Patent Publication Number: US-7912429-B2

Title: LO 2LO upconverter for an in-phase/quadrature-phase (I/Q) modulator

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of the filing date of co-pending U.S. Provisional Patent Application No. 60/946,259, filed on Jun. 26, 2007, entitled “LO-2LO Upconverter In IQ Modulators for SAW-less Transmitters,” the entire disclosure of which is hereby incorporated herein by reference; and is a continuation-in-part of U.S. patent application Ser. No. 11/220,030, filed on Sep. 6, 2005 now U.S. Pat. No. 7,398,073, entitled “Low Noise Mixer,” the entire disclosure of which is hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), WIFI transceivers, and other communication devices must be capable of communicating using a number of different frequency bands. For example, current portable communication devices may communicate in three, four, or more communication bands. 
     One of the challenges in designing a portable communication device that operates in multiple bands is providing isolation between the transmit and receive bands. For example, transmit energy in a particular transmit band must not overlap and interfere with a receive band. One way of ensuring this isolation is to implement a surface acoustic wave (SAW) filter at the output of each transmitter. A SAW filter reduces the noise of the transmitter output at the receiver frequency to prevent desensitizing the receiver. However, the SAW filter consumes valuable space on the integrated circuit and is costly to implement. Therefore, with the need for including more and more standards into a single radio frequency (RF) transceiver, eliminating the SAW filter in the transmitter design becomes highly attractive to aid in reducing cost and package size. 
     In order to eliminate the SAW filter from the transmit path, the noise of the transmitter should be low at the corresponding receive frequency. Unfortunately, it is not possible to reduce the noise of the transmitter to such low levels with available transmitter architectures. 
     In a modern RF transmitter, upconversion of baseband data is realized by multiplying, also referred to as mixing, the data signal with a carrier signal. Using the trigonometric identity, 
     
       
         
           
             
               
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     The baseband signal is upconverted into two sidebands in the frequency domain. Since only one of the generated sidebands is of interest, the unwanted sideband is rejected by using in-phase (I or sine) and quadrature-phase (Q or cos) signals both for baseband and LO signals. 
     In an in-phase (I) quadrature-phase (Q) (I/Q) transmitter that relies on a 90 degree phase separation between the in-phase signal and the quadrature-phase signal, one of the most significant circuit elements that limits the achievable performance is the local oscillator (LO) signal generation circuitry in the upconverter. The LO signal can be thought of as a reference signal that is used to upconvert the baseband information signal to a transmit signal. Therefore, phase noise of the LO signal at an offset frequency away from the main frequency directly contributes to the noise of the transmitter output. Specifically, for the offset frequencies which overlap the receive frequency of a different user channel, phase noise performance becomes highly critical. Typically, the circuitry that generates the LO signal occupies a large circuit area on an integrated circuit and consumes high power in order to maintain a low phase noise. 
     The baseband signal and the LO signal are combined in what is referred to as a “mixer core.” The mixer core is another critical element in the transmit chain. Similar to the circuitry that generates the LO signal, the mixer core also generates noise which directly contributes to noise in the transmitter output. An active mixer typically consumes high power in order to keep noise low. A passive mixer does not consume power, but provides limited isolation between I and Q baseband inputs. Good isolation between the I and Q baseband inputs is vital for most transmitters. 
     Therefore, it would be desirable to have an upconverter that achieves low phase noise, provides good sideband isolation and that consumes minimal circuit area and power. 
     SUMMARY 
     Embodiments of an upconverter include a switching architecture configured to receive an input signal, a first local oscillator (LO) signal, and a second local oscillator (2LO) signal that is at a frequency that is twice a frequency of the local oscillator (LO) signal, wherein the switching architecture is configured to switch the input signal on transitions of the second local oscillator (2LO) signal, and wherein the first local oscillator signal and the second local oscillator signal are combined to form combined LO 2LO switching signals. 
     Other embodiments are also provided. Other systems, methods, features, and advantages of the invention will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention can be better understood with reference to the following figures. The components within the figures are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is a block diagram illustrating a simplified portable transceiver. 
         FIG. 2  is a simplified schematic diagram illustrating an embodiment of an LO 2LO upconverter for an I/Q modulator. 
         FIG. 3A  is a schematic diagram illustrating an embodiment of the LO 2LO upconverter implemented in an active mixer arrangement. 
         FIG. 3B  is a schematic diagram illustrating an embodiment of the LO 2LO upconverter implemented in a passive mixer arrangement. 
         FIG. 4  is a graphical illustration of the LO and 2LO signals used in the mixer cores of  FIGS. 3A and 3B . 
         FIG. 5  is a schematic diagram illustrating a differential active mixer core constructed in accordance with an embodiment of the LO 2LO upconverter. 
         FIG. 6  is a schematic diagram illustrating a differential passive mixer core constructed in accordance with an embodiment of the LO 2LO upconverter. 
         FIG. 7  is a schematic diagram illustrating an alternative embodiment of a differential passive mixer core constructed in accordance with an embodiment of the LO 2LO upconverter. 
         FIG. 8  is a schematic diagram illustrating another alternative embodiment of a differential passive mixer core constructed in accordance with an embodiment of the LO 2LO upconverter. 
         FIG. 9  is a schematic diagram illustrating an example of circuitry that can generate the LO signals described herein. 
         FIG. 10  is a graphical illustration showing the LO signals used in the embodiment of the LO 2LO upconverter described in  FIG. 8 . 
         FIG. 11  is a flow chart describing the operation of an embodiment of the LO 2LO upconverter. 
     
