Patent Publication Number: US-2006007999-A1

Title: Method and system for enhancing image rejection in communications receivers using test tones and a baseband equalizer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE  
      This application makes reference to, claims priority to, and claims the benefit of U.S. Provisional Patent Application Ser. No. 60/586328 (Attorney Docket No. 15901 US01), filed on Jul. 8, 2004.  
      The above stated application is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION  
      Certain embodiments of the invention relate to communications receivers. More specifically, certain embodiments of the invention relate to a method and system for enhancing image rejection in communications receivers using test tones and a baseband equalizer.  
     BACKGROUND OF THE INVENTION  
      Conventional direct-conversion communications receivers generally rely on radio frequency (RF) or local oscillator (LO) signals that have been accurately split into quadrature components to produce in-phase (I) and quadrature (Q) baseband signals that may be used to reconstruct the message waveform. Similar principles are also utilized in low-IF receivers to achieve adequate image rejection. These types of receivers are increasingly popular because of their reduced reliance on RF or IF selectivity solely to achieve image rejection, leading to lower cost and size.  
      Based on design, some communication systems may require only a modest amount of image rejection, obtainable without trimming or calibrating the quadrature generation circuits. Other communication systems, for example, terrestrial broadcast systems or cable television, require very high levels of image rejection. In the case of terrestrial broadcast systems, there are often signals present in the image band of a receiver with much higher power levels than the desired carrier. This may result from close proximity to an interfering antenna, for example, and may be due to co-channel interference. In the case of analog cable television and other analog television broadcast systems, high levels of image rejection are required because the signal-to-interference (S/I) ratio must be very large for acceptable quality.  
      Achieving very high levels of image rejection or l-Q balance, for example, &gt;40 dB for 1 GHz signals, roughly, may require some form of trimming or calibration. A plurality of methods suitable for implementation in integrated circuits (IC&#39;s) have been proposed. For example, U.S. Pat. No. 6,714,776 entitled “System and Method for An Image Rejection Single Conversion Tuner With Phase Error Correction” provides one such method suitable for implementing in an IC. This invention discloses a single conversion tuner, which generally utilizes phase shifted in-phase and quadrature signal paths as an image rejection circuit. The entire signal bandwidth is processed within the tuner by utilizing broadband input low noise amplifier (LNA) and mixer circuits. The invention adds a test tone to the RF signal and compares the phase of the down-converted I and Q test tones to obtain an error signal, which is utilized to control the quadrature balance of the LO&#39;s. The I and Q channels may not have the same amplitude and may not be at perfect quadrature with respect to each other. As a result of imperfect I-Q matching, the performance of the receiver may deteriorate.  
      Tuning may involve translating signals in frequency. If a desired channel is to be translated to an IF by mixing with a local oscillator that is lower in frequency, then a channel two times the IF frequency below the desired channel may be translated to negative IF. Negative intermediate frequencies interfere with the desired channel at the positive IF. This interfering channel may be referred to as an image channel and may be rejected to a large degree for proper reception. Image rejection may be addressed with filters and/or with image-reject mixers. In a single-conversion tuner, a notch filter may be used to reject the image channel prior to frequency translation. The performance of such a filter may be limited to 50 to 60 dB, for example, in the UHF component of the TV band (470 MHz and up). Better performance may be possible with dual-conversion tuners, where the first IF filter may be adapted to suppress the image channel by an arbitrary amount, depending on the cost of the filter. For cost-effective, dual-conversion tuning systems, the preferred approach may be to use a reasonably priced surface-acoustic wave (SAW) filter at first IF to achieve around 40 to 50 dB, for example, by itself, and then to complement it with a specialized mixer called an image-reject mixer. Such a mixer may be adapted to achieve an additional 35 to 40 dB, for example, of suppression. The combination allows for consistent image rejection in the range of better than 70 dB, for example.  
      Other methods may infer the quadrature balance from the I and Q baseband signals and generate an error signal accordingly. One such method is described in “A Single-Chip tuner for DVB-T” by Dawkins et al, IEEE Journal for Solid State Circuits Vol. 38 No.8, August 2003 (IEEE publication 0018-9200/03). Another such method is described in “Direct Conversion—How to Make it Work in TV Tuners” by Aschwanden, IEEE Transactions on Consumer Electronics, Vol. 42, No. 3, Aug. 1996 (IEEE Publication No. 0098,3063/96). It has been proposed to use this error signal to control an equalizer, which then maintains I and Q balance.  
      Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.  
     BRIEF SUMMARY OF THE INVENTION  
      Certain embodiments of the invention provide a method for enhancing image rejection in communications receivers. A test tone signal may be injected into a receiver. A quadrature error in an in-phase (I) channel and a quadrature (Q) channel of the receiver may be estimated based on the injecting of the test tone signal into the receiver. A plurality of equalizer coefficients may be adjusted to correct the estimated quadrature error in the I channel and the Q channel of the receiver. A corrected I channel and a corrected Q channel may be generated, which corresponds to the I channel and the Q channel in the receiver based on the adjusting of the equalizer coefficients.  
      In an embodiment of the invention, the test tone signal may be generated by utilizing a direct digital frequency synthesizer. In this regard, the test tone signal generated by the direct digital frequency synthesizer may be converted into an analog signal injected at RF or a first IF frequency. The test tone signal may be injected at a first IF frequency having a narrow bandwidth to improve the quadrature accuracy of the second image reject mixer in a double conversion tuner. An amplitude error may be corrected in the I channel and the Q channel of the receiver. A phase error may be corrected in the I channel and the Q channel of the receiver. The I channel and the Q channel of the receiver may be filtered to allow the injected test tone signal. The injected test tone signal may have a high signal to noise ratio.  
      Another embodiment of the invention may provide a machine-readable storage, having stored thereon, a computer program having at least one code section executable by a machine, thereby causing the machine to perform the steps as described above in the method and system for enhancing image rejection in communications receivers using test tones and a baseband equalizer.  
      Another embodiment of the invention provides a system for enhancing image rejection in communications receivers. A test tone generator may be adapted to inject a test tone signal into a receiver. Circuitry may be adapted to estimate a quadrature error in an in-phase (I) channel and a quadrature (Q) channel of the receiver based on the injecting of the test tone signal into the receiver. Circuitry may be adapted to adjust a plurality of equalizer coefficients to correct the estimated quadrature error in the I channel and the Q channel of the receiver. The system may comprise circuitry that may be adapted to generate a corrected I channel and a corrected Q channel corresponding to the I channel and the Q channel in the receiver based on the adjusting of the equalizer coefficients.  
      A direct digital frequency synthesizer may be adapted to generate the test tone signal and a digital-to-analog converter may be adapted to convert the generated test tone signal from the direct digital frequency synthesizer into an analog signal. The test tone generator may be adapted to inject the test tone signal into the RF or first IF stages of the receiver. The test tone generator may be adapted to inject the test tone signal at a first IF frequency having a narrow bandwidth to improve the quadrature accuracy. Circuitry may be adapted to correct an amplitude error in the I channel and the Q channel of the receiver. Circuitry may also be adapted to correct a phase error in the I channel and the Q channel of the receiver. A low pass filter may be adapted to filter the I channel and the Q channel of the receiver to allow the injected test tone signal. The injected test tone signal may have a high signal to noise ratio.  
      These and other advantages, aspects and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings.  
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       FIG. 1  is a block diagram of a conventional direct-conversion receiver that may be utilized in connection with an embodiment of the invention.  
       FIG. 2  is a block diagram of an exemplary baseband equalizer that may be utilized for quadrature correction, in accordance with an embodiment of the invention.  
       FIG. 3  is a block diagram of an exemplary direct conversion tuner with quadrature correction, in accordance with an embodiment of the invention.  
       FIG. 4  is a block diagram of an exemplary test tone synthesizer, in accordance with an embodiment of the invention.  
       FIG. 5   a  is a block diagram of a double conversion tuner with quadrature correction and a complex mixer having separate I and Q components, in accordance with an embodiment of the invention.  
       FIG. 5   b  is a block diagram of a double conversion tuner with quadrature correction, where the test tone signals are injected at the first IF frequency, in accordance with an embodiment of the invention.  
       FIG. 6  is a flowchart illustrating exemplary steps for enhancing image rejection in communications receivers, in accordance with an embodiment of the invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Certain embodiments of the invention may provide a method for enhancing image rejection in communications receivers. A test tone signal may be injected into a receiver. A quadrature error in an in-phase (I) channel and a quadrature (Q) channel of the receiver may be estimated based on the injecting of the test tone signal into the receiver. A plurality of equalizer coefficients may be adjusted to correct the estimated quadrature error in the I channel and the Q channel of the receiver. A corrected I channel and a corrected Q channel may be generated corresponding to the I channel and the Q channel in the receiver based on the adjusting of the equalizer coefficients.  
