Abstract:
A signal receiver and a method for correcting frequency dependent IQ phase errors. The receiver uses a calibration tone generator for generating a calibration tone for providing in-phase (I) and quadrature phase (Q) tone components, I and Q filters for filtering the I and Q calibration tone components for issuing filtered I and Q output tones having undesired frequency dependent I/Q phase error, and a correlator for cross correlating the I and Q output tones for providing a correlation feedback signal. At least one of the I and Q filters has at least one adjustable pole and one adjustable zero. The correlation feedback signal adjusts the frequency of the adjustable poles and zeroes for reducing the frequency dependent I/Q phase error.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates generally to in-phase (I) and quadrature phase (Q) signal processing in signal receivers and more particularly to methods and apparatus for correcting I/Q phase errors that depend upon frequency of modulation.  
         [0003]     2. Description of the Prior Art  
         [0004]     In-phase (I) and quadrature phase (Q) signal processing is used in most modem radio signal receivers. The I and Q signals that are derived from an incoming modulated signal should have a phase difference (I/Q phase) of 90° or quadrature at the carrier frequency of the incoming signal and a gain ratio (I/Q gain) of unity. I/Q phase errors and I/Q gain errors degrade the bit rate (BER) performance of the receiver. Imperfections in the frequency downconversion circuitry are known to cause I(Q phase and I/Q gain errors that are independent of modulation frequency. There are several techniques that are known for correcting these frequency independent I/Q phase and I/Q gain errors. However, I/Q phase and I/Q gain errors that are dependent upon modulation frequency are not corrected by these techniques. For a given receiver, the frequency dependent errors typically increase as the modulation frequency increases. A common cause of these frequency dependent I/Q errors is a difference between the frequency responses of I and Q analog baseband filters.  
         [0005]     There is a need for a method and apparatus in a radio receiver for correcting frequency dependent I/Q phase error.  
       SUMMARY OF THE INVENTION  
       [0006]     It is therefore an object of the present invention to provide a method and apparatus in a signal receiver for correcting frequency dependent I/Q phase error.  
         [0007]     Briefly, in a preferred embodiment, a signal receiver of the present invention has a normal operation mode and a calibration mode. The receiver includes I and Q filters for providing filtered I and Q signal components in the normal operation mode. These filters introduce an undesired frequency dependent I/Q phase error. In the calibration mode the receiver uses a calibration tone generator for providing in-phase ( 1 ) and quadrature phase (Q) tone components to the I and Q filters and a correlator for cross correlating the filtered I and Q output tones for providing a correlation feedback signal. At least one of the I and Q filters is provided with an adjustable characteristic, such as cutoff frequency or phase delay, that can be controlled by adjusting poles and zeroes in the filter. The correlation feedback signal adjusts the adjustable characteristic to minimize the phase difference between the I and Q output tones in order to reduce the frequency dependent I/Q phase error.  
         [0008]     An advantage of the present invention is improved performance as a result of the reduction of frequency dependent I/Q phase error.  
         [0009]     These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various figures.  
     
    
     IN THE DRAWINGS  
       [0010]      FIG. 1  is a block diagram of an embodiment of a signal receiver of the present invention;  
         [0011]      FIG. 2  is block diagram of another embodiment of the signal receiver of the present invention;  
         [0012]      FIG. 3  is a block diagram of a variation on the signal receiver embodiments of  FIGS. 1 and 2 ;  
         [0013]      FIG. 4  is chart showing an adjustable cutoff frequency of an analog filter of the receiver of  FIG. 1 ;  
         [0014]      FIG. 5  is chart showing an adjustable phase delay of an allpass filter of the receiver of  FIG. 2 ; and  
         [0015]      FIG. 6  is phase plane chart of an adjustable pole-zero pair of the allpass filter of  FIG. 5 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]      FIG. 1  is a block diagram of a signal receiver  10 A of the present invention. The receiver  10 A includes an antenna  12 , a frequency downconverter  14 , a calibration mode switch  16 , I and Q analog filters  18 I and  19 Q, I and Q analog-to-digital converters (ADC)s  22 I and  22 Q, and an IQ digital signal processor  24 A. In normal operation, the antenna  12  converts an incoming modulated radio frequency (RF) signal from an airwave signal to a conducted signal and passes the conducted signal to the frequency downconverter  14 . The frequency downconverter  14  downconverts the RF conducted signal to I and Q signal components at baseband and passes the I and Q signal components through the calibration mode switch  16  (herein illustrated in a in switch state for a calibration mode) to the I and Q analog filters  18 I and  19 Q.  
         [0017]     The I and Q analog filters  18 I and  19 Q filter the I and Q signal components and pass the filtered analog I and Q signal components to the ADCs  22 I and  22 Q. The ADCs  22 I and  22 Q convert the filtered analog I and Q signal components to digital form and pass the digital I and Q signal components to the IQ digital signal processor  24 A. The IQ digital signal processor  24 A processes the digital I and Q signal components for providing data that is representative of the modulation on the incoming RF signal. For the lowest or best bit error rate (BER), the I and Q signal components should be in quadrature. The degree to which the I and Q signal components deviate from quadrature is termed I/Q phase error. An I/Q phase error that increases as the modulation frequency increases is termed frequency dependent I/Q phase error.  
