Bandpass filter with tunable resonator

[0001] This application is a continuation-in-part of U.S. application Ser. No. 10/981,814, filed Nov. 5, 2004, which is a continuation of U.S. application Ser. No. 10/076,171, filed Feb. 12, 2002, now U.S. Pat. No. 6,816,714, which claims the benefit of U.S. Provisional Application Ser. No. 60/283,093, filed Apr. 11, 2001, the disclosures of which are hereby incorporated by reference. In addition, this application relates to U.S. application Ser. No. 09/904,631 filed on Jul. 13, 2001, now U.S. Pat. No. 6,690,176, U.S. application Ser. No. 09/912,753 filed on Jul. 24, 2001, now U.S. Pat. No. 6,639,491, U.S. application Ser. No. 09/927,732 filed on Aug. 8, 2001, U.S. application Ser. No. 09/927,136, filed on Aug. 10, 2001, now U.S. Pat. No. 6,825,818, U.S. application Ser. No. 10/044,522, filed on Jan. 11, 2002, now U.S. Pat. No. 6,737,930, and U.S. application Ser. No. 10/077,654, filed on Feb. 14, 2002, the disclosures of which are hereby incorporated by reference.

[0002] This invention generally relates to radio frequency (RF) electronics and, more particularly, to a wireless communications bandpass filter with a tunable resonator section.

[0003] Filters, such as bandpass filters, have numerous applications in communications and electronics. In wireless multiple access communications, a given frequency band must accommodate many wireless users. To accommodate so many users, stringent bandpass filtering requirements are required to minimize interference from communications occurring in neighboring frequency bands or channels.

[0004] Conventionally, wireless handsets use fixed-tuned bandpass filters (BPFs) to meet their filtering specifications. The design of such filters is complicated because they must achieve the lowest possible passband insertion loss (IL), while simultaneously achieving a specified large out-of-band rejection. As a specific example, consider full band PCS CDMA handsets using fixed bandwidth filters. The PCS transmit (Tx) band should have no more than −3.5 dB IL inband (1850 to 1910 MHz in the US), while having at least a 38.0 dB out-of-band rejection in the receive (Rx) band (1930 to 1990 MHz range).

[0005] Further, this BPF is constrained by size limitations, as manufacturers are continually attempting to manufacture smaller wireless devices. A typical height constraint for a conventional handsets may be 4.0 mm, or less. To meet these demanding electrical requirements, yet possess the smallest possible size and height, high order fixed-tuned filters constructed from either individual coaxial resonator elements or monoblock structures are usually necessary. In addition, to satisfy out-of-band rejection requirements, a transmission zero is usually required, increasing IL at the band edge. Because of variations in ceramics and fabrication tolerances, vendors must individually adjust the characteristics of fixed-tuned filters during their manufacture, driving costs higher.

[0006] Moreover, if more than one frequency band is to be supported (e.g., supporting the PCS bands in the U.S., Korea, and India) multiple fixed-tuned BPFs are necessary, requiring extra switches that introduce additional losses. This is true, even if the power amplifier and low noise amplifier used have sufficient bandwidth to operate over these multiple bands.

[0007] A tunable BPF permits the BPF to be used over several bands. That is, a lower order filter with a narrow bandpass can be used to selectively tune within a larger passband. To provide tunability in a tunable BPF, a component capable of providing a variable capacitance is typically used.

[0008] Several structures are presently used to implement a variable capacitor. For example, movable parallel plates have been used for many years as the tuner in home radios. However, such plates are far too bulky, noisy, and impractical for use in most modern applications. Another alternative, the electronic varactor, is a semiconductor device that adjusts capacitance responsive to an applied voltage. Because the varactor is typically noisy and lossy, particularly in applications above 500 MHz, it is ineffective for high-frequency, low-loss applications where high performance is required.

[0009] Another alternative, a microelectromechanical system (MEMS) device can be used to switch between capacitors, responsive to an applied control signal. These devices have not yet proven practical for high-volume low-cost manufacturing. Further, such a mechanism still only provides discrete tuning, between a finite number of fixed capacitor values.

[0010] Ferroelectric tunable capacitors are another alternative. Ferroelectric (FE) materials are a class of materials, typically ceramic rare-earth oxides, whose prominent feature is that their dielectric constant (κ), and as a consequence, the electric permittivity (ε) changes in response to an applied slowly varying (DC or low frequency) electric field. The relationship of the dielectric constant and the electric permittivity of a material is given as follows:
ε=κε_O

[0011] where ε_O is the electric permittivity of a vacuum. At present, there are several hundred known materials that possess FE properties. In a typical FE material, one can obtain a change in dielectric constant as great as approximately 3:1. The DC voltage required to generate such changes depends, in one aspect, upon the dimensions of the FE material over which a DC control voltage is applied. As a result of their variable dielectric constant, one can make tunable capacitors using FE materials, because the capacitance of a capacitor depends on the dielectric constant of the dielectric proximate the capacitor conductors. Typically, a tunable FE capacitor is realized as a parallel plate (overlay), interdigital (IDC), or a gap capacitor.