       FIG. 1  is a block diagram of a conventional direct-conversion receiver that may be utilized in connection with an embodiment of the invention. Referring to  FIG. 1 , there is shown an amplifier  102 , a plurality of mixers  104  and  106 , a plurality of low pass filters  112  and  114 , a plurality of linear gain amplifiers  116  and  120 , a plurality of power detectors  118  and  122 , a phase splitter  108  and a phase locked loop (PLL)  110 .  
      The amplifier  102  may be adapted to receive an input signal and may generate an output signal that may be input to the plurality of mixers  104  and  106 . The mixers  104  and  106  may be adapted to downconvert the analog RF substreams to baseband. The phase splitter  108  may be adapted to ensure that the mixer local oscillator inputs are in quadrature, indicating that they are 90 degrees out of phase with respect to each other. Alternatively, one path may be shifted by positive (+) 45 degrees and the other path may be shifted by negative (−) 45 degrees, for example. The phase locked loop  110  may be adapted to drive the mixer local oscillator inputs and the phase splitter  108 . The plurality of low pass filters  112  and  114  may be adapted to filter the signals and may allow only a desired channel of frequencies. The plurality of linear gain amplifiers  116  and  120  may be adapted to maintain a constant amplitude and may be controlled by the plurality of power detectors  118  and  122 .  
       FIG. 2  is a block diagram of an exemplary baseband equalizer that may be utilized for quadrature correction, in accordance with an embodiment of the invention. Referring to  FIG. 2 , there is shown a plurality of summing nodes  202  and  208 , a phase correction block  204 , an amplitude correction block  206 , a loop filter block  210 , a plurality of filters  212  and  216  and a phase and amplitude detectors block  214 .  
      The summing node  202  may be adapted to receive a plurality of inputs from the I channel and the phase correction block  204  and generate an output to the summing node  208 . The phase correction block  204  may be adapted to receive a plurality of inputs from the Q channel and a phase coefficient signal ε φ  from the loop filter block  210 . The amplitude correction block  206  may be adapted to receive a plurality of inputs from the I channel and an amplitude coefficient signal ε A  from the loop filter block  210 . The summing node  208  may be adapted to receive a plurality of inputs from the I channel and a signal from the amplitude correction block  206  to generate a corrected amplitude and corrected phase I′ channel. The filters  212  and  216  may be adapted to filter and remove any out of band signals from the corrected I′ and Q′ channels and may generate outputs to the phase and amplitude detectors block  214 .  
      The phase and amplitude detectors block  214  may be adapted to estimate a phase error and an amplitude error which may be denoted by Δ φ  and Δ A  respectively. The loop filter block  210  may be adapted to integrate the phase and amplitude errors received from the phase and amplitude detectors block  214  and generate coefficient signals ε φ  and ε A  that may provide the necessary correction in phase and amplitude in the I and Q channels. The loop filter block  210  may comprise a plurality of loop filters. The generated coefficient signals may also be adapted to improve the characteristics of feedback and loop bandwidth. The phase error signal may be adapted to control the amount of residual Q (I) signal subtracted from the I (Q) signal. The amplitude error signal may be adapted to control the amplitude of the I (Q) path signal.  
       FIG. 3  is a block diagram of an exemplary direct conversion tuner with quadrature correction, in accordance with an embodiment of the invention. Referring to  FIG. 3 , there is shown an amplifier  302 , a summer  304 , a test tone generator  306 , a plurality of mixers  308  and  310 , a plurality of low pass filters  316  and  318 , a plurality of linear gain amplifiers  320  and  322 , a phase splitter  312 , a phase locked loop PLL  314  and a quadrature correction block  324 .  
      The amplifier  302  may be adapted to receive an input signal and may generate an output signal to the summer  304 . The summer  304  may be adapted to receive a plurality of inputs from the amplifier  302  and the test tone generator  306  and generate an output signal that may be input to the plurality of mixers  308  and  310 . The test tone generator  306  may comprise suitable logic and/or circuitry that may be adapted to generate an RF signal slightly different in frequency from the desired input signal that may be utilized to adjust the estimated quadrature error in the I channel and the Q channel of the receiver. The mixers  308  and  310  may be adapted to downconvert the analog RF substreams to baseband. The phase splitter  312  may be adapted to ensure that the mixer local oscillator inputs are  90  degrees out of phase with respect to each other. The phase locked loop  314  may be adapted to drive the mixer local oscillator inputs and the phase splitter  312 . The plurality of low pass filters  316  and  318  may be adapted to filter the signals and may be adapted to allow only the test tone signals generated by the test tone generator  306 . The plurality of linear gain amplifiers  320  and  322  may be adapted to maintain a constant amplitude and may be controlled by the quadrature correction block  324 .  
