Abstract:
A method and apparatus are provided for automatic alignment of a notch filter in a receiver. The method comprises the steps of determining a frequency of an interfering signal, monitoring an energy in the interfering signal, tuning the notch filter based on an initial tune value, detecting an energy content in the radio signal after the tuning step, incrementally tuning the notch filter away from the initial tune value while monitoring the energy in the interfering signal, repeating the step of detecting and the step of incrementally tuning until the energy in the interfering signal is minimized, and storing a new tune value as the initial tune value. The notch filter is configured to filter the radio signal. The new tune value indicates a minimized energy in the interfering signal.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to radio receivers, and more particularly relates to methods and receiver systems for attenuating undesired frequencies in an operational frequency band.  
       BACKGROUND OF THE INVENTION  
       [0002]     An interfering signal may decrease reception of a desired Radio Frequency (RF) signal. An RF signal usually includes a carrier signal with information modulated onto the carrier signal. The front-end of a radio receiver typically amplifies the RF signal and mixes the amplified signal to a base band or to an Intermediate Frequency (IF) for detection of the information. Detection of the information may be impeded by the presence of one or more interfering signals.  
         [0003]     Various types of filters have been used to remove the interfering signals from the desired RF signal. In one example, a notch filter is used to remove an interfering signal by selecting a notch filter having a pre-determined center frequency (e.g., a center frequency set at the time of manufacture) that is similar to the frequencies of known interfering signals. The notch filter may be affected by temperature, device aging, and other factors to alter the center frequency. For example, the accuracy of the notch filter is typically determined by the center frequency of the notch filter, which would represent the deepest loss in a transmission path, and a programmed notch frequency. This accuracy may be represented by (F programmed −F center )/(F center ), where F programmed  is an input frequency for the notch filter, and F center  is an actual center frequency of the notch filter.  
         [0004]     Accordingly, it is desirable to provide a method for aligning a notch filter to reject an interfering signal in a radio signal. In addition, it is desirable to provide a system for aligning a notch filter to reject an interfering signal in a radio signal. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     Methods and systems are provided for automatically aligning a notch filter to reduce interference during radio signal reception. In an exemplary embodiment, a method is provided for aligning a notch filter to reject an interfering signal in a radio signal. The method comprises the steps of determining a frequency of the interfering signal, monitoring an energy in the interfering signal, tuning the notch filter based on an initial tune value, detecting an energy content in the radio signal, incrementally tuning the notch filter away from the initial tune value while monitoring the energy in the interfering signal, repeating the step of detecting and the step of incrementally tuning until the energy in the interfering signal is minimized, and storing a new tune value as the initial tune value. The new tune value indicates a minimized energy in the interfering signal. The notch filter is configured to filter the radio signal.  
         [0006]     In another exemplary embodiment, a system is provided for reducing an interfering signal in a radio signal. The system comprises a first notch filter having first and second inputs and having an output, a signal analyzer having an input configured to receive an Intermediate Frequency (IF) signal and having an output, and a processing unit having an input coupled to the output of the signal analyzer and having an output coupled to the second input of the first notch filter. The first input of the first notch filter is configured to receive the radio signal, and the first notch filter is configured to produce a first filtered signal at the output of the first notch filter. The IF signal is based on the first filtered signal, and the signal analyzer is configured to detect an energy content in the radio signal. The processing unit is configured to: a) determine a frequency of the interfering signal and monitor an energy in the interfering signal; b) tune the first notch filter to an initial tune value; c) detect an energy content in the radio signal; d) tune the first notch filter away from the initial tune value while monitoring said energy in the interfering signal; e) repeat c)-d) until the energy in the interfering signal is at least partially minimized; and d) store a new tune value as the initial tune value. The new tune value indicates an at least partially minimized energy in the interfering signal.  
         [0007]     In yet another exemplary embodiment, a method is provided for reducing an interfering signal in a radio signal. The method comprises the steps of filtering the radio signal through a notch filter having a center frequency tuned to an initial value, translating the radio signal to an IF signal after the filtering step, detecting an energy content of the radio signal by fast Fourier transforming the IF signal, tuning the center frequency away from the initial value while monitoring an energy of the interfering signal, and repeating the detecting step and the tuning step until the energy in the interfering signal is at least partially minimized indicating a new value. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0009]      FIG. 1  is a schematic diagram of a portion of a receiver in accordance with an exemplary embodiment of the present invention;  
         [0010]      FIG. 2  is a more detailed schematic diagram of the receiver shown in  FIG. 1 ; and  
         [0011]      FIG. 3  is a flowchart of a method for aligning a notch filter to reject an interfering signal in a radio signal in accordance with an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.  