         [0018]     The frequency dependent I/Q phase error in the digital I and Q signals is caused primarily by mismatch between the phase responses of the I and Q analog filters  18 I and  19 Q. In order to reduce this error, the receiver  10 A uses a calibration tone generator  32 , a calibration IQ cross correlator  34 , and a digital to analog converter  36 . In the calibration mode, the calibration tone generator  32  generates a calibration signal or tone having quadrature tone components cosw o t (I) and sinw o t (Q). The calibration mode switch  16  is switched to the calibration mode state and the I and Q calibration tone components replace the normal I and Q signal components to the I and Q analog filters  18 I and  19 Q. The filtered I and Q calibration tone components are then digitized by the I and Q ADCs  22 I and  22 Q and passed as I and Q output signals or tones to the calibration IQ cross correlator  34 .  
         [0019]     The calibration IQ cross correlator  34  correlates the I and Q output tones from the I and Q ADCs  22 I and  22 Q for providing a cross correlation feedback signal. The cross correlation feedback signal is converted from a digital to an analog form and then used to control the frequency cutoff of the Q analog filter  19 Q. The I and Q analog filters  18 I and  19 Q have an approximate cutoff frequency in radians/second of w o . The action of the feedback adjusts the cutoff frequency of the Q analog filter  19 Q (or alternatively the I analog filter  18 I) to drive the cross correlation feedback signal near to zero by minimizing the phase difference between the I and Q output tones at the radian frequency w o  (see  FIG. 4 ). By minimizing the phase difference between the I and Q output tones at the radian frequency w o , the frequency dependent I/Q phase error of the receiver  10 A is reduced. It should be obvious that the Q analog filter  19 Q and the I analog filter  18 I are interchangeable for the adjustable purpose of the present invention and that either or both of the I and Q analog filters  18 I and  19 Q can be adjusted for the present invention.  
         [0020]     It should be noted that the frequency dependent I/Q phase error is reduced by adjusting the phase of the Q output tone to match the phase of the I output tone at the radian frequency w o  and that this is accomplished by adjusting the cutoff frequency of the Q analog filter  19 Q. Of course, there are other filter types and devices having other adjustable charateristics within the idea of the present invention.  
         [0021]      FIG. 2  is a block diagram of a signal receiver  10 B of the present invention. The receiver  10 B includes the antenna  12 , the frequency downconverter  14  and the calibration mode switch  16 , and uses the calibration tone generator  32  and the calibration IQ cross correlator  34  as described above.  
         [0022]     The receiver  10 B differs from the receiver  10 A by having I and Q mixed mode filters  42 I and  43 Q. The I mixed mode filter  42 I includes the I analog filter  18 I, the I ADC  22 I and a digital I allpass filter  44 I. Similarly, the Q mixed mode filter  43 Q includes a Q analog filter  18 Q, the Q ADC  22 Q and a digital Q allpass filter  45 Q. In the normal mode digital I and Q signal components from the I and Q ADCs  22 I and  22 Q are passed to the I and Q allpass filters  44 I and  45 Q. The I and Q allpass filters  44 I and  45 Q delay the digital  10  I and Q signal components and pass the delayed I and Q signal components to the IQ digital signal processor  24 B. The IQ digital signal processor  24 B processes the delayed I and Q signal components for providing data that is representative of the modulation on the incoming RF signal.  
         [0023]     For the calibration mode, the calibration tone generator  32  generates a calibration tone having quadrature tone components cosw o t (D and sinw o t (Q). The calibration mode switch  16  is switched to the calibration mode and the I and Q calibration tone components replace the normal I and Q signal components to the I and Q analog filters  18 I and  18 Q. The I and Q calibration tone components are filtered by the I and Q analog filters  18 I and  18 Q, digitized by the I and Q ADCs  22 I and  22 Q, and then delayed by the I and Q allpass filters  44 I and  45 Q for providing filtered I and Q output tones to the calibration IQ cross correlator  34 .  
         [0024]     The calibration IQ cross correlator  34  correlates the I and Q output tones from the I and Q allpass filters  44 I and  45 Q for providing the cross correlation feedback signal. The cross correlation feedback signal is used to control the delay (phase) in the Q allpass filter  45 Q at the radian frequency w o  (see  FIG. 5 ). The action of the feedback adjusts the phase delay of the Q allpass filter  45 Q (or alternatively the I allpass filter  44 I) to minimize the cross correlation feedback signal by minimizing the phase difference between the I and Q allpass calibration tone components at the radian frequency w o  (see  FIG. 5 ). Minimizing the phase difference between the I and Q output tones at the radian frequency w o  reduces the frequency dependent I/Q phase error of the receiver  10 B. It should be obvious that the Q allpass filter  45 Q and the I allpass filter  44 I are interchangeable for the adjustable purpose of the present invention and that either or both of the I and Q allpass filters  44 I and  45 Q can be adjusted for the present invention.  
