Frequency modulation tracking for band rejection to reduce dynamic range

Abstract

A method for reducing a dynamic range of a received radio frequency signal includes receiving a digital IQ signal corresponding to an in-phase component of the received radio frequency signal and a quadrature component of the received radio frequency signal. The method includes demodulating the digital IQ signal to generate an instantaneous frequency signal. The method includes selecting a center frequency of a selectable filter according to whether an interfering signal is detected in a target frequency band of the received radio frequency signal. The center frequency is selected from a predetermined frequency and an estimated center frequency determined using the instantaneous frequency signal. The method includes filtering the digital IQ signal using a selectable filter configured using the center frequency to generate an output digital IQ signal.

Description

[Technical field]

[0001] Background technology The present invention relates to communications technology, and more particularly, to communications using wireless technology. [Background technology]

[0002] Operation of a radio receiver in a noisy environment introduces noise to the received signal. In an exemplary radio application, a desired modulated radio frequency signal is corrupted by a substantial interfering signal (e.g., a constant tone or modulated signal in a target frequency band). For example, a constant tone may be introduced as a result of operating an electric motor in close proximity to a radio receiver, or a modulated interfering signal may be introduced into the received signal by operating a radio receiver near a radio using a different communication protocol. A substantial interfering signal in a target frequency band may increase the dynamic range of the received signal beyond the dynamic range required to represent the desired information in the received signal. Thus, transmission of the received signal with its increased dynamic range, for example to an external demodulator, may require the transceiver to operate at a higher data rate, use a faster clock signal, or communicate more information than is required to recover the desired information. Thus, techniques for reducing the dynamic range of a received radio frequency signal are desired. Summary of the Invention [Means for solving the problem]

[0003] Disclosure of embodiments of the invention In at least one embodiment of the present invention, a method for reducing a dynamic range of a received radio frequency signal includes receiving a digital IQ signal corresponding to an in-phase component of the received radio frequency signal and a quadrature component of the received radio frequency signal. The method includes demodulating the digital IQ signal to generate an instantaneous frequency signal. The method includes selecting a center frequency of a selectable filter according to whether an interfering signal is detected in a target frequency band of the received radio frequency signal. The center frequency is selected from a predetermined frequency and an estimated center frequency determined using the instantaneous frequency signal. The method includes filtering the digital IQ signal using a selectable filter configured using the center frequency to generate an output digital IQ signal.

[0004] In at least one embodiment, the receiver includes a receiver signal path configured to provide a digital IQ signal corresponding to an in-phase component of a received radio frequency signal and a quadrature component of the received radio frequency signal. The receiver includes a demodulator configured to provide an instantaneous frequency signal based on the digital IQ signal. The receiver includes a selectable filter configured to provide an output digital IQ signal based on the digital IQ signal and a center frequency. The receiver includes a selection circuit configured to select a center frequency of the selectable filter according to whether an interfering signal is detected in a target frequency band of the received radio frequency signal. The center frequency is selected from a predetermined frequency and an estimated center frequency determined using the instantaneous frequency signal.

[0005] In at least one embodiment, a program product is encoded on a tangible, machine-readable medium, the program product including instructions executable by a processor to receive a digital IQ signal corresponding to an in-phase component of a received radio frequency signal and a quadrature component of the received radio frequency signal, demodulate the digital IQ signal to generate an instantaneous frequency signal, select a center frequency of a selectable filter according to whether an interfering signal is detected in a target frequency band of the received radio frequency signal, the center frequency being selected from a predetermined frequency and an estimated center frequency determined using the instantaneous frequency signal, and filter the digital IQ signal using the selectable filter to generate an output digital signal.

