MEMS tunable filters

[0002] The present invention relates to filters. More particularly, the invention relates to a method and apparatus using micro electro mechanical system (MEMS) technology for tuning a filter.

[0003] Several types of filters are commonly used in electronic applications. These filters include, for example, high-pass filters, low-pass filters, band-pass filters, and band-stop filters. Each filter type provides a specific filtering function to meet a required performance characteristic.

[0004] The above-mentioned filters are well known in the art and will not be discussed in detail. Briefly, a high-pass filter has a passband from some frequency ω_pup upward, and a stopband from 0 to ω₅(where ω_s<ω_p). Conversely, a low-pass filter has a passband from 0 to ω_p, and a stopband from ω_supward (where ω_p<ω_s).

[0005] Band-pass and band-stop filters are similar to high-pass and low-pass filters, but include additional cutoff frequencies to accommodate the added filtering criteria. For example, a band-pass filter has a passband from ω_p1to ω_p2, and a stopband from 0 to ω_s1and ω_s2upward (where ω_s1<ω_p1<ω_p2<ω_s2). Conversely, a band-stop filter has a passband from 0 to ω_p1and from ω_p2upward, and a stopband from ω_s1to ω_s2(where ω_p1<ω_s1<ω_s2<ω_p2).

[0006] The need for a high-quality factor (Q), low insertion loss tunable filter pervades a wide range of microwave and RF applications, in both military, e.g., radar, communications and electronic intelligence (ELINT), and commercial fields such as in various communications applications, including cellular. For example, placing a sharply defined band-pass filter directly at the receiver antenna input will often eliminate various adverse effects resulting from strong interfering signals at frequencies near the desired signal frequency in such applications. Because of the location of the filter at the receiver antenna input, however, the insertion loss must be very low to not degrade the noise figure. In most filter technologies, achieving a low insertion loss requires a corresponding compromise in filter steepness or selectivity.

[0007] In many applications, particularly where frequency hopping is used, a receiver filter must be tunable to either select a desired frequency or to trap an interfering signal frequency. Thus, the insertion of a linear tunable filter between the receiver antenna and the first nonlinear element (typically a low-noise amplifier or mixer) in the receiver offers, providing that the insertion loss is very low, substantial advantages in a wide range of RF and microwave systems. For example, in radar systems, high amplitude interfering signals, either from “friendly” nearby sources, or from jammers, can desensitize receivers or intermodulate with high-amplitude clutter signal levels to give false target indications. In high-density signal environments, RADAR warning systems frequently become completely unusable.

[0008] Micro Electro-Mechanical Systems (MEMS) technology is currently implemented for the fabrication of narrow band-pass filters (high-Q filters) for various communication circuits (see U.S. Pat. No. 6,275,122 issued to Speidell et al.). These filters use the natural vibrational frequency of micro-resonators to transmit signals at very precise frequencies while attenuating signals and noise at other frequencies. A conventional MEMS band-pass filter device includes a semi-conductive resonator structure suspended over a conductive input structure, which is extended to a contact. By applying an alternating electrical signal on the input of the device, an image charge is formed on the resonator, attracting it and deflecting it downwards. If the alternating signal frequency is similar to the natural mechanical vibrational frequency of the resonator, the resonator may vibrate, enhancing the image charge and increasing the transmitted AC signal. The meshing of the electrical and mechanical vibrations selectively isolates and transmits desired frequencies for further signal amplification and manipulation.

[0009] Tuning the resonator frequency in the above described MEMS filter can be implemented by applying a DC bias voltage relative to the input contact, which will apply an internal stress to the resonator. Alternatively, a DC bias voltage can be applied relative to the output contact which will cause a current to flow through the resonator, thus increasing its temperature. Both types of bias change the modulus of elasticity of the resonator, resulting in a change of its fundamental natural vibrational frequency and therefore changing the filter characteristics.

[0010] A drawback to this approach of tuning the resonator frequency is that there are numerous variables that must be taken into consideration to determine the change in resonator frequency. These variables include, for example, the actual current injected into the device, the actual temperature rise of the device due to the injected current, elasticity variations of the resonator, and the ambient temperature. A slight error, for example, in the calculation of the temperature rise or in the effect of the ambient temperature may result in an error in the tuning frequency and thus less than optimal performance of the filter.

[0011] Tunable filters also have been implemented using a micro electro mechanical (MEMS) variable capacitor, wherein the capacitance is altered by changing the distance between the capacitor plates. In the simple vertical motion, parallel plate form of this device, a thin layer of dielectric separating normal metal plates (or a normal metal plate from very heavily doped silicon) is etched out in processing to leave a very narrow gap between the plates. The thin top plate is suspended on four highly compliant thin beams which terminate on posts (regions under which the spacer dielectric has not been removed). When a DC tuning voltage is applied between the plates, the small electrostatic attractive force, due to the high compliance of the support beams, causes substantial deflection of the movable plate toward the fixed plate or substrate, thus increasing the capacitance.

[0012] While the conventional MEMS variable capacitor structure is capable of improved Q values and avoids intermodulation problems of “tunable materials”, it has some potential problems. Because only the relatively weak electrostatic attraction between plates is used to drive the plate motion to vary the capacitance, the plate support “spider” structure must be extremely compliant to allow adequate motion with supportable values of bias voltage. A highly compliant suspension of even a small plate mass may render the device subject to microphonics problems (showing up as fluctuations in capacitance induced by mechanical vibrations or environmental noise). Having the electric field which drives the plates directly in the signal dielectric gap may cause another problem. In order to achieve a high tuning range (in this case, the ratio of the capacitance with maximum DC bias applied to that with no DC bias), the ratio of the minimum plate separation to the zero-bias plate separation must be large (e.g., 10 times would be desirable). Unfortunately, the minimum gap between the plates (maximum capacitance, and correspondingly, maximum danger of breakdown or “flash-over” failure between the plates) is achieved under exactly the wrong bias conditions: when the DC bias voltage is at a maximum.