         [0025]      FIG. 3  is a block diagram of a radio frequency (RF) variation, denoted by a general reference  50 , of the receivers  10 A and  10 B for the present invention. The receiver  50  includes the antenna  12 , a frequency downconverter  54 , and a calibration tone generator  62 . In normal operation, the antenna  12  converts the incoming modulated radio frequency (RF) signal from an airwave signal to a conducted signal and passes the conducted signal to the frequency downconverter  54 . The frequency downconverter  54  includes a low noise amplifier (LNA)  64 , a calibration mode switch  65 , I and Q frequency downconverters  66 I and  66 Q, and a local oscillator system (LO)  68  for frequency converting the RF conducted signal to the I and Q signal components as described above. The calibration tone generator  62  replaces the calibration tone generator  32  and the calibration mode switch  65  replaces the calibration mode switch  16 .  
         [0026]     The LNA  64  amplifies the RF conducted signal from the antenna  12  and passes the amplified signal through the calibration mode switch  65  (shown for the calibration mode) to the I and Q frequency downconverters  66 I and  66 Q. The I and Q downconverters  66 I and  66 Q use quadrature LO signals cosw c t and sinw c t from the LO  68  for downconverting the amplified RF signal to the I and Q signal components and passes the I and Q signal components to the I and Q analog filters  18 I and  19 Q for the receiver  10 A or  42 I and  43 Q for the receiver  10 B. The I and Q frequency downconverters  66 I and  66 Q include well known devices such as amplifiers, mixers, samplers, phase shifters and filters for one or more frequency up and down conversion stages with a net effect that the input frequency is downconverted to the output frequency. Each of the frequency conversion stages may use several frequency conversion devices in parallel.  
         [0027]     In the calibration mode the calibration tone generator  62  generates a calibration frequency offset tone cos(w c +w o )t. The calibration tone cos(w c +w o )t mixes with the quadrature LO signals cosw c t and sinw c t in the I and Q frequency downconverters  66 I and  66 Q for providing the quadrature I and Q tone components cosw o t and sinw o t as described above to the I and Q filters  18 I and  19 Q for the receiver  10 A or the I and Q filters  42 I and  43 Q for receiver  10 B.  
         [0028]     The calibration elements of the calibration mode switch  16  or  65 , the calibration tone generator  32  or  62 , and/or the calibration IQ cross correlator  34  may be built in to the receiver embodiments  10 A and  10 B and variation  50  or may be used for calibration and then removed.  
         [0029]      FIG. 4  is a chart illustrating amplitude versus frequency (denoted as frequency response) for the I analog filter  18 I and the Q analog adjustable filter  19 Q in the receiver  10 A. The frequency responses of the I and Q analog filters  18 I and  19 Q may have a cutoff frequency within less than about ten percent of w o . In a variation of the present invention, the radian frequency w o  of the I and Q calibration tone may be in a range of fifty percent to one hundred percent of the maximum modulation or data frequency. The frequency response of the Q analog adjustable filter  19 Q is adjusted by an adjustment that is controlled by the cross correlation feedback signal (so that the cross correlation feedback signal is about zero) for reducing the frequency dependent I/Q phase error. Such adjustment may be made by equally scaling all poles and zeros in the Q analog adjustable filter  19 Q. The poles and zeroes may be constructed using resistances and capacitances. In an integrated circuit having metal oxide silicon (MOS) field effect transistors (FET)s and capacitors, this may be accomplished by controlling the gate biases of the MOSFETs in order to control the channel resistances of the MOSFETs.  
         [0030]      FIG. 5  is a chart illustrating delay (phase) versus frequency (denoted phase response) for the I allpass filter  44 I and the Q adjustable allpass filter  45 Q in the receiver  10 B. The phase at the radia frequency w o  lags the phase at zero frequency. The amount of the lag in the Q adjustable allpass filter  45 Q is adjusted by an adjustment that is controlled by the cross correlation feedback signal so that the cross correlation feedback signal is driven to near zero, thereby reducing the frequency dependent I/Q phase error.  
         [0031]      FIG. 6  is a chart illustrating a complex phase plane for the I and Q allpass filters  44 I and  45 Q for the receiver  10 B. A pole-zero pair is illustrated with a pole “x” and a zero “o”. Radian frequency is represented by the angle around a unit circle from zero ( 0 ) frequency to the radian frequency w o  and beyond. The phase response of the I allpass filters  44 I (or the Q allpass filter  45 Q) is determined from the location of the pole x and the zero o with respect to the radian frequency on the unit circle. The pole x and zero o pair are geometrically centered about the unit circle on the negative real axis with the pole x inside the unit circle (for example when the pole x is 2/3 units, the zero o is 3/2 units). The adjustment is made by inversely scaling one or more pole-zero pairs in the Q adjustable allpass filter  45 Q (multiplying the frequency of the pole x and dividing the frequency of the zero o by the same factor). In an integrated circuit using metal oxide silicon (MOS) field effect transistors (FET)s and capacitors, this may be accomplished by controlling the gate biases of the MOSFETs in order to control the channel resistances.  
         [0032]     Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.