[0006] BRIEF DESCRIPTION OF THE DRAWINGS The present invention may be better understood, and its numerous objects, features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. [Brief description of the drawings]

[0007] [Figure 1] FIG. 2 is a functional block diagram of an example receiver signal path including a tracking and rejection filter consistent with at least one embodiment of the present invention. [Diagram 2] FIG. 2 is a functional block diagram of an example tracking and rejection filter including a selectable notch filter for reducing the dynamic range of a received signal consistent with at least one embodiment of the present invention. [Diagram 3] FIG. 2 is a functional block diagram of an example tracking and rejection filter portion including a selectable band reject filter consistent with at least one embodiment of the present invention. [Figure 4] FIG. 2 is a functional block diagram of a sample rate reduction portion of an example tracking and rejection filter consistent with at least one embodiment of the present invention. [Diagram 5] FIG. 2 is a functional block diagram of an exemplary receiver consistent with at least one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The use of the same reference numbers in different drawings indicates similar or identical items. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, an exemplary wireless receiver receives a radio frequency (RF) signal and detects one or more target signal carrier frequencies (e.g., one or more subcarriers) that carry information. An analog front end 102, which may include an impedance matching network coupled to a low noise amplifier, receives the RF signal from an antenna and amplifies the RF signal without substantially degrading the signal-to-noise ratio. In at least one embodiment, the analog front end 102 includes a frequency mixer that converts the RF signal frequency to a low intermediate frequency. The analog front end 102 amplifies and filters (e.g., using an image rejection filter) the signal and provides in-phase (I) and quadrature (Q) signals (i.e., IQ signals) as outputs. The IQ signals are analog time-domain signals. In at least one embodiment, an analog gain circuit 104 provides an amplified version of the IQ signal to an analog-to-digital converter (ADC) 106. The analog-to-digital converter 106 converts those versions of the IQ signal to digital IQ signals (i.e., IQ samples). Exemplary embodiments of the ADC 106 use a variety of signal conversion techniques, such as delta-sigma (or sigma-delta) analog-to-digital conversion.

[0009] The ADC 106 provides the digital IQ signal to the digital front end 108, which filters the digital received signal. In at least one embodiment, the digital front end 108 decimates the digital received signal after filtering it. When undesirable constant tones or other interference present in the desired bandwidth are relatively large compared to the desired signal, the dynamic range required to support communication is relatively large. Transmitting the entire signal to an external processor increases the number of bits required compared to transmitting the desired signal in the absence of undesirable constant tones or other interference, thereby requiring a higher clock rate and communicating more information than required. The tracking and rejection filter 110 detects RF noise and estimates the center frequency of the interfering signal. The tracking and rejection filter 110 includes a selectable filter (e.g., a notch filter or a band rejection filter) configured with a center frequency selected to attenuate that RF interference source. The digital automatic gain control 112 dynamically amplifies the digital signal to provide IQ samples at a signal amplitude suitable for the next receiver stage (e.g., an external demodulator, or a filter that combines signals from multiple antennas). In at least one embodiment, receiver 100 sends the IQ output to an external processor for demodulation (e.g., a demodulator compliant with National Radio System Coimmittee-5C, also known as HD™ Radio, Digital Audio Broadcasting, Digital Radio Module (DRM), or other digital radio technology).

[0010] In at least one embodiment, the tracking and rejection filter 110 reduces the bandwidth required to transmit the desired data over the communication channel. The tracking and rejection filter 110 attenuates interfering constant tones or interfering modulated signals from the received signal. In at least one embodiment, the interfering constant tones or modulated signals do not occur at a given frequency, and the location of such interference varies. Thus, the tracking and rejection filter 110 receives the IQ samples, detects any interfering signals, estimates the center frequency of the interfering signals, and attenuates the interfering signals to reduce the bandwidth required to communicate the desired data, in some embodiments, over the communication channel to the next stage of the receiver.

