Abstract:
A controllable filter arrangement with a voltage controlled device that subjects a signal to a predetermined impedance as part of the filtering process when the voltage controlled device is in an active state. In an inactive state, the voltage controlled device may subject the signal to an impedance that prevents all frequencies of the signal from passing. This configuration may advantageously increase frequency selectivity and reduce insertion loss, size, cost, and tuning complexity when compared with conventional filter designs.

Description:
TECHNICAL FIELD 
     The presently disclosed embodiments relate generally to signal processing, and more specifically to filtering a signal to obtain a desired frequency response. 
     BACKGROUND 
     Since the inception of signal transmission, there has been a struggle to reduce the amount of interference in the received signal. Interference can be caused by a number of factors. For example, various environmental factors (e.g., mountains, buildings, other man-made structures, etc.) can contribute to signal interference when they are located within the transmission path between a transmitter and a receiver. 
     Also, with the proliferation of many different methods and devices to transmit signals, there has been a significant increase in the amount of interference that can occur when signals interfere with each other. This can occur when signals propagate through the same medium (e.g., airspace, cable, wire, etc.) as other signals. 
     Various methods and techniques have been employed to minimize signal interference. For example, the receiving device may include various types of filters to remove unwanted interference from the received signal. These filters have been known to include various types of band pass filters (e.g., filters to permit signals within a predetermined frequency range to pass but removing all other portions of the signal) alone or in combination. 
       FIG. 1  shows one example of a switched filter bank, which has been used to remove unwanted interference from a received signal. Switched filter bank  100  includes input  102  to receive a signal to be filtered and output  104  to output the filtered signal. A controller (not shown) controls switches  106 ,  108 , and  110  between an open position and a closed position. In the closed position of the switch, the signal can move through the switch (e.g., switch  106   a ) and its respective filter (e.g., filter  112 ). In the open position, the signal may not pass through the switch or its respective filter. 
     For example, if switches  106   a  and  106   b  are closed, the signal can be filtered by filter  112 . Likewise, if switches  108   a  and  108   b  are closed, the signal can be filtered by filter  114 , and if switches  110   a  and  110   b  are closed, the signal can be filtered by filter  116 . Depending on the desired frequency response, one or more of filters  112 ,  114 , and  116  can be used to filter the signal before it is output via output  104 . 
     However, there are several drawbacks to using the switched filter bank. Specifically, the switches in the switched filter bank increase the insertion loss of the device, which degrades the sensitivity of the device. In addition, the switches in the switched filter bank increase the size and cost of the device. 
     Turning now to  FIG. 2 , a multiplexed filter is shown that has also been used to filter received signals. Multiplexed filter  200  includes input  202  and output  204 . Filters  206 ,  208 , and  210  are coupled in parallel to input  202  and output  204 . 
     In operation, multiplexed filter  200  utilizes filters  206 ,  208 , and  210  to pass all the desired channels (e.g., frequencies), similar to the switched filter bank, discussed above. However, multiplexed filter  200  uses input and output common node matching networks rather than switches. Although this feature reduces the insertion loss of multiplexed filter  200  when compared to the insertion loss of the switched filter bank, it also provides less selectivity than the switched filter bank since the multiplexed filter passes all frequencies in its combined pass band simultaneously. Thus, there is little interference rejection provided by the multiplexed filter. 
     Although not shown in the figures, tunable band pass filters have also been used, if they can meet the required performance specifications of a particular application, to filter out unwanted interference in a received signal. However, typical tunable band pass filters have been known to require complex tuning, alignment, and control functions. 
     There is therefore a need in the art for a device and method that can be used to filter out unwanted interference from a signal in a manner that provides lower insertion loss than the switched filter bank, better frequency selectivity than the multiplexed filter, and with less complex tuning, alignment, and control than the tunable filter. 
     SUMMARY 
     Embodiments disclosed herein address the above stated needs by providing a filter with a voltage variable impedance element that acts as a switch and as an impedance-contributing element (e.g., part of the filtering process) when the filter element with which the voltage variable impedance element is associated is used to filter the signal. 
     In one aspect, a plurality of the voltage variable impedance elements may be selectively placed in an active state, inactive state, or various combinations thereof to obtain a desired frequency response from the filter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a known switched filter bank; 
         FIG. 2  is a known multiplexed filter; 
         FIG. 3  is a schematic of a controllable filter arrangement with voltage controlled devices; and 
         FIG. 4  is a method of filtering a signal with a filter with voltage controlled devices. 
     
    
    
     DETAILED DESCRIPTION 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. In addition, references to “an,” “one,” “other,” or “various” embodiments should not be construed as limiting since various aspects of the disclosed embodiments may be used interchangeably within other embodiments. 