[0013] Some of the deficiencies of the MEMS variable capacitor described above have been addressed in U.S. Pat. No. 6,347,237. In particular, plate separation control has been improved by the addition of an independent mechanical actuator. Plate motion is provided by a mechanical driver, such as a piezoelectric device, which is coupled to one of the capacitor plates. A tuning signal is connected to the mechanical driver to provide control signals for controlling the plate separation. The mechanical driver eliminates the problems associated with microphonics and other external disturbances and thus, control of plate separation is much more precise.

[0014] While the mechanically driven MEMS variable capacitor provides extremely high Q values and increased immunity to external disturbances, these improvements come with a price. In particular, the piezoelectric material required for the mechanical driver is relatively large, having a length of approximately 5 mm. This length may be reduced to approximately 3 mm through folding of the piezoelectric material. The overall length, however, is significantly large when compared to other integrated components. Furthermore, the mechanical driver requires precision mechanical fabrication and assembly, thus adding cost and time to the manufacturing process.

[0015] Accordingly, there is a need in the art for a tunable filter that is compact in size. Additionally, it would be advantageous to provide such a filter with accurate and repeatable cutoff frequencies and low insertion losses. It would also be advantageous to provide such a filter that is easily manufactured.

[0016] In the light of the foregoing, one aspect of the invention relates to an integrated circuit tunable filter, which includes a substrate, an input line on the substrate, an output line on the substrate, a plurality of tuning stubs on the substrate and a plurality of resonators on the substrate. At least one resonator is operatively coupled to the input line and at least one resonator is operatively coupled to the output line, and the plurality of resonators include at least one MEMS switch, wherein the at least one MEMS switch connects and disconnects the resonator to at least one of the plurality of tuning stubs to adjust the center frequency of the tunable filter.

[0017] A second aspect of the invention relates to an integrated circuit tunable band-pass filter, which includes a substrate, an input line on the substrate, an output line on the substrate, a plurality of interdigitated stripline resonators on the substrate and a plurality of switch-capacitor groups on the substrate. At least one interdigitated stripline resonator is connected to the input line and at least one interdigitated stripline resonator is connected to the output line. Each switch-capacitor group includes a capacitor connected in series to a micro electro mechanical system (MEMS) switch, and each MEMS switch includes a control signal to connect or disconnect the respective switch-capacitor group from one of the plurality of interdigitated stripline resonators.

[0018] A third aspect of the invention relates to an integrated circuit tunable band-stop filter, which includes a substrate, an input line on the substrate, an output line on the substrate, a transmission line on the substrate, a plurality of switch-capacitor groups on the substrate, and a plurality of transmission line resonators on the substrate. The transmission line is operatively coupled to the input line and the output line, and each switch-capacitor group includes a capacitor connected in series to a micro electro mechanical system (MEMS) switch, and each MEMS switch includes a control signal to connect or disconnect the respective switch-capacitor group from the transmission line. Each transmission line resonator is coupled to the transmission line through one of the plurality of switch-capacitor groups.

[0019] To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

[0020] FIG. 1A is a block diagram of an exemplary MEMS switch that may be used in the present invention.

[0021] FIG. 1B is a cross section of the MEMS switch of FIG. 1A in an open position and taken along the line 1B—1B.

[0022] FIG. 1C is a cross section of the MEMS switch of FIG. 1A in a closed position and taken along the line 1C—1C.

[0023] FIG. 2 is a simplified equivalent circuit for several conventional microstrip coupled line filter configurations.

[0024] FIG. 3 illustrates a simplified equivalent circuit in relevant part of a two band switched tunable filter incorporating MEMS switches in accordance with one embodiment of the present invention.

[0025] FIG. 4A illustrates a simplified equivalent circuit in relevant part of a multiple band switched tunable filter in accordance with another embodiment of the present invention.

[0026] FIG. 4B illustrates a simplified equivalent circuit in relevant part of a multiple band switched tunable filter in accordance with another embodiment of the present invention.

[0027] FIG. 4C illustrates selectable capacitive input coupling in accordance with another embodiment of the present invention.

[0028] FIG. 5 is a strip line implementation of a switched tunable filter in accordance with an embodiment of the present invention.

[0029] FIG. 6A illustrates a switched tunable filter in which MEMS switches provide RF connections to tuning stubs for filter tuning and paths for control signals for downstream MEMS switches in accordance with another embodiment of the present invention.

[0030] FIG. 6B is a partial side view of the strip line implementation of FIG. 5.

[0031] FIG. 6C is a partial side view of a strip line implementation illustrating the encapsulation of the control signal layer in accordance with an embodiment of the present invention.

[0032] FIG. 7 illustrates a switched tunable filter implemented using an interdigitated structure in accordance with another embodiment of the presence invention.

[0033] FIG. 8 illustrates a switched tunable filter implemented using a microstrip end coupled filter structure in accordance with an embodiment of the present invention.

[0034] FIG. 9 illustrates an interdigitated switched tunable filter in accordance with an embodiment of the present invention.

[0035] FIG. 10 is an interdigitated thick film substrate implementation of the circuit of FIG. 9 in accordance with the present invention.

[0036] FIG. 11 illustrates a switched band-stop filter in accordance with an embodiment of the present invention.

[0037] FIG. 12 is a microstrip implementation of the band-stop filter of FIG. 11.

[0038] FIG. 13 illustrates a three band switched band-stop filter implemented using an interleaved structure in accordance with another embodiment of the present invention.

[0039] The following is a detailed description of the present invention with reference to the attached drawings, wherein like reference numerals will refer to like elements throughout.

[0040] A Micro Electro Mechanical System (MEMS) switch provides several advantages over a semiconductor switch (e.g., semiconductor transistors, pin diodes). In particular, a MEMS switch has a very low insertion loss (less than 0.2 dB at 45 GHz) and a high isolation when open (greater than 30 dB). In addition, the switch has a large frequency response and a large bandwidth compared to semiconductor transistors and pin diodes. These advantages provide enhanced performance and control when used in tunable filter designs.

[0041] Referring to FIG. 1A, a block diagram of a MEMS switch 2 that may be used in the present invention is illustrated. The MEMS switch 2 may be viewed as a single pole, single throw (SPST) switch device. In particular, the MEMS switch 2 may interrupt signal transmission by opening a conduction path between an input transmission line 4 and an output transmission line 6.

[0042] Also referring to FIG. 1B (illustrating a cross-section of the MEMS switch 2 in an open position) and FIG. 1C (illustrating a cross-section of the MEMS switch 2 in a closed position), features and characteristics of the MEMS switch 2 will be described below. Briefly, the MEMS switch 2 is a metal-to-metal contact series switch that exhibits relatively low insertion loss and high isolation through microwave and millimeter wave frequencies. Additional details of a suitable switching unit can be found in U.S. Pat. No. 6,046,659, the disclosure of which is herein incorporated by reference in its entirety.

[0043] The MEMS switch 2 includes an armature 8 affixed to a substrate 10 at a proximal end 11 of the armature 8. A distal end (or contact end 12) of the armature 8 is positioned over an input transmission line 4 and an output transmission line 6. A substrate bias electrode 13 can be disposed on the substrate 10 under the armature 8 and, when the armature 8 is in the open position, the armature 8 is spaced from the substrate bias electrode 13 and the lines 4 and 6 by an air gap.

[0044] A pair of conducting dimples, or contacts 14, protrude downward from the contact end 12 of the armature 8 such that in the closed position, one contact 14 contacts the input line 4 and the other contact 14 contacts the output line 6. The contacts 14 are electrically connected by a conducting transmission line 16 so that when the armature 8 is in the closed position, the input line 4 and the output line 6 are electrically coupled to one another by a conduction path via the contacts 14 and conducting line 16. Signals can then pass from the input line 4 to the output line 6 (or vice versa) via the MEMS switch 2. When the armature 8 is in the open position, the input line 4 and the output line 6 are electrically isolated from one another.

[0045] Above the substrate bias electrode 13, the armature 8 is provided with an armature bias electrode 18. The substrate bias electrode 13 is electrically coupled to a substrate bias pad 20 via a conductive line 22. The armature bias electrode 18 is electrically coupled to an armature bias pad 24 via a conductive line 26 and armature conductor 28. When a suitable voltage potential is applied between the substrate bias pad 20 and the armature bias pad 24, the armature bias electrode 18 is attracted to the substrate bias electrode 13 to actuate the MEMS switch 2 from the open position (FIG. 1B) to the closed position (FIG. 1C).

[0046] The armature 8 can include structural members 29 for supporting components such as the contacts 14, conducting line 16, bias electrode 18 and conductor 28. It is noted that the contacts 14 and conductor 16 can be formed from the same layer of material or from different material layers. In the illustrated embodiment, the armature bias electrode 18 is nested between structural member 29 layers.

[0047] Moving to FIG. 2, a simplified equivalent circuit 30 for various microstrip coupled line filter configurations is illustrated. A RF input connection 32 and a RF output connection 33 are coupled directly to an input inductor 34 and an output inductor 35 respectively. Coupling capacitors 36a, 36b, 36cprovide AC coupling between the RF input connection 32 and the RF output connection 33. A first parallel resonant circuit 38 is connected between the first coupling capacitor 36aand the second coupling capacitor 36b. Input tuning capacitor 39aforms a second parallel resonant circuit 38′ with the input inductor 34. Similarly, the output tuning capacitor 39bforms a third parallel resonant circuit 38″ with the output inductor 35. Accordingly, the circuit 30 has three parallel resonant circuits, 38, 38′, 38″. The center frequency of the circuit 30 is determined from the resonant frequency of the three parallel resonant circuits 38, 38′ 38″. The center frequency of the circuit 30 may be changed, for example, by simultaneously tuning the three parallel resonant circuits. Furthermore, constant bandwidth may be preserved by tuning the coupling capacitance 36a, 36b, 36c, the RF input connection 32 and the RF output connection 33.

[0048] A first embodiment of the present invention provides a MEMS switched microstrip filter circuit which achieves tunable center frequencies while maintaining constant bandwidth. The tunable filter can be used for applications with signal frequencies up to at least 12 GHz, for example.

[0049] Referring to FIG. 3, a simplified two band switched tunable filter 30′ in accordance with the invention is illustrated, in relevant part. The switched tunable filter 30′ incorporates MEMS switches to “tune” or alter the filter's characteristics. Tuning is implemented by changing the capacitance seen by the resonant circuits within the filter, thus changing their resonant frequency. For example, the capacitance seen by the resonant circuits may be changed using MEMS switches to connect and disconnect individual capacitors from the resonant circuits.

[0050] It is noted that control lines to command the each MEMS switch to “open” and “close” may or may not be shown in the diagrams. These control lines, however, would be evident to one skilled in the art.

[0051] In the tunable filter 30′ illustrated in FIG. 3, a first input MEMS switch 40aand a second input MEMS switch 40beach have one end connected to node 40 of a RF input connection 32′. The first input MEMS switch 40ahas its other end connected to an input inductor 34 at node 34a, and the second input MEMS switch 40bhas its other end connected to the input inductor 34 at node 34b. The input inductor 34 is connected between node 34dand ground. A coupling capacitor 36ais connected between node 34dand node 38a. A first parallel resonant circuit 38 is connected between node 38aand ground, and an input tuning capacitor 39ais connected between node 34dand ground, thus forming a second parallel resonant circuit 38′. A first tuning MEMS switch 42ais connected between node 34dand node 46a. A first tuning capacitor 44ais connected between node 46aand ground, and a second tuning capacitor 44bis connected between node 46band ground. A selectable coupling capacitor 46 is connected between node 46aand node 46b, and a second tuning MEMS switch 48ais connected between node 46band node 38a.

[0052] The input MEMS switches 40a, 40bselect between one of two possible input connections 32′ on the input inductor 34, thus providing the ability to alter the input coupling. For example, when the first input MEMS switch 40ais closed and the second input MEMS switch 40bis open, the input inductance seen at the input connection 32′ may be designated as L. Similarly, when the first input MEMS switch 40ais open and the second input MEMS switch 40bis closed, the input inductance may be designated as L′, where L′>L. Thus, the inductance seen at the input connection 32′ may be altered through the input MEMS switches 40a, 40b. In a similar manner, the output coupling (not shown) also may be adjusted using MEMS switches (not shown).

[0053] The capacitance of the circuit also may be altered using MEMS switches. For example, when the first tuning MEMS switch 42aand the second tuning MEMS switch 48aare closed, the first tuning capacitor 44ais connected in parallel to the second resonant circuit 38′ and the second tuning capacitor 44bis connected in parallel to the first resonant circuit 38. In addition, the selectable coupling capacitor 46 is connected in parallel to the first coupling capacitor 36a. It is noted that the first and second tuning MEMS switches 42a, 48aare opened and closed together, thus tuning the first and second resonant circuits 38, 38′ together.

[0054] FIG. 4A and FIG. 4B extend the concept shown in FIG. 3, and illustrate partial equivalent circuits with multiple band switching in accordance with the present invention. The switched tunable filter 30″ of FIG. 4A is similar to the switched tunable filter 30′ illustrated in FIG. 3 but includes additional tuning components which allow enhanced tuning of the tunable filter 30″. For example, a third input MEMS switch 40cis connected between node 40 and node 34c. A third tuning MEMS switch 42bis connected between node 34dand node 46a″. A fourth tuning MEMS switch 48bis connected between node 38aand node 46b″. A fifth tuning MEMS switch 42chas one end connected to node 34dand the other end connected to a tuning network (not shown). The tuning network may be, for example, a capacitor network similar to the capacitor network formed by the first tuning capacitor 44a, the second tuning capacitor 44band the selectable coupling capacitor 46 illustrated in FIG. 4A. A sixth tuning MEMS switch 48chas one end connected to node 38aand the other end connected to the tuning network (not shown). A third tuning capacitor 44a″ is connected to node 46a″ and ground, and a fourth tuning capacitor 44b″ is connected between node 46b″ and ground. A second selectable coupling capacitor 46″ is connected between node 46a″ and node 46b″. It is noted that while FIG. 4A illustrates three input coupling connections and three separate tuning networks, this may be expanded to include any number of input coupling connections and tuning networks and FIG. 4A is not intended to be limiting in any way.

[0055] Operation of the switched tunable filter 30″ is similar to the switched tunable filter 30′ of FIG. 3. The switched tunable filter 30″, in addition to the tuning selections available in FIG. 3, also offers additional tuning selections due to the additional MEMS switches. For example, the third input MEMS switch offers an additional input connection. Furthermore, the additional tuning MEMS switches 42b-42c, 48b-48callow additional tuning capacitors 44a″, 44b″ and coupling capacitor 46″ to be added to the tunable filter 30″ as well as the additional tuning network (not shown). Moreover, numerous combinations can be achieved depending on the state of each tuning MEMS switch 42a-42c, 48a-48c, the input MEMS switches 40a-40cand the output MEMS switches (not shown). As is the case for the circuit 30′ of FIG. 3, the MEMS switches are opened and closed in pairs, e.g., 42band 48b, 42cand 48c.

[0056] The switched tunable filter 30′″ of FIG. 4B is similar to the switched tunable filter 30″ of FIG. 4A. The configuration of the tuning MEMS switches, however, is slightly different and provides a different result. In FIG. 4A, the first, third and fifth tuning MEMS switches 42a, 42b, 42chave one end connected to node 34d, and the second, fourth and sixth tuning MEMS switches 48a, 48b, 48chave one end connected to node 38a. In FIG. 4B, only the first tuning MEMS switch 42ahas one end connected to node 34d, and only the second tuning MEMS switch 48ahas one end connected to node 38a. The third tuning MEMS switch 42bis connected between node 46aand node 46a″ and the fourth tuning MEMS switch is connected between node 46band node 46b″. The fifth tuning MEMS switch (not shown) has one end connected to node 46a″ and the other end connected to the tuning network (e.g., the tuning networked described in FIG. 4A). The sixth tuning MEMS switch (not shown) has one end connected to node 46b″ and the other end connected to the tuning network. The remainder of the switched tunable filter 30′″ is essentially the same as the switched tunable filter 30″ of FIG. 4A.

[0057] Operation of the filter 30′″ of FIG. 4B differs from the operation of the filter 30″ of FIG. 4A. In particular, each tuning MEMS switch in FIG. 4B requires the previous or “upstream” tuning MEMS switch to be closed before the “downstream” tuning MEMS switch may add capacitance to the tunable filter 30′″. For example, in the tunable filter 30″ of FIG. 4A, each tuning MEMS switch 42a-42c, 48a-48cmay add capacitance to the circuit regardless of the state of the other tuning MEMS switches. This is due to the common connection point for each group of MEMS switches (e.g., node 34dfor the first, third and fifth MEMS switches 42a, 42b, 42c, and node 38afor the second, fourth and sixth MEMS switches 48a, 48b, 48c). The tuning MEMS switches of the tunable filter 30′″ of FIG. 4B, however, are connected in a serial configuration (e.g., the output of the first MEMS switch 42ais connected to the input of the third MEMS switch 42b, etc.). If the first tuning MEMS switch 42ais open, all components connected to the output of the MEMS switch 42aare disconnected from the tunable filter 30′″. Thus, the third tuning MEMS switch 42bcannot add capacitance to the tunable filter until the first tuning MEMS switch 42ais closed. Similarly, the fifth tuning MEMS switch 42ccannot add capacitance to the tunable filter 30′″ until both the first tuning MEMS switch 42aand the third tuning MEMS switch 42bare closed.

[0058] Other types of filters, e.g., narrow bandwidth filters, may use capacitive input and output coupling, as is shown in the switched tunable filter 30″″ of FIG. 4C. Variable capacitive input coupling can be achieved by a slight variation of the concept shown in FIG. 3. Referring to FIG. 4C, an input capacitor 60 is connected between node 40 and ground. A first coupling capacitor 62 is connected between node 40 of the RF input connection 32′″ and node 34c. A first coupling MEMS switch 64 is connected to node 40 and to one end of a second coupling capacitor 66. A second coupling MEMS switch 68 is connected to node 34cand to the other end of the second coupling capacitor 66.

[0059] Initially, the coupling MEMS switches 64, 68 are open and the coupling capacitance seen at the RF input connection 32′″ is determined by the capacitance of the first coupling capacitor 62. Additional coupling capacitance may be added by closing the coupling MEMS switches 64, 68. When the coupling MEMS switches 64, 68 are closed, the second coupling capacitor 66 is connected in parallel with the first coupling capacitor 62, thus increasing the coupling capacitance of the tunable filter 30″″. The same approach may be applied to the output coupling (not shown) of the tunable filter 30″″.

[0060] A microstrip parallel coupled line implementation 69 of the tunable filter circuit 30′″ of FIG. 4B is illustrated in FIG. 5. Input and output connections to the filter are made at the RF input connection 32″ and the RF output connection 33″ respectively. Microstrip resonators 70 are located on a substrate 72, and tuning stubs 74 are located at the ends of each resonator 70. Through MEMS switches 76, the tuning stubs 74 may be connected to the resonator 70. Each resonator 70 includes a ground connection 78 which is used for control signal input, as will be discussed later.

[0061] The resonator 70 may be a half wavelength transmission line resonator which will resonate at a resonant frequency ω₀. As is well known by those skilled in the art, the resonant frequency of a transmission line resonator can be altered by changing the length of the transmission line resonator. The length of the resonator 70 can be increased by connecting the tuning stubs 74 to the end of the resonator 70 through MEMS switches 76. As the length of the resonator 70 is increased, the resonant frequency is decreased. The resonant frequency of the resonator 70 may be modeled using a parallel LC circuit. In a parallel LC circuit, the resonant frequency ω₀is determined from the formula

[0062] ω₀=1/(L*C)

[0063] where L is the inductance and C is the capacitance. Accordingly, the resonant frequency of the parallel LC circuit may be altered by changing the inductance (L) or the capacitance (C) of the transmission line. Similarly, the resonant frequency of a transmission line resonator may be altered by changing the length of the transmission line, e.g., by adding length to the resonator 70 through the addition of tuning stubs 74.

[0064] As was discussed previously, the tuning stubs 74 can be added to the resonator 70 through the MEMS switches 76. The additional transmission line length reduces the resonant frequency of the resonator and thus permits tuning of the filter. Moreover, the tuning stubs 74 also increase the capacitive coupling 79 between adjacent resonators. The additional capacitive coupling enables constant bandwidth tuning. Referring to the circuits of FIG. 4B and FIG. 5, the increase in the transmission line length (through the connection of the tuning stubs 74 to the resonator 70) may be modeled as adding the tuning capacitors 44a, 44b(FIG. 4B) to the equivalent circuit 30′″. The increase in capacitive coupling 79 (FIG. 5) between adjacent resonators due to the lengthening of the resonator 70 (FIG. 5) may be modeled as adding the coupling capacitor 46 (FIG. 4B) to the equivalent circuit 30′″. Furthermore, the input and output coupling can be adjusted using MEMS switches to compensate for filter center frequency shift.

[0065] Referring now to FIG. 6A, a switch control scheme 80 for a tunable filter is illustrated. The switch control scheme 80 serially connects several stubs, one after the other, to the end of a resonator. Each successive stub, when selected through a MEMS switch, increases the length of the resonator, thus decreasing the resonant frequency of the resonator and increasing the capacitive coupling to the adjacent resonator. Furthermore, in addition to selecting stubs, each MEMS switch may provide a DC control signal to a downstream MEMS switch to command the switch to open or close. In short, each MEMS switch may provide a RF connection to tuning stubs for filter tuning and a path for a control signal to control a downstream MEMS switch.

[0066] The switch control scheme 80 of FIG. 6A will now be discussed in detail using a four band filter as an example. It is noted, however, that the filter may have any number of bands, and the present example is not intended to be limiting in any way. Three MEMS switches 84, 86, 88, are located on the end of the resonator 70, each MEMS switch having a 2-terminal control signal connection and a SPST (single pole single throw) switch contact. A first control terminal 84a, 86a, 88aof each MEMS switch is connected to node 89, which is referred to as the return path. A second control terminal 84b, 86b, 88bof each MEMS switch is connected to node 90, which is referred to as Band 1 selector. The band selector nodes 90, 91, 92 provide a signal to control the state of each bank of MEMS switches (e.g., open or close) on the resonator and each respective stub. The resonator ground connection 78 (FIG. 5) is connected to ground to provide a path to route the control signals out of the resonator 70 as will be discussed in more detail later. The resonator also includes four bypass capacitors 93, 94, 95, 96. The first bypass capacitor 93 is connected between node 89 and ground, the second bypass capacitor 94 is connected between node 90 and ground, the third bypass capacitor 95 is connected between node 91 and ground, and the fourth bypass capacitor 96 is connected between node 92 and ground.

[0067] The first MEMS switch 84 on the resonator 70 has a first terminal 84cconnected to node 89, and a second terminal 84dconnected to node 100aon an adjacent first stub 98.

[0068] The second MEMS switch 86 on the resonator 70 has a first terminal 86cconnected to node 91 and a second terminal 86dconnected to node 106aon the adjacent first stub 98.

[0069] The third MEMS switch 88 on the resonator 70 has a first terminal 88cconnected to node 92 and a second terminal 88dconnected to node 108aon the adjacent first stub 98.

[0070] The first stub 98 includes three bypass capacitors 100, 106, 108 and two MEMS switches 102, 104. The first bypass capacitor 100 is connected between node 100aand ground, the second bypass capacitor 106 is connected between node 106aand ground, and the third bypass capacitor 108 is connected between node 108aand ground. The first MEMS switch 102 on the first stub 98 has a first control terminal 102aconnected to node 100a, and a second control terminal 102bconnected to node 106a. The First MEMS switch also has a first terminal 102cwhich is connected to node 100a, and a second terminal 102dis connected to node 112aon an adjacent second stub 110. The second MEMS switch 104 on the first stub 98 has a first control terminal 104aconnected to node 100aand a second control terminal 104bconnected to node 106a. The second MEMS switch 104 also has a first terminal 104cwhich is connected to node 108a, and a second terminal 104dis connected to node 116aon the adjacent second stub 110.

[0071] The second stub 110 includes two bypass capacitors 112, 116 and one MEMS switch 114. The first bypass capacitor 112 on the second stub 110 is connected between node 112aand ground, and the second bypass capacitor 116 is connected between node 116aand ground. The MEMS switch 114 on the second stub 110 has a first control terminal 114aconnected to node 112a, and a second control terminal 112bconnected to node 116a. The MEMS switch also has a first terminal 114cconnected to ground, and a second terminal 114dconnected to ground on an adjacent third stub 118.

[0072] The operation of the circuit illustrated in FIG. 6A will now be discussed. Referring briefly to FIG. 6B, the microstrip resonator 70 is constructed from a metallization layer 120 on top of a dielectric substrate 122. The back side of the dielectric substrate 122 also includes a metallization layer 124. Thus, the two metallization layers 120,124 separated by a dielectric layer 122 form a transmission line. The three stubs 98, 110, 118 are constructed in the same manner illustrated in FIG. 6B and thus may be viewed as short transmission lines. By adding stubs to the resonator 70, the length of the resonator is increased and thus the resonant frequency of the resonator 70 is decreased.

[0073] To route control signals out of the MEMS switches, a multilayer substrate may be used, as illustrated in FIG. 6C. For example, the control conductors may be placed above the resonator metal 120 on an insulating layer 126. An additional insulation layer 127 and metal layer 128 may be applied above the control signal layer 126 to encapsulate the control signals to prevent them from interacting with the RF circuit.

[0074] Referring back to FIG. 6A, the band select signals 90, 91, 92 are assumed initially to be at logic 0 (low). Accordingly, all MEMS switches are in an open state and no additional stubs are added to the resonator 70. When Band 1 selector 90 is set to logic 1 (high), the control signal at each MEMS switch 84, 86, 88 on the resonator 70 is at logic 1 and the switches close. The Return connection 89, which is connected to the resonator ground and the Band select signals 2 and 3 are passed to the adjacent first stub 98 through the first, second and third MEMS switches 84, 86, 88 respectively. Furthermore, RF signals are passed through the same MEMS switches 84, 86, 88 and the bypass capacitors 93-96, 100, 106, 108. The bypass capacitors appear as short circuits to RF signals, and thus provide a means of connecting the resonator to stubs while isolating the control signals to the MEMS switches from the resonator and/or stubs. The length of the resonator 70 is increased through the connection to the adjacent first stub 98 (the metallization layer 120 of the resonator 70 is connected to the metallization layer (not shown) of the first stub 98). Accordingly, the resonant frequency of the resonator is decreased. Moreover, due to the increased resonator length, the capacitive coupling between adjacent resonators is increased. The increased capacitive coupling permits constant bandwidth of the filter throughout the tuning range

[0075] Additional stubs may be added to the resonator 70 through Band 2 selector 91. For example, when Band 2 selector is set to logic 1, the control signal at the first and second MEMS switch 102, 104 on the first stub 98 is at logic 1 and the switches close. When the two switches 102, 104 are closed, the metallization layer (not shown) of the first stub 98 is connected to the metallization layer (not shown) of the second stub 110 which increases the length of the resonator 70. Accordingly, the resonant frequency of the resonator is decreased and the capacitive coupling between adjacent resonators is increased. Furthermore, Band 3 selector 92 is passed to the second stub 110 through the second MEMS switch 104.

[0076] In the same manner, the resonant frequency may be decreased again by setting the Band 3 selector 92 to logic 1, thus closing the MEMS switch 114 on the second stub 110. When the MEMS switch 114 is closed, the metallization layer (not shown) of the second stub 110 is connected to the metallization layer (not shown) of the third stub 118, which increases the length of the resonator 70. Accordingly, the resonant frequency of the resonator is decreased and the capacitive coupling between adjacent resonators is increased.

[0077] It is noted that in the present example if Band 2 selector 91 or Band 3 selector 92 is set to logic 1 while Band 1 selector 90 is set to logic 0, the length of the resonator 70 will not change. Band 2 and Band 3 signals are passed to the adjacent stubs only when the MEMS switches 84, 86, 88 on the resonator 70 are closed. Since the MEMS switches on the resonator 70 are controlled by the Band 1 selector 90, no signal will be passed to the adjacent stubs if Band 1 is at logic 0. Effectively, this configuration operates in the same manner as the tunable filter illustrated in FIG. 4B, which was discussed previously.

[0078] In an alternative embodiment, the filter may be implemented using a microstrip interdigitated structure 130, as illustrated in FIG. 7. Resonators 132 are formed parallel to each other on a substrate (not shown). One end 134 of the resonator is grounded to provide a path to route the control signals out of the resonator. The other end 136 of the resonator has a plurality of MEMS switches (not shown) linking the resonator 132 to tuning stubs 138 to tune the frequency and bandwidth. A RF input connection 140 and a RF output connection 142 also may include MEMS switches to adjust the input and output coupling, including, for example, direct coupling and/or capacitive coupling, as was discussed previously.

[0079] Another embodiment includes a microstrip end coupled filter structure 150, as is illustrated in FIG. 8. Coupling between resonators 152 is accomplished by capacitive coupling 153 between the resonators. Tuning stubs 154 are selected by MEMS switches (not shown) and load the ends of the resonators 152, lowering the resonant frequency. Appropriate geometry of the stubs 154 provides the required additional coupling capacitance to achieve constant bandwidth. The geometry of the tuning stubs 154 may be determined using electromagnetic simulation software, which is well known by those skilled in the art. Using the electromagnetic simulation software, a structure is designed that adds the correct amount of capacitance to tune the resonator 152 to the desired frequency and at the same time increases the coupling capacitance 153 to the adjacent resonator to achieve the desired bandwidth. A resonator grounding section 156 is provided for bias input as was implemented in the parallel coupled line filter shown in FIG. 5. The stubs 154 can be selected individually or together via MEMS switches to select three bands.

[0080] Referring now to FIG. 9, a schematic diagram of a four-band switchable band-pass filter 200 is illustrated. The filter 200 is a four-section interdigitated stripline design. A first MEMS switch 202ahas one end connected to node 204a. A first capacitor 206ahas one end connected to the first MEMS switch 202aand the other end connected to ground. A second MEMS switch 202bhas one end connected to node 204a. A second capacitor 206bhas one end connected to the second MEMS switch 202band the other end connected to ground. A RF input connection 208 is connected to node 204a, and a first resonator 210ahas one end connected to node 204aand the other end connected to ground. A third MEMS switch 202chas one end connected to node 204b. A third capacitor 206chas one end connected to the third MEMS switch 202cand the other end connected to ground. A fourth MEMS switch 202dhas one end connected to node 204b. A fourth capacitor 206dhas one end connected to the fourth MEMS switch 202dand the other end connected to ground. A second resonator 210bhas one end connected to node 204band the other end connected to ground. A fifth MEMS switch 202ehas one end connected to node 204c. A fifth capacitor 206ehas one end connected to the fifth MEMS switch 202eand the other end connected to ground. A sixth MEMS switch 202fhas one end connected to node 204c. A sixth capacitor 206fhas one end connected to the sixth MEMS switch 202fand the other end connected to ground. A third resonator 210chas one end connected to node 204cand the other end connected to ground. A seventh MEMS switch 202ghas one end connected to node 204d. A seventh capacitor 206ghas one end connected to the seventh MEMS switch 202gand the other end connected to ground. An eighth MEMS switch 202hhas one end connected to node 204d. An eighth capacitor 206hhas one end connected to the eighth MEMS switch 202hand the other end connected to ground. A fourth resonator 210dhas one end connected to node 204dand the other end connected to ground, and a RF output connection 212 is connected to node 204d.

[0081] The operation of the switched tunable bandpass filter 200 will now be described. Initially, all MEMS switches 202a-202hare assumed to be open. RF signals enter the filter 200 at the RF input connection 208. Signals which have a frequency substantially equivalent to the resonant frequency of the resonators 210a-210hpass through the filter, while signals with frequencies substantial different from the resonant frequency are rejected.

[0082] The pass band of the filter may be altered by changing the resonant frequency of the resonators. As was detailed previously, the resonator may be modeled as an LC circuit, and the resonant frequency of an LC circuit is determined from the inductance and capacitance of the resonant circuit (ω₀=1/(L*C)). Accordingly, by adding capacitance to the resonators 210a-210h, the resonant frequency may be altered and thus the pass band of the filter 200 may be controlled.

[0083] For example, closing the first MEMS switch 202aconnects capacitor 206ato the first resonator 210a. The additional capacitance reduces the resonant frequency of the first resonator and thus the pass band of the filter 200. Similarly, capacitor 206bmay be added to the first resonator 210aby closing MEMS switch 202b. By selectively enabling the capacitors 206a-206hthrough the MEMS switches 202a-202h, the pass band of the filter 200 may be precisely controlled. It is noted that as a particular capacitor is added to a resonator, a corresponding capacitor should be added to the remaining resonators. For example, if the first MEMS switch 202ais closed, thus adding the first capacitor 206ato first resonator 210a, then the third MEMS switch 202cshould be closed to add the third capacitor 206cto the second resonator 210b; the fifth MEMS switch 202eshould be closed to add the fifth capacitor 206eto the third resonator 210c; and the seventh MEMS switch 202gshould be closed to add the seventh capacitor 206gto the fourth resonator 210d.

[0084] FIG. 10 shows an illustration of the interdigitated thick film substrate 220. The substrate may be formed from a high-K dielectric ceramic material. The high-K dielectric material allows for a compact stripline design. In one embodiment, the dielectric ceramic material has a K of approximately 65. The conductors (not shown) are thick film etchable gold and two substrates 222, 224 are fired together using thick film dielectric paste to form the stripline. Connections between the resonators 210a-210dand the topside circuitry (not shown) are made through vias 228. The ceramic structure is externally metallized using thick film gold to provide the stripline ground.

[0085] A four section band-stop filter 240 is illustrated in FIG. 11. Quarter wavelength transmission line resonators 242a-242dare capacitively coupled to a transmission line 244 at approximately quarter wavelength intervals 246. The circuit provides a narrow stop band at the resonant frequency of the quarter wave resonators. The width of the stop band is determined by the amount of capacitive coupling between the resonators 242a-242dand the transmission line 244.

[0086] The band-stop filter 240 has a RF input connection 248 connected to node 249a. A first quarter wavelength resonator 242ahas one end connected to a first MEMS switch 252aand the other end connected to ground. A first capacitor 254ahas one end connected node 249aand its other end connected to the first MEMS switch 252a. Between the first capacitor 254aand the first MEMS switch 252ais a short section of transmission line 243a. A transmission line 244 is connected between node 249aand node 249d. In one embodiment the transmission line has an impedance of 50 ohms. A second quarter wavelength resonator 242bhas one end connected to a second MEMS switch 252band the other end connected to ground. A second capacitor 254bhas one end connected node 249band its other end connected to the second switch 252b. Between the second capacitor 254band the second MEMS switch 252bis a short section of transmission line 243b. A third quarter wavelength resonator 242chas one end connected to a third MEMS switch 252cand the other end connected to ground. A third capacitor 254chas one end connected node 249cand its other end connected to the third MEMS switch 252c. Between the third capacitor 254cand the third MEMS switch 252cis a short section of transmission line 243c. A fourth quarter wavelength resonator 242dhas one end connected to a fourth MEMS switch 252dand the other end connected to ground. A fourth capacitor 254dhas one end connected node 249dand its other end connected to the fourth MEMS switch 252d. Between the fourth capacitor 254dand the fourth MEMS switch 252dis a short section of transmission line 243d. A RF output connection 256 is connected to node 249d.

[0087] As can be seen in FIG. 11, each MEMS switch 252a-252dis located part way between each coupling capacitor 254a-254dand the grounded end of each resonator. Due to its design, the MEMS switch inherently has a small amount of series capacitance while in the “open” state, which may cause a parasitic resonance when the MEMS switch is open. To reduce the effects of the parasitic resonance, each MEMS switch 252a-252dis positioned such that the parasitic resonant frequency, when the switch is open, is a frequency that is well above the band of interest. Locating the switch too far from the coupling capacitor places the MEMS switch in a low impedance area of the circuit and the switch loss becomes a significant factor. Furthermore, the rejection skirt widens out into the pass band area. In selecting the location of the MEMS switch, a trade off exists between moving the parasitic stop band far enough away from the band of interest and degrading performance of the filter due to switch loss. Electromagnetic simulation software may be used to determine the optimum location for each MEMS switch 252a-252d.

[0088] When all of the MEMS switches 252 are in the open state, the circuit provides a low loss thru-path for signals within the band of interest. Signals significantly above the band of interest, however, are prevented from passing through the filter 240 due to the parasitic resonance described previously. Since the parasitic resonance occurs above the band of interest, it does not present a problem for signals within the band of interest. When all of the MEMS switches 252a-252dare closed, a narrow stop band is formed at the resonant frequency of the resonator, thus preventing signals having a frequency within the stop band from passing through the filter 240. Multiple stop bands may be achieved by connecting multiple filters together in a cascade configuration, wherein each filter is designed for a different stop band. By selecting one or more cascaded filters, precise control of the stop band is achieved.

[0089] The band-stop filter 240 may be implemented using a microstrip structure 240′ as illustrated in FIG. 12. As was discussed above with regard to FIG. 11, the microstrip structure 240′ includes a transmission line 244, wherein resonators 242a-242dare spaced along the transmission line 244 at quarter wavelength intervals 246. The resonators 242a-242dare coupled to a transmission line 244 through MEMS switches 252a-252dand coupling capacitors 254a-254drespectively. A RF input connection 248 and a RF output connection 256 provide signal input and output points to the filter 240′. In addition, control input terminals 270a-270deach feed control signals to each MEMS switch 252a-252d. The control signal provides the command to open or close each MEMS switch 252a-252d. Control input bypass capacitors 272a-272dshort out any RF frequencies that may find their way into the control circuitry. Ground vias 274a-274dprovide a ground connection to the resonators 242a-242d.

[0090] FIG. 13 illustrates an alternative embodiment of the band-stop filter. In particular, FIG. 13 illustrates a three stop band filter 280 implemented using an interleaved structure. The band-stop filter 280 includes a transmission line 244′ and resonators 242a-242lcoupled to the transmission line 244′ through MEMS switches 252a-252land coupling capacitors 254a-254l. The resonators are placed on both the top 282 and bottom 284 of the transmission line 244′, thus allowing more resonators to be placed along the transmission 244′. An RF input connection 248 and a RF output connection 256 provide signal input and output points to the filter. Control input terminals 270a-270lfeed control signals to each MEMS switch 252a-252lto command the respective switch to open or close, and ground vias 272a-272lprovide a ground connection to each resonator 242a-242l.

[0091] While particular embodiments of the invention have been described in detail, it is understood that the invention is not limited correspondingly in scope, but includes all changes, modifications and equivalents coming within the spirit and terms of the claims appended hereto.