Abstract:
A quadrature switching mixer is provided for mixing a received RF signal and a local oscillator signal, while rejecting an image signal associated with the RF signal. Input signal components in quadrature, that is, I and Q input components derived from the received RF signal, are respectively coupled through first and second input paths to corresponding commuting switches in a configuration of switches. Each of the switches operates to multiply respective quadrature components of RF and local oscillator signals to provide quadrature output signal components. A unidirectional device, such as a buffer amplifier included in a signal splitter, is placed in each input path to prevent any portion of an output signal component from leaking backward through one of the input paths to the other input path, and thus to the other output signal component.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application for patent claims the benefit of priority from, and hereby incorporates by reference the entire disclosure of, co-pending U.S. Provisional Application for Patent No. 60/370,322, filed Apr. 4, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Technical Field of the Invention  
           [0003]    The invention disclosed and claimed herein generally pertains to an apparatus for use in connection with a quadrature switching mixer configured to reject an image signal associated with a received RF signal. More particularly, the invention pertains to an apparatus of the above type which substantially reduces leakage between the in phase (I) and quadrature (Q) output branches of the mixer, and thereby improves the conversion gain of the mixer and avoids deterioration of the mixer output signal.  
           [0004]    2. History of the Related Art  
           [0005]    In radio equipment, the signals transmitted through the air occupy one frequency band, whereas the signals that are processed occupy a considerably lower frequency band. Accordingly, a mixer is used to translate or convert the radio frequency signals to an intermediate frequency (IF). The mixing process, or heterodyning, is multiplicative, that is, the input signal is multiplied by a local oscillator signal (in the time domain). As a result of the multiplication, however, the output of the mixer may include contributions from the desired signal as well an undesired image signal. Those of ordinary skill in the radio reception art know that the image signal is a signal whose frequency is capable of being converted, via the mixing process, to the same IF as the desired signal.  
           [0006]    Image rejection mixers have been developed which use the principle of canceling to reduce the contribution of the image signal at the mixer output. In one commonly used type of image rejection mixer, the desired input signal (RF) is split into two signal components: an in-phase (I) component (RF I ) and a quadrature (Q) component (RF Q ). The quadrature (Q) signal is delayed 90 degrees relative to the in-phase (I) signal, that is: RF Q (ωt)=RF I (ωt−π/2). The local oscillator signal is also split into a quadrature signal LO Q  which is delayed 90 degrees relative to the in-phase oscillator signal LO I . Mathematically, in complex notation, the image rejection mixer works as a multiplication of the input signal RF I +jRF Q  with the local oscillator signal LO I +jLO Q .  
           [0007]    As described hereinafter in further detail, the multiplication is usually implemented with commuting switches. Each commuting switch has a pair of complementary switches SW and {overscore (SW)}. When the SW switch is closed, the output signal of the commuting switch has the same polarity as the input signal, and when the SW switch is closed, the output signal of the commuting switch has a different polarity than the input signal. Summing junctions are then provided for cancellation of the components representing the undesired image signal. This type of image rejection mixer is referred to as a quadrature switching mixer.  
           [0008]    The components for a quadrature switching mixer of the type described above can be fabricated using any suitable semiconductor technology such as CMOS, BJT, and the like. This provides certain important advantages when the mixers are used in wireless receivers for small portable electronic devices, such as mobile phones and the like for use in UMTS (Universal Mobile Telecommunications System), Bluetooth, and other wireless communication systems. CMOS based quadrature switching mixers, however, have a serious drawback in that the I and Q output signal components of the mixer, OUT I  and OUT Q  respectively, tend to short together. That is, a portion of one of the quadrature branch outputs may flow backward through one of the switches in the mixer, and enter the input of the other branch. This leakage can cause a conventional quadrature image rejection mixer of the type mentioned above to become less useful in practice, because of the resulting large conversion loss. Moreover, the leakage between the two outputs OUT I  and OUT Q  deteriorates the cumulative mixer output signal and leads to poor image rejection ratios.  
           [0009]    One presently used approach to reduce the leakage between the I and Q output signals of a quadrature switching mixer is to add resistors in series with the switches, thereby effectively isolating the I and Q branches from each other. A switch, however, should have low resistance when it is closed to minimize any signal loss. Adding isolation resistors introduces an additional loss. This loss can be significant, even though it is usually less than the loss resulting from leakage between the I and Q branch output signals. The added resistors also cause an undesirable voltage drop, especially if the signal is in current mode, which means that the input impedance of the mixer should be kept low. Therefore, it is undesirable to add any further resistors to the quadrature switching mixer.  