[0011] 1 and 2, in at least one embodiment, the tracking and rejection filter 110 receives the digital IQ signal 109 (e.g., a sampled frequency modulated (FM) signal) from the digital front end 108. The tracking and rejection filter 110 tracks non-modulated signals (e.g., a carrier or a constant tone) in the digital IQ signal and attenuates or rejects the non-modulated signals to provide a digital IQ output. The CORDIC and FM demodulator 204 receives the digital IQ signal and processes the samples to generate an instantaneous frequency signal. In at least one embodiment, the CORDIC and FM demodulator 204 converts the digital IQ signal from a Cartesian representation to a polar representation (i.e., instantaneous phase and instantaneous amplitude in radians) and uses a polar discriminator technique on successive complex-valued baseband FM samples to obtain the instantaneous frequency signal of the digital IQ signal.

[0012] In at least one embodiment, the CORDIC and FM demodulator 204 includes a coordinate rotation digital computer (CORDIC), which may be dedicated to demodulation or shared with other operations of the receiver. In general, the CORDIC implements known techniques for performing calculations including trigonometric functions and complex multiplication without the use of multipliers. The only operations that the CORDIC uses are addition, subtraction, bit shifting, and table lookup operations to implement the arctangent function to convert a Cartesian representation of a signal to a polar representation of the signal, and in some embodiments also converts a polar representation of a signal to a Cartesian representation. In other embodiments of the tracking and rejection filter 110, instead of using a CORDIC, a complex multiplier calculates a complex multiplication of the current digital IQ signal with the complex conjugate of the most recent previous digital IQ signal. In other embodiments, a digital signal processor running firmware performs the conversion to polar representation and FM demodulation. The CORDIC and FM demodulator 204 converts the phase to a signal equivalent to a frequency offset from a 0 Hz signal at the input to the CORDIC. The output of the CORDIC and FM demodulator 204 is an audio signal equivalent to the polar domain signal and has units of radians Hz.

[0013] In at least one embodiment, the low pass filter 206 reduces noise in the instantaneous frequency signal and reduces the sample rate to reduce the number of cycles required for subsequent calculations. Reducing the sample rate reduces the precision with which the deviation of the FM signal is measured, but in at least some embodiments, high precision is not required to track the deviation. In other embodiments, the low pass filter 206 is omitted and the operation of the low sample rate filter 230 is performed at the sample rate of the digital IQ signal 109.

[0014] In at least one embodiment, the low sample rate filter 230 low pass filters the low sample rate signal using the low pass filter 208. The carrier detector 210 provides the selection circuit 219 with an indication of the frequency of the interfering carrier in the received signal. The storage location 218 provides the selection circuit 219 with a predetermined value corresponding to the Nyquist frequency (i.e., fs / 2). In at least one embodiment, the difference circuit 216 provides the confidence level generated by calculating the difference between the peak frequency deviation (i.e., the maximum value of the frequency above the modulation frequency) provided by the peak tracking filter 212 and the valley frequency deviation (i.e., the minimum value of the frequency below the modulation frequency) provided by the valley filter 214. In at least one embodiment of the low sample rate filter 230, the peak tracking filter 212 and the valley tracking filter 214 are dual time constant low pass filters. The difference circuit 216 outputs a control signal indicative of the difference between the output of the peak tracking filter 212 and the output of the valley tracking filter 214, indicative of the frequency deviation of the received signal. If the deviation is relatively large, the received signal is unlikely to contain a constant tone interferer. If the deviation is relatively small, the received signal is likely to contain a constant tone interferer.

[0015] In at least one embodiment, the selection circuit 219 compares a predetermined threshold (e.g., 75 Hz) to the control signal generated by the difference circuit 216. The control signal indicates a level of confidence that an interfering carrier or interfering constant tone will be detected in the target frequency band of the received signal. If the confidence level is equal to or greater than the predetermined threshold, the selection circuit 219 provides an indication of the frequency location of the interfering carrier as the output filter center frequency. If the confidence level is less than the predetermined threshold, the selection circuit 219 provides a predetermined frequency stored in the storage element 218, e.g., the Nyquist frequency (i.e., fs / 2, where fs is the sample rate), as the center frequency fc of the notch filter 224.

