Patent Publication Number: US-8990279-B2

Title: Filter shaping using a signal cancellation function

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application a continuation of U.S. patent application Ser. No. 12/413,454, now U.S. Pat. No. 8,380,771, and filed on Mar. 27, 2009, entitled “FILTER SHAPING USING A SIGNAL-CANCELLATION FUNCTION”, which is related to U.S. patent application Ser. No. 12/652,281, now U.S. Pat. No. 8,116,084, and filed on Jan. 5, 2010, entitled “CALIBRATION OF ADJUSTABLE FILTERS,” both of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This application relates in general to electronic signal processing systems and more specifically to creating signal filters with desired characteristics. 
     This application relates in general to electronic signal processing systems and more specifically to creating configurable filters with desired characteristics. This application relates in general to electronic signal processing systems and more specifically to creating configurable filters with desired characteristics. 
     Filters are used extensively in signal processing systems to modify or shape a signal&#39;s frequency components. For example, low pass, high pass, bandpass and other types of filters attempt to attenuate and/or amplify or enhance specific ranges of frequencies in an input signal. Signal filters can be employed in various demanding applications, including cellular telephone transceiver circuits, Global Positioning System (GPS) receivers, Wireless Local Area Network (WLAN) transceivers, Blue Tooth transceivers, sound processing circuits, electronic noise cancellation circuits, analog and digital television tuners (e.g., terrestrial, network, and satellite), satellite and cable radios, WiMAX (Worldwide Interoperability for Microwave Access) transceivers, and so on. 
     Desired types of filter characteristics may be difficult, expensive, require additional components, or be otherwise problematic to achieve. Thus, it is desirable to provide techniques and systems for implementing filters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a first example of selective spectrum self-canceler filter capable of wide ranging adjustments of the accompanying filter frequency response. 
         FIG. 2  is a more detailed diagram of the selective spectrum self-canceler filter of  FIG. 1 . 
         FIG. 3  illustrates a second example embodiment of a selective spectrum self-canceler filter. 
         FIG. 4  illustrates a third example embodiment of a selective spectrum self-canceler filter. 
         FIG. 5  is a graph depicting example frequency responses of the selective-spectrum self-canceler filters of  FIGS. 1-4 . 
         FIG. 6  is a diagram of a fourth example adjustable filter incorporating instances of the first example selective spectrum self-canceler filter of  FIGS. 1-2 . 
         FIG. 7  is a first example frequency response of the adjustable filter of  FIG. 6 . 
         FIG. 8  is a second example frequency response of the adjustable filter of  FIG. 6 . 
         FIG. 9  is a third example frequency response of an adjustable filter incorporating several selective spectrum self-canceler filters to create a desired frequency response. 
         FIG. 10  is a flow diagram of a first example method for making the adjustable filter of  FIG. 6 . 
         FIG. 11  is a flow diagram of a second example method implemented via the selective spectrum self-canceler filters of  FIGS. 1-4 . 
         FIG. 12  is a diagram of a simplified fifth example adjustable filter incorporating an instance of the first example selective spectrum self-canceler filter of  FIGS. 1-2  and adapted to implement the method of  FIG. 11 . 
         FIG. 13  is a diagram of a sixth example adjustable filter incorporating nested instances of an adjustable selective spectrum self-canceler filter. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     An example adjustable filter includes a first mechanism for receiving a first signal. A second mechanism employs one or more modified representations of the first signal to cancel, suppress, or reduce one or more frequency components of the first signal, yielding an output signal in response thereto. 
     In a more specific embodiment, the first mechanism includes a splitter for receiving the first signal and splitting the first signal onto a first path and a second path. The second mechanism further includes one or more delay modules and one or more phase shifters in the first path and/or the second path. One or more controllable amplifiers are additionally included in the first path and/or the second path or may be merged into the splitter or other block. 
     In another embodiment, one or more delay modules, phase shifters, or amplifiers are responsive to one or more control signals from a controller, called an alignment controller. The alignment controller may include a local reference oscillator to facilitate selectively controlling the one or more controllable amplifiers, phase shifters, or delay modules in the first path and/or the second path. The alignment controller is adapted to modify the behavior of the second mechanism so that filter is characterized by a desired frequency response. In this embodiment, the filter implements a controllable notch filter that is controllable, in part, via one or more controllable Variable Gain Amplifiers (“VGAs,” also including voltage controlled amplifiers (VCAs), mixers, etc.) included in the second mechanism. A suitable VGA might be implemented with a mixer like structure, but other suitable implementations are possible. The one or more controllable VGAs affect alignment of a modified representation of the first signal relative to the first signal so that when the modified representation of the first signal on the second path is combined with the first signal on the first path, the resulting output signal exhibits desired spectral characteristics. 
     In an illustrative embodiment, the filter is a notch filter that is implemented in a combined filter. The combined filter includes one or more instances of the notch filter and a second filter. A transfer function of the combined filter is characterized by an asymmetrical frequency response curve. 