           [0010]    Accordingly, it would be desirable to be able to reduce the leakage between the I and Q output signals of a quadrature switching mixer, and to be able to do so without introducing additional loss to the mixer such as from isolation resistors.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention provides an effective and comparatively simple technique for substantially reducing leakage between the outputs of the I and Q branches of a quadrature switching mixer. In accordance with the invention, a unidirectional device, such as a component that is part of a signal splitter, is inserted into the input path of each switch of the mixer. Each of the unidirectional devices acts to prevent an output signal from one of the branches from leaking backward through an input path to the other branch output. Deterioration of the I and Q signals outputted from the quadrature switching mixer is thus significantly reduced. As a result, a passive quadrature switching mixer is made available that is highly linear, has low noise mixing capabilities, and can be efficiently used for image rejection mixing.  
           [0012]    In one embodiment, the invention is directed to switching mixer apparatus for mixing an RF signal and a local oscillator signal. The apparatus comprises a configuration of switching devices, each of the switching devices configured to multiply respective inphase (I) and quadrature (Q) components of the RF and LO signals to provide I and Q output signal components. The apparatus further comprises a first RF input path for coupling an I component of the RF signal to at least one of the switching devices, a second RF input path for coupling a Q component of the RF signal to at least one other of the switching devices, and a unidirectional signal processing device placed in at least one of the input paths for preventing an output signal component from passing backward through one of the input paths to the other input path.  
           [0013]    In the above embodiment, each of the unidirectional devices usefully comprises a buffer amplifier or other component of a signal splitter. Usefully, respective components of the mixer apparatus are implemented in CMOS or other suitable technology for use in a portable electronic device such as a wireless telephone terminal.  
           [0014]    A further embodiment of the invention is directed to a method for mixing RF and local oscillator signals to provide an IF signal from which the image frequency signal has been rejected. The method comprises the steps of processing the RF signal to provide corresponding I and Q input signal components, and coupling the I and Q input components through first and second input paths, respectively, to first and second pluralities of switching devices in a configuration of switching devices. The method further comprises operating each of the switching devices to multiply its received I or Q RF input component with an I or Q component, selectively, of the local oscillator signal to generate I and Q mixer output signal components. A unidirectional device is placed in each of the switching device input paths to prevent an output signal component associated with one of the input paths from passing backward into the other input path.  
           [0015]    It should be emphasized that the term comprises/comprising, when used in this specification, is taken to specify the presence of stated features, integers, steps, or components, but does not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    A more detailed understanding of the method and system of the present invention may be had by reference to the following detailed description when taken in conjunction with the drawings, wherein:  
         [0017]    [0017]FIG. 1 is a schematic diagram showing a quadrature switching mixer of the prior art adapted for rejection of image signals;  
         [0018]    [0018]FIG. 2 is a schematic diagram showing an embodiment of the invention;  
         [0019]    [0019]FIG. 3 is a block diagram showing an embodiment of the invention used to provide an IF stage in a receiver; and  
         [0020]    [0020]FIG. 4 is a schematic diagram showing components of the embodiment of FIG. 3 in further detail. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Following is a detailed description of the invention with reference to the drawings wherein reference numerals for the same or similar elements are carried forward.  
         [0022]    As mentioned previously, the present invention provides an effective and comparatively simple technique for substantially reducing leakage between the outputs of the I and Q branches of a quadrature switching mixer. Referring now to FIG. 1, there is shown a quadrature switching mixer  10  of conventional design, wherein mixer  10  is capable of image signal rejection and is provided with quadrature input terminals  12   a  and  12   b.  Terminal  12   a  is adapted to receive the in-phase input RF signal component RF I , and terminal  12   b  is adapted to receive the quadrature input RF signal component RF Q , where RF Q (ωt)=RF I (ωt−π/2). Mixer  10  is also provided with I and Q output terminals  14   a  and  14   b,  which respectively provide I and Q mixer output signal components OUT I  and OUT Q .  