[0016] In at least one embodiment, the smoothing filter 220 receives the output of the selection circuit 219 and provides an output with reduced random fluctuations as a center frequency fc of the notch filter 224, and uses the center frequency fc to place a corresponding notch (e.g., a second order complex notch) centered about that frequency. In at least one embodiment, the notch filter 224 is a first order notch filter, and uses a convolution of a predetermined notch filter with the center frequency fc to vary the frequency that is attenuated. In at least one embodiment, the notch filter 224 attenuates frequencies at the center frequency of the signal path and provides a filtered output as the digital IQ signal 111. Note that in the embodiment of FIG. 2, the notched filter still operates when no jamming carrier is detected, but the notch is located at or near the Nyquist frequency fs / 2. Positioning the notch filter 224 around the Nyquist frequency allows the filter to process the digital IQ signal 111 without substantially affecting the digital IQ signal 109 in the absence of interference, reducing the introduction of discontinuities in the digital IQ signal 111 (e.g., phase transitions or other nonlinearities) compared to implementations that disable the notch filter 224 in the absence of interference.

[0017] 1, 3, and 4, in at least one embodiment of the tracking and rejection filter 110, rather than detecting only the interfering carrier and attenuating the detected interfering carrier, the tracking and rejection filter 110 detects the interfering carrier or the interferometric modulation data in the target frequency band and applies a band rejection filter to the received signal to attenuate the interfering signal in the target frequency band. The CORDIC and FM demodulator 304 receives the digital IQ signal and processes the samples. In at least one embodiment, the CORDIC and FM demodulator 304 converts the digital IQ samples from a Cartesian representation to a polar representation (i.e., instantaneous phase and instantaneous amplitude in radians) and uses a polar discriminator technique on the successive complex-valued baseband FM samples to obtain the instantaneous frequency of the sampled FM signal.

[0018] In at least one embodiment, the CORDIC and FM demodulator 304 includes a CORDIC, which may be dedicated to demodulation or shared with other operations of the receiver. The CORDIC and FM demodulator 304 converts the phase to a signal equivalent to the frequency offset from the 0 Hz signal at the input to the CORDIC. The output of the CORDIC and FM demodulator 304 is an audio signal equivalent to the polar domain signal, with units of radians Hz. In at least one embodiment, the low pass filter 306 reduces noise in the instantaneous frequency signal and reduces the sample rate to reduce the number of cycles required for subsequent processing. Reducing the sample rate reduces the accuracy with which deviations in the FM signal are detected, but in at least some embodiments, high accuracy is not required to track the deviations. In other embodiments, the low pass filter 306 does not reduce the sample rate, and the processing of the low sample rate filter 330 is performed at the sample rate fs. Thus, interference tracking at the sample rate fs to identify the corresponding center frequency of the notch filter or band-stop filter is a more expensive implementation (e.g., requires more cycles of the processor).

[0019] The RMS filter 308 performs a root mean square operation on the digital IQ signal 109 to provide a value corresponding to the total power of the received signal. The upper sideband bandpass filter 312 is centered on the upper sideband of the digital IQ signal 109. The RMS filter 314 estimates the power of the upper sideband signal by performing a root mean square operation on the output of the bandpass filter 312. The lower sideband bandpass filter 316 is centered on the lower sideband of the digital IQ signal 109. The RMS filter 318 estimates the power of the lower sideband signal by performing a root mean square operation on the output of the bandpass filter 316. The minimum function circuit 320 provides a value corresponding to the minimum of the lower sideband power and the upper sideband power as an indication of the power of the target received signal.

