Abstract:
A mixer includes a transduction circuit, a first and a second switch circuit, and a first and a second load circuit. The transconductor circuit is for generating a differential current signal according to a differential voltage signal. The first switch circuit and the first load circuit are connected in series, and the first switch circuit is used to regulate the differential current signal in response to a first oscillator signal. The second switch circuit and a second load circuit are connected in series, and the second switch circuit is used to regulate the differential current signal in response to a second oscillator signal. The first load circuit and the second load circuit are connected at a common node to reduce harmonic interferences.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This patent application is based on Taiwan, R.O.C. patent application No. 98106350 filed on Feb. 27, 2009. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a mixer, and more particularly, to a mixer capable of improving signal quality. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a wireless transmitter or a wireless receiver, a mixer is widely used as a frequency conversion device.  FIG. 1  shows a conventional wireless transmitter  10  capable of converting a base-band transmitting signal to a radio frequency (RF) transmitting signal to be transmitted via an antenna. The wireless transmitter  10  comprises filters  11  and  12 , programmable gain amplifiers  13  and  14 , mixers  15  and  16 , and a power amplifier  17 . After removing needless frequency components from a base-band transmitting signal I by the filter  11 , the base-band transmitting signal I is amplified by the programmable gain amplifier  13  and then sent to the mixer  15 , where the base-band transmitting signal I is converted to an RF I signal according to a local oscillator signal LO I  generated by a local oscillator (not shown). A base-band transmitting signal Q is similarly converted to an RF Q signal, which is sent to the power amplifier  17  together with the RF I signal, so as to amplify the RF I and Q signals for wireless transmission. 
         [0004]      FIG. 2  is a conventional wireless receiver  20  capable of converting an RF receiving signal to a base-band receiving signal for subsequent signal processes. The wireless receiver  20  comprises a low noise amplifier  21 , mixers  22  and  23 , filters  24  and  25 , and programmable gain amplifiers  26  and  27 . After being amplified by the low noise amplifier  21 , frequencies of in-phase and quadrature-phase signals of the RF receiving signal are converted into base-band frequencies respectively by the mixers  22  and  23  according to local oscillator signals LO I  and LO Q  generated by a local oscillator (not shown). After removing needless frequency components by the filters  24  and  25  and amplifying by the programmable gain amplifiers  26  and  27 , base-band receiving signals I and Q are generated. Hence, signal quality of the wireless communication depends largely on the frequency conversion of the mixer  15  and  16 ,  22  and  23 . 
         [0005]      FIG. 3  is a circuit diagram of a conventional mixer. Referring to  FIG. 3 , a Gilbert mixer  30  comprises a transconductor circuit  31 , a switch circuit  32  and a load circuit  33  having loads  331  and  332 . Each of the loads  331  and  332  has its one end coupled to a voltage source Vcc and its other end serving as an output end. The switch circuit  32  comprises n-channel transistors M 3 , M 4 , M 5  and M 6 . The transistors M 3  and M 5  have their drains coupled to one end of the load  331 , and the transistors M 4  and M 6  have their drains coupled to one end of the load  332 . Moreover, the transistor M 3  and the transistor M 6  have their gates coupled to each other, the transistor M 4  and the transistor M 5  have their gates coupled to each other. The gates of the transistors M 3  and M 4  are capable of receiving a local oscillator signal LO. The transistor M 3  and the transistor M 4  are coupled to each other to form a first current path, and the transistor M 5  and the transistor M 6  are coupled to each other to form a second current path. 
         [0006]    The transconductor circuit  31  comprises n-channel transistors M 1  and M 2 . The transistor M 1  has its drain coupled to the first current path of the switch circuit  32 , and the transistor M 2  has its drain coupled to the second current path of the switch circuit  32 . Gates of the transistors M 1  and M 2  respectively receive differential voltage signals Vin +  and Vin − . The sources of the transistors M 1  and M 2  are coupled to each other. Moreover, an n-channel transistor MS is coupled between the source of the transistor M 1  and a ground terminal; and a fixed voltage is inputted into a gate of the n-channel transistor MS such that the n-channel transistor MS forms a current source. 
