Abstract:
A zero intermediate frequency (ZIF) radio frequency (RF) digital mixer ( 200 ) includes a low noise amplifier (LNA) ( 203 ), a first RF mixer stage ( 205 ) for mixing an RF signal from the at least one LNA with a first local oscillator signal operating a first predetermined frequency. A second RF mixer stage ( 207 ) is then utilized for mixing in-phase (I) and quadrature (Q) digital signals from the first RF mixer stage ( 205 ) with a plurality of second local oscillator signals (LO) operating at a second predetermined frequency. The invention significantly improves current drain by eliminating the need for a linear transconductance stage while still maintaining high degrees of isolation and linearity.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates in general to radio frequency (RF) receivers and more particularly to down conversion RF mixers used with the front-end receiver.  
         BACKGROUND  
         [0002]    In the past few years, the growing demand for low voltage, low power, low cost, high performance mobile communications equipment has changed the way wireless receivers are designed. BiCMOS technology has become a practical contender for use in receiver design, especially because it lends itself to easier integration with digital integrated circuits (ICs), as well as analog circuits. However, usage of submicron ceramic metal-oxide semi-conductor (CMOS) technologies imposes an upper limit on the supply voltages, therefore it is important to focus on low voltage design when designing RF CMOS circuits. Low voltage designs also reduce the average current drain for digital integrated circuits. However, this may not always have the same application to analog circuitry. Moreover, communication products are becoming increasingly complex. By improving on existing circuitry and systems, a reduction in power consumption will enable the operation of ever more complicated communication products without sacrificing product battery life performance. Most modem mixers used in current wireless receivers are based on the classical Gilbert cell. However, one improvement that can always be made to present Gilbert cell designs is a reduction in current drain.  
           [0003]    As seen in prior art FIG. 1, a typical Gilbert cell mixer  100  generally located at the front-end of a zero intermediate frequency (ZIF) receiver includes an input  101  that typically feeds into a low noise amplifier (LNA)  103 . The LNA  103  is used to amplify the input signal without generating an inordinate amount of noise and distortion signals. This enables the receiver to maintain a significantly high signal-to-noise ratio (SNR). The output of the LNA  103  is fed in a differential fashion to the inputs of transconductance amplifier  105  and transconductance amplifier  107 . Both transconductance amplifiers  105 ,  107  are linear gain stages that provide a high degree of gain while isolating an in-phase (I) and quadrature (Q) local oscillator (LO) mixer input signals  113 ,  117  from interfering with the output of LNA  103 . Without these stages the LO signals  113 ,  117  can degrade receiver performance by introducing imbalance between I signal  121 , Q signal  123 , and by creating DC offsets. As is known in the art, the transconductance amplifier is used to convert the RF input voltage into a current. The output of the transconductance amplifiers  105 ,  107  are then directed to an in-phase LO driven Gilbert cell mixer  109  and a quadrature LO driven Gilbert mixer cell  111 . They work with a local oscillator input signals  113 ,  117  to mix each respective RF input signal from transconductance stages  105 ,  107  to produce in-phase (I) and quadrature (Q) output signals  121 ,  123 . The local oscillator signals  113  and  117  for the first Gilbert cell mixer  109  and second Gilbert cell mixer  111  are generated by an RF oscillator operating at two times (2×) the oscillator frequency. This is accomplished by operating a voltage controlled oscillator (VCO)  115  whose output duty cycle is 50% with a divide by two circuit producing two local oscillator (LO) signals for both in-phase (I) and quadrature (Q) inputs  113  (LO i ) and  117  (LO q ). The LO i  and LO q  are then mixed with the input RF signal where they are mixed down to a lower intermediate frequency (IF) or baseband to provide an in-phase (I) output signal  121  and a quadrature (Q) output signal  123 .  
           [0004]    One major drawback associated with this type of mixer topology is the high current drain required in the transconductance stages  105 ,  107 . When placed on-chip these stages operate with a high degree of isolation and linearity, however this comes at the cost of high power consumption or current drain. Obviously this plays a critical role by ultimately shortening the operating time of any portable battery operated device. If the output of the LNA  103  were connected directly to the input of each respective mixer by removing the transconductance stages, there would be an inadequate amount of isolation between the various mixing signals and would be detrimental to receiver performance. However, this would reduce the current drain substantially. Thus, the need exists to innovate an alternative mixer topology that reduces the current drain by removing the transconductance stages while maintaining a high degree of in-phase and quadrature IF isolation and linearity.  
         SUMMARY OF THE INVENTION  
         [0005]    Briefly, according to the invention, there is provided a two Gilbert cell mixer circuit generally used in a zero intermediate frequency (ZIF) receiver operating without typical linear amplification stages. The invention includes a low noise amplifier (LNA), voltage controlled oscillator (VCO) operating at twice the desired local oscillator (LO) frequency with 50% duty cycle on output signal, a divide by two stage to generate the in-phase and quadrature LO signals, a low current drain highly linear dynamic power splitter which provides a high degree of isolation between in-phase (I) and quadrature (Q) LO signals. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:  
         [0007]    [0007]FIG. 1 is a block diagram of a prior art radio frequency (RF) receiver front-end using a low noise amplifier (LNA) and transconductance stages;  
         [0008]    [0008]FIG. 2 is a block diagram of a Gilbert cell mixer according to the preferred embodiment of the invention where the transconductance stage has been eliminated; and  
         [0009]    [0009]FIG. 3 is a block diagram of a Gilbert cell mixer of FIG. 2 showing operation of the dynamic power transfer circuit according to the preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]    While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward.  
