Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/06—Receivers
  • H04B1/16—Circuits
  • H04B1/1646—Circuits adapted for the reception of stereophonic signals
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
  • H04L25/03006—Arrangements for removing intersymbol interference
  • H04L25/03012—Arrangements for removing intersymbol interference operating in the time domain
  • H04L25/03019—Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception
  • H04L25/03057—Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception with a recursive structure
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/10—Frequency-modulated carrier systems, i.e. using frequency-shift keying
  • H04L27/14—Demodulator circuits; Receiver circuits

Definitions

• This disclosure relates generally to radio, and more specifically, to a radio receiver having a channel equalizer and method therefor.
• a constant modulus algorithm 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.
• the CMA can cause unintended effects in the FM receiver.
• single-frequency tones spurs
• 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.
• FIG. 1 illustrates an FM receiver in accordance with one embodiment.
• FIG. 2 illustrates various signals of the FM receiver of FIG. 1.
• FIG. 3 illustrates a method for operating the FM receiver of FIG. 1.
• 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.
• a radio receiver 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.
• FIG. 1 illustrates an FM receiver 10 in accordance with one embodiment.
• FM receiver 10 includes antenna 12, mixer 14, local oscillator 16, analog-to-digital (A/D) converter 18, automatic gain control (AGC) circuit 20, constant modulus algorithm (CMA) channel equalizer 22, FM demodulator 24, down sampler 26, down sampler 28, Direct current (DC) lowpass filter (LPF) 30, coefficient generator 32, multiplex (MPX) blanker 34, and stereo decoder 36.
• APC automatic gain control
• CMA constant modulus algorithm
• LPF Direct current
• MPX multiplex
• Antenna 12 is coupled to a first input of mixer 14.
• Mixer 14 has a second input coupled to a local oscillator 16 for receiving a local oscillator signal, and an output coupled the input of A/D converter 18.
• the mixer 14 and local oscillator 16 are used to convert radio frequency (RF) signals from antenna 12 to FM signals in an intermediate frequency (IF) band of about 10.8 MHz. In other embodiments the IF may be different.
• the antenna 12, mixer 14, and local oscillator 16 are part of a receiver portion known as a "front-end". There are other parts of the front-end that are not illustrated in FIG. 1.
• the front-end may have circuits that amplify and broadband filter the received FM signals.
• a switch (not shown) may be connected between the antenna 12 and mixer 14 in other embodiments.
• RF front-end design is known in the art and will not be further described.
• A/D converter 18 converts 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 converter 18 has an output coupled to an input of AGC circuit 20 for providing I and Q quadature signals at a sample rate of 480 kilo samples per second (KS/s).
• the AGC circuit 20 then provides a gain controlled signal labeled "GAIN CONTROLLED SIGNAL 21 " to CMA equalizer 22.
• CMA equalizer 22 has an output for providing a constant modulus signal labeled "EQUALIZED SIGNALS 23" to FM demodulator 24.
• the CMA equalizer performs an equalization on the gain controlled signal 21 to produce equalized signals 23 having a relatively constant amplitude.
• FM demodulator 24 has an output for providing demodulated MPX signals 25 (DEMODULATED MPX SIGNAL 25) to down sampler 26.
• Down sampler 26 down samples demodulated MPX signals 25 by two to reduce the sample rate to 240 KS/s. The down sampled signals are then provided to MPX blanker 34 and to down sampler 28. Down sampler 28 down 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) LPF 30 has an input coupled to the output of down sampler 28, and an output for providing an offset signal labeled "OFFSET 31" to an input of coefficient generator 32. The DC LPF 30 receives the demodulated and down sampled signal from down sampler 28 and provides the offset signal 31 as a DC signal having a voltage corresponding to the frequency of a detected spur.
• MPX blanker 34 has an output coupled to an input of stereo decoder 36.
• Stereo decoder 36 has left and right outputs labeled "L” and “R”, respectively, for providing a stereo audio signal corresponding to the received FM signal.
• Coefficient generator 32 has a first output for providing COEFFICIENTS 33 to a control input of CMA equalizer 22, and a second output for providing a control signal labeled "BLANKER CONTROL 35" to a control input of MPX blanker 34.
• FIG. 2 illustrates various signals of the receiver 10 of FIG. 1 in the frequency domain useful for understanding the illustrated embodiment. The operation of receiver 10 will be discussed referring to FIGs. 1 and 2.
• the receiver 10 is tuned to a predetermined frequency, or station, and an FM signal is received and processed by antenna 12, mixer 14, local oscillator 16 and A/D converter 18 to produce quadrature signals I and Q.
• the I and Q signals are processed by AGC 20 to change the signal to produce gain controlled signal 21 having 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 SIGNAL 21 of AGC 20 as illustrated in FIG. 2.
• CMA equalizer 22 is used to equalize the amplitude of the gain controlled signal 21 including the spur. As illustrated in FIG.
• the CMA equalizer 22 is 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 SIGNAL 23.