         [0007]      FIG. 4  is a schematic diagram of signals associated with the Gilbert mixer  30 . The transconductor circuit  31  converts input differential voltage signals such as the Vin +  and Vin −  to a current signal Ib. When flowing through the first current path and the second current path of the switch circuit  32 , the current signal Ib, being driving by an oscillator signal LO, becomes a frequency-converted current signal, e.g., the current signal Ib is converted from a base-band frequency to a radio frequency as illustrated in  FIG. 4 . After that, the frequency-converted current signal is converted by the load circuit  33  so that an output voltage is generated at the output end. 
         [0008]    However, transistors of a conventional mixer are not completely ideal. For example, the transistors have nonlinear characteristics, due to which harmonic interferences are generated at an output voltage of the mixer, and thus signal quality of a frequency conversion is reduced. 
         [0009]    In addition, in the conventional mixer, bias points of a transconductor circuit and a switch circuit are correlative rather than being independent so that linearity of the transconductor circuit is influenced. For example, in the Gilbert mixer  30  illustrated in  FIG. 3 , correlation exists between bias points of the transconductor circuit  31  and the switch circuit  32 . When the bias point of the switch circuit  32  is too low, the bias point of the transconductor circuit  31  becomes too low such that the transistors M 1  and M 2  can not operate in a saturation region and the linearity of the transconductor circuit  31  is swayed. When the bias point of the transconductor circuit  31  or the switch circuit  32  is shifted higher in order to avoid the foregoing problem, the switch circuit  32  may not operate normally. 
       SUMMARY OF THE INVENTION 
       [0010]    In view of the foregoing issues, one object of the present invention is to provide a mixer capable of reducing harmonic interferences to improve signal quality during a frequency conversion performed by the mixer. 
         [0011]    Another object of the present invention is to provide a mixer capable of independently biasing a transconductor circuit and a switch circuit inside the mixer to ensure linearity of the transconductor circuit, so as to improve signal quality during a frequency conversion performed by the mixer. 
         [0012]    A mixer is provided according to the present invention. The mixer comprises a transconductor circuit, a first switch circuit, a first load circuit, a second switch circuit and a second load circuit. The transconductor circuit is for generating a differential current signal according to a differential voltage signal. The first switch circuit and the first load circuit are connected in series, and the first switch circuit is used to regulate the differential current signal in response to a first oscillator signal. The second switch circuit and a second load circuit are connected in series, and the second switch circuit is used to regulate the differential current signal in response to a second oscillator signal. The first load circuit and the second load circuit are connected at a common node to reduce harmonic interferences. 
         [0013]    A mixer is further provided according to the present invention. The mixer comprises a transconductor circuit, a switch circuit, and a load circuit. The transconductor circuit generates a differential current signal according to a differential voltage signal. The switch circuit and the load circuit are connected in series, and the switch circuit is used for regulating the differential current signal in response to an oscillating signal. The transconductor circuit and the switch circuit are biased independently. 
         [0014]    In an embodiment, the foregoing mixer further comprises a capacitance unit coupled between the transconductor circuit and the switch circuit. The capacitance unit is for separating biases provided for transconductor circuit and the switch circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a schematic diagram of a conventional wireless transmitter. 
           [0016]      FIG. 2  is a schematic diagram of a conventional wireless receiver. 
           [0017]      FIG. 3  is a circuit diagram of a conventional mixer. 
           [0018]      FIG. 4  is a schematic diagram of signals associated with a conventional mixer. 
           [0019]      FIG. 5  is a circuit diagram of a mixer in accordance with a first embodiment of the present invention. 
           [0020]      FIG. 6  is a circuit diagram of a mixer in accordance with a second embodiment of the present invention.’ 