[0012] Conventional FE variable capacitors use a layer of an appropriate FE material, such as barium strontium titanate, Ba_xSr_1-xTiO₃ (BSTO), disposed adjacent to one or both conductors of a capacitor. Depending upon the strength of the electric field applied to the FE material and the intrinsic properties of the FE material selected, the capacitance changes. Typically, below the Curie temperature (T_C) of the FE film, the FE material is in the ferroelectric state and will exhibit hysteresis in its response to a changing electric field. Above T_C, the FE material is in the paraelectric state and will not exhibit hysteresis. Thus, an FE material is generally chosen that has a T_C lower than the expected operating temperature so as to operate in the paraelectric state, avoiding the hysteresis effects of the ferroelectric state.

[0013] However, conventional FE variable capacitors have proven to be too lossy for use in insertion-loss-sensitive applications such as in the RF circuits of handsets. Moreover, these devices often perform unpredictably, preventing optimal design, construction, and use of FE tunable filters.

[0014] Duplexers are used in wireless telephone technology to separate the Tx and the Rx frequencies into their respective signal paths. Duplexers typically comprise two bandpass filters. Each filter “selects” either the Tx or the Rx frequency signal to be passed. The filters are coupled together at one end, forming a common port. This common port is typically coupled to an antenna or a diplexer for sending transmit signals and receiving receive signals.

[0015] Strict insertion loss and out-of-band rejection requirements are the primary requirements that influence the design of duplexers for use in loss-sensitive applications, for example, in wireless handsets. Other electrical and mechanical specifications must also be satisfied, such as, for example, size and height requirements.

[0016] It would be advantageous if a bandpass filter could be made tunable, to operate at a number of channels within a frequency band.

[0017] It would be advantageous if the tunable bandpass filter could be tuned to operate in multiple frequency bands.

[0018] It would be advantageous if the above-mentioned bandpass filter could be fabricated using an FE tunable capacitor.

[0019] It would be advantageous if a duplexer could be made using two tunable bandpass filters.

[0020] In wireless handsets, strict insertion loss and out-of-band rejection requirements generally mandate high order (multiple pole) filters for use in duplexers. The inband insertion loss requirements generally apply to a frequency band that is wider than any particular channel in the frequency band that may be used. This means that a fixed tuned filter must have a broader passband than would a tunable filter operating in the same passband. Because the tunable filter can have a smaller (tunable) passband, it can have a lower order (fewer poles), taking up less space and likely have a lower insertion loss. The invention provides for a bandpass filter that is made tunable using a resonator that includes a FE tunable capacitor. The tunable bandpass filter can be used to make a tunable duplex.

[0021] Thus, a low insertion loss tunable bandpass and duplexer are provided, which are smaller and have less insertion loss than a fixed tuned bandpass filter covering the same passband. The space savings in a wireless handset can be used to provide other desired functions and properties, or their use can simply reduce the size, weight, or cost of the handset. Additionally, the savings in insertion loss result in a longer talk time and battery life.

[0022] More specifically, a tunable bandpass filter is provided that comprises a first shunt-connected ferroelectric (FE) tunable tank circuit having a first node to accept an input signal. A second shunt-connected FE tunable tank circuit has a second node to supply a bandpass filtered signal. A first capacitor is connected in series between the first and second nodes.

[0023] In one aspect, the first tank circuit comprises a first resonator having an input connected to the first node and an output. A fourth capacitor has an input connected to the first resonator output, and has an output connected to a reference voltage. The fourth capacitor is a tunable FE capacitor. Typically, a fifth capacitor has an input connected to the first node and an output connected to the reference voltage.

[0024] Likewise, the second tank circuit comprises a second resonator having an input connected to the second node and an output. A sixth (FE) capacitor has an input connected to the second resonator output, and has an output connected to the reference voltage. A seventh capacitor has an input connected to the second node and an output connected to the reference voltage.

[0025] Additional details of the above-described bandpass filter and a tunable duplexer are provided below.

[0026] FIG. 1 is a schematic diagram of a tunable bandpass filter.

[0027] FIG. 2 is a schematic diagram illustrating a first variation of the bandpass filter of FIG. 1.