         [0023]    [0023]FIG. 1 further shows quadrature switching mixer  10  provided with commuting or commutating switches  16 - 22 , which respectively comprise pairs of complementary switches  16   a - b ,  18   a - b,    20   a - b  and  22   a - b.  The input signal component RF I  is coupled to switches  16  and  20  by means of input paths  24   a  and  24   b,  respectively. Similarly, input signal component RF Q  is coupled to switches  18  and  22  by means of input paths  26   a  and  26   b.  An I component LO I  of a local oscillator signal is also coupled to commuting switches  16  and  18 , and a component LO Q  of the local oscillator signal is coupled to commuting switches  20  and  22 . The quadrature signal component LO Q  is delayed 90 degrees relative to the in-phase oscillator signal component LO I . As stated above, each of the commuting switches is operable to multiply its received RF input signal component and its received local oscillator signal component. By providing each switch with complementary switches SW I  and {overscore (SW I )}, or SW Q  and {overscore (SW Q )}, the output signal from a switch is equal to the input signal to the switch when the non-complemented switch is closed, and the polarity of the signal is changed when the complemented switch is closed (i.e., OUT(t)=sgn {LO(t)}·RF(t)).  
         [0024]    Referring still to FIG. 1, there are shown the outputs of switches  16  and  18  coupled to a summing junction  28   a  (Σ 1 ) and the outputs of switches  20  and  22  coupled to a summing junction  28   b  (Σ Q ), to provide OUT I  and OUT Q , respectively.  
         [0025]    In accordance with the image rejection feature of mixer  10 , respective output components of the switches derived from the undesired image signal are cancelled out at the summing junctions. Accordingly, the combined output of quadrature switching mixer  10 , that is, OUT I +OUT Q  in complex form is: 
         OUT I   +j OUT Q =RF I ·LO I −RF Q ·LO Q   +j (RF Q ·LO I +RF I ·LO Q )  Eqn. (1) 
         [0026]    In the prior art device shown in FIG. 1, leakage can occur between the output signal components OUT I  and OUT Q , so that the output terminals  14   a  and  14   b  are effectively shorted together. For example, FIG. 1 shows a component L I  of OUT I  which may leak backwards, through switch  16 . Component L I  could then be coupled forward through switch  20  to output terminal  14   b.  Similarly, a component L 2  of OUT Q  may leak backward through switch  22  and then move through switch  18  to become part of output OUT I .  
         [0027]    Referring to FIG. 2, there is shown a quadrature switching mixer  30  constructed in accordance with an embodiment of the invention. The mixer  30  is able to overcome the above output leakage problem of prior art devices, while at the same time perform image signal rejection as described above in connection with FIG. 1. Mixer  30  includes switches  16 - 22 , terminals  12   a - b  and  14   a - b  and input paths  24   a - b  and  26   a - b , which are identical or very similar to their respective same-numbered components shown in FIG. 1.  
         [0028]    Referring further to FIG. 2, there is shown a mixer  30  provided with unidirectional buffer amplifiers  32 - 38 , respectively. The unidirectional buffer amplifiers  32 - 38  together form a signal splitter  40 , as described hereinafter in further detail. In accordance with embodiments of the invention, buffers  32  and  34  are inserted into input paths  24   a  and  24   b,  respectively, and thus receive the I input signal component RF I . RF I  is coupled to switches  16  and  20  through respective buffers  32  and  34 , designated as B II  and B IQ . Similarly, buffers  36  and  38 , designated as B QI  and B QQ , are inserted into input paths  26   a  and  26   b , respectively. The quadrature input signal component RF Q  is then coupled through the buffers  36  and  38  to switches  18  and  22 , respectively.  
         [0029]    Since the buffers of the signal splitter  40  are unidirectional devices, they effectively prevent leakage between the I and Q branches of switching mixer  30 . That is, portions of output signal components OUT I  and OUT Q  cannot be connected backward to the input of the other branch. This reduces deterioration of the output signals provided by the mixer  30  and enhances conversion gain thereof. Accordingly, the quadrature switching mixer  30  may be readily used to take advantage of highly linear and low noise mixing characteristics, and at the same time provide efficient image signal rejection mixing.  
         [0030]    Referring to FIG. 3, there is shown the switching mixer  30  and the signal splitter  40  configured to serve as an intermediate frequency (IF) stage in a radio receiver. Such a radio receiver can be found in wireless devices such as mobile phones and the like that use UMTS, Bluetooth, and other wireless communication systems. The RF input signal is applied to a low noise amplifier (LNA)  42 , and coupled therethrough to the signal splitter  40 . The signal splitter  40 , described hereinafter in further detail, is operable to supply both the RF input components RF I  and RF Q , which are respectively coupled to the mixer  30 .  
