Abstract:
An RF receiver image rejection scheme. The RF is received and mixed in two quadrature channels allowing separation of the undesired image portion within the RF signal from the desired portion. The two channels can be summed to allow the image portions to cancel out and form a signal which is predominantly based on the desired portion. Another sum of the two channels can also be made to provide a signal which is primarily based on the image portion. Since there are some components of the image portion even in the compensated desired signal, that signal indicative of the image portion is used to compensate for that undesired portion.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 10/891,672, filed Jul. 15, 2004. The entire disclosure of U.S. patent application Ser. No. 10/891,672 is incorporated herein by reference. 
    
    
     BACKGROUND 
     Reception of a low level RF signal often benefits from low noise and high precision operation. Noise and imprecision in such circuits has many different origins. 
     One common undesired signal is called an image frequency. The image signal is symmetrical to the desired frequency, relative to the local oscillator frequency. 
     The undesired component of the image frequency may cause image frequency interference, and this may produce undesirable content in the eventual received signal. Image frequency interference may reduce the amount of information that the channel can carry. 
     Different techniques have been used to reduce the effect of the image frequency interference. 
     SUMMARY 
     The present system teaches a new technique for reducing the effect of image frequency interference. One aspect defines a digital filter for a receiver. The filter can be digital or analog. The filter operates to form a first signal that mostly indicates the desired signal, and a second signal that mostly indicates the image signal, and uses the second signal to compensate the first signal. 
     A specific aspect describes a mixer, that has an in-phase mixer branch, forming an in-phase version of a complex signal, and a quadrature mixer branch maintaining a quadrature version of the same said complex signal, one of said branches including a phase rotator which rotates a first portion of said signal to relative to a second portion of said signal, wherein one of said first and second portions represents a desired portion of the signal and the other of said first and second portions represents an image portion of the signal; and first and second adders, where the first adder obtains a difference between said in-phase and quadrature signals to obtain a first signal which is primarily based on said first portion, and the second adder obtains a sum between said in-phase and quadrature signals to obtain a second signal which is primarily based on the second portion. A compensation part which uses one of said first and second signals to compensate the other of said first and second signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects will now be described in detail with reference to the accompanying drawings, wherein: 
         FIG. 1  shows a block diagram of a typical superheterodyne receiver using quadrature mixing; 
         FIGS. 2A-2C  show exemplary frequency domain transformations which occur in the  FIG. 1  device; 
         FIG. 3  shows a block diagram of a direct conversion receiver; 
         FIG. 4  shows a block diagram of a low IF conversion receiver; 
         FIG. 5  shows a block diagram of an image rejection quadrature mixer; 
         FIG. 6A  shows a block diagram of an improved image rejection mixer with additional compensation signals; 
         FIG. 6B  shows a digital filter which can carry out the function of the improved image rejection mixer in  FIG. 6A ; 
         FIGS. 7A and 7B  show an analog filter that operates using similar techniques to that described relative to  FIGS. 6A and 6B ; and 
         FIG. 8  shows a block diagram of an exemplary adaptive compensation device. 
     
    
    