[0028] FIG. 3 is a schematic diagram illustrating a second variation of the bandpass filter of FIG. 1.

[0029] FIG. 4 is a schematic diagram illustrating a third variation of the bandpass filter of FIG. 1.

[0030] FIG. 5 is a schematic block diagram of a wireless telephone device with a tunable duplexer.

[0031] FIG. 6.is a schematic block diagram of a wireless telephone device with a tunable filter.

[0032] FIG. 7 is a plan view of a distributed element gap capacitor.

[0033] FIG. 8 is a cross-sectional view of an overlay capacitor.

[0034] FIG. 9 is a plan view of an interdigital (IDC) capacitor.

[0035] FIG. 10 is a schematic diagram depicting a variation of the bandpass filter of FIG. 2.

[0036] FIG. 11 is a flowchart illustrating a method for bandpass filtering a signal.

[0037] FIG. 1 is a schematic diagram of a tunable bandpass filter. The filter 100 comprises a first shunt-connected ferroelectric (FE) tunable tank circuit 102 having a first node 104 to accept an input signal. A second shunt-connected FE tunable tank circuit 106 has a second node 108 to supply a bandpass filtered signal. A first capacitor 110 is connected in series between the first node 104 and the second node 108.

[0038] FIG. 2 is a schematic diagram illustrating a first variation of the bandpass filter of FIG. 1. In this variation the first tank circuit 102 comprises a first resonator 200 having an input connected to the first node 104 and an output connected to a reference voltage 202. For example, the reference voltage can be an AC or DC ground. A second capacitor 204 has an input connected to the first node 104, an output connected to the reference voltage 202, and an input to accept a control signal on line 206. The second capacitor 206 includes an FE material with a dielectric constant responsive to the control voltage on line 206. Although the control voltage is shown being introduced via a separate line, in some aspects the control voltage is introduced to the FE capacitors through one of the capacitor terminals. For example, the control voltage used to control second capacitor 206 may be introduced at node 104.

[0039] The second tank circuit 106 comprises a second resonator 208 having an input connected to the second node 108 and an output connected to the reference voltage 202. A third capacitor 210 has an input connected to the second node 108, an output connected to the reference voltage 202, and an input to accept a control signal on line 212. Again, the third capacitor 210 includes an FE material with a dielectric constant responsive to the control voltage on line 212.

[0040] FIG. 3 is a schematic diagram illustrating a second variation of the bandpass filter of FIG. 1. The first tank circuit 102 comprises a first resonator 200 having an input connected to the first node 104 and an output on line 302. A fourth capacitor 300 has an input connected to the first resonator output on line 302, an output connected to a reference voltage 202, and an input to accept a control signal on line 304. The fourth capacitor 300 includes an FE material with a dielectric constant responsive to the control voltage on line 304. A fifth capacitor 306 has an input connected to the first node 104 and an output connected to the reference voltage 202.

[0041] The second tank circuit 106 comprises a second resonator 208 having an-input connected to the second node 108 and an output on line 308. A sixth capacitor 310 has an input connected to the second resonator output on line 308, an output connected to a reference voltage 202, and an input to accept a control signal on line 312. The sixth capacitor 310 includes an FE material with a dielectric constant responsive to the control voltage on line 312. A seventh capacitor 314 has an input connected to the second node 108 and an output connected to the reference voltage 202.

[0042] In one aspect (as shown), the fifth capacitor 306 is an FE tunable capacitor having an input to accept a control signal on line 316. The fifth capacitor 306 includes an FE material with a dielectric constant responsive to the control voltage on line 316. Likewise, the seventh capacitor 314 is an FE tunable capacitor having an input to accept a control signal on line 318. The seventh capacitor 314 includes an FE material with a dielectric constant responsive to the control voltage on line 318.

[0043] FIG. 4 is a schematic diagram illustrating a third variation of the bandpass filter of FIG. 1. This variation is the same as the bandpass filter of FIG. 3, with additional components. Here, the first tank circuit 102 further comprises an eighth capacitor 400 having an input and output interposed between the output of the fifth capacitor 306 and the reference voltage 202. The eighth capacitor 400 has an input on line 402 to accept a control signal. The eighth capacitor 400 includes an FE material with a dielectric constant responsive to the control voltage on line 402. The second tank circuit 106 further comprises a ninth capacitor 404 having an input and output interposed between the output of the seventh capacitor 314 and the reference voltage 202, and an input to accept a control signal on line 406. The ninth capacitor 404 includes an FE material with a dielectric constant responsive to the control voltage on line 406.