[0020] In at least one embodiment, the low sample rate filter 330 calculates the ratio of the power of the target received signal to the total power of the received signal. The comparator 402 calculates the ratio and compares it to a predefined filter enable threshold to output a decision signal. The smoothing filter 404 filters the output stream of bits provided by the comparator 402 and provides the filtered output stream to a tracking selection percentage filter 406, which calculates the percentage likelihood that the received signal includes an interfering signal. The percentage is used to generate a control signal SEL that selects the center frequency of the band-stop filter 340. If the decision signal provided by the smoothing filter 404 indicates that the power ratio is less than the predefined filter enable threshold level, the output signal is an active signal level (i.e., effectively enabling the selectable band-stop filter). If the power ratio is greater than the predefined filter enable threshold level plus the history value, the output signal is an inactive signal level (i.e., effectively disabling the band-stop filter). If the power ratio is between or equal to the predefined filter enable threshold level and the predefined filter enable threshold level plus the history value, the output signal retains its previous value.

[0021] In at least one embodiment of the low sample rate filter 330, the peak tracking filter 408 is a dual time constant low pass filter that tracks the outer deviations of the FM signal (i.e., the maxima in frequency above the modulation frequency), and the valley tracking filter 412 is a dual time constant low pass filter that tracks the inner deviations of the FM signal (i.e., the minima in frequency below the modulation frequency). The low pass filters 410 and 414 attenuate noise at the output of the peak tracking filter 408 and the output of the valley tracking filter 412, respectively. The center calculator 416 estimates the nominal center or carrier frequency of the interferometric modulation signal, for example, by calculating the difference between the frequency output of the low pass filter 410 and the frequency output of the low pass filter 414. In at least one embodiment of the low sample rate filter 330, the center calculator 416 uses a wrapping calculation (i.e., circular arithmetic) to account for aliasing, since the frequency outputs of the low pass filters 410 and 414 relate to reduced sample rate signals. If the likelihood that the received signal includes an interfering signal is less than a predetermined percentage (e.g., 80%), the selection signal SEL causes the selection circuit 420 to provide a predetermined frequency stored in the storage element 218, e.g., the Nyquist frequency (i.e., fs / 2), as the center frequency fc of the band-stop filter 340. If the percentage is greater than or equal to the predetermined percentage (e.g., 80%), the selection circuit SEL causes the selection circuit 420 to provide an estimated center frequency determined by the output filter center frequency of the center calculator 416.

[0022] In at least one embodiment, a smoothing filter 422 receives the output of the selection circuit 420 and provides an output having reduced random fluctuations in center frequency fc and provides that value as the center frequency fc of the band reject filter 340. In at least one embodiment, rather than dynamically calculating the filter coefficients of a band reject filter having a center frequency fc, the low sample rate filter 330 includes a difference circuit 424, which calculates the difference between the Nyquist frequency fs / 2 and the center frequency fc and provides the difference fs / 2-fc to the band reject filter 340, which includes a mixer 322, a selectable low pass filter 324, and a mixer 326. In at least one embodiment, the selectable low pass filter 324 has a selectable rejection bandwidth configured to be a wide rejection bandwidth when rejecting an interfering signal or a narrow rejection bandwidth (e.g., rejecting only a relatively small amount of signals around the Nyquist frequency) when there is no interfering signal. In at least one embodiment, the selectable low pass filter 324 has a narrow rejection bandwidth in a pass band of 0-300 kHz and is selectively configurable to attenuate the input signal by 100 dB from 362.75 kHz to 375 kHz in response to a first value of the selection signal SEL, and has a wide rejection bandwidth in a pass band of 0-245 kHz and is selectively configurable to attenuate the input signal by 50 dB from 263.35 kHz to 375 kHz in response to a second value of the selection signal SEL. The mixer 322 is a complex mixer that rotates the received signal by fs / 2-fc. The low pass filter 324 attenuates interference signals in the stop band of the low pass filter 324, which may include removing only a relatively small amount of signals around a predetermined frequency in the absence of an interferer, or removing a larger amount of signals around an estimated center frequency in the presence of an interferer. Mixer 326 is a complex mixer that de-rotates the IQ signal output by low pass filter 324 to baseband (eg, a low intermediate frequency) and provides digital output IQ signal 111.

