Abstract:
Methods and apparatus, including computer program products, are provided for RF filtering. Related apparatus, systems, methods, and articles are also described.

Description:
FIELD 
       [0001]    The subject matter described herein relates to filters. 
       BACKGROUND 
       [0002]    A radio receiver may handle large out-of-band interferers (blockers) and recover a weak desired signal in presence of other in-band and out-of-band interferers. As such, the receiver may need to be able to process the desired channel while sufficiently rejecting strong-in-band and out-of-band interferers. These interfering signals near the desired signal may need to be suppressed. To that end, a band selection filter may provide attenuation for out-of-band signals, and a subsequent baseband lowpass channel filter may provide channel selection. Some currently available filters may not be able to provide channel selection at the radio frequency (RF)-level because of the extremely high quality (high-Q) requirements required to perform channel selection. Moreover, some band selection filters may not be tunable across a plurality of bands. 
       SUMMARY 
       [0003]    In some example embodiments, there is provided an apparatus. The apparatus may include a first bandpass filter to receive a differential signal, wherein the first bandpass filter comprises a resistive-capacitive configuration of N-path filters tunable to a first center frequency; a first notch filter to receive a first output of the first bandpass filter, wherein the first notch filter comprises a capacitive-resistive configuration of N-path filters tunable to a first notch center frequency; a second bandpass filter to receive a second output of the first notch filter, wherein the second bandpass filter comprises a resistive-capacitive configuration of N-path filters tunable to the first center frequency; a second notch filter to receive a third output of the second bandpass filter, wherein the second notch filter comprises a capacitive-resistive configuration of N-path filters tunable to a second notch center frequency; and a third bandpass filter to receive a fourth output of the second notch filter and to provide a filtered output of the apparatus, wherein the third bandpass filter comprises a resistive-capacitive configuration of N-path filters tunable to the first center frequency. 
         [0004]    In some variations, one or more of the features disclosed herein including the following features can optionally be included in any feasible combination. The first notch center frequency may be offset in frequency at least one of above and below the first center frequency. The second notch center frequency may be offset in frequency at least one of above and below the first center frequency. The first notch filter may include a first feedforward transconductance amplifier and a second feedback transconductance amplifier configured to provide frequency shifting at the first notch filter. The second notch filter may further include a first feedforward transconductance amplifier and a second feedback transconductance amplifier configured to provide frequency shifting at the second notch filter. The first transconductance amplifier may amplify the first output provided to the first bandpass filter. The second transconductance amplifier may amplify the second output provided by the first notch filter. A balun may be coupled to the first bandpass filter to provide the differential signal input to the first bandpass filter. 
         [0005]    The above-noted aspects and features may be implemented in systems, apparatus, methods, and/or articles depending on the desired configuration. The details of one or more variations of the subject matter described herein are set forth in the accompanying drawings and the description below. Features and advantages of the subject matter described herein will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]    In the drawings, 
           [0007]      FIG. 1  depicts an example of a tunable bandpass filter, in accordance with some example embodiments; 
           [0008]      FIGS. 2A-2B  depict an example of a tunable bandpass filter, in accordance with some example embodiments; 
           [0009]      FIG. 3  depicts an example of a transconductance amplifier cell implemented with an inverter, in accordance with some example embodiments; 
           [0010]      FIG. 4  depicts an example of a transconductance amplifier cell which may be used for frequency shifting, in accordance with some example embodiments; 
           [0011]      FIG. 5  depicts an example of back-to-back inverters configured to provide a negative resistance, in accordance with some example embodiments; 
           [0012]      FIGS. 6-8  depict example frequency spectrum plots for tunable bandpass filters, in accordance with some example embodiments; and 
           [0013]      FIG. 9  depicts an example of a transceiver, in accordance with some example embodiments. 
       