• the initial EQUALIZED SIGNAL 23 is demodulated to produce DEMODULATED MPX SIGNAL 25.
• the demodulated signal 25 includes 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 demodulator 24 as illustrated in FIG. 2. In other embodiments, the frequency band of demodulated signal 25 may be unchanged.
• the demodulated FM signal and spur are provided to DC LPF 30.
• DC LPF 30 removes substantially the entire demodulated FM signal leaving only the DC voltage from the spur as voltage OFFSET 31.
• OFFSET 31 is provided to coefficient generator 32. If the spur voltage OFFSET 31 is 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.
• the second set of coefficients causes the CMA equalizer 22 to 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.
• f spu r is the corresponding frequency for spur voltage OFFSET 31
• Fs is the sample rate
• K is an array of real integer numbers from 1 to the number of equalizer taps.
• Coefficients prototype is a set of coefficients related to a predetermined prototype filter having a notch at a notch frequency of f no tct ⁇
• f notCh is 50 KHz. A frequency different than 50 KHz may be used in the above equation in other embodiments.
• the frequency used for f n otc h can be randomly chosen within the frequency band of interest.
• the frequency f notCh is near the center frequency of the frequency band of interest.
• a prototype filter has a notch at the frequency f notch . Using the center frequency for the prototype filter minimizes how far the prototype notch is moved to correspond to the spur frequency.
• Coefficients new is the new second set of generated coefficients. The new coefficients are then used with a proper gain factor to re-initialize CMA equalizer 22. The new coefficients are used to create a notch in the equalized signal 23 at the frequency of the spur.
• the RESULTING EQUALIZED SIGNAL 23 is illustrated in FIG. 2.
• the notch in RESULTING EQUALIZED SIGNAL 23 is at the spur frequency, thus removing the spur from the received FM signal.
• CMA 22 After being re-initialized, CMA 22 will adaptively update without reinitializing again.
• signal BLANKER CONTROL 35 from coefficient generator 32 is used to control the MPX blanker 34 to lower a noise detection threshold associated with MPX blanker 34 if OFFSET 31 is 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 CMA 22 is allowed to operate without re-initialization. That is, the above equation is not used to reinitialize the coefficients.
• 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.
• DSP digital signal processor
• the FM receiver is part of a digital IF automobile radio having a CMA based channel equalizer.
• the described embodiment may be implemented as hardware or software or a combination of hardware and software.
• 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.
• the described embodiment can reduce electromagnetic interference (EMI) caused by a nearby class D amplifier.
• EMI electromagnetic interference
• FIG. 3 illustrates a method for operating the FM receiver 10 of FIG. 1.
• receiver 10 is tuned to receive an FM signal.
• the CMA equalizer is initialized as an all pass filter.
• the FM signal is initially received and a spur is detected by the CMA equalizer 22 within the frequency band, the CMA equalizer 22 is run at step 54.
• the CMA equalizer 22 locks onto the spur instead of the FM signal to be equalized and removes substantially the entire FM signal.
• the FM signal is demodulated using FM demodulator 24. After being down sampled by down samplers 26 and 28, DC lowpass filter
• step 58 is used in step 58 to lowpass filter the demodulated signal, detect the spur, and generate signal OFFSET 31.
• the signal OFFSET 31 is compared to a predetermined threshold. If OFFSET 31 is less than or equal to the threshold, then the NO path is taken to step 62, indicating no spur was detected, and CMA 22 continues to equalize the FM signal without being re-initialized by the above equation. If OFFSET 31 is greater than the threshold at decision step 60, then the YES path is taken to step 64. At step 64 OFFSET 31 is translated to determine the frequency of the spur using coefficient generator 32.
• Signal OFFSET 31 is translated by using a look-up table (not shown) in coefficient generator 32 to determine the frequency of the spurious signal based on OFFSET 31.
• OFFSET 31 may be translated using a linear equation instead of a look-up table.
• new COEFFICIENTS 33 are generated by coefficient generator 32 using the above equation for new coefficients and used to re-initialize CMA 22 and the method continues to step 62.
• the BLANKER CONTROL 35 is used to control the MPX blanker 34 to lower a noise detection threshold associated with MPX blanker 34 if OFFSET
• the method is repeated whenever the FM receiver is tuned to a new station.
• any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components.
• any two components so associated can also be viewed as being “operably connected,” or “operably coupled,” to each other to achieve the desired functionality.
• Coupled is not intended to be limited to a direct coupling or a mechanical coupling.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Power Engineering (AREA)
• Noise Elimination (AREA)
• Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)

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• 2007
  • 2007-05-22 US US11/751,771 patent/US7796688B2/en not_active Expired - Fee Related
• 2008
  • 2008-04-22 WO PCT/US2008/061145 patent/WO2008147606A1/en not_active Ceased
  • 2008-04-22 JP JP2010509418A patent/JP5133404B2/ja not_active Expired - Fee Related

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