     DETAILED DESCRIPTION 
     A number of different receiver topologies may be used to reduce the effect of the image signal. 
       FIG. 1  shows a block diagram of a first receiver. An RF signal  100  is received by an antenna  105 . The signal is filtered by an RF filter  110 , which may be, for example, a filter which allows the desired channel to pass. The filtered signal  111  is amplified by a low noise amplifier  115 . The amplified signal  116  is then mixed in a mixer  120  with a local oscillator frequency from source  125 . The output  121  of the mixer is applied to an IF filter  125  which may be a surface acoustic wave “SAW” device. The output  126  of the SAW filter is then mixed with sine and cosine components, to form in-phase  131  and quadrature  132  signals, in a second mixer  130 . Each of the in-phase  131  and quadrature  132  signals are low pass filtered and converted to digital by respective LPF and ADC blocks  135 ,  136 . The resultant signals are digital baseband signals  137 . These signals are processed by a digital baseband processor  140 . 
     An exemplary frequency domain version of the signals is shown in  FIGS. 2A-2C . 
       FIG. 2A  shows the output  116  of the low noise amplifier  115 . This includes both a desired signal f c (signal  201 ) and its mirror image signal f image (signal  203 ), which is symmetrical to f c  relative to the local oscillator frequency f LO (signal  202 ). 
       FIG. 2B  shows how the output signal  126  from IF filter includes a combination of the desired signal f c , and the image signal f image . 
       FIG. 2C  shows the resultant baseband-converted output signal  137 , showing that this is a combination of the desired signal f c  and the image signal f image . It may be desirable to reject the image frequency to eliminate the image component from the final signal. A low pass filter may also be used to filter out adjacent channel components in  FIG. 2A . 
     In certain receivers, it may be desirable to carry out more of the processing in digital, to provide the advantages of digital electronics including improved noise rejection. A block diagram of a direct conversion receiver is shown in  FIG. 3 . This includes the LNA  115  as in  FIG. 1 , but no IF stages. Instead the RF signal output  305  of the LNA is directly converted to in-phase and quadrature signals in mixer  310 . These in phase and quadrature signals are each applied to respective filter and A/D converter blocks  315 ,  316 , and are directly filtered and converted from analog to digital. No SAW filter is used in this circuit. Also, since there is no IF, no image signal is formed. 
     Circuit related low frequency noise and offsets exist in the direct conversion signal, e.g., based on noise and nonlinearities and/or by RF leakage added by the circuit components. Because the signal remains at relatively low frequency, it may be very difficult to distinguish the low frequency noise from the desired signal itself  FIG. 4  shows an alternative system in which low IF conversion is used. The low IF converter  400  may use similar circuitry to that described in  FIG. 1 , but uses a lowered frequency f LO . This lower frequency allows the signal to be digitized directly after the IF stage. The function of the second mixer is therefore done within the baseband processor  410 , in the digital domain. Since this system uses a signal spectrum that does not overlap the low frequency noise, the low frequency noise can be separated from the remaining parts of the signal, using a filter. 
     The IF frequency f I  needs to be sufficiently low that an A/D converter can be used to digitize the IF signal. As an example, for a 2.4 GHz channel, a typical IF signal would be in range of hundreds of megahertz. A low IF signal, in contrast, would be around 40 MHz, hence reduced by approximately a decade. While this circuit allows certain processing to be done in the digital domain, the image frequency f image  will be close to the local oscillator frequency and to the desired frequency. The value f I  in  FIG. 2A , representing the spacing between f LO  and f image , gets smaller. As f I  gets small, it becomes more difficult to filter the image frequency from the desired frequency. 
     An image rejection mixer may be used to reduce the image signal using a complex signal representation to distinguish the image signal component from the desired signal component. An image rejection mixer is shown in  FIG. 5 . The input RF shown as  500  is mixed with an in-phase mixing signal  502  to form an in-phase branch, and mixed with a quadrature signal  504  to form a quadrature branch  506 . Each of the branches are then filtered by a bandpass filter  508  and digitized by an A/D converter  510 . The quadrature branch is also rotated by 90° by a phase rotation element T, element  512 . The phase rotation element can be a hardware device, which causes a 90 degree phase clockwise inversion. This forms two branches, where the first branch has an output signal
 
 T ( T ( F   desired )+ T   −1 ( F*   image ))
 
where T represents a 90° clockwise rotation,
 
=− F   desired   F*   image  
 
Where F* image  represents the conjugate of the signal, and the second branch has a signal
 
 F   desired   +F*   image  
 
The two branches are digitally summed at  514  to produce an “IF signal” that includes summed desired frequencies from both branches and cancelled image frequencies from both branches.
 
 IF= 2 F   desired   +O*F   image   (1)
 
     Ideally, this system will reject the entire image signal (F image ). However, imperfections in the mixers, the rotator and the channels, will cause distortion. Magnitude and phase mismatches between the I and Q mixers will cause the I and Q channels to have slightly different signal handling characteristics, leading to distortion and crosstalk. Therefore, while equation (1) represents the output for a perfect channel, the actual IF output signal can be expressed as
 
 IF:{circumflex over (F)}   desired   =W   distort   *F   desired   +W   cross   *F   image  
 
Where F^represents the actual value of F desired . Defining this in terms of inverse of channel distortion (a number close to 1) gives
 
 IF:W   distort   −1   *{circumflex over (F)}   desired   =F   desired   +W   distort   −1   *W   cross   *F   image  
 
Compensating for mismatch between the mixers can improve the performance. However, it is often not practical to remove all mismatch between the I and Q channels. Rejection of 20-30 db is typical.
 
       FIG. 6A  shows an alternative image rejection filter which can reduce errors caused by distortion and crosstalk. The filter in  FIG. 5  subtracts signals to form a signal that is primarily the desired signal, but has image components also, in amounts based on imperfections in the circuit. In  FIG. 6 , an additional adder  601  adds the I and Q signals. The output of the adder is an additive signal, which is primarily image signal, but also includes components of the desired signal, in amounts based on circuit imperfections. 
     The output from the first summer  514  (I-Q) is the signal, as above
 
 IF   desired   =W 1 distort   F   desired   +W 1 cross   *F*   image   (3)
 
where W1 cross →0, and W1 distort →1. Hence, IF desired  represents mostly the desired signal.
 
     The additional IF image signal from adder  601  is called IF image , and corresponds to the signal
 
 {circumflex over (F)}   image   =IF   image   =W 2 distort   F*   image   +W 2 cross   *F   desired   (4)
 
where W2 distort →1 and W2 cross →0. This signal is mostly dominated by the image component. W1 distort ; W1 cross , W2 distort  and W2 cross  can be estimated, or adaptively or otherwise determined. The value of IF image  from equation (4) is then used, along with values of at least W1, in equation (3) to solve for a compensated version of IF desired, as:
 
               IF   desired     =         F   ^     desired     ≈       IF   desired     -         W   ⁢           ⁢     1   cross         W   ⁢           ⁢     2   distort         *     IF   image                 
This compensated signal removes more of the undesired components.
 