    
    
       [0014]    Like labels are used to refer to same or similar items in the drawings. 
       DETAILED DESCRIPTION 
       [0015]      FIG. 1  depicts a block diagram of a tunable bandpass filter  100 , in accordance with some example embodiments. The tunable bandpass filter  100  may include a splitter  102 , a tunable bandpass filter  104 , a transconductance amplifier  106 , a tunable notch filter  108 , a second transconductance amplifier  110 , a tunable bandpass filter  112 , a tunable notch filter  114 , and a tunable bandpass filter  116 . The tunable bandpass filter  100  may be used for band selection and/or channel-selection. Moreover, tunable bandpass filter  100  may, in some example embodiments, have a high stop-band rejection. Furthermore, tunable bandpass filter  100  may, in some example embodiments, be based on an elliptic filter technology, although other filter technologies maybe used as well. 
         [0016]    The splitter  102  may, in some example embodiments, receive an input signal, Vin,  101  which may be a single-ended signal being filtered by tunable bandpass filter  100 . The splitter  102  may, in some example embodiments, output a differential output signal  120 . 
         [0017]    In some example embodiments, the differential output signal  120  may be provided as a differential input signal to a tunable bandpass filter  104 , such as a tunable bandpass filter comprising N-path filters. The output of the tunable bandpass N-path filter  104  may then be provided to a transconductance amplifier  106  (labeled g m -cell). The transconductance amplifier  106  may provide gain, which may facilitate noise reduction in subsequent stages. The output  140  of the transconductance amplifier  106  may be coupled to an input of a tunable notch filter  108 , which provides output signal  150 . 
         [0018]    In some example embodiments, the center frequency of the notch filter  108  (as well as notch filter  114  described below) may be shifted to an offset frequency different from the center frequency of the bandpass filter  104 . For example, the center frequency of notch filters  108  and/or  114  may be 20 MHz below (or above) the center frequency of the bandpass filter  104 . The notch filters (which are offset in frequency) may sharpen the frequency response of filter  100  around the center frequency of the bandpass filters  104 ,  112 , and  116 , which may increase the stopband rejection. A complex impedance may be realized by using two identical real impedances, as an example a capacitor in this case and two transconductors. The complex impedance may receive a complex current and may generate a complex response voltage. A voltage dependent current source may sense the imaginary voltage and may inject a current to a real port (and vice versa) which realizes frequency shifting (Δf) of the baseband complex impedance at a notch filter. Depending on whether Gm is positive or negative, the frequency shifting is negative or positive, respectively. The tunable notch N-path filter  204  may be used to up-convert this frequency-shifted complex impedance to the filter clock frequency (fclk) as shown in for example  FIG. 2A  where, fclk=1/Tclk, thereby the notch is created at (fclk+Δf) or (fclk−Δf) depending on the sign of gm cell. As such, the notch filters  108  and/or  114  are offset in frequency with respect to the center frequency of the bandpass filters  104 ,  112 , and  116  (which in this example is at fclk), and this offset effectively provides a higher-Q filter. Although the previous example describes a notch filter offset frequency of 20 MHz below the center frequency of the bandpass filter, other offsets may be realized as well. 
         [0019]    In some example embodiments, a second transconductance amplifier  110  (labeled Gm-cell) may receive output  150  and provide amplified output  160 . This second transconductance amplifier  110  may be placed after the notch filter  108 , and may serve to provide gain and/or enable noise reduction in subsequent stages. 
         [0020]    In some example embodiments, a second tunable bandpass filter  112 , such as an N-path bandpass filter, may be coupled to the output  160  of second transconductance amplifier  110 . The output  170  of N-path bandpass filter  112  may then be provided as an input to a tunable notch filter, such as N-path notch filter  114 . The N-path notch filter  114  may be configured with a center frequency different than bandpass filter  112 . For example, N-path notch filter  114  may have a center frequency 20 MHz above the center frequency of the bandpass filter  112 . The notch frequency may be shifted using feedforward and feedback gm cells  292 A-B and C 4 . By changing the polarity of the gm cells (as an example input and/or output), the center frequency of the notch filter  114  may be shifted to the right. 
         [0021]    In some example embodiments, a tunable bandpass filter  116 , such as a tunable N-path filter, may receive the output  180  of notch filter  114  and provide filtered output, Vout  190 . 