     Note that a given notch sub-filter of a combined filter may be characterized by an asymmetrical frequency response. Use of notch sub-filters with asymmetrical frequency responses may significantly reduce the number of sub-filters needed to obtain a desired combined frequency response, e.g., to selectively shape, attenuate, or cut the left or right side of a frequency response of the overall filter as desired. 
     In a more specific implementation, the second filter includes a bandpass filter. A controller is coupled to the one or more instances of the notch filter. The controller includes instructions for calibrating the combined filter to yield a combined desired transfer function or frequency response based on one or more predetermined inputs, such as a temperature input, to the controller. Other example inputs include supply voltage, process corner, and channel number. 
     The novelty of certain embodiments discussed herein is facilitated by use of a signal or version thereof to selectively cancel certain portions or frequency components of the signal by selectively adjusting and temporally aligning the signal with a version of the signal. The filter may implement frequency-dependent subtraction or cancellation of desired spectral components of an input signal. Accordingly, the filter is also called a selective spectrum self-canceler filter. For the purposes of the present discussion, a selective spectrum self-canceler filter may be any filter that employs one or more portions or versions of a signal to selectively cancel one or more portions, such as frequency components, of the same signal. 
     Use of a self-cancellation mechanism, such as the second mechanism discussed above, enables the frequency response characteristics of the filter to be radically adjusted by simply changing the way one portion of a signal is shifted, delayed, and amplified before it is combined with the original signal. Such modifications, which may include increasing or decreasing the Quality factor (Q) of a filter, may readily be performed by controlling VGAs in phase shifting modules and/or by controlling circuit component values, such as the capacitance or inductance of a voltage controlled capacitor (varactor), switched-capacitor array (also called a switched-capacitor bank), or, e.g., the current in a controllable inductor, as discussed more fully below. 
     Certain embodiments discussed herein may exhibit a highly controllable notched transfer function or frequency response, such that when notched transfer function is combined with other filters and transfer functions, the resulting combined transfer function or frequency response exhibits excellent controllability. Such controllability is particularly effective in designing adaptive filters whose transfer functions may be adjusted to meet the needs of changing signal environments without the need to use additional switches or filters. Furthermore, noise or other interfering signals which are close to the frequency band of a desired signal and that are often difficult to remove with conventional filters, may be readily removed via filters with asymmetrical frequency responses implemented in accordance with the present teachings. 
     While certain embodiments are discussed herein with respect to methods for creating a notch filter, those skilled in the art may readily adapt certain embodiments to create other types of filters without departing from the scope of the present teachings. 
     For clarity, certain well-known components, such as power supplies, local oscillators, mixers, clocks, digital-to-analog converters, analog-to-digital converters, detectors, and so on, have been omitted from the figures. However, those skilled in the art with access to the present teachings will know which components to implement and how to implement them to meet the needs of a given application. 
       FIG. 1  illustrates a first example of selective spectrum self-canceler filter  10  capable of significant adjustments to the accompanying filter frequency response. For the purposes of the present discussion, the frequency response of a filter represents the behavior of a filter or the effects that a filter has on a given input signal as a function of frequency. A frequency response may be represented via a curve or mathematical representation of the behavior of the filter with respect to frequency. The frequency response may also refer to a graphical depiction of a transfer function of a filter. For the purposes of the present discussion, a transfer function of a filter may be any mathematical representation of filter behavior. 
     The selective spectrum self-canceler filter  10  is also called a canceler filter herein. The canceler filter  10  includes a splitter  12 , which splits an input signal onto a first path  14  and a second path  16 . The second path  16  is called the canceler path. The canceler path  16  includes a frequency-dependent aligner  18  coupled between the splitter  12  and an output adder  20 . The output adder  20  may also be called a signal combiner  20 . 
     In the present specific embodiment, the first path  14  is input directly to the output adder  20 . An output of the adder  20  represents the output of the filter  10 . Note that while the first path  14  is shown directly coupling an output of the splitter  12  to an input of the output adder  20 , other circuit components may be included in the first path  14  without departing from the scope of the present teachings. The splitter  12  may be implemented via one or more various well known components, such as passive resistors, capacitors, active splitters, transformers, and so on. 
     A controller  22  receives input from an output of the splitter  12  and an output of the output adder  20  and provides control input to the frequency-dependent aligner  18 . The controller  22  optionally receives input from a temperature sensor  24  and a computer  26 . The computer  26  may provide external control signals to the controller  22  to facilitate controlling the controller  22  and accompanying canceler filter  10 . The computer  26  may include control software and a user interface for facilitating configuring the controller  22  and accompanying filter  10 . The alignment controller  22  may include a local reference oscillator to facilitate adjusting the frequency-dependent aligner  18 . Other example inputs to the alignment controller may include supply voltage, process corner, channel number, and so on. 