      The quadrature correction block  324  may be adapted to receive the I and Q channel inputs that may comprise a combination of a desired signal and a test tone signal. The test tone signals are almost in quadrature with respect to the desired signal and have a high signal to noise ratio, which may increase the bandwidth and the convergence rate of the feedback loop. The I and Q channels may be digitized by analog-to-digital converters (ADCs) in the quadrature correction block  324 . This has the advantage of permitting arbitrarily accurate measurement and correction of the quadrature errors, limited only by digital precision. The test tone signal may be injected between the quadrature mixers  308  and  310  and a front-end block, such as an amplifier  302 , for example, which may provide significant reverse isolation preventing unwanted leakage of the test tone signal backwards into the communication medium.  
      Distinct from conventional systems such as that which is described in U.S. Pat. No. 6,714,776, both the phase and amplitude of the baseband I-Q signals may be corrected based on a comparison of the I and Q test tone signals. When compared to the systems described by, for example, Dawkins et al, and Aschwanden, which depend on features of the received signal to identify quadrature imbalance, the present invention is independent of the signal characteristics. In this regard, the present invention provides a method and system that may rapidly converge to the correct equalizer setting under all conditions, because the test tone signals may be strong enough to overcome received noise or interference.  
       FIG. 4  is a block diagram of an exemplary test tone synthesizer, in accordance with an embodiment of the invention. Referring to  FIG. 4 , there is shown a digital-to-analog converter DAC  402 , a direct digital frequency synthesizer  404 , a low pass filter  406 , a summer  410 , a frequency divider  408 , a loop filter  412  and a voltage controlled oscillator  414 .  
      The direct digital frequency synthesizer  404  may comprise suitable logic and/or circuitry that may be adapted to generate a low frequency test tone signal in response to receiving an input frequency command signal. The test tone signal may be generated in an integrated circuit utilizing the direct digital frequency synthesizer  404  and optionally a PLL to frequency multiply and filter the output of the DDFS  404 . This technique may produce a test tone signal with very fine frequency resolution, good spectral purity, and tunability over a wide range with a small amount of circuitry. The digital-to-analog converter  402  may be adapted to convert the digital test tone signal received from the direct digital frequency synthesizer  404  to an analog signal. The low pass filter  406  may be adapted to receive the analog signal from the DAC  402  and remove the DAC image to generate a smooth signal to the summer  410 . The loop filter  412 , the voltage controlled oscillator  414  and the frequency divider  408  may be a part of a traditional phase locked loop PLL. The frequency divider  408  may be adapted to divide an incoming frequency by a suitable number N. The summer  410  may be adapted to receive a plurality of inputs from the low pass filter  406  and the frequency divider  408  and generate an output to the loop filter  412 . The voltage controlled oscillator  414  may be adapted to receive a signal from the loop filter  412  and generate an output test tone signal to the summer  304  [ FIG. 3 ]. The phase locked loop may be adapted to multiply the frequency generated by the direct digital frequency synthesizer  404  up to a RF frequency, which may be the test tone output signal.  
      In operation, the direct digital frequency synthesizer  404  may be adapted to receive an input frequency command signal and generate a low frequency test tone signal to the digital-to-analog converter DAC  402 . The digital-to-analog converter DAC  402  may be adapted to receive the low frequency digital test tone signal and convert it to an analog signal and transmit it to the low pass filter  406 . The low pass filter  406  may be adapted to receive the analog signal from the DAC  402  and remove the DAC image to generate a smooth signal to the summer  410 . The summer  410  may be adapted to receive a plurality of input signals from the low pass filter  406  and the frequency divider  408  and generate an output to the loop filter  412 . The loop filter  412  may be adapted to receive an input signal from the summer  410  and generate an output signal to the voltage controlled oscillator  414 . The voltage controlled oscillator  414  may be adapted to receive a signal from the loop filter  412  and generate an output test tone signal to the summer  304  [ FIG. 3 ] and the frequency divider  408 .  