         [0013]     Referring now to the drawings,  FIG. 1  is a schematic diagram of a portion of a receiver  10  in accordance with an exemplary embodiment of the present invention. The receiver  10  comprises an antenna  12 , a notch filter  14  having a first input coupled to the antenna  12 , a first amplifier  16  having an input coupled to an output of the notch filter  14 , a mixer  18  having a first input coupled to an output of the first amplifier  16 , an oscillator  20  coupled to a second input of the mixer  18 , a low pass filter  22  having an input coupled to an output of the mixer  18 , a second amplifier  24  having an input coupled to an output of the low pass filter  22 , and an alignment processing unit  26  having an input coupled to an output of the second amplifier  24  and having an output coupled to a second input of the notch filter  14 . Although the receiver  10  has a single notch filter  14  on the input side of the mixer  18  and has a single filter  22  following the mixer  18 , a different number of notch filters may be coupled to the input or output side of the mixer  18  and a different number of other filters (e.g., low pass filters) may be coupled to the output side of the mixer  18  along with additional amplifiers to maintain a desired signal strength. The receiver  10  may be incorporated as a component of a radio, such as an avionics radio or other radio.  
         [0014]     The receiver  10  receives a Radio Frequency (RF) signal via the antenna  12  and rejects interfering signals from the RF signal by filtering the RF signal through one or more notch filters  14 . Each of the notch filters  14  has a center frequency that may be varied by an input control voltage provided by the alignment processing unit  26 . Initially, the center frequency of the notch filter  14  is calibrated or aligned to a pre-determined frequency. During an initial tune, the pre-determined frequency is an approximated notch filter value or an initial tune value. The term initial tune refers to aligning the notch filter  14  prior to an interfering signal being applied to the receiver  10 . The initial tune value is used as an initial starting value for the notch filter during an adjusted tune. The term adjusted tune refers to improving the alignment state of the notch filter  14  after the receiver  10  has been in use. The number of notch filters may vary based on the number of pre-determined operational frequencies of the receiver  10  (e.g., operational frequencies associated with communications channels). Other factors such as minimizing the complexity of the receiver  10  may also limit the number of notch filters in the receiver  10 . In one example, the notch filter  14  comprises a series capacitor and inductor, and varactor diodes may be used to tune the center frequency.  
         [0015]     After filtering the RF signal through the notch filter  14 , the first amplifier  16  amplifies the filtered RF signal, and the mixer  18  translates the RF signal to an Intermediate Frequency (IF) signal using an oscillation signal provided by the oscillator  20 . The IF signal is filtered through the low pass filter  22  and amplified by the second amplifier  24 . The low pass filter  22  is preferably a wide-band filter allowing for a measurement of a full-range of undesired signals. Additional filters and amplifiers may be included following the low pass filter  22  to further condition the IF signal. A variety of signal processing (e.g., demodulation, data detection, decryption, and the like) may be performed on the filtered IF signal after amplification by the second amplifier  24 .  
         [0016]     The alignment processing unit  26  analyzes the filtered IF signal to detect an energy content of the RF signal, after being filtered by the notch filter  14 . Additionally, the alignment processing unit  26  locates the frequency of an interfering signal and monitors the energy of the interfering signal based on the energy content of the RF signal. By adjusting the center frequency of the notch filter  14  above and below the initial tune value and comparing the energy of the interfering signal before and after the adjustment to the center frequency, the alignment processing unit  26  tunes the notch filter  14  to a tune value that maximizes attenuation of the interfering signal. In an alternative embodiment, the alignment processing unit  26  tunes the notch filter  14  to a tune value that produces an attenuation of the interfering signal that is offset by a pre-determined value from the maximum attenuation of the interfering signal. The alignment processing unit  26  stores one or more tune values, each corresponding to the minimization of an interfering signal, for each notch filter. Additionally, although the mixer  18  is used to produce an IF signal, by multiplying the signal filtered by the notch filter  14  with an oscillation signal, the receiver  10  may directly provide the signal filtered by the notch filter  14  to the alignment processing unit in an alternative embodiment. In this alternative embodiment, the alignment processing unit  26  performs the digital signal processing on the RF signal filtered by the notch filter  14 .  