         [0022]    In some example embodiments, the center frequency of individual bandpass filters  104 ,  112 , and  116  may be similar or the same. Moreover, the center frequency of bandpass filters  104 ,  112 , and  116  may define the center frequency of tunable bandpass filter  100 . The center frequencies of the notch filters  108  and  114  may be configured with center frequencies different from the bandpass filters  104 ,  112 , and  116  as noted above. The notch filters  108  and  114  may each be configured to create zeros in the transfer function of the filter at offset frequencies of the filter. By creating zeros outside the bandwidth of the filter, the stopband rejection of the filter may be increased compared with an all pole filter. By choosing the offset frequencies (with respect to the clock frequency fclk) of the notch filters  108  and  114 , the overall filter shape, bandwidth and attenuation may be controlled around the clock frequency fclk. Although the previous example describes the same frequency offset at notch filters  104  and  114 , different offsets may be used at each filter as well. Moreover, the example values of 20 MHz are merely examples, as other values may be used as well. 
         [0023]    In some example embodiments, the center frequency of the notch filters  108  and  114  may be independently selected irrespective of the center frequency of the bandpass filters  104 ,  112 , and  116 . Depending on the desired bandwidth and sharpness of attenuation, the offset frequencies of notch filter  108  and  114  may be chosen. 
         [0024]      FIGS. 2A-B  depict an example implementation of a tunable bandpass filter  200 , in accordance with some example embodiments. A signal to be filtered by tunable bandpass filter  200  may be received at  296 A as an input (V in ) and then filtered by tunable bandpass filter  200 , yielding differential output signal  296 B. 
         [0025]    The tunable bandpass filter  200  may include a splitter  201 , a tunable bandpass filter  202 , a transconductance amplifier  203 , a tunable notch filter  204 , a second transconductance amplifier  205 , a tunable bandpass filter  206 , a tunable notch filter  207 , and a tunable bandpass filter  208 . The tunable bandpass filter  200  may tune over one or more frequency bands and/or may be used for channel-selection. In some example embodiments, tunable bandpass filter  200  may be based on an elliptic filter technology, although other filter technologies maybe used as well. 
         [0026]    In some example embodiments, a balun  201  may split an input signal  101  into to differential voltage signals. The differential voltage output signal of balun  201  may, in accordance with some example embodiments, be coupled to a tunable bandpass filter  202 , such as a resistive-capacitive (R-C) configuration of an N-path filter. The tunable bandpass filter  202  output may couple to the differential input of transconductance amplifier  203 . 
         [0027]    In some example embodiments, the transconductance amplifier  203  may be implemented as two transconductance (g m ) cells of the same or similar type given the differential architecture of filter  200 . 
         [0028]      FIG. 3  depicts an example implementations of transconductance amplifier cell  203  implemented with self-biased inverters as shown in  FIG. 3 . Lower threshold voltage devices may be used for the input transistors, M 1  and M 2 , to provide lower parasitic capacitance (which may reduce noise figure). The feedback resistor, R, may hold the output voltage level, Vout, as V DD /2. In some example embodiments, a plurality of identical transconductance (g m ) amplifier cells  300  may be used to implement transconductance amplifier  203  in order to provide gain and/or a noise figure reduction. 
         [0029]    Referring again to  FIG. 2 , the differential output of transconductance amplifier  203  may be provided to a tunable notch filter  204 , which may be implemented as a capacitive-resistive (C-R) configuration of N-path filters. 
         [0030]    In some example embodiments, the resonant (or center) frequency of the notch filter  204  may be shifted by using two transconductance, g m , cells  220 A-B (labeled g m2  and g m3 ). The g m  cells  220 A-B may be implemented differently when compared to the g m  cell  300 . The differential input voltage of transconductor g m5  may provide current output in the quadrature phase part of capacitor C 4 . Similarly, the input voltage of g m6  may provide current output to the quadrature part of capacitor C 4 . This may effectively result in the frequency shift of the notch. 
         [0031]      FIG. 4  depicts an example of a g m  cell  400 , which may be used at each of g m  cells  220 A-B (g m2  and g m3 ) to provide frequency shifting noted with respect to tunable notch filter  204 . The frequency shifting provided by g m  cells  220 A-B may be determined in accordance with the following: 
         [0000]    
       