         [0011]    Referring now to FIG. 2, a RF front-end zero intermediate frequency (ZIF) mixer  200  according to the preferred embodiment of the invention includes an RF input  201  that provides an RF input signal to a low noise amplifier (LNA)  203 . The LNA  203  is used to provide a high degree of signal gain while introducing little or no noise and/or spurious signal components to the amplified signal. A first mixer stage  205  is used to dynamically split the power of the RF output signal from the LNA  203  into two signals  218 ,  219 . This is accomplished by using a uniquely configured switch in the first mixer stage  205  which is driven by a voltage controlled oscillator (VCO) signal  255  operating at two times (2×) a predetermined local oscillator frequency (2×LO). It is essential that the VCO output signal  255  be conditioned with a 50% duty cycle; i.e. the signal is in a “high” state for the same duration it is in a “low” state. This fits well into the system because the divide by two quadrature generator will require this condition as well. Thus, the invention will require no additional modifications to the combined VCO and quadrature generating circuitry.  
         [0012]    The output of the divide-by-two circuit  221  provides both in-phase (I) and quadrature (Q) signal components  216 ,  217  respectively, that are provided to a second mixer stage  207 . The second mixer stage  207  is configured with both an in-phase driven Gilbert cell  209  and a quadrature driven Gilbert cell  211  that work to mix the power from signals dynamically split by the first mixer stage  205  with the separate local oscillator signals  216 ,  217 . The second mixer stage  207  provides both an in-phase output signal  213  and quadrature output signal  215  of the down converted RF signal. The down converter frequency called the intermediate frequency (IF) will be equal to the RF frequency minus the LO frequency. For example, a zero IF receiver will have its RF frequency and LO frequency equal to each other.  
         [0013]    In FIG. 3, specific details of the dynamic power sharing method  300  are illustrated where a radio frequency (RF) input signal  301  is input to a low noise amplifier (LNA)  303  operating to provide high gain while adding no noise or distortion to the components in the amplified signal. As noted in FIG. 2, the output of the LNA  303  is directed to a dynamically power splitting stage  305  which is mixed with the VCO signal which acts as a local oscillator (LO)  340  operating at two-times the frequency provided to the second mixer stage  331  (discussed hereinafter). The dynamic power splitter stage  305  operates using a plurality of switches  307 ,  309 ,  311 ,  313  which work to split the power of the RF input signal to a plurality of outputs  355 ,  356 ,  357 ,  358 . The switches  307 ,  309 ,  311  and  313  are switched in such a fashion so as to provide a high degree of isolation between the switch outputs  355 ,  356 ,  357  and  358 . This prevents interference between the two Gilbert cells  370 ,  371  facilitated by the LO signals  380 ,  381  in view of the high RF signal amplitude and close proximity on-chip.  
         [0014]    The splitter stage  305  operates using switches  307 ,  309  that alternately pass the LNA output signals  390 ,  391  to the respective paired switches  315 ,  317  and  323 ,  325 . When switch  307  is “ON,” the signal  390  passes to switch pair  315 ,  317 . During this time switch  309  is “OFF”, so there is no continuous connection between Gilbert cell  370 ,  371 . Similarly, when switch  309  is “ON” switch  307  is “OFF” and the signal  390  is passed to Gilbert cell switch pair  323 ,  325 . Switches  311 ,  313  perform the same function to signal  391  as switches  307 ,  309 . Only the signal  391  is split between switches  319 ,  321 ,  327  and  329 . As will be recognized by those skilled in the art, the splitter stage  305  requires much less current and can maintain the same linearity as the circuit topology shown in the prior art from FIG. 1. Thus, the splitter stage  305  substantially reduces current drain while maintaining comparable mixer performance.  
         [0015]    The second mixer stage  331  includes both an in-phase driven Gilbert cell  370  and quadrature driven Gilbert cell  371  that work to mix the signals provided from the splitter stage  305  with a predetermined local oscillator (LO) signal operating at one-half the VCO frequency. Separate LO signals  316 ,  318  are provided to the in-phase Gilbert cell  370  and quadrature Gilbert cell  371 . Thus, at the input of I Gilbert cell  370  and Q Gilbert cell  371  this may be represented in Equation (1) as: 
           Gil   in(RF)   =VCO (2 ×LO )− RF;   (1) 
         [0016]    The in-phase (I) Gilbert cell  370  includes a plurality of switches  315 ,  317 ,  319 ,  321 , while the quadrature (Q) Gilbert cell  371  includes switches  323 ,  325 ,  327 ,  329  that work to mix the RF signal from splitter stage  305  with a local oscillator (LO) signal operating at some predetermined frequency to provide an intermediate frequency (IF) signal from the second mixer stage represented by Equation (2): 
           Gil   out(IF)   =LO−RF   ( 2 ) 
         [0017]    Both the switches in the in-phase Gilbert cell  370  and quadrature Gilbert cell  371  switch in such a fashion as to provide an in-phase (I) output signal  380  and quadrature (Q) output signal  381  at the output of the second mixer stage  331 .  
         [0018]    As will be recognized by those skilled in the art, the present invention is directed to zero intermediate frequency (ZIF) radio frequency (RF) mixer that includes a dynamic power splitter that eliminates an amplifier transconductance stage for providing gain and isolation. The addition of the dynamic power splitter stage still offers a high degree of isolation between the in-phase and quadrature Gilbert cell mixer signals while providing a high degree of linearity.  
         [0019]    While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.