         [0031]    Referring further to FIG. 3, there is also shown an oscillator  44  generating the local oscillator signal LO, which is coupled to a phase shifter  46 . A square-wave drive of the local oscillator  44  may be desirable in order to improve linearity and noise performance. However, a square-wave drive is hard to achieve at RF; instead the local oscillator signal may be sinusoidal, with a large amplitude to steepen the slope of the wave form. A phase shifter  46  provides both the in-phase local oscillator component LO I  and the quadrature local oscillator component LO Q . Phase shifter  46  usefully comprises an RC-CR network for generating the phase difference of 90 degrees needed to achieve the quadrature signal LO Q .  
         [0032]    In FIG. 3, component  30   a  of the mixer  30  represents the commuting switches  16  and  18  and summing junction Σ I , which collectively produce the in-phase mixer output signal OUT I . Similarly, component  30   b  of the mixer  30  represents the switches  20  and  22  and summing junction Σ Q , which collectively produce the quadrature output signal OUT Q . Accordingly, RF I  and LO I  are coupled to component  30   a,  and RF Q  and LO Q  are coupled to component  30   b.  FIG. 3 further shows mixer outputs OUT I  and OUT Q  coupled to buffer amplifier components  48   a  and  48   b,  respectively.  
         [0033]    Referring to FIG. 4, there is shown LNA  42  comprising inductively degenerated common-source stage M 5 , followed by a common-gate stage M 6 . The LNA design provides low noise, about 1.6 dB, at a low current consumption. Usefully, a 1.6 dB noise figure corresponds to a drain current of 800 μA and an aspect ratio of 112 μm/0.1 μm (W/L). This low drain current implies that stage M 5  works neat to the region of weak inversion, where the transconductance per unit of drain is at maximum, thus reducing the power consumption of the stage.  
         [0034]    Referring further to FIG. 4, there is shown the signal splitter  40  receiving the RF input, amplified by LNA  42 , to provide the in-phase signal component RF I  and the quadrature signal component RF Q . FIG. 4 further shows splitter  40  coupling the RF I  and RF Q  components to both mixer components  30   a  and  30   b.    
         [0035]    Components of splitter  40  located above voltage line V SS  as viewed in FIG. 4, and generally referenced collectively as  40   a , cooperate to provide the in-phase input component RF I . The splitter component  40   a  acts as a current amplifier. It includes a common-source stage M 3  that is connected to a second stage involving two identical transistors, M 4   a  and M 4   b,  which provide two identical output currents. These output currents are fed back via two identical resistive networks, (R 2   a , R 1   a ) and (R 2   b,  R 1   b ), to the input of the M 3  stage. The output of transistor M 4   a  provides the RF I  signal coupled to mixer component  30   a , and the output of transistor M 4   b  provides the RF I  signal coupled to mixer component  30   b.    
         [0036]    Signal splitter component  40   b  comprises a configuration of components located below voltage line V SS  as viewed in FIG. 4, which is very similar to the configuration of splitter component  40   a.  Splitter component  40   b  provides the quadrature input component RF Q , which is shifted 90 degrees in phase from RF I  and is coupled to both mixer components  30   a  and  30   b.    
         [0037]    Switching mixer  30  is passive, double-balanced, and based on, for example, CMOS switches. FIG. 4 shows these switches realized as two complementary MOS transistors in pair, M 1  and M 2 . Thus, each of the switches  16   a - b  through  22   a - b  comprises a pair of switches M 1  and M 2 . The advantage with such a complementary switch, compared to a signal-transistor switch, is reduced on-resistance, which improves noise performance. In addition, charge injection from the local oscillator signals to the input signal and the output signal of mixer  30  is reduced. Moreover, because there is no DC current through the CMOS switches, flicker noise is reduced at the mixer  30  output. This consideration is particularly important in an embodiment with a low IF architecture, where low frequency is of concern. It should be noted, however, that other technology besides CMOS may also be used, such as BJT switches, without departing from the invention.  
         [0038]    Referring further to FIG. 4, there are shown buffer amplifier components  48   a  and  48   b  coupled to receive the outputs of mixer components  30   a  and  30   b,  respectively. Usefully, buffer components  48   a  and  48   b  are respective components of a single-stage transimpedence amplifier.  
         [0039]    Obviously, many other modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the disclosed concept, the invention may be practiced otherwise than as has been specifically described.