       FIG. 6B  shows this operation being done in the digital domain using an inverse matrix filter, having the transfer function: 
     
       
         
           
             H 
             = 
             
               
                 [ 
                 
                   
                     
                       
                         W 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           1 
                           distort 
                         
                       
                     
                     
                       
                         W 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           1 
                           cross 
                         
                       
                     
                   
                   
                     
                       
                         W 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           2 
                           cross 
                         
                       
                     
                     
                       
                         W 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           2 
                           distort 
                         
                       
                     
                   
                 
                 ] 
               
               
                 - 
                 1 
               
             
           
         
       
     
     A channel characteristic storage device  710  may be used to determine characteristics of the channel. The uncompensated values F desired  and F image  are input to the filter, and the transformation is used to {circumflex over (F)} desired  estimate and {circumflex over (F)} image  using equations (3) and (4), and channel characteristics. 
     The characteristics of the channel can be found using any of a number of different techniques. A first open loop technique can be used in which the characteristics of the channel are found e.g. by calibration, and stored into the channel characteristic storage device  710 . 
     A different open loop technique can store a table with a number of different W1 and W2 values, and select the best set of W1, W2 values on power up. 
     Alternatively, an adaptive compensation scheme can be used as shown with reference to  FIG. 8 .  FIG. 8  shows the desired frequency being input to a slicer  800  which is a two-dimensional slicer that determines an error signal  802  between the actual signal received, and a decision made about the signal. The error signal  802  is fed back as part of the desired frequency, and also input to an adaptive compensator  810 . The adaptive compensator uses a least mean squares approach to determine the channel characteristics from the calculated values. 
     The signals are taken as corresponding with the signals as follows: 
                 F   ~     desired     =         [     H   ⁢           ⁢   11   ⁢           ⁢   H   ⁢           ⁢   12     ]     *       [             F   ^     desired                 F   ^     image           ]     ⁢     
     ⁢           [           H   ⁢           ⁢   11   ⁢     (     k   +   1     )                 H   ⁢           ⁢   12   ⁢     (     k   +   1     )             ]       =             [           H   ⁢           ⁢   11   ⁢     (   k   )                 H   ⁢           ⁢   12   ⁢     (   k   )             ]     ⁢           -           ⁢     μ   ×     e   ⁡     (   k   )       ×     [               F   ^     desired   *     ⁢           ⁢     (   k   )                     F   ^     image   *     ⁡     (   k   )             ]                   
where “*” denotes conjugate
 
e(k) is the estimated error which is
 
 e ( k )= {tilde over (F)}   desired −(decision on  {tilde over (F)}   desired )
 
and feeds the error signal into that equation to find the channel characteristics.
 
     In the equation above, H(k+1) is the adaptive compensation values for a current time, H(k) is the adaptive compensation values for a previous time, e(k) is the error signal in that previous time, μ is a scaling factor used to prevent overcompensation during any specific time interval, and is the actual received values for that previous time. The adaptive compensation scheme therefore adaptively determines the channel values and allows them to settle towards the proper values at each specific time. 
     No matter how determined, characteristics of the channel which are stored in the store  710 . 
       FIGS. 6A and 6B  show how this filtering can be done in a digital filter.  FIGS. 7A and 7B  show an alternative configuration in which the filtering is done in the analog domain. In  FIG. 7A , the bandpass filter  508  feeds its output signal  714  directly to the clockwise rotator device  512 . Similarly, in the quadrature branch, the output of the band pass filter  716  remains in the analog domain. Therefore, in  FIG. 7A , the adder  750  is actually an analog adder, which simply sums the voltages at the nodes, while the subtracter  752  is an analog subtracter that subtracts the voltages applied to the nodes. The output of the adder, representing IF desired , is coupled to an A/D converter  755  which produces a digital output signal  756 . Analogously, the IF image  signal is coupled to an A/D converter  760  that produces an output signal  761 . In figure  7 B, the filter  770  is actually an analog filter that operates based on analog signals, and both F desired  and F image  are output to respective A/D converters  772 ,  774 . 
     The techniques described above have described one form of compensation, but it should be understood that the same techniques can be used to carry out other compensation, using even more compensation. For example, a recursive solution of equations 3 and 4 can be carried out. 
     The filter described above can be embodied in a number of different ways. For example, it may be preferred to embody this filter as part of an integrated circuit on a single piece of silicon, where one or many circuits may be formed on a single silicon substrate, and other digital components used for the communication may also be formed on the substrate. In addition, however, this may be embodied as discrete components, e.g. defined using hardware definition language, or by a suitably programmed digital signal processor, or in software executed by a general purpose processor. The processor may filter the signal according to the filter transfer function shown in  FIG. 7 . The processor may also be configured to simulate the results of the filter, e.g., as part of a simulation program such as Matlab™. 
     In addition, other modifications are possible. For example, while the above describes one way of using this system as part of an image rejection mixer, it should be understood that this system can analogously be used for other kinds of noise rejection. Moreover, while this describes the compensation being done in the digital domain, it should be understood that this could also be done in the analog domain. 
     All such modifications are intended to be encompassed within the following claims.