       FIG. 5   a  is a block diagram of a double conversion tuner with quadrature correction and a complex mixer having separate I and Q components, in accordance with an embodiment of the invention. Referring to  FIG. 5 , there is shown an amplifier  502 , a summer  504 , a test tone generator  506 , a plurality of mixers  508  and  510 , a plurality of band pass filters  516  and  518 , a plurality of voltage controlled oscillators  514  and  520 , a complex mixer  522 , a phase splitter  512  and a quadrature correction block  524 .  
      The amplifier  502  may be adapted to receive an input signal and may generate an output signal to the summer  504 . The summer  504  may be adapted to receive a plurality of inputs from the amplifier  502  and the test tone generator  506  and generate an output signal that may be input to the plurality of mixers  508  and  510 . The test tone generator  506  may comprise suitable logic and/or circuitry that may be adapted to generate an RF signal slightly different in frequency from the desired input signal that may be utilized to adjust the estimated quadrature error in the I channel and the Q channel of the receiver. The mixers  508  and  510  may be adapted to downconvert the analog RF substreams to baseband. The phase splitter  512  may be adapted to ensure that the mixer local oscillator inputs are in quadrature, that is, they are 90 degrees out of phase with respect to each other.  
      The voltage controlled oscillator  514  may be adapted to drive the mixer local oscillator inputs and the phase splitter  512 . The plurality of band pass filters  516  and  518  may be adapted to filter the signals and may be adapted to allow only the test tone signals generated by the test tone generator  506 . The complex mixer  522  may be driven by the voltage controlled oscillator  520  and may receive the I and Q channel inputs from the band pass filters  516  and  518  respectively and generate a plurality of outputs to the quadrature correction block  524 . The quadrature correction block  524  may be adapted to receive the I and Q channel inputs that may comprise a combination of a desired signal and a test tone signal. The test tone signals are almost in quadrature with respect to the desired signal and have a high signal to noise ratio, which may increase the bandwidth and the convergence rate of the feedback loop. The down converter mixers  508  and  510  may be adapted to downconvert the analog RF substreams to a first IF frequency. The complex mixer  522  may be adapted to downconvert the first IF frequency to a baseband frequency. The second mixer may be a part of a carrier tracking loop, which may be adapted to remove residual phase and frequency errors.  
       FIG. 5   b  is a block diagram of a double conversion tuner with quadrature correction, where the test tone signals are injected at the first IF frequency, in accordance with an embodiment of the invention. Referring to  FIG. 5   b,  there is shown a plurality of amplifiers  550  and  558 , a plurality of mixers  552 ,  564  and  566 , a plurality of local oscillators  554  and  570 , a band pass filter  556 , a summer  560 , a test tone generator  562 , a phase splitter  568  and a quadrature correction block  572 .  
      The amplifier  550  may be adapted to receive an input signal and may generate an output signal to the mixer  552 . The voltage-controlled oscillator  554  may be adapted to drive the mixer  552 . The mixer  552  may receive a plurality of inputs from the amplifier  550  and the voltage-controlled oscillator  554  and generate an output to the band pass filter  556 . The band pass filter  556  may be adapted to receive an input signal from the mixer  552  and generate an output signal to the amplifier  558 . The amplifier  558  may be adapted to receive an input signal from the band pass filter  556  and may generate an output signal to the summer  560 . The summer  560  may be adapted to receive a plurality of inputs from the amplifier  558  and the test tone generator  562  and generate an output signal that may be input to the plurality of mixers  564  and  566 . The test tone generator  562  may comprise suitable logic and/or circuitry that may be adapted to generate an RF signal slightly different in frequency from the desired input signal that may be utilized to adjust the estimated quadrature error in the I channel and the Q channel of the receiver. The mixers  564  and  566  may be adapted to downconvert the analog RF substreams to baseband and generate a plurality of output signals to the quadrature correction block  572 . The phase splitter  568  may be adapted to ensure that the mixer local oscillator inputs are in quadrature, that is, they are 90 degrees out of phase with respect to each other.  
      The voltage-controlled oscillator  570  may be adapted to drive the mixer local oscillator inputs and the phase splitter  568 . The quadrature correction block  572  may be adapted to receive the I and Q channel inputs that may comprise a combination of a desired signal and a test tone signal. The test tone signals are almost in quadrature with respect to the desired signal and have a high signal to noise ratio, which may increase the bandwidth and the convergence rate of the feedback loop. The down converter mixer  552  may be adapted to down convert the analog RF substream to a first IF frequency. The down converter mixers  564  and  566  may be adapted to upconvert the first IF frequency to a second IF frequency. By utilizing a double conversion architecture, the image channel interference may be significantly suppressed.  