[0044] Although the following components are described in context of the circuit of FIG. 4, it should be understood that they could also be used with the circuits of FIGS. 1, 2, and 3. One skilled in the art having the benefit of this disclosure would understand their applicability, and an explicit discussion of modifications to FIGS. 1, 2, and 3 is bypassed in the interest of brevity. The filter 100 further comprises an input node 408 to accept the input signal. A tenth capacitor 410 has an input connected to the input node 408 and an output connected to the first node 104. An output node 412 supplies the filtered signal. An eleventh capacitor 414 has an input connected to the second node 108 and an output connected to the output node 412.

[0045] In one aspect, the first capacitor 110 is a FE tunable capacitor having an input an input on line 416 to accept a control signal. The first capacitor 110 includes-an FE material with a dielectric constant responsive to the control voltage on line 416. The tenth capacitor 410 may also be an FE tunable capacitor having an input on line 418 to accept a control signal. Likewise, the eleventh capacitor 414 may be a FE tunable capacitor having an input to accept a control signal on line 420.

[0046] A few examples are given to illustrate the enablement of the above-described passband filter. In one aspect the first capacitor 110, the tenth capacitor 410, and the eleventh capacitor 414 each have a capacitance in the range of 0.5 and 0.7 picofarads (pF). The fourth capacitor 300 and the sixth capacitor 310 each have a capacitance in the range of 2 and 4.1 pF. The fifth capacitor 306 and the seventh capacitor 314 each have a capacitance in the range of 0.5 and 1 pF. The first resonator 200 and second resonator 208 each have an inductance of about 2.6 nanoHenrys (nH). The first resonator 200 and the second resonator 208 may be coaxial, stripline, microstrip, or monoblock resonators. However, the filter is not limited to any particular resonator type.

[0047] In this example, it is assumed that the eighth capacitor 400 and ninth capacitor 404 have been removed, or have a very low impedance, so that the fifth capacitor 306 and seventh capacitor 314 are connected directly to the reference voltage 202. Alternately, the values given below for the fifth capacitor 306 may be considered to be the total capacitance value representing the combination of the fifth capacitor 306, series connected to the eighth capacitor 400. Likewise, the values given below for the seventh capacitor 314 may be considered to be the total capacitance value representing the combination of the seventh capacitor 314, series connected to the ninth capacitor 404.

[0048] As understood by one skilled in the art having the benefit of this disclosure, the above-mentioned FE capacitors may represent series-connected combinations of FE capacitors, parallel-connected FE capacitors, FE capacitors series connected with fixed value capacitors, and FE capacitors parallel connected with fixed value capacitors. These combinations may be desired so that practical FE capacitor values can be transformed for use in desired capacitance ranges. The combinations may make the tuning of an FE either more, or less sensitive, depending on the application. Further, fixed value capacitors may be used to isolate an FE capacitor control voltage from DC grounds in the circuit, or from the control voltage of a neighboring FE capacitor.

[0049] The above-mentioned component values and circuit configuration permit the bandpass filter 100 to be both broadly tunable between two different frequency bands, as well as finely tunable within each frequency band. For example, the Korean PCS (KPCS) wireless telephone system operates at 1750-1780 megahertz (MHz) (Tx) to 1840-1870 MHz (Rx). The United States PCS (USPCS) system operates at 1850-1910 MHz (Tx) to 1930-1990 MHz (Rx). The exemplary filter 100 of FIG. 4 is able to tune between 1710 and 1910 MHz. Thus, it can be used to tune within both the KPCS and USPCS transmit bands. Therefore, the bandpass filter 100 could be the transmitter section of a duplexer. This tunable duplexer permits the same wireless telephone to operate in both Korea and the US. There are a number of frequency bands, associated with CDMA, GSM, and WCDMA telephone systems, operating in the frequencies between approximately 2.4 gigahertz and 500 MHz. One skilled in the art having the benefit of this disclosure would be able to tailor the bandpass filter design of FIG. 4 to operate anywhere within this range of frequencies.

[0050] In the US, the cellular band (AMPS), at around 850 megahertz (MHz), and the PCS (Personal Communication System) band, at around 1900 MHz, are used. Other communication bands include the PCN (Personal Communication Network) and DCS at approximately 1800 MHz, the GSM system (Global System for Mobile communication) at approximately 900 MHz, and the JDC (Japanese Digital Cellular) at approximately 800 and 1500 MHz. Other bands of interest are GPS signals at approximately 1575 MHz, Bluetooth at approximately 2400 MHz, and wideband code division multiple access (WCDMA) at 1850 to 2200 MHz.

[0051] Generally, the capacitors in series with the resonators (fourth capacitor 300 and sixth capacitor 310) provide the-coarse tuning. The fine tuning is provided by the capacitors in series with the resonators (fifth capacitor 306 and seventh capacitor 314), and by the coupling capacitors (first capacitor 110, tenth capacitor 410, and eleventh capacitor 414).

[0052] To continue the example, in one aspect the first capacitor 110 has a capacitance of 0.7 pF, the fourth and sixth capacitors 300/310 each have a capacitance of 3.45 pF, the fifth and seventh capacitors 306/314 each have a capacitance of 0.53 pF, and the tenth and eleventh capacitors 410/414 each have a capacitance of 0.65 pF. Then, the filter 100 is tuned to optimally pass signals from the input node 408 to the output node 412 having a frequency of about 1750 megahertz (MHz). That is, the filter has a minimum insertion loss at 1750 MHz.

[0053] In a second example, the first capacitor 110 has a capacitance of 0.5 pF, the fourth and sixth capacitors 300/310 each have a capacitance of 2 pF, the fifth and seventh capacitors 306/314 each have a capacitance of 1 pF, and the tenth and eleventh capacitors 410/414 each have a capacitance of 0.59 pF. Then, the filter 100 is tuned to optimally pass signals having a frequency of about 1850 MHz.

[0054] In a third example, the first capacitor 110 has a capacitance of 0.7 pF, the fourth and sixth capacitors 300/310 each have a capacitance of 1.72 pF, the fifth and seventh capacitors 306/314 each have a capacitance of 0.78 pF, and the tenth and eleventh capacitors 410/414 each have a capacitance of 0.59 pF. Then, the filter 100 is tuned to optimally pass signals having a frequency of about 1910 MHz.

[0055] In a fourth example, the first capacitor 110 has a capacitance of 0.67 pF, the fourth and sixth capacitors 300/310 each have a capacitance of 4.1 pF, the fifth and seventh capacitors 306/314 each have a capacitance of 1 pF, and the tenth and eleventh capacitors 410/414 each have a capacitance of 0.65 pF. Then, the filter 100 is tuned to optimally pass signals having a frequency of about 1700 MHz.

[0056] Details of the FE capacitors are provided below. Generally, the above-described FE capacitors have a quality factor greater than about 80 in the temperature range between −50 and 100 degrees C. The quality factor is related to the measures of loss and quality factor (Q). The FE capacitor types are typically an interdigital capacitor (IDC), overlay, or gap capacitor design.

[0057] FIG. 5 is a schematic block diagram of a wireless telephone device with a tunable duplexer. The device 500 comprises a receiver 502 having an input on line 504, a transmitter 506 having an output on line 408, and an antenna 510 having an antenna port on line 412. A duplexer 514 comprises an input node connected to the antenna port on line 408, a first bandpass filter 100, and a second bandpass filter 516. The first bandpass filter may be the same as the filters shown in FIGS. 1 through 4. Therefore, details of the filter 100 are not shown in FIG. 5. In the interest of brevity, the description of filter 100 is not repeated here.

[0058] The design of the second bandpass filter 516 is essentially the same as the first bandpass filter 100. Note, the actual component values the first bandpass filter 100 may vary from the second bandpass filter 516, as they typically do not tune exactly the same frequency bands. However, in one aspect the two filters are identical and designed to tune within multiple Rx and Tx bands. Such as design simplifies manufacturing by permitting identical bandpass filter assemblies to be used for both the Rx and Tx sections of the duplexer 500.

[0059] The second bandpass filter 516 comprises a twelfth capacitor 520 connected between the input node 412 and a third node 522. A third tank circuit 524 comprises a third resonator 526 having an input connected to the third node 522 and an output. A thirteenth capacitor 528 has an input connected to the third resonator output, an output connected to a reference voltage 202, and an input to accept a control signal on line 530. The thirteenth capacitor 528 includes an FE material with a dielectric constant responsive to the control voltage on line 530. A fourteenth capacitor 532 has an input connected to the third node 522 and an output connected to the reference voltage. As in filter 100, a nineteenth capacitor 534 can be inserted in series between the fourteenth capacitor 532 and the reference voltage 202, for greater tunability. In one aspect, the fourteenth and nineteenth capacitors 532/534 can be FE capacitors responsive to control signals.

[0060] A fourth tank circuit 536 comprises a fourth resonator 538 having an input connected to a fourth node 540 and an output. A fifteenth capacitor 542 has an input connected to the fourth resonator output, an output connected to a reference voltage 202, and an input to accept a control signal on line 544. The fifteenth capacitor 542 includes an FE material with a dielectric constant responsive to the control voltage on line 544. A sixteenth capacitor 546 has an input connected to the fourth node 540 and an output connected to the reference voltage 202. A twentieth capacitor 548 can be series-connected between the sixteenth capacitor 546 and the reference voltage 202, for greater tunability. In one aspect, the sixteenth and twentieth capacitors 546/548 can be FE capacitors responsive to control signals.

