Abstract:
A technique includes providing a plurality of local oscillator signals such that each of the local oscillator signals has a different phase. The technique includes providing scaling units to scale the input signal pursuant to different scaling factors to generate scaled input signals. The scaling factors are selected on a periodic function of the phases. The technique also includes providing mixing circuits to mix the local oscillator signals with the scaled input signals to generate mixed signals and providing an adder to combine the mixed signals to generate an output signal.

Description:
BACKGROUND 
   The invention generally relates to a harmonic rejection mixer. 
   A conventional wireless receiver system may include at least one mixer, for purposes of downconverting the frequency of the incoming wireless signal. More specifically, the mixer typically multiplies the incoming wireless signal with a local oscillator signal to produce a signal, which has spectral energy that is distributed at sums and differences of the local oscillator and incoming signals&#39; frequencies. If the local oscillator signal is a pure sinusoid that has its spectral energy concentrated at a fundamental frequency, then ideally, it is relatively easy to filter out unwanted spectral energy so that the spectral energy of the filtered signal is generally located at the desired intermediate frequency. However, for certain mixing applications, the local oscillator signal may be a non-sinusoidal, such as a square wave signal, which contains spectral energy that is located at a fundamental frequency and additional spectral energy that is located at harmonic frequencies. Mixing the incoming signal with such a local oscillator signal typically results in undesired spectral energy being located close enough to the desired spectral energy to make the undesired spectral energy relatively difficult to remove by filtering. 
   Thus, there exists a continuing need for a mixer that rejects harmonic frequencies that may be introduced by a local oscillator signal that is not a pure sinusoid. 
   SUMMARY 
   In an embodiment of the invention, a technique includes providing a plurality of local oscillator signals such that each of the local oscillator signals has a different phase. The technique includes providing scaling units to scale the input signal pursuant to different scaling factors to generate scaled input signals. The scaling factors are selected on a periodic function of the phases. The technique also includes providing mixing circuits to mix the local oscillator signals with the scaled input signals to generate mixed signals and providing an adder to combine the mixed signals to generate an output signal. 
   Advantages and other features of the invention will become apparent from the following drawing, description and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a schematic diagram of a wireless receiver system. 
       FIG. 2  is a waveform of a square wave signal. 
       FIG. 3  is an illustration of spectral content of an input signal to a mixer of  FIG. 1 . 
       FIG. 4  is an illustration of spectral energy of a local oscillator signal used by the mixer of  FIG. 1 . 
       FIG. 5  is an illustration of spectral energy of an output signal provided by the mixer of  FIG. 1 . 
       FIG. 6  is a schematic diagram of a mixer according to an embodiment of the invention. 
       FIGS. 7 ,  8  and  9  illustrate local oscillator signals received by the mixer according to an embodiment of the invention. 
       FIG. 10  depicts a circuit level implementation of the mixer according to an embodiment of the invention. 
       FIG. 11  is a table depicting harmonics rejected by the mixer for different implementations of the mixer according to embodiments of the invention. 
       FIG. 12  is a schematic diagram of a mixer according to another embodiment of the invention. 
       FIG. 13  is a schematic diagram of a wireless receiver system according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a receiver system  10  may include a mixer  26  that frequency translates an incoming signal (called “x(t)”) to produce a frequency translated signal (called “z(t)”) by multiplying the x(t) signal with a local oscillator signal (called “y(t)”). As an example, the x(t) signal may be a modulated signal that is provided by an amplifier  24 , in response to a signal (an AM or FM signal, for example) that is received from an antenna  22 . Due to the frequency translation by the mixer  26 , the receiver system  10  may further process the z(t) signal to remove unwanted spectral energy, such as processing that includes passing the z(t) signal through a lowpass filter (LPF)  28  to recover for purposes of producing an audio signal that may be played over a speaker  30 . A particular challenge may arise if the y(t) local oscillator signal is a square wave, which has spectral energy that is located at fundamental and harmonic frequencies. 
   More particularly, referring to  FIG. 2  in conjunction with  FIG. 1 , the y(t) signal may be a square wave signal that has a fundamental frequency (called “f LO ”) and harmonic frequencies, which introduce undesirable spectral energy in the z(t) signal. To illustrate this problem,  FIG. 3  depicts the spectral content of the x(t) signal, where the x(t) signal is assumed to be of the following form:
 
 x ( t )= a ·cos(ω RFt ),   Eq. 1
 
where “ω RF ” represents a radian radio frequency (RF) (2π·f Rf ). The spectral content of the x(t) signal for this example is depicted in  FIG. 3 . As shown, the spectral content includes components  52  and  50  that are located at positive and negative RF frequencies, respectively.
 