     In operation, the splitter  12  splits an input signal into a first signal traveling on the first path  14  and a second signal travelling on the second path  16 . The first signal is representative of the signal input to the splitter  12 . The second signal is selectively modified by the frequency dependent aligner  18  so that when the resulting modified second signal is combined with the first signal via the output adder  20 , the resulting output signal exhibits desired spectral characteristics. The modified second signal is selectively modified so that the filter  10  exhibits a desired transfer function and accompanying frequency response. The signal-dependent aligner  18  in combination with the output adder  20  is said to implement a signal-cancellation function, whereby desired components in the input signal are cancelled, resulting in an output signal with desired frequencies cancelled or suppressed. 
     In one implementation, the frequency-dependent aligner  18  modifies a predetermined spectral component of the second signal, such as by delaying a predetermined frequency band and then phase shifting the entire signal, so that when the output adder  20  combines the modified second signal with the first signal, it implements frequency-dependent subtraction. Portions of the modified second signal destructively couple or add, effectuating signal subtraction or cancellation, resulting in an output signal with desired frequency components thereof suppressed or cancelled. 
     For the purposes of the present discussion, a frequency component of a signal is also called a spectral component of a signal. A spectral component of a signal may be any portion of the signal characterized by a particular frequency or range of frequencies. For example, a certain signal may include various sinusoidal signals of different frequencies coupled together in one signal. Each sinusoidal signal of a given frequency may be considered a spectral component of the signal into which the sinusoidal signal is combined. Note that generally, many signals of practical interest may be decomposed into a sum of sinusoidal signal components. For periodic signals, such a decomposition is known in the art as a Fourier series. 
     For the purposes of the present discussion, a frequency dependent aligner may be any module that is adapted to selectively modify a first signal and then temporally align the first signal relative to a second signal. A first signal is said to be aligned with the second signal if the first signal and second signal are characterized by amplitudes at particular times such that a combination of the first signal with the second signal at the particular times results in desired complete or partial cancellation of a portion of the first signal or the second signal. Hence, alignment may be considered a type of misalignment to result in the selective destructive combination of a first signal with a second signal. 
     The controller  22  includes one or more routines or instructions for adjusting the frequency dependent aligner  18 , such as by controlling VGA multiplication values, varactor capacitance, switched-capacitor array capacitance values, and so on, to selectively adjust the transfer function or frequency response of the filter  10 . The controller  22  may receive feedback from the filter  10  via the output of the filter  10 , and may compare it with the input to the filter  10  to determine how modifications to the frequency dependent aligner  18  affect the frequency response of the filter  10 . 
     The controller  22  may also include a pilot tone generator, such as a voltage controlled oscillator, for inputting desired test signals (characterized by the pilot tone or frequency) into the filter  10  to test the response of the filter  10 . The controller  22  may also include instructions for adjusting the frequency response of the filter  10  based on temperature sensed by the temperature sensor  24  and/or based on other inputs. Since filter behavior may be affected by temperature, filter adjustments in response to certain temperature changes may be desirable. 
     In one implementation, the controller  22  may include a Look-Up Table (LUT) that stores parameters to be used for certain components of the frequency dependent aligner  18 . Different sets of parameters may be associated with different temperatures. When the temperature changes by a predetermined threshold, a different set of parameters, as specified in the LUT, may be input from the controller  22  to the frequency dependent aligner  18  to make appropriate adjustments to the frequency response of the filter  10 . 
     The controller  22  may be used to initially calibrate the filter  10  for a predetermined application. The controller  22  may also be used to dynamically change the frequency response of the filter  10  based on changing signal environments, such as, for example, in response to the detection of a given interference signal; in response to a predetermined signal level or amplitude detected at the input of the filter  10 ; in response to a signal indicating a channel has been changed in the accompanying device, or a particular adjacent antenna has been activated, and so on. Those skilled in the art may incorporate additional detectors, signal inputs, and so on, and may configure the control algorithms of the controller  22  appropriately to meet the needs of a given implementation without undue experimentation. 
     Examples of suitable control algorithms, filter-calibration methods, and accompanying systems are discussed more fully in the co-pending U.S. Patent Application referenced above. 
     The computer  26 , which may be implemented, for example, by a cellular telephone baseband processor, firmware, or other devices, may implement a mechanism to facilitate controlling the controller  22 . In certain implementations, the computer  26  may include a user interface to allow user adjustments of the transfer function of the filter  10  by enabling adjustments to circuit component parameters included in the frequency dependent aligner  18 . 
       FIG. 2  is a more detailed diagram of the selective spectrum self-canceler filter  10  of  FIG. 1 . The frequency dependent aligner  18  is shown including a controllable delay module  30  coupled to a controllable phase shifter  32 . An output of the controllable phase shifter  32  is coupled to a controllable amplifier  34 . An input to the controllable delay module  30  is coupled to an output of the splitter  12 . An output of the controllable amplifier  34  is coupled to an input of the output adder  20 . 
     The controllable delay module  30  includes controllable components  36 , which may be variable resistors, voltage-controlled capacitors and inductors, and so on. The values of the circuit components may be selectively set by one or more control signals from the controller  22 . The exact implementation of the control signals, such as the number of bits to use; whether pulse code modulation is used; whether Direct Current (DC) offsets are used, and so on, are application specific. Those skilled in the art with access to the present teachings may implement a controller to meet the needs of a given application without undue experimentation. 