         
           
             
               g 
               
                 
                   m 
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                   2 
                 
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                 3 
               
             
             
               2 
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                 C 
                 BB 
               
             
           
         
       
     
         [0000]    wherein C BB  is combination of C 2  and the parasitic capacitances of the transistor switches.
 
By adjusting the values of cells g m2,3  ( 220 A-B) and C 2 , the center frequency is shifted of the N-path implementation of the notch filter  204  with feed-forward and feedback gm cells  220 A-B (for example 20 MHz lower than the center frequency of the filter).
 
         [0032]    To compensate the loss from the notch filter  204  and to reduce the noise from subsequent stages, another transconductance amplifier (g m  cell)  205  may be coupled to the output of notch filter  204 . The transconductance amplifier  205  may be implemented in the same or similar manner as the transconductance amplifier  300  (described further below). 
         [0033]    The next bandpass filter  206  may, in some example embodiments, be implemented by tunable N-path filters in an R-C configuration  206 . The negative resistances (R neg1 ) may be implemented with for example back-to-back inverters  500  as shown in  FIG. 5 . The negative resistor (R neg1 ) may have a separate supply voltage with for example a nominal value of 1.2 volts, although other values may be used as well. The parasitic capacitance at each node of the filter modifies the equivalent resistance of that node which is frequency dependent. As the clock frequency decreases, the Q-factor of the filter  200  increases leading to higher ripples in the pass-band of the filter  200  for low clock frequencies. As such, the supply voltage of the negative resistors may be reduced for low clock frequencies to reduce these ripples. 
         [0034]    The tunable second notch filter  207  may, in some example embodiments, be implemented as N-path filters configured in a C-R configuration as shown at  FIG. 2B . The center frequency of the notch filter  207  may be shifted by for example 20 MHz above the center frequency of the bandpass filters  202 ,  206 , and  208  by changing the connections (as an example, input and output) of the gm cells  292 A-B (labeled g m5  and g m6 ). The frequency shifting may be determined by the following: 
         [0000]    
       
         
           
             
               g 
               
                 
                   m 
                    
                   
                       
                   
                    
                   5 
                 
                 , 
                 6 
               
             
             
               2 
                
               
                   
               
                
               π 
                
               
                   
               
                
               
                 C 
                 BB 
                 ′ 
               
             
           
         
       