           [0021]      FIG. 7  is a circuit diagram of a mixer in accordance with a third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]      FIG. 5  is a circuit diagram of a mixer in accordance with a first embodiment of the present invention. A mixer  50  comprises a transconductor circuit  51 , a first switch circuit  52 , a first load circuit  53 , a second switch circuit  54 , and a second load circuit  55 . The transconductor circuit  51  receives differential voltage signals Vin +  and Vin −  and correspondingly outputs differential current signals I +  and I − , which are respectively referred to as a first current signal and a second current signal in the following description. The first switch circuit  52  comprises four p-channel transistors M 1 , M 2 , M 3  and M 4 . Whether to allow the passing of the first current signal is controlled by the transistors M 1  and M 2  according to a local oscillator LO I , and whether to allow the passing of the second current signal is controlled by the transistors M 3  and M 4  according to the local oscillator signal LO I . Accordingly, the first switch circuit  52  converts a frequency of the differential current signal into a summation of the frequency of the differential current signal and a frequency of the local oscillator signal LO I . Similarly, the second switch circuit  54  comprises four p-channel transistors M 7 , M 8 , M 9  and M 10 . Whether to allow the passing of the second current signal is controlled by the transistors M 7  and M 8  according to a local oscillator signal LO Q , and whether to allow the passing of the first current signal is controlled by the transistors M 9  and M 10  according to the local oscillator signal LO Q . Therefore, the second switch circuit  54  converts a frequency of the differential current signal to a summation of the frequency of the differential current signal and a frequency of the local oscillator signal LO Q . It is to be noted that, a phase difference between the local oscillator signal LO I  and the local oscillator signal LO Q  is about 90 degrees. 
         [0023]    The first load circuit  53  comprises n-channel transistors M 5  and M 6 , and resistors R 1  and R 2 . Gates of the transistors M 5  and M 6  are coupled to each other, and the transistor M 5  has its drain coupled to drains of the transistors M 1  and M 3 . The transistor M 6  has its drain coupled to drains of the transistors M 2  and M 4 . The coupling points between the transistors M 5  and M 6  and the first switch circuit  52  are first differential output ends of the mixer  50 , and differential output voltages of which are represented by V I   +  and V I   − . The resistor R 1  is coupled between the drain and the gate of the transistor M 5 , and resistor R 2  is coupled between the drain and the gate of the transistor M 6 . Similarly, the second load circuit  55  comprises n-channel transistors M 11  and M 12 , and resistors R 3  and R 4 . Gates of the transistors M 11  and M 12  are coupled to each other, and the transistor M 11  has its drain coupled to drains of the transistors M 7  and M 9 . The transistor M 12  has its drain coupled to drains of the transistors M 8  and M 10 . The coupling points are second differential output ends of the mixer  50 , and differential output voltages of which are represented by V Q   +  and V Q   − . The resistor R 3  is coupled between the gate and the drain of the transistor M 11 , and the resistor R 4  is coupled between the gate and the drain of the transistor M 12 . 
         [0024]    In the first load circuit  53 , a direct current signal (being a bias signal) only flows through the transistor M 5  but not the resistor R 1 . Therefore, the output voltages V I   +  and V I   −  comprising direct and alternating current components only need to respectively activate the transistors M 5  and M 6  to operate in a saturation region. That is, the output voltages V I   +  and V I   −  need not to be too large so that operating voltages of the first switch circuit  52  and the transconductor circuit  51  shall not be out of an appropriate range, thus keeping the mixer  50  uninfluenced. Further, since only an alternating current signal (being a data signal carried in the bias signal) flows through the resistors R 1  and R 2 , which are then given larger resistance values for generating a larger gain value for the alternating current signal. Similarly, in the second load circuit  55 , the resistors R 3  and R 4  also can have larger resistance values for generating a larger gain value for the alternating current signal. Consequently, the first load circuit  53  and the second load circuit  55  are capable of providing larger signal gain values without causing too much voltage drop. 
         [0025]    It is to be noted that, referring to  FIG. 5 , the gates of the transistors M 5 , M 6 , M 11  and M 12  are mutually coupled to reduce harmonic interferences. The reason is analyzed below. 
