BACKGROUND OF THE INVENTION 
     This invention relates to radio communications equipment and more specifically to radio frequency (RF) mixers used in RF equipment. 
     Radio communications equipment use mixers to convert a signal from a low frequency to a high frequency or a high frequency to a low frequency by mixing the signal with a local oscillator signal. The local oscillator frequency can be above or below the frequency of the desire signal to produce a sum and a difference frequency one of which is the frequency of interest. 
     Mixer performance is critical to the overall performance of the RF equipment. Mixer performance is especially critical in receiver applications where the dynamic range of the receiver is limited by the mixer. Mixers must have good linearity or intermodulation performance characteristics, wide dynamic range, low power consumption, low noise figure, and offer conversion gain in some applications. 
     Direct conversion or homodyne receivers convert the RF signal directly to baseband or a zero intermediate frequency (IF). Mixers for this application require low second or even-order distortion products, good balance, and low 1/f noise. Diode ring mixers are not suitable for direct conversion receiver applications since they have poor balance, require high injection levels, and have a conversion loss. Active mixers such as the Gilbert cell fabricated in gallium arsenide (GaAs) heterojunction bipolar transistor (HBT) technology offer some performance advantages over passive diode mixers. However these active mixer designs are expensive and suffer from 1/f noise problems and other low frequency discrete noise spurs due to GaAs processing issues. These mixers also have high power consumption and larger than desired noise figures. 
     What is needed for direct conversion and other receiver applications is a mixer that has no anomalous low frequency spurs, has low distortion, has significantly reduced power consumption, lower noise figure, good balance, and low cost. 
     SUMMARY OF THE INVENTION 
     An active commutated double balanced mixer is disclosed. The active commutated double balanced mixer comprises a transformer with a primary connected to an RF input signal to be up or down converted and a secondary with a center tap connected to ground. A first resistor is used with a first end connected to a first end terminal of the transformer secondary. A second resistor is used with a first end connected to a second end terminal of the transformer secondary. A quad transistor core is connected to a second end of the first resistor and a 180 degree out of phase RF input connected to a second end of the second resistor, a local oscillator input, a power supply input, and an intermediate frequency output. 
     It is an object of the present invention to provide a mixer that has low intermodulation distortion, reduced power consumption, good balance, and good amplitude modulation (AM) demodulation performance. 
     It is an advantage of the present invention that bipolar junction transistor and field effect transistor technology can be used. 
     It is an advantage of the present invention that silicon bipolar junction transistor technology is utilized to lower cost. 
     It is another advantage of the present invention using silicon bipolar junction transistor technology that spurious low frequency noise is reduced. 
     It is a feature of the present invention to provide the performance needed for direct conversion receiver applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is diagram of a standard Gilbert cell mixer known in the art; 
     FIG. 2 is a diagram of mixer according to the present invention; and 
     FIG. 3 is a diagram the RF differential half-circuit of the quad transistor cell shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION 
     Radio frequency (RF) mixers are know in the art for up-converting a baseband signal to a high frequency or down-converting a high frequency signal to baseband. There are many mixer types such as unbalanced, single and double balanced. Mixers can be passive diode or active transistor types using various semiconductor technologies such as silicon and gallium arsenide with bipolar junction transistors (BJT), field effect transistors (FET), or other variations of these types. 
     One type of mixer known in the art is the Gilbert cell mixer  100  shown in FIG.  1 . The Gilbert cell mixer  100  provides conversion gain, good rejection at the RF and LO ports, and a differential IF output. The Gilbert cell mixer  100  is the preferred choice for integrated circuit mixer designs and is shown in FIG. 1 implemented in bipolar junction transistors. The Gilbert cell mixer  100  is a double balanced switching or commutating mixer using a differential pair of transistor  105  and  106  in the RF differential cell  110  to drive the mixer quad transistor core  120  consisting of transistors  101 ,  102 ,  103 , and  104  and collector resistors  125  and  126 . Transistors  101  and  102  form a differential pair as do  103  and  104 . 
     In FIG. 1, the RF input signal to be up or down converted is fed into the bases of the differential pair  105  and  106  in the RF differential cell  110 . The RF signal (RF) at the input terminal  111  connected to the base of  105  is 180 degrees out of phase to RF signal (RFN) at the input terminal  112  connected to the base of  106 . A transformer (not shown) can provide this phase shift. The RF signal and the 180-degree phased shifted RF signal from the output of the differential pair  105  and  106  are then applied to the mixer quad transistor core  120  at the in-phase and out-of-phase inputs at the emitters of the transistor differential pairs  101 / 102  and  103 / 104  respectively. The local oscillator signal (LO) to be mixed with the RF signal is connected to input terminal  117  and fed to the base of transistor  101  and  104  and the 180-degree phase shifted local oscillator (LON) is connected to input terminal  118  and fed to the bases of transistor  102  and  103 . Another transformer (not shown) can provide this phase shift input. The up or down converted IF signal is taken from terminal  121  (IF) connected to the collectors of transistor  101  and  103  and the 180-degree IF signal (IFN) from terminal  122  connected to the collectors of transistor  102  and  104 . Power is supplied to the mixer at Vdd terminal  123  and bias through current source Ib  119 . 
     The mixer quad transistor core  120  and the RF differential cell  110  determine the overall intermodulation performance. The dominant cause of distortion is due to the RF differential cell  110 . To improve the linearity of the mixer, the emitter degeneration resistors  115  and  116  must increase in value or the current Ib  119  must increase. However increasing the emitter resistors  115  and  116  increases the noise and increasing the current in the current source Ib  119  increases the power consumption. 
     A novel active commutated double balanced mixer topology is created by eliminating the non-linearity of the RF differential cell  110  by eliminating the RF differential cell  110  consisting of transistors  105  and  106 . The mixer of the present invention is shown in FIG. 2 as  200  implemented in bipolar junction transistors. Transistor  105  and  106  of FIG. 1 are eliminated and a transformer  210  is added as shown in FIG.  2 . The RF input is connected to terminal  211  of the primary of transformer  210 . The other primary terminal of transformer  210  is connected to ground. The first secondary terminal of transformer  210  is connected to the first end of first resistor  115 , the second secondary terminal of transformer  210  is connected to first end of second resistor  116  and the center tap is connected to ground. With the elimination of transistor  105  and  106 , the second ends of resistor  115  and  116  are now connected directly to the emitters or first terminals of transistors  101  and  102  and  103  and  104  respectively of the mixer quad transistor core  120 . The local oscillator (LO) and intermediate frequency (IF) connections to the mixer quad are as in the standard Gilbert cell mixer  100  of FIG.  1 . The LO input (LO) is connected to terminal  117 , which is connected to the bases or second terminals of transistors  101  and  104 . The 180-degree phase shifted LO input (LON) is connected to terminal  118 , which is connected to the base or second terminals of transistor  102  and  103 . The IF output (IF) at terminal  121  is taken from the collectors or third terminals of transistors  101  and  103 . The 180-degree phase shifted IF output (IFN) at terminal  122  is taken from the collectors or third terminals of transistors  102  and  104 . The first end of the third resistor  125  is connected to the collectors of transistors  101  and  103 . The second end of the third resistor  125  is connected to power source Vdd at terminal  123 . The first end of the fourth resistor  126  is connected to the collectors of transistors  102  and  104 . The second end of the fourth resistor  126  is connected to the power source Vdd at terminal  123 . 
     The mixer of the present invention  200  is shown in FIG. 2 as being implemented in bipolar junction transistors and these transistors can be silicon or some other similar semiconductor material. The first resistor  115  and second resistor  116  are emitter resistors and the third resistor  125  and fourth resistor  126  are collector resistors when bipolar junction transistors are used. The transistors  101 ,  102 ,  103 , and  104  can also be heterojunction bipolar transistors (HBT) implemented in gallium arsenide (GaAs) or some other similar semiconductor material. The transistors  101 ,  102 ,  103 , and  104  can also be field effect transistors where the sources are connected in place of the emitters as the first terminal, the gates are connected in place of the bases as the second terminal, and the drains are connected in place of the collectors as the third terminal of the corresponding bipolar junction transistors. The field effect transistors can be silicon junction field effect transistors, gallium arsenide field effect transistors (GaAsFET), metallic semiconductor field effect transistors (MESFET), pseudomorphic high electron mobility transistors (PHEMT). The first resistor  115  and second resistor  116  are source resistors and the third resistor  125  and fourth resistor  126  are drain resistors when field effect transistors are used. 
     The RF differential half circuit of the mixer is shown in FIG. 3 implemented in bipolar junction transistors for this example. The RF signal from transformer  210  in FIG. 2 is applied in-phase (RF) at terminal  311  and out-of-phase (RFN) at terminal  312  in FIG.  3 . The LO in-phase signal (LO) is connected to the base of transistor  101  and  104  at terminal  117  in FIG.  3 . The LO out-of-phase signal (LON) is connected to the base of transistor  102  and  103  at terminal  118  as shown in FIG.  2 . The bases of  101  and  104  are at AC ground when the LO signal is high thus  101  and  104  appear to be a common base stage. The out-of-phase LON signal turns off the other transistor pair  102  and  103 . The input impedance at the emitter of  101  or  104  is 1/gm and the input impedance to the RF signal at terminals  311  and  312  is 1/gm+Re, where gm is the transistor transconductance and Re is the value of the emitter resistors  115  and  116 . The input impedance becomes approximately equal to Re in high linearity applications. The gain Vg of the half circuit is given by equation 1 below.              Vg   =     +       gmR1     1   +   gmRe       ~       +   R1     Re                 Equation                 1                                
     R1 is the value of the collector resistors  125  and  126 . The new circuit still has the same common mode rejection of a differential amplifier as with the Gilbert cell mixer. The resistive feedback provided by emitter resistors  115  and  116  provides good linearity. Power is greatly reduced with the removal of the differential amplifiers  105  and  106  and the current source Ib  119  of FIG.  1 . 
     It is believed that the active commutated double balanced mixer of the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.
A transistor commuted double balanced mixer that uses a Gilbert cell mixer quad transistor core. The RF input differential pair is deleted from the standard Gilbert cell mixer configuration. A transformer is used to apply the RF input through resistors to the Gilbert cell mixer quad transistor core.