     The filter devices and methods described below can be used in any device, apparatus, or system that could benefit from signal filtering, including, for example, channelized receivers, mobile/cellular telephones, multi-band radios and/or transceivers (e.g., wired or wireless). As used herein, the term “filter” may be used to describe a device through which a signal may be passed in order to remove unwanted components of the signal, which may include, for example, certain frequencies, artifacts, and interference. The term “filter element” is used herein to describe a component of a filter that is capable of actively participating in the filtering process (e.g., removing unwanted signal components) and exhibit varying impedance over frequency. 
     When selected filter elements are connected in a particular arrangement, the arrangement forms a filter that has a particular filter response dependent on the selected filter elements. The response of the filter formed by the arrangement of filter elements may have a band pass filter response where signals within a desired frequency band are attenuated less than frequencies outside the desired frequency band. Also, the filter may have a stop-band filter response where signals within a stop band are attenuated more than frequencies outside the desired frequency band. The filter may have low pass filter response where signals below a selected frequency are attenuated less than frequencies above the frequency. Where signals below a selected frequency are attenuated more than frequencies above the frequency, the filter has a high pass filter response. 
     Examples of filter elements may include, among others, a capacitive element such as a capacitor, an inductive element such as an inductor, and a resistive element such as a resistor. Some filter elements may be voltage controlled devices that have characteristics that depend on an applied control voltage. In one aspect, a voltage controlled device is a voltage variable impedance element that has an impedance that is varied by the control voltage (e.g., control signal). For example, as discussed below, a voltage controlled device may be a voltage variable capacitor or a voltage variable inductor. 
     In operation, a voltage controlled device may, in an active state, contribute to the filtering process by subjecting the signal to a predetermined impedance from the voltage controlled device. As used herein, an “active state” of the voltage controlled device refers to a state in which the voltage controlled device is set to participate in the filtering of a signal, either alone or in combination with one or more other filter elements (e.g., band pass filter elements, stop-band filter elements, low pass filter elements, high pass filter elements, other voltage controlled devices). Conversely, a voltage controlled device may be placed in an “inactive state,” which is a state in which the voltage controlled device subjects the signal to an impedance that prevents all frequencies of the signal from passing. None of the known techniques discussed in the Background section utilize this type of configuration. 
       FIG. 3  is a block diagram of a controllable filter arrangement example. The controllable filter arrangement  300  includes input  302  to receive a signal to be filtered and output  304  to provide the filtered signal. The input  302  is connected to the inputs of three filters  306 ,  308 ,  310  that are connected in parallel and have output connected to the output  304 . Each filter  306 ,  308 ,  310  includes a plurality of filter elements  312 ,  314 ,  316 . 
     For this example, the controllable filter arrangement includes three filters where each plurality of filter elements forming a filter includes a voltage controlled device  318 ,  320 ,  322 . The filter arrangement may include a different number of filters and each filter may include any number of voltage controlled devices. The arrangement, however, includes at least two filters where at least one of the filters includes a voltage controlled device. Each plurality of filter elements forming a filter includes at least one other filter element that is not voltage controlled. 
     Accordingly, the first filter  306  is formed by a first plurality of filter elements  312  that includes a voltage controlled device  318  and other filter elements  324 . The second filter  308  is formed by a second plurality of filter elements  314  that includes a voltage controlled device  320  and other filter elements  326 . The third filter  310  is formed by a third plurality of filter elements  316  that includes a voltage controlled device  322  and other filter elements  328 . 
     Each filter, however, is only formed when the voltage controlled device is in an active state. In the active state, the voltage controlled device forms a filter element of the plurality of filter elements. In the inactive state, the voltage controlled device is a relatively high impedance that minimizes any path through the other filter elements of the filter. Although the voltage controlled devices are illustrated as directly connected to the input, the voltage controlled devices may be connected anywhere within the filter. For the examples herein, the voltage controlled devices are series elements of the filter. 
     For the example, each filter is used to pass a different frequency range. The different frequency ranges of the filters may or may not overlap. As will be seen below, additional filters can provide enhanced frequency selectivity or otherwise change performance. In some circumstances, the benefits of improved frequency selectivity from adding additional filters should be balanced against the increased size and cost of adding the additional filters. 
     In the example shown in  FIG. 3 , voltage controlled devices  318 ,  320 , and  322  are each coupled in series with filter elements  324 ,  326 , and  328 , respectively. In operation, each of voltage controlled devices  318 ,  320 , and  322  are capable of subjecting a signal to a different impedance. For example, controller  330 , which is coupled to each of voltage controlled devices  318 ,  320 , and  322 , can send a control signal (e.g., a control voltage) to voltage controlled device  318  to place voltage controlled device  318  in an active state or an inactive state. Similarly, controller  330  can send control signals to voltage controlled devices  320  and  322  to place voltage controlled devices  320  and  322  in an active state or an inactive state. 
     In the active state, voltage controlled device  318  subjects the signal to an active state impedance, which contributes to the filtering process. In the example of  FIG. 3 , the signal may also pass through filter element  324  after passing through voltage controlled device  318 . The resultant signal that emerges from filter element  324  is a signal within a predetermined frequency range that is associated with voltage controlled device  318  and filter element  324 , which form the first filter. 
     In the inactive state, the impedance of voltage controlled device  318  is higher than the impedance of voltage controlled device  318  in the active state. For example, the impedance of voltage controlled device  318  is sufficiently high in the inactive state that voltage controlled device  318  may appear as an open circuit, preventing the signal from passing through filter element  324 . 
     In various embodiments, voltage controlled device  318  is a voltage variable capacitor (“VVC”) and may have (1) an active state in which its capacitance is initially adjusted to meet the needs of a specific filter design application, and (2) an inactive state in which the VVC is purposely driven to its extreme lowest capacitance. At its lowest capacitance, the impedance is highest for the VVC. Depending on the application, the capacitance of the VVC in an active state may be 0.36 pF, and the capacitance of the VVC in an inactive state may be 0.06 pF. This active/inactive capacitance ratio (e.g., 6:1) may be used in a wide range of applications. However, other capacitance values may be used for the active and inactive states, and other active/inactive capacitance ratios may be used. 
     In some circumstances, voltage controlled device  318  may be a voltage variable inductor (“VVI”) instead of a VVC. It is also possible that voltage controlled elements  318 ,  320 , and  322  may be a combination of VVCs and VVIs. For the exemplary embodiments discussed herein, the on/off ratio is maximized. As a result the voltage variable device need not be precisely controlled, or tuned, in its inactive state. In some circumstances, however, lower on/off ratios may be used requiring at least some adjustment of the voltage variable device in the inactive state. An example of a suitable on/off ratio is at least 3:1. For the examples, the on/off ratio is at least 6:1. 
     In various embodiments, filter elements  326  and  328  have their own unique predetermined frequency ranges that are different from that of filter element  324  and different from each other. By varying which combination of filter elements  324 ,  326 , and  328  are used to filter the signal, different frequency responses may be obtained and output from controllable filter arrangement  300  via output  304 . For example, a first frequency response could be obtained from controllable filter arrangement  300  when voltage controlled device  318  is in an active state and voltage controlled devices  320  and  322  are both in an inactive state. Other frequency responses could be obtained by only placing voltage controlled device  320  or voltage controlled device  322  in an active state. Although these examples describe only one voltage controlled device in an active state, other frequency responses could be obtained by placing two or more voltage controlled devices in an active state. Accordingly, a combined frequency response can be created by placing the voltage controller devices of two filters in the active states. 
     Referring now to  FIG. 4 , a method of filtering a signal with a controllable filter arrangement with voltage controlled devices is shown. Method  400  begins with inputting a signal to a controllable filter arrangement that includes voltage controlled devices (e.g., step  402 ). For example, the controllable filter arrangement may be similar to controllable filter arrangement  300  shown in  FIG. 3 . 
     At step  404 , one of a first frequency response and a second frequency response is selected. For example, if method  400  is implemented with controllable filter arrangement  300  of  FIG. 3 , the first frequency response may be selected when voltage controlled device  318  is in an active state (e.g., and has an active state impedance) and voltage controlled devices  320  and  322  are in an inactive state (e.g., and have an inactive state impedance). In this example, the resultant signal (e.g., first frequency response) corresponds with the frequencies allowed to pass through voltage controlled device  318  and filter element  324 . 
     Alternatively, a second frequency response may be selected when voltage controlled device  320  is in an active state (e.g., and has an active state impedance) and voltage controlled devices  318  and  322  are in an inactive state (e.g., and have an inactive state impedance). In this example, the resultant signal (e.g., second frequency response) corresponds with the frequencies allowed to pass through voltage controlled device  320  and filter element  326 . 
     Once the desired frequency response is selected, it may be output from the controllable filter arrangement. Thus, according to method  400 , the signal is either filtered by the first voltage controlled device and the first filter element (e.g., when the first voltage controlled device is active), or the signal is prevented from passing through the first filter element (e.g., when the first voltage controlled element is inactive). 
     Similarly, the signal is either filtered by the second voltage controlled device and the second filter element (e.g., when the second voltage controlled device is active), or the signal is prevented from passing through the second filter element (e.g., when the second voltage controlled device is inactive). 
     Although not shown in  FIG. 4 , method  400  may additionally include a step in which a control signal (e.g., voltage control signal) is sent to one or more voltage controlled devices to switch the voltage controlled devices between the active state and the inactive state or vice versa. 
     Method  400  has been described in terms of only two frequency responses in order to simplify the description. However, as described above, a greater number of potential frequency responses can be selected when a greater number of voltage controlled devices and filter elements are used (e.g., by mixing the number and configuration of voltage controlled devices that are placed in an active state). Although the steps of  FIG. 4  have been discussed and depicted within a particular order, one of skill in the art should understand that the steps can be performed in a different order or otherwise interchanged without departing from the scope of the various embodiments. 
     Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention. 
     The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. Further, the controller or processor may be integrated directly with the filter block, it may be stand-alone, or it may be integrated as part of another integrated circuit device. 
     The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal. 
     In one or more exemplary embodiments, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.