   Referring also to  FIG. 4 , for this example, the y(t) signal, being a square wave signal, has spectral components  60  that are located not only at the fundamental frequency, ω LO , but are also located at odd harmonic frequencies ω LO . Similarly, the y(t) signal has spectral components  64 , which are located at the negative ω LO  fundamental frequency and odd harmonics thereof. 
   As a result of the harmonics that are present in the y(t) signal, the resultant z(t) signal has undesired spectral components  84  and  88 , which are depicted in  FIG. 5 . More specifically, the multiplication of the y(t) and x(t) signals by the mixer  26  produces desired spectral energy  80 , due to the fundamental frequency component of the y(t) signal and also produces the unwanted spectral components  84  and  88  due to the harmonics of the y(t) signal. The spectral components  84  and  88  may be relatively difficult to remove from the z(t) signal. 
   To overcome the problems that are set forth above for a square wave or other non-pure sinusoid local oscillator signal,  FIG. 6  depicts a harmonic rejection mixer  100  in accordance with embodiments of the invention. The mixer  100  includes N mixers  104  (mixers  104   0 ,  104   1  . . .  104   N−1 , being depicted as examples in  FIG. 6 ), each of which multiplies a scaled version of the x(t) signal with a square wave local oscillator signal. More specifically, each of the mixers  104 , in accordance with embodiments of the invention described herein, multiplies a scaled version of the x(t) input signal with a square wave oscillator signal that has a different phase. 
   Referring also to  FIGS. 7 ,  8  and  9 , the square wave oscillator signals include a square signal (y(t)) ( FIG. 7  ) that has a phase of zero and other square wave signals (such as exemplary square wave signals called 
           “     y   ⁡     (     t   -       T   0     N       )       ”         
( FIG. 8 ) and
 
           “     y   ⁡     (     t   -         N   -   1     N     ⁢     T   0         )       ”         
( FIG. 9 )). More particularly, the mixer  104   0  receives the local oscillator signal y(t), which has a phase of zero, and each of the other mixers  104   1  . . .  104   N−1  receives a phase shifted version of the y(t) signal. The output signals that are produced by the mixers  104  are combined by an adder  105  to produce the z(t) signal.
 
   Each of the mixers  104  receives a different scaled version of the x(t) signal. In this regard, the mixer  100  includes scaling units, or amplifiers  103 , each of which is associated with a different one of the mixers  104 . Each amplifier  103  scales the x(t) signal by a different factor, or degree, to produce the resultant scaled signal that is provided to the associated mixer  104 . More specifically, the amplifier  103  for the mixer  104   0  multiples the x(t) by a coefficient called “a 0 ,” to produce a signal that is provided to the mixers  104   0 , the amplifier  103  multiplies the x(t) signal by a coefficient called “a 1 ” to produce a signal that is provided to the mixer  104   1 , etc. As described further below, the coefficients a 0 , a 1  . . . a N−1  are selected to cancel harmonics in the z(t) signal. 
   The Fourier transform of the z(t) signal may be described as follows:
 
 Z ( j ω)= Y ( j ω)·α(ω),   Eq. 2
 
where “Y(jω)” represents the Fourier transform of the square wave signal y(t), and “α(ω)” represents a scaling factor in the frequency domain, which varies with frequency, as described below:
 
   
     
       
         
           
             
               
                 
                   
                     α 
                     ⁡ 
                     
                       ( 
                       ω 
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         k 
                         = 
                         0 
                       
                       
                         N 
                         - 
                         1 
                       
                     
                     ⁢ 
                     
                       
                         a 
                         k 
                       
                       ⁢ 
                       e 
                       ⁢ 
                       
                         
                           
                             - 
                             jω 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           kT 
                         
                         N 
                       
                     
                   
                 
                 , 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 3 
               
             
           
         
       
     
   
   By choosing a k  (wherein “k” is 0 to N−1) to be equal to a sinusoid that is function of the square wave phase, nulls are created in the spectral frequency of the z(t) signal due to the α(ω) scaling factor becoming zero at certain frequencies. More specifically, in accordance with some embodiments of the invention, the a k  coefficients are selected based on the following periodic function of the square wave phase: 
   
     
       
         
           
             
               
                 
                   a 
                   k 
                 
                 = 
                 
                   
                     sin 
                     ⁡ 
                     
                       ( 
                       
                         
                           
                             2 
                             ⁢ 
                             π 
                           
                           N 
                         
                         ⁢ 
                         k 
                       
                       ) 
                     
                   
                   . 
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 4 
               
             
           
         
       
     
   