         [0017]      FIG. 2  is a more detailed schematic diagram of the receiver  10  shown in  FIG. 1 . In this exemplary embodiment, the receiver  10  additionally comprises a second notch filter  32  and a second filter  34 . The second notch filter  32  has a first input coupled to the output of the first amplifier  16 , a second input, and an output coupled to the mixer  18 , and a filter  34  having an input coupled to the output of the second amplifier  24  and having an output. The center frequency of the second notch filter  32  is different from the center frequency of the first notch filter  14  such that each notch filter  14 ,  32  may be aligned to a different interfering signal to reject the same. The second filter  34  may further filter the IF signal prior to analysis of the energy content of the RF signal by the alignment processing unit  26 .  
         [0018]     The alignment processing unit  26  comprises an analog-to-digital converter (ADC)  36  having a first input coupled the output of the second filter  34 , a clock unit  38  coupled to a second input of the ADC  36 , a signal analyzer  40  has an input coupled to an output of the ADC  36 , a centering unit  42  having an input coupled to an output of the signal analyzer  40 , and a digital-to-analog-converter (DAC)  44  having an input coupled to an output of the centering unit  42  and having an output coupled to the second inputs of the notch filters  14 ,  32 .  
         [0019]     The ADC  36  digitizes the filtered IF signal in sequence with a clock signal transmitted by the clock unit  38 . In an exemplary embodiment, the signal analyzer  40  comprises a unit that applies a Fast Fourier Transform (FFT) function to the digitized signal from the ADC  36  and detects an energy content of the RF signal for a particular instant in time. Using the detected energy content of the RF signal, the centering unit  42  adjusts the center frequency of each notch filter  14 ,  32  away from the initial tune value and monitors the energy of the interfering signal as captured in the detected energy content of the RF signal. The centering unit  42  may tune or adjust the center frequency in relatively small, pre-determined increments above and/or below the initial tune value.  
         [0020]     The centering unit  42  repeats tuning the center frequency away from the initial tune value and monitoring the energy of the interfering signal until the energy of the interfering signal is attenuated to a pre-determined attenuation (e.g., a maximum attenuation or an offset from the maximum attenuation). In an exemplary embodiment, the centering unit  42  compares the energy of the interfering signal prior to tuning the center frequency to a different tune value with the energy of the interfering signal after tuning the center frequency to the different tune value. At a tune value when the energy of the interfering signal is attenuated to the pre-determined attenuation, the centering unit  42  stores this tune value. As previously mentioned, this tune value may replace the prior initial tune value for a particular notch filter  14 ,  32 , or may be stored as one of a number of initial tune values for future retrieval during an adjusted tune.  
         [0021]      FIG. 3  is a flowchart of a method for aligning a notch filter to reject an interfering signal in a radio signal in accordance with an exemplary embodiment of the present invention. Referring to  FIGS. 2 and 3 , the method begins at step  100 . The centering unit  42  determines the frequency of the interfering signal at step  105 . The centering unit  42  monitors the energy of the interfering signal at step  110 . The centering unit  42  tunes at least one notch filter (e.g., notch filters  14 ,  16 ) based on an initial tune value at step  115 . For example, one or more notch filters may be used to reject one or more interfering signals. The transform unit  40  detects the energy content in the radio signal after tuning the notch filter  14  at step  120 . The centering unit  42  incrementally tunes the notch filter  14  up or down from the initial tune value while monitoring the energy in the interfering signal at step  125 . The centering unit  42  determines if the energy in the interfering signal is at least partially minimized at step  130 . In one exemplary embodiment, the centering unit  42  determines if the energy in the interfering signal is attenuated to a maximum degree. In another exemplary embodiment, the centering unit  42  determines if the energy in the interfering signal is attenuated to a pre-determined offset from the maximum degree. In the event the energy in the interfering signal is not at least partially minimized, the centering unit  42  repeats steps  120  and  125  until the energy in the interfering signal is at least partially minimized. In the event the energy in the interfering signal is at least partially minimized, the centering unit  42  stores the new tune value associated with the minimized energy of the interfering signal. In one embodiment, the centering unit  42  replaces the prior initial tune value with the new tune value. In another embodiment, the centering unit  42  save the new tune value as one of a number of tune value for use as an initial tune value in future alignments of the notch filters  14 ,  32 .  
         [0022]     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.