[0061] A seventeenth capacitor 550 connects the third node 522 to the fourth node 540. An eighteenth capacitor 552 connects the fourth node 540 and the receiver on line 412. In one aspect, the eighteenth capacitor 552, the seventeenth capacitor 550, and the twelfth capacitor 520 are FE capacitors having a dielectric constant, and therefore a capacitance value, responsive to a control signal.

[0062] FIG. 6 is a schematic block diagram of a wireless telephone device with a tunable filter. The device 600 comprises a transceiver 602 having an interface port on line 412, an antenna 604 having an antenna port on line 408, and a tunable bandpass filter 100. The bandpass filter 100 can be one of the filters described in FIGS. 1, 2, 3, or 4. Therefore, details of the filter are omitted in the interest of brevity. The transceiver 602 may represent a receiver, transmitter, or both. In some aspects, the filter 100 is part of a duplexer (not shown).

[0063] Functional Description

[0064] In designing a tunable bandpass filter, out-of-band rejection, passband insertion loss, size, weight and other mechanical, environmental and electrical requirements should be considered. Typically, the passband is defined by the points where the bandpass filter response falls to 3.0 dB below the mid-band, or band-center insertion loss. Higher order (more resonators) bandpass filters improve the out-of-band rejection, at the cost of greater insertion loss.

[0065] Tunable BPFs have the best chance of replacing fixed-tuned BPFs in those cases where the fixed-tuned BPF covers a system bandwidth that is greater than that required for the transmission or reception of a single channel. For example, a fixed-tuned BPF in a handset for operation in the US CDMA PCS band covers such a BW. It will be understood that this is also true of US cellular CDMA and many other standards. The techniques, methods and devices taught herein are applicable to many standards besides US CDMA PCS. US CDMA PCS is discussed as an example only.

[0066] As mentioned earlier, in the U.S. PCS band, 60 MHz is allocated for Tx (1850 to 1910 MHz) and 60 MHz for Rx (1930 to 1990 MHz). The CDMA standard is a full duplex system, meaning the handset is capable of simultaneously transmitting and receiving. To accomplish this, a duplexer filter is needed to separate the bands and prevent interference. While the PCS band is 60 MHz wide, the individual CDMA channel is only 1.25 MHz wide. Current system architecture, however, forces CDMA PCS bandpass filters and multiplexers (including duplexers) to have a BW of greater than 60 MHz, to allow the system to accommodate operation of any 1.25 MHz channel in any region of the 60 MHz band.

[0067] A tunable PCS band filter alters this situation by meeting the worst case rejection specifications, while providing a lower order BPF of simpler topology that occupies a smaller physical area. Assuming high-Q components are used, such a lower order filter necessarily provides lower insertion loss, as is well understood in the art.

[0068] In the replacement of a high-order fixed-tuned BPF with a low-order tunable BPF, three factors are presented. First, the bandwidth (Q1) of the low-order BPF and the chosen topology must be such that the worst case rejection specification is met. Because Q1=f0/BW, as the 3 dB bandwidth (BW) decreases, the IL increases. Thus, if BW is too small relative to f0, the resulting BPF has an unacceptably high IL.

[0069] Second, the low-order tunable filter must be tunable to cover the entire BW, just as with a fixed-tuned filter. Finally, the tunable capacitor used within the low-order tunable filter should be of sufficiently low loss. The total loss of a capacitor, Lt, whether tunable or not, is given by a ratio of its dissipated to stored energy, where the energy is stored in the electric field and dissipated in resistance, i.e., Lt=(dissipated energy)/(stored energy). The inverse of this loss is the quality factor, Q. For a capacitor, Lt may be given by the quantity (ω*Rs*C), where ω is the frequency in radians, Rs is the total series resistance of the capacitor, and C is the capacitance.

[0070] The importance of determining the total loss given by an FE capacitor in a resonant circuit can be seen from the following equations:
Lc=1/Qc and 1/Qt=1/Qc+1/Qu, where,

• • [0071] Lc=the loss of the capacitor;
  • [0072] Qt=the total Q of the FE capacitor and the resonator or inductor combined;
  • [0073] Qc=the Q of the capacitor; and
  • [0074] Qu=t he Q of the unloaded resonator or alternatively, the Q of an inductor used to create a parallel resonant circuit.