     The various controllable components  36  of the delay module  30  may be adjusted to affect which frequency components of the second signal on the second path  16  are delayed and by what amount. Note that the delay module  30  acts to selectively delay one or more spectral components of the second signal traveling along the second path  16  relative to the first signal travelling along the first path  14 . The Quality factor (Q) of the filter  10  may be adjusted by controlling the delay imparted by the delay module  30 . Generally, larger delays are obtained from a larger Q, and smaller delays are obtained from a smaller Q. Adjusting the Q of the filter  10  will typically make the resulting notched frequency response narrower or wider. Wider notches are associated with lower Qs and have a shallower depth, whereas smaller notches are associated with higher Qs and are usually deeper. 
     Those skilled in the art will appreciate that the delay module  30  may be implemented via a delay line, allpass filter, or a high Q (Quality factor) bandpass filter, called a Q-enhancer. Suitable delay modules for a particular application may be readily implemented by those skilled in the art with access to the present teachings without undue experimentation. 
     The delay module  30  outputs a signal with selectively delayed spectral components to the phase shifter  32 . The phase shifter  32  then temporally shifts the resulting signal by a predetermined phase angle, thereby selectively aligning the second signal with the first signal to result in desired destructive signal canceling in the output adder  20 . 
     The phase shifter  32  may be implemented via a conventional phase shifter, but with controllable VGAs  38  used in place of conventional fixed mixers or amplifiers therein. The controllable VGAs may be implemented via mixers or via other components without departing from the scope of the present teachings. Alternatively, the phase shifter  32  and accompanying amplifier  34  may be replaced with an (In phase/Quadrature) IQ modulator or quadrature modulator, as discussed more fully below. Suitable controllable VGAs may be readily developed or obtained by those skilled in the art without undue effort. 
     By selectively controlling the VGAs  38  of the phase shifter  32  and the circuit components  36  of the delay module  30 , the filter  10  may be tuned to exhibit a wide variety of transfer functions or frequency responses. Use of the canceler path  16  and accompanying frequency dependent aligner  18  enables drastic changes to the filter transfer function and frequency response that are not currently obtainable via conventional filters. Such controllability is particularly useful in noise-canceling applications in crowded signal environments, such as mobile computing applications involving mobile television, Global Positioning System (GPS) and WiFi (Wireless Fidelity) or Wi-MAX (Worldwide Interoperability for Microwave Access) features. However, uses of the filter  10  are not limited to such applications. 
     The controllable amplifier  34  then amplifies the resulting delayed and phase-shifted signal output by the phase shifter  32  by a predetermined factor, the factor of which depends on the desired transfer function of the filter  10 . 
     The controller  22  is adapted to control the behavior of the delay module  30 , phase shifter  32 , and amplifier  34  by setting circuit-component values (e.g., of the components  36 ) thereof and/or by adjusting one or more VGAs (e.g., of the VGAs  38 ) thereof, and/or by controlling DC offsets or other parameters. For illustrative purposes, the controller  22  is shown receiving a clock input from a clock  40  for an accompanying circuit. Note that in digital implementations of the controller  22 , a clock signal will be used by the controller to synchronize its internal circuits. Additional inputs, such as inputs indicating voltage changes (ΔV) or channel changes in accompanying circuits that might affect the desired filter transfer function, and other external inputs, such as enable signals (En), data signals (Data), and so on, may be input to the controller  22  for use by the accompanying control algorithm. Exact details of the control algorithm are application specific and may vary depending on the implementation. 
     Note that various additional or fewer components may be added to or removed from the filter  10  without departing from the scope of the present teachings. For example, a local oscillator may be employed to sample the analog output signal from the output adder  20  for input to the controller  22 , and digital-to-analog converters may be positioned at outputs of the controller  22 . Furthermore, in certain implementations, the amplifier  34  may be omitted. In addition, the delay module  30  and/or the phase shifter  32  may be implemented on separate paths, such that, for example, the delay module  30  is positioned in the first path  14 , and the phase shifter  32  is positioned in the second path  16 . Furthermore, while the filter  10  is shown implemented for analog applications, digital implementations of the filter  10  may be constructed without departing from the scope of the present teachings. 
       FIG. 3  illustrates a second example embodiment of a first alternative selective spectrum self-canceler filter  50 . The alternative filter  50  includes the splitter  12 , which splits an input signal onto a top path  68  and a bottom path  66 . The top path  68  includes a first phase shifter  52  coupled to a first amplifier  54 , an output of which is input to the output adder  20 . The second path includes a first delay module  60  coupled to a second phase shifter  62 , which is coupled to a second amplifier  64 . An output of the second amplifier  64  is input to the output adder  20 . 