   The choice of N (the number of mixers  104 ) determines the harmonics that are cancelled by the mixer  100  (i.e., the frequency at which nulls occur). 
   If N is an odd, problems may arise when the duty cycle of the y(t) square wave signal is not exactly 50 percent. Therefore, in accordance with some embodiments of the invention, N is chosen to be even. With this selection, a the number of harmonics increases with N. 
   In this regard,  FIG. 11  depicts a table  200 , which illustrates a relationship between N (in column  202 ) and the harmonics rejected (in column  204 ). As shown, for N equal to four, all even harmonics of the z(t) signals are rejected, for N equal to six, all even and the third harmonics are rejected. For N equal to eight, all even, third and fifth harmonics are rejected. Lastly, as depicted in table  200 , for N equal to ten, all even, third, fifth and seventh harmonics are rejected. 
   As a more specific example,  FIG. 10  depicts a mixer  150  in accordance with embodiments of the invention. In particular, the mixer  150  implements paths that scale and frequency translate the x(t) signal, similar to the paths that are depicted in the mixer  100  of  FIG. 6 . Each of the paths include a current scaling transistor  180  (an n-channel metal oxide-semiconductor field-effect-transistor (NMOSFET), for example) and a square wave switching pair  170 . The switching pair  170  connects the drain of the transistor  180  to either a positive output node  190  or a negative output node  192 , depending on the plurality of the received square wave local oscillator signal. Because all of the switching pairs  174  are connected to the output terminals  190  and  192 , the currents that are provided to the nodes  190  and  192  from the switching pairs  170  are summed to provide the collective z(t) output signal. As shown in  FIG. 10 , resistors  194  and  196  may be coupled between the nodes  190  and  192 , respectively, and ground. 
   In accordance with some embodiments of the invention, the scaling for each path is provided by the current scaling transistor  180 . In this regard, the transistors  180  have aspect ratios that are scaled with respect to each other to establish the different a k  values. As shown by way of specific example in  FIG. 10 , the a k  values may be different values obtained from the sinusoidal function (see Eq. 4) for the particular square wave phase. 
   The mixers that are described herein may be used in a variety of applications, including applications in which orthogonal signals are processed. In this regard, in accordance with some embodiments of the invention, the techniques and systems that are described herein may be applied to a mixer  250 , which is depicted in  FIG. 12 . The mixer  250  frequency translates the incoming x(t) signal to produce two orthogonal signals: an in-phase signal (called “I(t)”) and a quadrature signal (called “Q(t)”). The mixer  250  includes an in-phase mixer  254  that has a similar design to the mixers  100  and  150  described above. In this regard, the mixer  254  receives a set of phase-shifted square wave signals and provides the I(t) in-phase signal. 
   The mixer  250  also includes a mixer  256  that provides the Q(t) quadrature signal and receives the same set of phase-shifted square wave signals as the mixer  254 . Unlike the mixer  254 , the mixer  256  has a k  coefficients that are derived from a cosine function of the square wave phase (instead of a sine function), as set forth below: 
   
     
       
         
           
             
               
                 
                   a 
                   k 
                 
                 = 
                 
                   
                     cos 
                     ⁡ 
                     
                       ( 
                       
                         
                           
                             2 
                             ⁢ 
                             π 
                           
                           N 
                         
                         ⁢ 
                         k 
                       
                       ) 
                     
                   
                   . 
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 5 
               
             
           
         
       
     
   
   Referring to  FIG. 13 , as an example of a possible application of the mixers described herein, the mixers  100  and  250  may be used in a wireless system  300 . In this regard, the wireless system  300  may include, for example, an FM receive path  310  that includes the mixer  250  and may also include an AM receive path  320  that includes the mixer  100 . In this regard, the FM  310  and AM  320  receive paths that may be part of a semiconductor package  350  that provides either an FM signal or an AM signal to an amplifier  330  that drives a speaker  370 . Thus, a switch  324  may, in an FM receive mode of the package  350  couple the input terminal of the amplifier  330  to the output terminal of the FM receive path  310 ; and in an AM receive mode of the package  350 , the switch  328  may alternatively connect the output terminal of the AM receive path  320  to the input terminal of the amplifier  330 . Among its features, the wireless system  300  may include antennae  360  and  364  that are coupled to the FM  310  and AM  320  receive paths, respectively. In some embodiments of the invention, the semiconductor package  350  may also include an FM transmitter, which may be enabled or disabled, depending on the particular application in which the package  350  is used. In other embodiments of the invention, the FM  310  and AM  320  receive paths may be formed on the same die, may be formed on separate dies, and may be parts of separate semiconductor packages. Thus, many variations are possible and are within the scope of the appended claims. 
   While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.