[0075] As Qc increases, it will affect the Qt less. If Qc is infinite, it has no affect on Qt. For practical purposes, this is also true if Qc is approximately 10*Qu. The converse is true too. As Qu becomes higher relative to Qc, Qu has less effect on Qt. In either case, the highest practical Qc is desired.

[0076] For example in the PCS band, for a 1.0 pF tunable capacitor to have a Qc=250 at 2.0 GHz, Rs is about 0.32 Ω (ohms). To minimize loss (obtain a low Rs), requires an accounting of all loss mechanisms present and an elimination of these loss mechanisms if possible.

[0077] For FE devices, the total loss is governed by summing each source contribution as follows:
Lt=Lgeom+Lattach+Lmetal+Lsub+Lrad+Lmeas+LFE;

• • [0078] where Lgeom is derived from the topology of the capacitor,
  • [0079] Lattach is loss due to device attachment,
  • [0080] Lmetal is the total metal loss,
  • [0081] Lsub is the base substrate loss (if present),
  • [0082] Lrad is the radiation loss, both desired and undesired,
  • [0083] Lmeas is the total loss arising from measurement errors, and
  • [0084] LFE is the FE loss tangent. A more detailed treatment of this
    subject can be found in U.S. Pat. No. 6,639,491, which is incorporated herein by reference.

[0085] FE film properties and fabrication play a significant role in overall capacitor loss. Many techniques exist to mitigate and minimize FE film loss. One feature of FE films is that FE film loss and tunability usually have an inverse relationship. The greater the FE dielectric constant tuning range, the greater is the FE loss in most cases.

[0086] Thus, even though FE materials can achieve a dielectric constant tuning range of about 3 to 1, less tuning may be acceptable for a given filter application. In that case, less tuning would be chosen, with the benefit of less loss. For example, in the US PCS CDMA band, the tuning requirement in the transmit band is from 1850 MHz to 1910 MHz, or about 4%. Even if the KPCS and USPCS is combined (1750 to 1910 MHz, or about 8%), the FE material can have significantly less tunability than 3 to 1.

[0087] For example, an FE gap capacitor with 0.6 pF at 0V DC bias, needs to tune 33%, (from 0.6 pF down to 0.4 pF) to tune over the PCS transmit band. The actual tuning range depends on the BPF topology and the band over which the BPF is tuned. The required tuning voltage to provide the 33% tuning in this example depends on the FE capacitor geometry, including FE film thickness, and the FE film characteristics. The effect of dielectric constant tunability on frequency tunability is determined by the filter topology.

[0088] With respect to Lgeom for a gap capacitor, the major contributions to loss are the four corners formed by the gap. These losses can be reduced by rounding the corners. In comparison to gap and interdigital capacitors, an overlay capacitor has the lowest Lgeom. An overlay capacitor is an example of a parallel plate geometry where the plate dimensions (length and width) are much greater than the plate separation. Given such a geometry, most of the electric field between the plates is uniform except for fringing along the edges. The fringing effect can be reduced significantly by the use of a guard band. Thus, the geometric loss from a parallel plate capacitor is quite low. In addition, parallel plate geometries can provide high capacitances along with high tuning from small control voltage swings.

[0089] FIG. 7 is a plan view of a distributed element gap capacitor. Compared to an IDC, the gap capacitor has a better Q, but lower capacitance per unit cross section (W). The IDC's capacitance is greater due to the use of a number of fingers per unit cross section. For many communication filter applications, however, large capacitance (C≧4.0 pF) is not needed. Thus, a gap capacitor often can provide adequate capacitance. The inherently high value of κ for most FE films helps provide relatively high capacitance per unit cross-section, W, compared to a conventional gap capacitor.

[0090] FIG. 8 is a cross-sectional view of an overlay capacitor. In comparison to gap and interdigital capacitors, an overlay capacitor has the lowest Lgeom. An overlay capacitor is an example of a parallel plate geometry where the plate dimensions (length and width) are much greater than the plate separation. Given such a geometry, most of the electric field between the plates is uniform except for fringing along the edges. The fringing effect can be reduced significantly by the use of a guard band. Thus, the geometric loss from a parallel plate capacitor is quite low. In addition, parallel plate geometries can provide high capacitances along with high tuning from small control voltage swings.

[0091] FIG. 9 is a plan view of an interdigital (IDC) capacitor. For a given cross-sectional area, an IDC can provide a higher capacitance than a gap capacitor. Loss increases as the gap spacing decreases. Similarly, loss increases as finger width decreases. The finger length also affects loss, with loss increasing as finger length increases; especially in a microstrip realization of an IDC, as the odd mode loss dominates in such a structure. In addition, loss increases as the number of fingers increases, due to loss introduced from the additional sharp corners. Note that an increase in the number of fingers typically increases the capacitance of an IDC.