         [0026]    Suppose that alternating current components of the output voltages V I   + , V I   − , V Q   +  and V Q   −  are cos ωt, −cos ωt, cos(ωt+90° and cos(ωt+90°), respectively, wherein ω is the frequency of the local oscillator signals LO I  and LO Q . For that the transistors M 5 , M 6 , M 11  and M 12  have a non-linearity, the foregoing output voltages generate harmonic components between the gates of the transistors. The output voltages V I   + , V I   − , V Q   +  and V Q   −  are expanded based on Fourier series, such as: 
         [0000]        V   I   +   →a 1 cos ω t+a 2 cos 2 ωt+a 3 cos 3 ωt+   
         [0000]        V   I   −   →a 1 cos ω t+a 2 cos 2 ωt−a 3 cos 3 ωt+   
         [0000]        V   Q   +   →a 1 cos(ω t+ 90°)+ a 2 cos(2 ωt+ 180°)+ a 3 cos(3 ωt+ 270°)+ 
         [0000]        V   Q   −   →a 1 cos(ω t+ 90°)+ a 2 cos(2 ωt+ 180°)− a 3 cos(3 ωt+ 270°)+ 
         [0027]    where a1, a2, a3 and so on are weighting coefficients of the harmonic components. The presence of the harmonic components imposes severe interference on the gate-source voltages (V GS ) between the transistors M 5 , M 6 , M 11  and M 12 . Hence, the output voltages V I   + , V I   − , V Q   +  and V Q   −  are undesirably affected and signal quality of the mixer  50  is also deteriorated. Therefore, in  FIG. 5 , the gates of the transistors M 5 , M 6 , M 11  and M 12  are mutually coupled, and a voltage at a Y point is: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0028]    wherein odd-numbered harmonic components cancel out one another, and even-numbered harmonic components such as two-order or six-order harmonic components also cancel out one another, so that even-numbered harmonic components such as four-order, eight-order harmonic components are remained. Since the harmonic components remained behind are not too large, the gate-source voltages V GS  of the transistors M 5 , M 6 , M 11  and M 12  are only influenced by a small amount of harmonic interferences such that the output voltages V I   + , V I   − , V Q   +  and V Q   −  are kept stable while also increasing the signal quality of the mixer  50 . 
         [0029]      FIG. 6  is a circuit diagram of a mixer in accordance with a second embodiment of the present invention. A mixer  60  comprises a transconductor circuit  61 , a switch circuit  62 , a load circuit  63 , a capacitance unit  64  and a bias circuit  65 . The transconductor circuit  61  comprises a bias circuit  611 , and the capacitance unit  64  comprises capacitors C 1  and C 2 . The mixer  60  performs a frequency conversion of an input voltage Vin according a local oscillator signal LO to generate an output voltage Vout. The main characteristic of the mixer  60  is that the transconductor circuit  61  and the switch circuit  62  have their own bias circuits to bias independently. That is, the transconductor circuit  61  and the switch circuit  62  respectively decide bias points as needed, so that an issue of swaying the linearity of the transconductor circuit in the prior art, due to the non-independent bias points of the transconductor circuit and the switch circuit, is eliminated. In addition, the capacitors C 1  and C 2  of the capacitance unit  64  are coupled between the transconductor circuit  61  and the switch circuit  62 , thus prohibiting the communication between the transconductor  61  and the switch circuit  62  to further ensure that bias provided by the bias circuits  611  and  65  are independent from each other. In another embodiment, the bias circuit  611  is designed as being outside the transconductor circuit  61 . The bias circuits  611  and  65  can be current sources or voltage supplies. 
         [0030]      FIG. 7  is a circuit diagram of a mixer in accordance with a third embodiment of the present invention. The main characteristic of the second embodiment is applied to the first embodiment. Compared with the mixer  50  of the first embodiment, a mixer  70  of the third embodiment further comprises bias circuits  511  and  72 , and a capacitance unit  71 . The bias circuit  511  provides a bias to the transconductor circuit  51 . The bias circuit  72  comprises current sources I 1  and I 2  for providing biases to the first switch circuit  52  and the second switch circuit  54 . The current source I 1  is coupled to the transistors M 3 , M 4 , M 7 , and M 8 , and the current source I 2  is coupled to the transistors M 1 , M 2 , M 9 , and M 10 . The transconductor circuit  51  comprises two current output ends for outputting differential current signals. The capacitance unit  71  comprises capacitors C 3  and C 4 . The capacitor C 3  has its one end coupled to one current output end of the transconductor circuit  51 , and its other end coupled to the transistors M 1 , M 2 , M 9 , and M 10 . The capacitor C 4  has its one end coupled to the other current output end of the transconductor  51 , and its other end coupled to the transistors M 3 , M 4 , M 7 , and M 8 . The capacitance unit  71  allows the biases provided by the bias circuits  511  and  72  to be independent from each other. 
         [0031]    In a practical application, the mixers  50  and  70  respectively illustrated in  FIGS. 5 and 7  are applied to the wireless transmitters  10  and  20  to replace the mixers respectively illustrated in  FIGS. 1 and 2 , so as to achieve the object of improving the signal quality. 
         [0032]    While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not to be limited to the above embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.