     
         [0000]    wherein C′ BB  is the combination of C 4  and the parasitic capacitances of the transistor switches. Although the previous example as well as other examples herein refer to a specific offset, other offset values may be used as well. 
         [0035]    In some example embodiments, tunable bandpass filter  208  may be implemented using N-path filters in an R-C configuration. Two buffers  294 A-B may be provided at the differential output  296 B of the filter  200 . 
         [0036]      FIG. 6  depicts an example plot of magnitude (in decibels, dB) versus frequency (in Gigahertz, GHz) for the tunable filter  200 .  FIG. 7  depicts a zoomed-in version of  FIG. 6 . As can be seen, the tunable filter  200  may provide a band and/or channel selection filter. Moreover, the pass band ripple, in some example implementations, may be relatively small (for example, less than about 0.4 dB). Unlike passive filters, the tunable filter  200  may provide a gain (for example, 15.5 dB) as shown in  FIGS. 6 and 7 . In some example implementations, the tunable filter  100  or  200  may have about a 44 dB rejection at a 20 MHz offset and have at least 52.54 dB rejection at 45 MHz offset from the center frequency as shown in  FIG. 6 , although other results may be obtained as well. 
         [0037]      FIG. 8  depicts the tunablity of filter  200  from 0.2 GHz to 2 GHz. The tuning moves the pass band across the spectrum, while substantially maintaining stop-band rejection over the wide frequency band as shown in  FIG. 8 . The filter may be tuned by changing the frequency of a four-phase clock. The four-phase clock is shown at the bottom in  FIG. 2A . The stop-band rejection may be higher when compared with an 8 th  order all pole Butterworth filter. The high stop-band rejection may be due in part to the notches created outside the bandwidth of the filter  100 / 200 . The notches may also be independently tuned. 
         [0038]    In some example embodiments, the tunable filter  100 / 200  disclosed herein may be integrated into a RF transceiver, such as at a receiver and/or a transmitter. When this is the case, filter  100 / 200  may enable elimination of several band-specific, bulky, and costly SAW filters and the elimination of fixed band/channel selection filters from for example cellular transceivers as the filter  100 / 200  may be used for band tuning and/or channel selection. Moreover, filter  100 / 200  may be a tracking filter, so filter  100 / 200  may be used as part of a reconfigurable multiband radio with variable bandwidth, which may be utilized in cellular transceivers. 
         [0039]      FIG. 9  illustrates a block diagram of an apparatus  10 , in accordance with some example embodiments. For example, apparatus  10  may comprise a radio, such as a user equipment, a smart phone, mobile station, a mobile unit, a subscriber station, a wireless terminal, a tablet, a wireless plug-in accessory, a wireless access point, a base station, and/or or any other device with device having a transceiver. 
         [0040]    The apparatus  10  may include at least one antenna  12  in communication with a transmitter  14  and a receiver  16 . Alternatively transmit and receive antennas may be separate. 
         [0041]    In some example embodiments, the tunable filters  100 / 200  disclosed herein may be used in the transmitter  14  and/or receiver  16  for RF band selection and/or channel selection. Moreover, the processor  20  may execute code stored in memory to control the tuning of the tunable filters  100 / 200 , in accordance with some example embodiments. 
         [0042]    The apparatus  10  may also include a processor  20  configured to provide signals to and receive signals from the transmitter and receiver, respectively, and to control the functioning of the apparatus. Processor  20  may be configured to control the functioning of the transmitter and receiver by effecting control signaling via electrical leads to the transmitter and receiver. Likewise, processor  20  may be configured to control other elements of apparatus  10  by effecting control signaling via electrical leads connecting processor  20  to the other elements, such as a display or a memory. The processor  20  may, for example, be embodied in a variety of ways including circuitry, at least one processing core, one or more microprocessors with accompanying digital signal processor(s), one or more processor(s) without an accompanying digital signal processor, one or more coprocessors, one or more multi-core processors, one or more controllers, processing circuitry, one or more computers, various other processing elements including integrated circuits (for example, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), and/or the like), or some combination thereof. Accordingly, although illustrated in  FIG. 9  as a single processor, in some example embodiments the processor  20  may comprise a plurality of processors or processing cores. 
         [0043]    Signals sent and received by the processor  20  may include signaling information in accordance with an air interface standard of an applicable cellular system, and/or any number of different wireline or wireless networking techniques, comprising but not limited to Wi-Fi, wireless local access network (WLAN) techniques, such as Institute of Electrical and Electronics Engineers (IEEE) 802.11, 802.16, and/or the like. In addition, these signals may include speech data, user generated data, user requested data, and/or the like. 