[0092] In general, a gap capacitor is easiest to fabricate. An IDC is next easiest, and an overlay capacitor is hardest of these three. Compared to an IDC, the gap capacitor has a better Q, but lower capacitance per unit cross-section. The IDC's capacitance is greater due to the use of a number of fingers per unit cross section. For many communication filter applications, however, large capacitance (C greater than 4.0 pF) is not needed. Thus, a gap capacitor often can provide adequate capacitance. The inherently high value of dielectric constant for most FE films helps provide relatively high capacitance per unit cross section, compared to a conventional gap capacitor.

[0093] The highest possible Qu for a resonator should be used to minimize IL, given size and height constraints. To define Qu, a stage can defined, which is formed by a resonator series connected to a capacitor. The resonator may be grounded quarter wavelength resonator, but the length of the resonator can be selected to obtain a particular inductance value. Volumetric resonators (e.g., coaxial, stripline, and monoblock) are often preferred, as they provide the highest Qu and smallest area and height at a minimal price, as compared to planar, i.e., microstrip, alternatives. For high volume applications, such as wireless handsets, transverse electromagnetic (TEM) wave volumetric resonators may be preferred. Such volumetric resonators can be ceramic loaded coaxial resonators, slabline (monoblock) or stripline, to name the three most common realizations.

[0094] FIG. 10 is a schematic diagram depicting a variation of the bandpass filter of FIG. 2. A two-stage top capacitively coupled (TCC) tunable BPF 1000 is illustrated. Each stage of bandpass filter 1000 comprises a resonator 1004 and 1008 and FE capacitor 1010a and 1010b. The resonators 1004 and 1008 are shown as ¼ wavelength short-circuited resonators but may also be ½ wavelength open circuit resonators.

[0095] A variable DC voltage, or control signal is applied to the FE capacitors 1010a and 1010b, to tune the bandpass filter 1000. This is an example of where the control signal is introduced to the capacitor terminal. The FE capacitors 1010a and 1010b couple to ground through DC blocking capacitors 1012a and 1012b, since the resonators are shorted in this example.

[0096] An RF signal is received at input port 1002 and output at output port 1006. Note that input port 1002 and output port 1006 are interchangeable. In addition to input capacitor 1034a and output capacitor 1034b, an additional capacitor 1032 is provided as an impedance and admittance inverter between the resonators 1004 and 1008 to create the desired BPF response. It will be appreciated that capacitor 1032 can also be a discrete element or implemented through aperture coupling between resonators 1004 and 1008.

[0097] FIG. 11 is a flowchart illustrating a method for bandpass filtering a signal. Although the method is depicted as a sequence of numbered steps for clarity, no order should be inferred from the numbering unless explicitly stated. It should be understood that some of these steps may be skipped, performed in parallel, or performed without the requirement of maintaining a strict order of sequence. The method starts at Step 1100.

[0098] Step 1102 provides a first tank circuit, with a resonator and parallel capacitor, capacitively coupled to a second tank circuit with a resonator and parallel capacitor. Step 1104 accepts an input signal. Step 1106 ferroelectrically tunes the resonators of the first and second tank circuits. Step 1108 bandpass filters the input signal in response to ferroelectrically tuning the resonators.

[0099] In one aspect, providing first and second tank circuits in Step 1102 comprises providing a ferroelectric (FE) capacitor series connected with each resonator, where each FE capacitor is made with a ferroelectric material having a dielectric constant responsive to a control signal. Then, ferroelectrically tuning the resonators of the first and second tank circuits in Step 1106 comprises supplying control signals to the FE capacitors connected in series to the resonators.

[0100] In another aspect, providing first and second tank circuits in Step 1102 comprises providing a FE capacitor in parallel with each resonator, where each FE capacitor is made with a ferroelectric material having a dielectric constant responsive to a control signal. Then, ferroelectrically tuning the resonators of the first and second tank circuits in Step 1106 comprises supplying control signals to the FE capacitors connected in parallel with the resonators.

[0101] In one aspect, Step 1102 provides a FE capacitor in parallel with each resonator and in series with each resonator, where each FE capacitor is made with a ferroelectric material having a dielectric constant responsive to a control signal. Then, Step 1106 supplies control signals to the FE capacitors connected in parallel and in series with the resonators.

[0102] A tunable bandpass filter has been provided. Specific filter topologies have been provided to illustrate the invention. Further, a duplexer application and particular frequency bands have been provided as examples. However, the invention is not limited to merely these illustrations and examples. Other variations and embodiments of the invention will occur to those skilled in the art having the benefit of this disclosure.