[0023] Note that the bandwidth of the rejection filter may vary depending on the sample rate and the specified IQ mask of the target application. In at least one embodiment, the band rejection filter 340 has one bandwidth instead of a selectively narrow or selectively wide bandwidth. While the band rejection filter 340 of FIG. 3 includes only one low pass filter with a selectively wide or selectively narrow bandwidth, in other embodiments, the band rejection filter 340 may include multiple filters coupled in series and configure them to match the width of the interferer and use additional mixers.

[0024] With reference to FIG. 5, one or more of the structures included in the tracking and rejection filter 110 may be implemented using software (including firmware) executed on a processor or by a combination of software and hardware. The software described herein may be encoded on at least one tangible (i.e., non-transitory) computer-readable medium. As referred to herein, a tangible computer-readable medium includes at least a disk, tape, or other magnetic, optical, or electronic storage medium (e.g., random access memory, read-only memory). For example, FIG. 5 shows a radio 502, which includes a receiver analog front-end 514 coupled to an antenna. The processor 504, which may be a digital signal processor or other processing circuit, performs complex data processing, such as filtering and modulation, by executing instructions fetched from a memory 508. The receiver analog front-end 514 and associated antenna receive electromagnetic signals over the air and provide analog signals to a custom hardware circuit implementation of an analog-to-digital converter 510 that provides digital data to the processor 504. The processor 504 performs complex data processing, such as demodulation, filtering, or other signal processing, which may include at least some functionality of the tracking and rejection filter 110, by executing instructions fetched from the memory 508.

[0025] Thus, techniques are provided for attenuating large modulated or constant tone interfering signals in a target frequency band to reduce the required dynamic range of a desired signal. With reference to FIG. 1, in at least one embodiment, the accuracy of digital IQ signal 109 is equal to or greater than the accuracy of digital IQ signal 113. The embodiment of tracking and rejection filter 110 described above with reference to FIGS. 2-4 provides digital output IQ signal 111 to digital automatic gain control 112, which adjusts the band reject output IQ signal to the range of available bits of digital IQ signal 113. In at least one embodiment, tracking and rejection filter 110 provides digital IQ signal 111 having a reduced dynamic range compared to digital IQ signal 109 in response to detecting and reducing an interference source in the target frequency band.

[0026] The description of the invention set forth herein is illustrative and is not intended to limit the scope of the invention as set forth in the following claims. Terms such as "first", "second", "third", etc., used in the claims are intended to distinguish between different items of the claims, unless otherwise clear from the context, and do not otherwise indicate or imply an order of time, place or quality. For example, "first received signal", "second received signal" does not indicate or imply that the first received signal occurs in time prior to the second received signal. Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein without departing from the scope of the invention as set forth in the following claims.

Claims

1. 1. A method for reducing the dynamic range of a received radio frequency signal, comprising: receiving digital IQ signals corresponding to an in-phase component of the received radio frequency signal and a quadrature component of the received radio frequency signal; demodulating the digital IQ signal to generate an instantaneous frequency signal; selecting a center frequency of a selectable filter according to whether an interfering signal is detected in a target frequency band of the received radio frequency signal, the center frequency being selected from a predetermined frequency and an estimated center frequency determined using the instantaneous frequency signal; filtering the digital IQ signal using the selectable filter configured using the center frequency to generate an output digital IQ signal; A method comprising:

2. The method of claim 1 , further comprising low pass filtering the instantaneous frequency signal to generate a low sample rate instantaneous frequency signal that is used to estimate the estimated center frequency of the interfering signal.