         [0044]    The apparatus  10  may be capable of operating with one or more air interface standards, communication protocols, modulation types, access types, and/or the like. For example, the apparatus  10  and/or a cellular modem therein may be capable of operating in accordance with various first generation (1G) communication protocols, second generation (2G or 2.5G) communication protocols, third-generation (3G) communication protocols, fourth-generation (4G) communication protocols, Internet Protocol Multimedia Subsystem (IMS) communication protocols (for example, session initiation protocol (SIP) and/or the like. For example, the apparatus  10  may be capable of operating in accordance with 2G wireless communication protocols IS-136, Time Division Multiple Access TDMA, Global System for Mobile communications, GSM, IS-95, Code Division Multiple Access, CDMA, and/or the like. In addition, for example, the apparatus  10  may be capable of operating in accordance with 2.5G wireless communication protocols General Packet Radio Service (GPRS), Enhanced Data GSM Environment (EDGE), and/or the like. Further, for example, the apparatus  10  may be capable of operating in accordance with 3G wireless communication protocols, such as Universal Mobile Telecommunications System (UMTS), Code Division Multiple Access 2000 (CDMA2000), Wideband Code Division Multiple Access (WCDMA), Time Division-Synchronous Code Division Multiple Access (TD-SCDMA), and/or the like. The apparatus  10  may be additionally capable of operating in accordance with 3.9G wireless communication protocols, such as Long Term Evolution (LTE), Evolved Universal Terrestrial Radio Access Network (E-UTRAN), and/or the like. Additionally, for example, the apparatus  10  may be capable of operating in accordance with 4G wireless communication protocols, such as LTE Advanced and/or the like as well as similar wireless communication protocols that may be subsequently developed. 
         [0045]    It is understood that the processor  20  may include circuitry for implementing audio/video and logic functions of apparatus  10 . For example, the processor  20  may comprise a digital signal processor device, a microprocessor device, an analog-to-digital converter, a digital-to-analog converter, and/or the like. Control and signal processing functions of the apparatus  10  may be allocated between these devices according to their respective capabilities. The processor  20  may additionally comprise an internal voice coder (VC)  20   a , an internal data modem (DM)  20   b , and/or the like. Further, the processor  20  may include functionality to operate one or more software programs, which may be stored in memory. In general, processor  20  and stored software instructions may be configured to cause apparatus  10  to perform actions. For example, processor  20  may be capable of operating a connectivity program, such as a web browser. The connectivity program may allow the apparatus  10  to transmit and receive web content, such as location-based content, according to a protocol, such as wireless application protocol, WAP, hypertext transfer protocol, HTTP, and/or the like. 
         [0046]    Apparatus  10  may also comprise a user interface including, for example, an earphone or speaker  24 , a ringer  22 , a microphone  26 , a display  28 , a user input interface, and/or the like, which may be operationally coupled to the processor  20 . The display  28  may, as noted above, include a touch sensitive display, where a user may touch and/or gesture to make selections, enter values, and/or the like. The processor  20  may also include user interface circuitry configured to control at least some functions of one or more elements of the user interface, such as the speaker  24 , the ringer  22 , the microphone  26 , the display  28 , and/or the like. The processor  20  and/or user interface circuitry comprising the processor  20  may be configured to control one or more functions of one or more elements of the user interface through computer program instructions, for example, software and/or firmware, stored on a memory accessible to the processor  20 , for example, volatile memory  40 , non-volatile memory  42 , and/or the like. The apparatus  10  may include a battery for powering various circuits related to the mobile terminal, for example, a circuit to provide mechanical vibration as a detectable output. The user input interface may comprise devices allowing the apparatus  20  to receive data, such as a keypad  30  (which can be a virtual keyboard presented on display  28  or an externally coupled keyboard) and/or other input devices. 