     The principle of operation of the alternative filter  50  is similar to the principle of operation of the filter  10  of  FIGS. 1-2  with the exception that, for illustrative purposes, the alternative filter  50  is shown lacking a controller, and both the signal traveling on the first path  68  and the signal traveling on the second path  66  are selectively modified before being combined via the output adder  20 . 
     The present alternative filter  50  represents an example static implementation, where no control over the filter&#39;s performance is needed. Design parameters of the various modules  52 ,  54 , and  60 - 64  are chosen so that the signal output by the second amplifier  64  selectively combines with the output of the first amplifier  54  in the output adder  20  so that desired spectral components cancel each other, yielding a desired output signal from the output adder  20 . Design parameters and circuit configurations of various modules  52 ,  54 , and  60 - 64  are chosen to result in a desired frequency response of the filter  50 . 
     Note that while  FIG. 3  shows the splitter  12  splitting an input signal onto an upper path with the first phase shifter  52  and amplifier  54  and onto a lower path with a delay module  60 , second phase shifter  62 , and second amplifier  64 , the various modules  52 ,  54 ,  60 ,  62 ,  64  may be positioned on different paths than shown, and certain modules may be omitted. For example, the first phase shifter  52  and amplifier  54  may be omitted from the upper path. Alternatively, the delay  60  may be positioned on the upper path. Alternatively, the second phase shifter  62  and amplifier  64  may be removed from the lower path. Note that additional variations other than those discussed here are also possible, e.g., one or more of the amplifiers  54 ,  64  may be omitted, and so on. In addition, the adjustable filter  10  may be made more complex or may be nested in another adjustable filter constructed according to the present teachings. 
       FIG. 4  illustrates a third example embodiment of a second alternative selective spectrum self-canceler filter  60 . The second alternative filter  60  includes the splitter  12  at the input of the filter  60 . The splitter  12  splits the input signal onto an upper path  78  and a lower path  76 . The upper path includes a delay module  66  for selectively delaying a desired frequency component of the signal on the upper path  78 . The lower path includes an IQ modulator  68  for selectively phase shifting and amplifying the signal on the lower path so that the resulting signals from the upper path  78  and lower path  76  combine at the output adder  20 , yielding an output signal with desired spectral characteristics. 
     For illustrative purposes, the IQ modulator  68  is shown including a quadrature splitter  70 , which splits a signal output by the splitter  12  and delivered to the lower path  76  into two separate signals, including an In-phase (I) signal and a Quadrature (Q) signal, that are approximately ninety degrees out of phase. The signals (I, Q) output from the quadrature splitter  70  are said to be in quadrature. The quadrature splitter  70  may be implemented via various types of circuits, such as an LC, RC, LR, capacitive only, allpass, or polyphase filter. 
     The (I) signal is input to a first controllable amplifier  80 , and the (Q) signal is input to a second controllable amplifier  82 , the outputs of which are added via a summation circuit  84 , yielding a phase-shifted and amplified signal as output. The resulting phase-shifted and amplified signal is input to one terminal of the output adder  20 . Another terminal of the output adder  20  is coupled to an output of the upper-path delay module  66 . 
     The behaviors of the upper-path delay module  66  and the amplifiers  80 ,  82  of the IQ modulator  68  are adjustable via controls signals output from an alternative controller  72 . Adjustments made to the delay module  66  and the IQ modulator  68  result in changes to the transfer function of the filter  60 . The adjustable amplifiers  80 ,  82  may be implemented via voltage controlled analog amplifiers, digitally controlled switch type amplifiers, or other suitable controllable amplifiers. Note that one or more of the amplifiers  80 ,  82  or the upper-path delay module  66  may be non-controllable without departing from the scope of the present teachings. 
     In general, the second alternative filter  60  operates on a similar principle as the filters  10 ,  50  of  FIGS. 1-3 , whereby signal cancellation or selective destructive or constructive combining one or more modified versions of a signal is used to yield an output signal with desired spectral characteristics in accordance with a desired filter frequency response. 
       FIG. 5  is a graph depicting example frequency responses  90  of the selective-spectrum self-canceler filters  10 ,  50 ,  60  of  FIGS. 1-4 . The example frequency responses  90  are plotted as a function of amplitude  92  versus frequency  94 . The example frequency responses  90  include a first notch  96 , a second notch  98 , a third notch  100 , and a fourth notch  102 . The first notch  96  illustrates a low quality factor Q and a lower center frequency than the other notches  98 - 102 . The second notch  98  illustrates larger quality factor than the first notch  96 . The third notch  100  is asymmetrical with a relatively steep right side, while the fourth notch  102  is asymmetrical with a relatively steep left side. 
     The various frequency response curves  96 - 102  are merely illustrative of some frequency response curves that may be obtained via one or more filters constructed in accordance with one or more embodiments discussed herein. Filters with such frequency responses may be selectively combined with other types of filters, such as low pass, high pass, bandpass, or allpass filters to achieve a desired combined transfer function, as discussed more fully below. For example, a filter characterized by the fourth frequency response curve  102  may be used with a bandpass filter to suppress blockers or transmitters near the upper end of the passband of the filter. 
       FIG. 6  is a diagram of a fourth example adjustable filter  110  incorporating a first instance  10  and a second instance  10 ′ of the first example selective spectrum self-canceler filter  10  of  FIGS. 1-2 . The example adjustable filter  110  represents a combined filter, which includes canceler filters  10 ,  10 ′ and a bandpass filter  114  coupled therebetween. The combined filter  110  includes an input Low Noise Amplifier (LNA) with an integrated bandpass filter (BPF), which is labeled LNA/BPF  124 . 
     In the present specific embodiment, the LNA  124  is coupled to the first canceler filter  10  via a first single pole double throw controllable switch  126 . An output of the first canceler filter  10  is coupled to an input of the bandpass filter  114  via a second single pole double throw switch  128 . An output of the bandpass filter  114  is coupled to the second canceler filter  10 ′ via a third single pole double pole switch  130 . An output of the second canceler filter  10 ′ is coupled to an input of an output buffer  120  via a fourth single pole double throw switch  132 . Second output poles of the first controllable switch  126  and the second controllable switch  128  are coupled to an input and an output, respectively, of a fifth switch  133 . The output of the fifth switch  133  represents a first node  134  that connects an output of fifth switch  133 , an input of a sixth switch  135 , and a second output pole of the second controllable switch  128 . 
     Similarly, second output poles of the third controllable switch  130  and the fourth controllable switch  132  are coupled to an input and an output, respectively, of a seventh switch  137 . The input of the seventh switch  137  represents a second node that connects the input of the seventh switch  137 , an output of the sixth switch  135 , and a second output pole of the third controllable switch  130 . 
     Note that the fifth switch  133 , the sixth switch  135 , and the seventh switch  137  are controllable switches, which receive control input from a controller  122 . These switches  133 ,  135 ,  137  may be implemented via a single switch without departing from the scope of the present teachings. 
     The output of the seventh switch  137  is coupled to an input of a second output buffer  118 . The controller  122  receives input from the input of the LNA  124 ; receives input from the output of the filter  110 ; and provides control signals to the switches  126 - 132 ,  133 ,  135 ,  137 , the canceler filters  10 ,  10 ′, and the bandpass filter  114 . 
     In operation, the controller  122  initially calibrates parameters of the various filters  10 ,  10 ′,  114  by employing the switches  126 - 132 ,  133 ,  135 ,  137  to selectively switch the different sub-filters  10 ,  10 ′,  114  into and out of the combined filter  110 . Use of the switches  126 - 132 ,  133 ,  135 ,  137  enables algorithms running on the controller  122  to calibrate each of the sub-filters  10 ,  10 ′,  114  individually and to then calibrate the entire filter  110 . Additional details of a suitable calibration algorithm are discussed more fully in the above-identified co-pending U.S. patent application, Ser. No. 12/652,281, entitled “CALIBRATION OF ADJUSTABLE FILTERS,” the teachings of which are incorporated by reference herein. 
     The canceler filters  10 ,  10 ′ act as notch filters and are used to selectively notch the frequency response of the bandpass filter  114  at desired frequencies, thereby creating a desired asymmetrical frequency response of the overall filter  110 , as discussed more fully below. 
       FIG. 7  is a first example frequency response  140  of the combined adjustable filter  110  of  FIG. 6 . The frequency response  140  is plotted as signal amplitude  142  versus frequency  144 . 
     With reference to  FIGS. 6 and 7 , the frequency response  140  includes a peak  146  corresponding to the center frequency (Fpeak) of the passband of the bandpass filter  114 . For illustrative purposes, the first canceler filter  10  is tuned to produce a first notch  148 , and the second canceler filter  10 ′ is tuned to produce a second notch  150 , which are positioned left of the peak response  146 . The notches  148 ,  150  may be used, for example, to filter noise or interference signals occurring close to the frequency of the desired signal (Fpeak). 
     Note that conventionally, to create the frequency response  140  would require substantially more hardware, such as filters and sub-filters, than the combined filter  110  of  FIG. 6 . Furthermore, unlike the combined filter  110  of  FIG. 6 , existing filters for creating desired frequency responses, especially strategically asymmetrical frequency responses, are not widely controllable. 
       FIG. 8  is a second example frequency response  160  of the combined adjustable filter  110  of  FIG. 6 . The second example frequency response  160 , which is plotted as amplitude  142  versus frequency  144 , includes a first asymmetrical notch  162  positioned to the left of the peak frequency response  166 , and a second asymmetrical notch  164  occurring to the right of the peak frequency response  164 . 
     With reference to  FIGS. 6 and 8 , the notches  162 ,  164  may be selectively created and positioned in the overall frequency response  160  by selectively tuning the first canceler filter  10  and the second canceler filter  10 ′. Accordingly, the canceler filters  10 ,  10 ′ may be employed to shape the overall filter frequency response  160  as desired for a particular application. 
