to the drawings and obscure the features which are described. In effect, the cross-sectional figures may be thought of as “slice” figures, representing a slice of an apparatus.
Referring now to
Referring now also to
The exemplary circuit component 100 also includes a second (or lower) structure 200, having a first surface 205a and a second surface 205b. The conductive material disposed upon the first surface 205a of the lower structure 200 is structured to provide an input coupling capacitor plate 210a, a tank capacitor plate 220 and an output coupling capacitor plate 210b. Thus, three parallel-plate capacitors may be formed between the input plate 210a, the tank plate 220 and the output plate 210b and the movable top plate 120. Conductive vias 211a and 211b provide an electrical contact path to the second conductive layer 205b. Input and output striplines, 212a and 212b respectively, are used to couple electrical power into and out of the coupling capacitor plates 210a and 210b. A bottom conductive material is disposed upon the surface 205b and patterned to define input and output striplines 212a and 212b, respectively, and a ground plane 215. The tank capacitor plate 220 may preferably be electrically grounded. Additional ground vias (not shown) may couple the top ground plane regions 209a, 209b and 220 to the bottom ground plane 215, thereby decreasing any parasitic coupling between input and output striplines 212a and 212b respectively.
In one exemplary embodiment, the force F can be provided by piezoelectric element 300 coupled to the second surface 101b of the first structure 101. In such an embodiment, in response to a signal provided thereto, the piezoelectric element may provide a force upon the first structure 101 in the lever regions formed by the side structure 140. While the piezoelectric element 300 is shown, in other embodiments, an external piezoelectric stack or any suitable electrostatic or electromechanical actuator can be provided in place of, or in addition to, the piezoelectric element 300 to provide the force F upon the second surface 130.
In one particular embodiment, the first structure 101 may be made from metal, for example copper metal, using electroforming techniques, and the second structure 200 may be made from ceramic, such as for example, Aluminum Nitride, Aluminum Oxide, or Pyrex™ with conductive regions disposed and patterned thereupon using conventional circuit processing techniques that are widely known in the art. In another embodiment, the first structure 101 may be made from a metal alloy, for example “Alloy 42”, whose composition of Nickel and Iron may be adjusted such that the metal alloy has a coefficient of thermal expansion that is closely matched to the ceramic of the second structure 200. Furthermore, the inner surface 101a of the first structure 101 can have a thin layer (1-3 microns) of non-ferromagnetic material such as copper or gold disposed upon it to desirably reduce the level of third-order intermodulation at RF frequencies.
As described above, in other embodiments, the force F can equally well be applied with another type of actuator in place of or in addition to the piezoelectric element 300. For example, in other embodiments, the force F can be applied with an external electromechanical actuator or piezoelectric stack actuator (not shown).
Because the gap δ of the circuit component 100 has a high aspect ratio, i.e., a major axis or a diameter d much greater than the gap δ, which can be precisely controlled, the circuit component 100 can form a capacitor having a relatively wide range of achievable capacitance values. A tuning ratio can be defined as the largest capacitance value which can be achieved divided by the smallest capacitance value which can be achieved, and the capacitor 100 is provided having a relatively high tuning ratio. In one particular embodiment, the tuning ratio may be 10, although values up to at least about 100 may be achieved. With addition of an integral inductor as described more fully below, a tunable LC resonator circuit, or LC tank circuit, may operate from, for example, UHF (Ultra-High Frequency) to SHF (Super-High Frequency) and may be capable of band selection over a wide frequency range. It should, however, be appreciated that the structures and techniques described herein may also be applied to frequency ranges which are lower than and higher than UHF and SHF.
Referring now to
Referring now to
Referring now to
The electrical response characteristics of the circuit component 100 may be analyzed by first assuming that a current flows into the conductive region 160 and out the conductive region 220, by also assuming that current distributes evenly, forming a surface current Kf in the closed conductor 190, by also assuming that magnetic flux lines (not shown) are contained inside the effective toroid 150 formed by the conductive regions 190 and 180 respectively, and by assuming that an H field is zero directly outside of the closed conductor. A boundary condition, n×(Ha-Hb)=Kf, may be used, where Ha is inside the toroid and Hb is outside. Therefore, in such case, the H field inside the toroid is Ha=Kf.
The surface current Kf is a function of the radius r is:
The flux density is thus
To calculate inductance, the total flux in the toroid may be calculated. This is done by integrating the flux density across a cross-sectional area of the toroid. Dividing the flux-linkage by the current gives the inductance,
Capacitance between the conductive regions 160 and 220 respectively, derived by inspection, is written below, taking into account the effect of a higher permittivity, ε1, of the oxide layer 131 and the thickness δ1 of the oxide layer 131:
The resistance of the toroid, i.e., effective resistance in series with the inductor formed by the conductive regions 190 and 180 respectively, is calculated below. A skin depth wAu is a function of resonant frequency. The calculated resistance below does not take into account dielectric hysteresis, radiation, charge relaxation time constants, and leakage through first structure 101, all of which tend to reduce the Q of the tank circuit.
Referring now to
and the capacitance corresponding to the coupling circuit 210a may be represented by:
Referring now to
In one particular embodiment R1 is 2.5 mm, R2 is 5.8 mm, d is 3 mm, the thickness of the insulating layer 131 is 100 nm, the variable gap δ can be varied in a range between about 1 μm and 20 μm (although the desired range could be from about 100 μm to 10 nm), the closed conductor 191 may comprised of gold having a skin depth of 1.61 μm, a calculated inductance of the toroid 150 is 505 pico-Henries (pH), a calculated equivalent series resistance of the toroid is 8.2 mΩ, a capacitance of the capacitor formed by the conductive regions 160, 170, respectively, varies between 173 pico-Farads (pF) and 8.69 pF as the variable gap is varied in the above range. The coupling capacitor regions are each 0.75 mm×0.5 mm, thus the coupling capacitance varies between 0.16 pF and 3.3 pF. The resonant frequency of resonant cavity varies between 534 Mhz and 2.38 GHz as the variable gap is varied in the above range, and the loaded Q varies between 26.7 and 198 as the variable gap is varied in the above range, and the 3 dB bandwidth of the resonance, given 50-Ohm input and output coupling, is between 20 Mhz and 12 Mhz as the variable gap is varied in the above range. However, in other embodiments, other dimensions and characteristics can be selected in order to provide a circuit component having another capacitance range, another inductance, another bandwidth, another range of resonant frequencies, and another range of Qs.
Referring now to
To electrically isolate the variable capacitor from the actuation circuitry, an RF choke 815 may be connected between the conductive structure 701 and the ground 816, with a wire 817. Likewise, an RF choke 811 may be connected with a wire 813 to the top surface 301 of the piezoelectric element 300. The RF choke 811 may be connected to the variable voltage supply 812, which provides a control voltage to the piezoelectric bimorph actuator, thus varying the gap δ, in a manner similar to that employed in the tunable resonator device described earlier.
The variable capacitors 841 and 842 are electrically connected in series, thus their equivalent capacitance is:
All references cited herein are hereby incorporated herein by reference in their entirety.
Having described preferred embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may be used. It is felt therefore that these embodiments should not be limited to disclosed embodiments, but rather should be limited only by the spirit and scope of the appended claims.
This is a Continuation Application of U.S. patent application Ser. No. 11/392,980, filed on Mar. 28, 2006, and entitled, “A Variable Electrical Circuit Component.”
Number | Date | Country | |
---|---|---|---|
Parent | 11392980 | Mar 2006 | US |
Child | 12378889 | US |