         [0047]    As shown in  FIG. 9 , apparatus  10  may also include one or more mechanisms for sharing and/or obtaining data. For example, the apparatus  10  may include a short-range radio frequency (RF) transceiver and/or interrogator  64 , so data may be shared with and/or obtained from electronic devices in accordance with RF techniques. The apparatus  10  may include other short-range transceivers, such as an infrared (IR) transceiver  66 , a Bluetooth (BT) transceiver  68  operating using Bluetooth wireless technology, a wireless universal serial bus (USB) transceiver  70 , a Bluetooth Low Energy transceiver, a ZigBee transceiver, an ANT transceiver, a cellular device-to-device transceiver, a wireless local area link transceiver, and/or any other short-range radio technology. Apparatus  10  and, in particular, the short-range transceiver may be capable of transmitting data to and/or receiving data from electronic devices within the proximity of the apparatus, such as within 10 meters, for example. The apparatus  10  including the WiFi or wireless local area networking modem may also be capable of transmitting and/or receiving data from electronic devices according to various wireless networking techniques, including 6LoWpan, Wi-Fi, Wi-Fi low power, WLAN techniques such as IEEE 802.11 techniques, IEEE 802.15 techniques, IEEE 802.16 techniques, and/or the like. 
         [0048]    The apparatus  10  may comprise memory, such as a subscriber identity module (SIM)  38 , a removable user identity module (R-UIM), an eUICC, an UICC, and/or the like, which may store information elements related to a mobile subscriber. In addition to the SIM, the apparatus  10  may include other removable and/or fixed memory. The apparatus  10  may include volatile memory  40  and/or non-volatile memory  42 . For example, volatile memory  40  may include Random Access Memory (RAM) including dynamic and/or static RAM, on-chip or off-chip cache memory, and/or the like. Non-volatile memory  42 , which may be embedded and/or removable, may include, for example, read-only memory, flash memory, magnetic storage devices, for example, hard disks, floppy disk drives, magnetic tape, optical disc drives and/or media, non-volatile random access memory (NVRAM), and/or the like. Like volatile memory  40 , non-volatile memory  42  may include a cache area for temporary storage of data. At least part of the volatile and/or non-volatile memory may be embedded in processor  20 . The memories may store one or more software programs, instructions, pieces of information, data, and/or the like which may be used by the apparatus for performing the functions the apparatus including providing and/or controlling the tunable filters  100 / 200 . The memories may comprise an identifier, such as an international mobile equipment identification (IMEI) code, capable of uniquely identifying apparatus  10 . The functions may include one or more of the operations disclosed herein with providing and/or controlling the tunable filters  100 / 200  and the like. The memories may comprise an identifier, such as an international mobile equipment identification (IMEI) code, capable of uniquely identifying apparatus  10 . In the example embodiment, the processor  20  may be configured using computer code stored at memory  40  and/or  42  to operations disclosed herein with respect to tunable filters  100 / 200  (for example, providing one or more aspects of the tunable filters  100 / 200  and/or controlling the tuning of the filters  100 / 200 ). 
         [0049]    Some of the embodiments disclosed herein may be implemented in software, hardware, application logic, or a combination of software, hardware, and application logic. The software, application logic, and/or hardware may reside on memory  40 , the control apparatus  20 , or electronic components, for example. In some example embodiment, the application logic, software or an instruction set is maintained on any one of various conventional computer-readable media. In the context of this document, a “computer-readable medium” may be any non-transitory media that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer or data processor circuitry, with examples depicted at  FIG. 9 , computer-readable medium may comprise a non-transitory computer-readable storage medium that may be any media that can contain or store the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer. For example, the computer-readable medium may include computer program code which when executed by processor circuitry provides control of the tuning of tunable band pass filter disclosed herein. 
         [0050]    Without in any way limiting the scope, interpretation, or application of the claims appearing below, a technical effect of one or more of the example embodiments disclosed herein is a tracking filter that may be used in cellular radios. 
         [0051]    If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described functions may be optional or may be combined. Although various aspects of the invention are set out in the independent claims, other aspects of the invention comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims. It is also noted herein that while the above describes example embodiments, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications that may be made without departing from the scope of the present invention as defined in the appended claims. Other embodiments may be within the scope of the following claims. The term “based on” includes “based on at least.” The use of the phase “such as” means “such as for example” unless otherwise indicated.