       FIG. 9  is a third example frequency response of an adjustable filter incorporating several selective spectrum self-canceler filters, such as instances of the canceler filter  10  of  FIGS. 1-2 , to create a desired frequency response. The frequency response  170  depicts use of multiple notches  172  to the left of the peak frequency  176  of a bandpass filter response, and further depicts use of multiple notches  174  to the right of the peak frequency  176 . The notches  172 - 174  are used to shape the overall frequency response  170  as desired. Those skilled in the art with access to the present teachings may readily implement a combined filter to yield the box-like frequency response  170  shown in  FIG. 9  without undue experimentation. 
       FIG. 10  is a flow diagram of a first example method  180  adapted for making the adjustable filter  110  of  FIG. 6 . 
     The first example method  180  includes a first step  182 , which includes determining a first notch frequency response. The first notch frequency response may correspond to the frequency response of a canceler filter, such as the canceler filter  10  of  FIG. 1 . Note that the canceler filter  10  of  FIG. 1  is also called a selective spectrum self-canceler filter. 
     A second step  184  includes determining a first bandpass frequency response. The bandpass frequency response may be analogous to the frequency response of the bandpass filter  114  of  FIG. 6 . 
     A third step  186  includes determining or adjusting a desired total frequency response. Examples of total frequency responses are shown in  FIGS. 7-9 . 
     A fourth step  188  includes selectively combining one or more controllable notch filters characterized by one or more versions of the first notch frequency response with one or more bandpass filters characterized by one or more versions of the bandpass frequency response to yield a combined filter, such as the filter  110  of  FIG. 6 , characterized by a version of the desired total frequency response, such as the frequency response  140  of  FIG. 7 . 
       FIG. 11  is a flow diagram of a second example method  200  implemented via the selective spectrum self-canceler filters  10  of  FIGS. 1-4 . 
     The method  200  includes a first step  202 , which includes receiving an input signal. 
     A second step  204  includes splitting the input signal onto a first path and a second path. 
     A third step  206  includes selectively delaying and/or phase shifting and/or adjusting the gain (also called gain-adjusting) of a signal on the first path and/or the second path so that when signals on the first path and second path are combined, the resulting output signal exhibits desired spectral characteristics. 
       FIG. 12  is a diagram of a simplified fifth example combined adjustable filter  210  incorporating an instance of the first example selective spectrum self-canceler filter  10  of  FIGS. 1-2  and adapted to implement the method of  FIG. 11 . 
     The adjustable filter  210  represents a combined filter that includes a first notch filter  10  and a second filter  114 , both of which are optionally controllable via the controller  122 . 
     With reference to  FIGS. 1 ,  2  and  12 , the first filter  10  represents an example adjustable filter that includes a first mechanism  12  for receiving a first signal. A second mechanism  18 ,  20  employs one or more modified representations of the first signal to cancel, suppress, or reduce one or more frequency components of the first signal, yielding an output signal in response thereto. The first mechanism  12  includes a splitter  12  for receiving the first signal and splitting the first signal onto a first path  14  and a second path  16 . The second mechanism  18 ,  20  further includes one or more delay modules  30  and one or more phase shifters  32  in the first path  14  and/or the second path  16 . One or more controllable amplifiers  34  are optionally included in the first path  14  and/or the second path  16 . 
     The second filter  114  may be implemented via a bandpass, lowpass, allpass, high pass, notch filter, or other suitable filter or combination of filters. Furthermore, note that the second filter  114  may be included before the first filter  10 . Furthermore, while the second filter  114  is shown as a single module, the second filter  114  may be implemented as plural filters positioned at different locations within the overall adjustable filter  210 . For example, a first portion of the second filter  114  may be included before the first filter  10 , while a second portion of the second filter  114  may be included after the first filter  10  without departing from the scope of the present teachings. 
     The exact choice of the second filter  114  is application specific and depends on the desired frequency response of the overall adjustable filter  210 . For example, the second filter  114  may include a bandpass filter and an additional notch filter. In this example, the resulting combined filter  210  may exhibit the example asymmetrical frequency response  140  of  FIG. 7 , or the combined filter  210  may exhibit another frequency response, such as the response  170  of  FIG. 9  or the response  160  of  FIG. 8 . 
     The controller  122  may include instructions for calibrating the combined filter  210  to yield a combined desired transfer function or frequency response based on one or more predetermined inputs, such as a temperature input, to the controller  122 . 
       FIG. 13  is a diagram of a sixth example adjustable filter  220  incorporating nested instances  222 ,  224  of an adjustable selective spectrum self-canceler filter. A first instance  222  includes the delay module  60 , phase shifter  62 , amplifier  64 , and first adder  20  on a second path and includes a first path from the splitter  242  directly to the first adder  20 . A second instance  224  includes another delay  230 , phase shifter  232 , and amplifier  234 , and second adder  240 . A three-way input splitter  242  splits the input signal onto three separate paths, including a first path directly to the first adder  20 , a second path, which includes components of the first instance  222 , and a third path, which includes components of the second instance  224 . 