      The test tone generator  562  may comprise suitable logic and/or circuitry that may be adapted to generate a test tone signal at the first IF frequency, which may be within a narrow frequency range. The first IF frequency may be 50 MHz, for example, while the input signal to the amplifier  550  may be a broadband signal in the frequency range of 50 MHz to 1 GHz, for example. As a result, the test tone generator  562 , the plurality of mixers  564  and  566  may be adapted to operate at a narrower frequency range reducing hardware complexity and simplifying the generation of accurate quadrature balanced I and Q channels.  
       FIG. 6  is a flowchart illustrating exemplary steps for enhancing image rejection in communications receivers, in accordance with an embodiment of the invention. Referring to  FIG. 6 , there is shown, after start step  602 , in step  604 , a test tone signal may be generated by a direct digital frequency synthesizer. In step  606 , the generated test tone signal from the direct digital frequency synthesizer may be converted into an analog signal by a digital-to-analog converter DAC. In step  608 , the generated test tone signal may be injected into a receiver. In step  610 , it may be checked if the injected test tone signal to the receiver is at first IF frequency. If the injected test tone signal is not at first frequency, in step  612 , the test tone signal may be down converted to the first IF frequency from the RF substream and then in step  614 , the I and Q channels may be filtered by low pass filters to allow only the test tone signals to pass through. If the injected test tone signals are injected at the first IF frequency, the mixers and the test tone generator may be adapted to operate at a narrower frequency range reducing hardware complexity and simplifying the generation of accurate quadrature balanced I and Q channels. In step  616 , a quadrature error in the I channel and the Q channel of the receiver may be estimated based on the injecting of the test tone signal into the receiver. In step  618 , a plurality of equalizer coefficients may be adjusted to correct the estimated quadrature error in the I channel and the Q channel of the receiver. In step  620 , a corrected I channel and a corrected Q channel may be generated corresponding to the I channel and the Q channel in the receiver based on the adjusting of the equalizer coefficients.  
      Another embodiment of the invention provides a system for enhancing image rejection in communications receivers. A test tone generator  306  [ FIG. 3 ] may be adapted to inject a test tone signal into a receiver. Circuitry may be adapted to estimate a quadrature error in an in-phase (I) channel and a quadrature (Q) channel of the receiver based on the injecting of the test tone signal into the receiver. Circuitry may be adapted to adjust a plurality of equalizer coefficients to correct the estimated quadrature error in the I channel and the Q channel of the receiver. The system may comprise circuitry that may be adapted to generate a corrected I channel and a corrected Q channel corresponding to the I channel and the Q channel in the receiver based on the adjusting of the equalizer coefficients.  
      A direct digital frequency synthesizer  404  [ FIG. 4 ] may be adapted to generate the test tone signal and a digital-to-analog converter DAC  402  may be adapted to convert the generated test tone signal from the direct digital frequency synthesizer  404  into an analog signal. The test tone generator  306  [ FIG. 3 ] may be adapted to inject the test tone signal into the I channel of the receiver and/or the Q channel of the receiver. For example, the test tone generator  306  may also be adapted to inject the test tone signal at any IF frequency. The test tone generator may be adapted to inject the test tone signal at a first IF frequency having a narrow bandwidth to improve the quadrature accuracy. Circuitry may be adapted to correct an amplitude error in the I channel and the Q channel of the receiver. Circuitry may also be adapted to correct a phase error in the I channel and the Q channel of the receiver. A low pass filter  316  [ FIG. 3 ] and  318  may be adapted to filter the I channel and the Q channel of the receiver respectively, to allow the injected test tone signal. The injected test tone signal may have a high signal to noise ratio.  
      Accordingly, the present invention may be realized in hardware, software, or a combination of hardware and software. The present invention may be realized in a centralized fashion in at least one computer system, or in a distributed fashion where different elements are spread across several interconnected computer systems. Any kind of computer system or other apparatus adapted for carrying out the methods described herein is suited. A typical combination of hardware and software may be a general-purpose computer system with a computer program that, when being loaded and executed, controls the computer system such that it carries out the methods described herein.  
      The present invention may also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods described herein, and which when loaded in a computer system is able to carry out these methods. Computer program in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation; b) reproduction in a different material form.  
      While the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims.