Radio receiver having a channel equalizer and method therefor

A radio receiver for receiving a signal is provided. The radio receiver comprises an equalizer configured to perform a constant modulus algorithm initialized using a first set of coefficients on the received signal and for generating an equalized signal. The radio receiver further comprises a demodulator coupled to the equalizer for demodulating the equalized signal. The radio receiver further comprises a lowpass filter coupled to the demodulator for lowpass filtering the demodulated signal to detect a spurious signal and to generate an offset signal. The radio receiver further comprises a coefficient generator coupled to the lowpass filter and configured to compare the offset signal to a predetermined threshold, and if the offset signal satisfies a predetermined condition in relation to the predetermined threshold, then to generate a second set of coefficients for re-initializing the constant modulus algorithm.

BACKGROUND

This disclosure relates generally to radio, and more specifically, to a radio receiver having a channel equalizer and method therefor.

2. Related Art

A constant modulus algorithm (CMA) is commonly used to provide a channel equalizer function in a digital FM (frequency modulation) receiver. The channel equalizer is used to correct or mitigate the effects of multi-path noise or adjacent channel interference. However, in some cases, the CMA can cause unintended effects in the FM receiver. For example, single-frequency tones (spurs) may be generated within the frequency band of interest by defects in an RF (radio frequency) front-end. Also, spurs may be generated by the switching of an operating class D digital amplifier. If the spurs have a greater amplitude than the desired FM signal (signal-of-interest or SOI), the CMA based channel equalizer may lock onto the spurs while suppressing the SOI.

Therefore, it would be desirable to have an FM radio receiver that removes the spurs while still having the ability to equalize a relatively wide band SOI.

DETAILED DESCRIPTION

In one aspect there is provided, a radio receiver for receiving a signal, the radio receiver comprises an equalizer, a demodulator, a lowpass filter, and a coefficient generator. The equalizer is configured to perform a constant modulus algorithm initialized using a first set of coefficients on the received signal and for generating an equalized signal. The demodulator is coupled to the equalizer for demodulating the equalized signal. The lowpass filter is coupled to the demodulator for lowpass filtering the demodulated signal to detect a spurious signal and to generate an offset signal. The coefficient generator is coupled to the lowpass filter and configured to compare the offset signal to a predetermined threshold, and if the offset signal satisfies a predetermined condition in relation to the predetermined threshold, then to generate a second set of coefficients for re-initializing the constant modulus algorithm.

In another aspect, a radio receiver is provided for receiving a signal that comprises: an equalizer, a demodulator, a lowpass filter, and a coefficient generator. The equalizer is configured to perform a constant modulus algorithm initialized using a first set of coefficients on the received signal and for generating an equalized signal. The demodulator is coupled to the equalizer for demodulating the equalized signal. The lowpass filter is coupled to the demodulator for lowpass filtering the demodulated signal to detect a spurious signal and to generate an offset signal. The coefficient generator is coupled to the lowpass filter and configured to compare the offset signal to a predetermined threshold, and if the offset signal satisfies a predetermined condition in relation to the predetermined threshold, then to generate a second set of coefficients for re-initializing the constant modulus algorithm, wherein the second set of coefficients for the constant modulus algorithm are generated based on a frequency of the spurious signal and a set of coefficients related to a predetermined prototype filter.

In yet another aspect, there is provided a method in a radio receiver for receiving a signal, where the method comprises: performing a constant modulus algorithm initialized using a first set of coefficients on the received signal to generate an equalized signal; demodulating the equalized signal; filtering the demodulated signal to detect a spurious signal and to generate an offset signal; and comparing the offset signal to a predetermined threshold, and if the offset signal satisfies a predetermined condition in relation to the predetermined threshold, then generating a second set of coefficients for re-initializing the constant modulus algorithm.

Antenna12is coupled to a first input of mixer14. Mixer14has a second input coupled to a local oscillator16for receiving a local oscillator signal, and an output coupled the input of A/D converter18. The mixer14and local oscillator16are used to convert radio frequency (RF) signals from antenna12to FM signals in an intermediate frequency (IF) band of about 10.8 MHz. In other embodiments the IF may be different. The antenna12, mixer14, and local oscillator16are part of a receiver portion known as a “front-end”. There are other parts of the front-end that are not illustrated inFIG. 1. For example, the front-end may have circuits that amplify and broadband filter the received FM signals. Also, in other embodiments there may be more than one antenna connected to mixer14. Also, a switch (not shown) may be connected between the antenna12and mixer14in other embodiments. RF front-end design is known in the art and will not be further described. A/D converter18converts the analog output signal of the front-end circuit to a digital signal and moves the FM signals from the IF frequency to a base band frequency.

A/D converter18has an output coupled to an input of AGC circuit20for providing I and Q quadature signals at a sample rate of 480 kilo samples per second (KS/s). The AGC circuit20then provides a gain controlled signal labeled “GAIN CONTROLLED SIGNAL21” to CMA equalizer22. CMA equalizer22has an output for providing a constant modulus signal labeled “EQUALIZED SIGNALS23” to FM demodulator24. The CMA equalizer performs an equalization on the gain controlled signal21to produce equalized signals23having a relatively constant amplitude. FM demodulator24has an output for providing demodulated MPX signals25(DEMODULATED MPX SIGNAL25) to down sampler26. Down sampler26down samples demodulated MPX signals25by two to reduce the sample rate to 240 KS/s. The down sampled signals are then provided to MPX blanker34and to down sampler28. Down sampler28down samples by five and has an output for providing a down sampled signal at a sample rate of 48 KS/s. Note that in other embodiments the sample rates may be different. DC (direct current) LPF30has an input coupled to the output of down sampler28, and an output for providing an offset signal labeled “OFFSET31” to an input of coefficient generator32. The DC LPF30receives the demodulated and down sampled signal from down sampler28and provides the offset signal31as a DC signal having a voltage corresponding to the frequency of a detected spur. MPX blanker34has an output coupled to an input of stereo decoder36. Stereo decoder36has left and right outputs labeled “L” and “R”, respectively, for providing a stereo audio signal corresponding to the received FM signal. Coefficient generator32has a first output for providing COEFFICIENTS33to a control input of CMA equalizer22, and a second output for providing a control signal labeled “BLANKER CONTROL35” to a control input of MPX blanker34.

FIG. 2illustrates various signals of the receiver10ofFIG. 1in the frequency domain useful for understanding the illustrated embodiment. The operation of receiver10will be discussed referring toFIGS. 1 and 2. In operation, the receiver10is tuned to a predetermined frequency, or station, and an FM signal is received and processed by antenna12, mixer14, local oscillator16and A/D converter18to produce quadrature signals I and Q. The I and Q signals are processed by AGC20to change the signal to produce gain controlled signal21having a relatively fixed signal strength. Due to unintended effects of, for example, the receiver front-end circuits, one or more spurs may be generated with the FM signal that appear as a single tone signal in the frequency band of interest in the GAIN CONTROLLED SIGNAL21of AGC20as illustrated inFIG. 2. In accordance with the illustrated embodiment, CMA equalizer22is used to equalize the amplitude of the gain controlled signal21including the spur. As illustrated inFIG. 2, the CMA equalizer22is initialized by a first set of coefficients, which normally represents an all pass filter, to detect and lock onto the spur and attenuate the FM signal to produce an initial EQUALIZED SIGNAL23. The initial EQUALIZED SIGNAL23is demodulated to produce DEMODULATED MPX SIGNAL25. The demodulated signal25includes the spur and the demodulated FM signal. The spur is moved to DC and the frequency band of the FM demodulated signal is reduced by demodulator24as illustrated inFIG. 2. In other embodiments, the frequency band of demodulated signal25may be unchanged. After being down sampled by down samplers26and28, the demodulated FM signal and spur are provided to DC LPF30. DC LPF30removes substantially the entire demodulated FM signal leaving only the DC voltage from the spur as voltage OFFSET31. OFFSET31is provided to coefficient generator32. If the spur voltage OFFSET31is higher than a predetermined threshold, then the following equation is used to determine a second set of equalizer coefficients to re-initialize the CMA and remove the spur.
Coefficientsnew=Coefficientsprototype*ei*2π*(fspur−fnotch)/Fs*K
Generally, the second set of coefficients causes the CMA equalizer22to create a notch in the frequency band of the FM signal at the frequency of the spur. The notch effectively removes the spurious single frequency signal. In the above equation fspuris the corresponding frequency for spur voltage OFFSET31, FSis the sample rate, and K is an array of real integer numbers from 1 to the number of equalizer taps. Coefficientsprototypeis a set of coefficients related to a predetermined prototype filter having a notch at a notch frequency of fnotch. In the illustrated embodiment, fnotchis 50 KHz. A frequency different than 50 KHz may be used in the above equation in other embodiments. The frequency used for fnotchcan be randomly chosen within the frequency band of interest. In a preferred embodiment, the frequency fnotchis near the center frequency of the frequency band of interest. A prototype filter has a notch at the frequency fnotch. Using the center frequency for the prototype filter minimizes how far the prototype notch is moved to correspond to the spur frequency. Coefficientsnewis the new second set of generated coefficients. The new coefficients are then used with a proper gain factor to re-initialize CMA equalizer22. The new coefficients are used to create a notch in the equalized signal23at the frequency of the spur. The RESULTING EQUALIZED SIGNAL23is illustrated inFIG. 2. The notch in RESULTING EQUALIZED SIGNAL23is at the spur frequency, thus removing the spur from the received FM signal. After being re-initialized, CMA22will adaptively update without re-initializing again. Also, signal BLANKER CONTROL35from coefficient generator32is used to control the MPX blanker34to lower a noise detection threshold associated with MPX blanker34if OFFSET31is greater than a predetermined threshold.

A spur may not be present or a spur may not be detected if the spur has an amplitude that is less than the amplitude of the FM signal. The RF function of the front-end circuits may produce a spurious signal in the IF signal for some tuned channels and not for others. If no spur is detected when the FM signal is initially received, then the CMA22is allowed to operate without re-initialization. That is, the above equation is not used to re-initialize the coefficients. Note that the illustrated embodiment includes an FM signal. However, in other embodiments, signals of other modulation types that are processed using a CMA may be substituted for the FM signal.

The described embodiment is intended to be implemented in software or firmware of a digital signal processor (DSP) in an integrated circuit. The FM receiver is part of a digital IF automobile radio having a CMA based channel equalizer. However, the described embodiment may be implemented as hardware or software or a combination of hardware and software. Also, the described embodiment may be used in another type of radio receiver in another environment. The described embodiment removes a detected spur from an FM signal that may be caused by, for example, defects in a receiver front-end circuit. Removing the spur as described improves audio quality of the FM signal while still allowing the channel equalizer to equalize a constant modulus SOI. In addition, the described embodiment can reduce electromagnetic interference (EMI) caused by a nearby class D amplifier.

FIG. 3illustrates a method for operating the FM receiver10ofFIG. 1. At step50, receiver10is tuned to receive an FM signal. At step52, the CMA equalizer is initialized as an all pass filter. When the FM signal is initially received and a spur is detected by the CMA equalizer22within the frequency band, the CMA equalizer22is run at step54. The CMA equalizer22locks onto the spur instead of the FM signal to be equalized and removes substantially the entire FM signal. At step56, the FM signal is demodulated using FM demodulator24. After being down sampled by down samplers26and28, DC lowpass filter30is used in step58to lowpass filter the demodulated signal, detect the spur, and generate signal OFFSET31. At decision step60, the signal OFFSET31is compared to a predetermined threshold. If OFFSET31is less than or equal to the threshold, then the NO path is taken to step62, indicating no spur was detected, and CMA22continues to equalize the FM signal without being re-initialized by the above equation. If OFFSET31is greater than the threshold at decision step60, then the YES path is taken to step64. At step64OFFSET31is translated to determine the frequency of the spur using coefficient generator32. Signal OFFSET31is translated by using a look-up table (not shown) in coefficient generator32to determine the frequency of the spurious signal based on OFFSET31. In another embodiment, OFFSET31may be translated using a linear equation instead of a look-up table. At step66, new COEFFICIENTS33are generated by coefficient generator32using the above equation for new coefficients and used to re-initialize CMA22and the method continues to step62. Also, the BLANKER CONTROL35is used to control the MPX blanker34to lower a noise detection threshold associated with MPX blanker34if OFFSET31is greater than a predetermined threshold to remove any residual noise. The method is repeated whenever the FM receiver is tuned to a new station.

By now it should be appreciated that there has been provided a method and receiver for removing spurs from a received FM signal. The spurs are removed by generating new coefficients for a CMA equalizer. The new coefficients create a notch at the frequency of the spur. This removes the spur without affecting the rest of the bandwidth of the CMA. Also, an MPX blanker is used to remove any residual noise from the spurious signal.