     The operation of the sixth adjustable filter  220  in  FIG. 13  may be similar to the operation of the adjustable filter  10  of  FIG. 1  with the exception that the second filter instance  224  acts to further adjust the output of the first adder  20  by selectively combining the output thereof with another delayed, shifted, and amplified version of the input signal to the splitter  242  via the second adder  240 . 
     Note that the first adder  20  and the second adder  240  may be combined into a single adder without departing from the scope of the present teachings. Furthermore, one or more components of the first instance  222  on the second path or the second instance  224  on the third path may be moved to the first path, i.e., upper path. 
     Although embodiments of the invention are discussed primarily with respect to analog filters for reducing or suppressing undesirable signal components in a signal environment, embodiments of the invention are not limited thereto. For example, filters discussed herein may be digital filters or hybrid digital and analog filters. Furthermore, such filters may be applicable to other fields, such as use in feedback control of dynamic systems. 
     Arrowheads shown on signal paths between various modules are for illustrative purposes only. For example, various communication paths or connecting lines, which appear to be unidirectional in the drawings, may be bidirectional without departing from the scope of the present invention. 
     Although a process of embodiments discussed herein may be presented as a single entity, such as software or hardware executing on a single machine, such software can readily be executed on multiple machines. That is, there may be multiple instances of a given software program, a single program may be executing on two or more processors in a distributed processing environment, parts of a single program may be executing on different physical machines, etc. Furthermore, two different programs, such as a convergence algorithm, a controller, and a noise-pattern analyzer can be executing in a single module, or in different modules. 
     Although the invention has been discussed with respect to specific example embodiments thereof, these embodiments are merely illustrative, and not restrictive, of the invention. In the description herein, numerous specific details are provided, such as examples of components and/or methods, to provide a thorough understanding of discussed example embodiments. One skilled in the relevant art will recognize, however, that certain embodiments can be practiced without one or more of the specific details, or with other apparatus, systems, assemblies, methods, components, materials, parts, and/or the like. In other instances, well-known structures, materials, or operations are not specifically shown or described in detail to avoid obscuring aspects of the example embodiments discussed herein. 
     A “machine-readable medium” or “computer-readable medium” may be any medium that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, system or device. The computer readable medium can be, by way of example only but not by limitation, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, system, device, propagation medium, or computer memory. 
     A “processor” or “process” includes any human, hardware and/or software system, mechanism or component that processes data, signals or other information. A processor can include a system with a general-purpose central processing unit, multiple processing units, dedicated circuitry for achieving functionality, or other systems. Processing need not be limited to a geographic location, or have temporal limitations. For example, a processor can perform its functions in “real time,” “offline,” in a “batch mode,” etc. Portions of processing can be performed at different times and at different locations, by different (or the same) processing systems. A computer may be any processor in communication with a memory. 
     Reference throughout this specification to “one embodiment”, “an example embodiment”, and “illustrative embodiment”, or “a specific embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment not necessarily included in all possible example embodiments. Thus, respective appearances of the phrases “in one embodiment”, “in an embodiment”, “illustrative embodiment”, or “in a specific embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics of any specific embodiment or example embodiment discussed herein may be combined in any suitable manner with one or more other embodiments. It is to be understood that other variations and modifications of the embodiments described and illustrated herein are possible in light of the teachings herein, and the variations are to be considered as part of the spirit and scope of the present invention. 
     Example embodiments discussed herein may be implemented in whole or in part by using a programmed general purpose digital computer; by using application specific integrated circuits, programmable logic devices, optical, chemical, biological, quantum or nanoengineered systems or mechanisms; and so on. In general, the functions of various embodiments can be achieved by any means as is known in the art. Distributed or networked systems, components, and/or circuits can be used. Communication, or transfer of data may be wired, wireless, or by any other means. 
     It will also be appreciated that one or more of the elements depicted in the drawings/figures can also be implemented in a more separated or integrated manner, or even removed or rendered as inoperable in certain cases, as is useful in accordance with a particular application. It is also within the spirit and scope of the present invention to implement a program or code that can be stored in a machine-readable medium to permit a computer to perform any of the methods described above. 
     As used in the description herein and throughout the claims that follow “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. Furthermore, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. 
     The foregoing description of illustrated example embodiments, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed herein. While certain example embodiments are described herein for illustrative purposes only, various equivalent modifications are possible within the spirit and scope of the present invention, as those skilled in the relevant art will recognize and appreciate. As indicated, these modifications may be made in light of the foregoing description of illustrated example embodiments and are to be included within the spirit and scope of the present invention. 
     Thus, while example embodiments have been described herein, a latitude of modification, various changes and substitutions are intended in the foregoing disclosures, and it will be appreciated that in some instances some features of embodiments will be employed without a corresponding use of other features without departing from the scope and spirit of the invention. Therefore, many modifications may be made to adapt a particular situation or material to the essential scope and spirit of the present invention. It is intended that the invention not be limited to the particular terms used in following claims and/or to a particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include any and all embodiments and equivalents falling within the scope of the appended claims.