3. 3. The method of claim 2, wherein selecting the center frequency of the selectable filter comprises selecting between the predetermined frequency and the estimated center frequency to determine a stop band of the selectable filter based on a first frequency of highest deviation of the low sample rate instantaneous frequency signal and a second frequency of lowest frequency deviation of the low sample rate instantaneous frequency signal.

4. The method of claim 3 , wherein the predetermined frequency is the Nyquist frequency.

5. The method of claim 1 , wherein a first dynamic range of the output digital IQ signal is smaller than a second dynamic range of the output digital IQ signal.

6. The method of claim 1 , wherein the selectable filter is a band-stop filter having selectable stop bands.

7. The method of claim 6 , further comprising selecting a width of the selectable stop band based on a ratio of a power of a desired signal to a total power of the digital IQ signal.

8. The method of claim 1 , wherein the selectable filter is a notch filter having a selectable notch frequency.

9. Selectively updating the selectable filters includes: estimating the total power of the digital IQ signal; estimating a power of a desired signal of the digital IQ signal using a first bandpass filter centered on an upper sideband of the digital IQ signal and a second bandpass filter centered on a lower sideband of the digital IQ signal; generating a ratio of the power of the desired signal to the total power of the digital IQ signals; The method of claim 1 , comprising:

10. The filtering of the digital IQ signal comprises: mixing the digital IQ signal with the center frequency to generate a rotated signal; low pass filtering the rotated signal to generate a filtered IQ signal; mixing the filtered IQ signal with the center frequency to provide the output digital IQ signal; The method of claim 1 , comprising:

11. a receiver signal path configured to provide digital IQ signals corresponding to an in-phase component of a received radio frequency signal and a quadrature component of the received radio frequency signal; a demodulator configured to provide an instantaneous frequency signal based on the digital IQ signal; a selectable filter configured to provide an output digital IQ signal based on the digital IQ signal and a center frequency; a selection circuit configured to select the center frequency of the selectable filter according to whether an interfering signal is detected in a target frequency band of the received radio frequency signal; wherein the center frequency is selected from a predetermined frequency and an estimated center frequency determined using the instantaneous frequency signal.

12. The receiver of claim 11 , wherein a first dynamic range of the output digital IQ signal is smaller than a second dynamic range of the output digital IQ signal.

13. 12. The receiver of claim 11, wherein the selectable filter is a notch filter.

14. The selectable filter may include: a first mixer configured to provide a rotated digital IQ signal based on the digital IQ signal and the center frequency; a low pass filter configured to provide a filtered IQ signal based on the rotated digital IQ signal; a second mixer configured to provide the output digital IQ signal based on the rotated digital IQ signal and the center frequency; and 12. The receiver of claim 11, wherein the selectable bandstop filter comprises:

15. 15. The receiver of claim 14, wherein the bandwidth of the low pass filter is selectable.

16. The receiver of claim 11 , wherein the interfering signal is a constant tone at the center frequency.

17. The receiver of claim 11 , wherein the interfering signal is a frequency modulated signal having the center frequency.

18. A program product encoded on a tangible machine-readable medium, comprising: receiving digital IQ signals corresponding to an in-phase component of a received radio frequency signal and a quadrature component of the received radio frequency signal; demodulating the digital IQ signal to generate an instantaneous frequency signal; selecting a center frequency of a selectable filter according to whether an interference signal is detected in a target frequency band of the received radio frequency signal, the center frequency being selected from a predetermined frequency and an estimated center frequency determined using the instantaneous frequency signal; filtering the digital IQ signal using the selectable filter to generate an output digital signal; 4. A program product comprising instructions executable by a processor for:

19. 20. The program product of claim 18, further comprising additional instructions executable by the processor for updating a width of a selectable passband of the selectable filter based on a ratio of a power of a modulated signal to a total power of the digital IQ signal.

20. 20. The program product of claim 18, wherein the center frequency is selected based on a first frequency of maximum deviation of the instantaneous frequency signal and a second frequency of minimum frequency deviation of the instantaneous frequency signal.

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