    
    
     DETAILED DESCRIPTION 
     Although described with particular reference to a portable transceiver, the LO 2LO upconverter for an I/Q modulator can be used in any device that uses signal upconversion in a transmitter. The LO 2LO upconverter for an I/Q modulator can be implemented in a passive mixer, or can be implemented in an active mixer. Further, the LO 2LO upconverter for an I/Q modulator can be implemented in the mixer core, or can be implemented in the LO generation circuitry with a conventional mixer core. 
     A passive mixer offers low power consumption, low noise and high linearity, but requires better performance from the following stages. The most problematic drawback of a passive mixer is the lack of isolation between I and Q inputs. This lack of isolation significantly reduces the sideband rejection, and limits the effective use of a passive mixer in many applications. An active mixer provides gain, and hence, reduces the noise contribution of the following stages to the overall system noise. However, an active mixer tends to consume a significant amount of current to achieve good noise and linearity performance. 
     The generation of quadrature I and Q LO signals is also important to mixer operation. Since both phase noise and I and Q matching of the quadrature LO signals are important, the circuitry related to the generation of the LO signal typically consumes high power and occupies a large area on the circuit. 
     The LO 2LO upconverter for an I/Q modulator overcomes most of the challenges described above. The LO 2LO upconverter for an I/Q modulator significantly reduces the contribution of LO chain phase noise to the overall system noise. The LO 2LO upconverter for an I/Q modulator also reduces the effect of IQ imbalance in the LO chain that degrades the sideband rejection of the upconverter. 
     Further, by improving the isolation between the I and Q inputs for a passive mixer, the LO 2LO upconverter for an I/Q modulator allows the use of a passive mixer in numerous applications without degrading sideband rejection performance. 
     The LO 2LO upconverter for an I/Q modulator can be implemented in hardware, software, or a combination of hardware and software. When implemented in hardware, the LO 2LO upconverter for an I/Q modulator can be implemented using specialized hardware elements and logic. When the LO 2LO upconverter for an I/Q modulator is implemented partially in software, the software portion can be used to precisely control the various components when generating the LO and 2LO signals. The software can be stored in a memory and executed by a suitable instruction execution system (microprocessor). The hardware implementation of the LO 2LO upconverter for an I/Q modulator can include any or a combination of the following technologies, which are all well known in the art: discrete electronic components, a discrete logic circuit(s) having logic gates for implementing logic functions upon data signals, an application specific integrated circuit having appropriate logic gates, a programmable gate array(s) (PGA), a field programmable gate array (FPGA), etc. 
     The software for the LO 2LO upconverter for an I/Q modulator comprises an ordered listing of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. 
     In the context of this document, a “computer-readable medium” can be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer readable medium can be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non-exhaustive list) of the computer-readable medium would include the following: an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory) (magnetic), an optical fiber (optical), and a portable compact disc read-only memory (CDROM) (optical). Note that the computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via for instance, optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory. 
       FIG. 1  is a block diagram illustrating a simplified portable transceiver  100 . Embodiments of the LO 2LO upconverter for an I/Q modulator can be implemented in any RF transmitter or RF transceiver, and in this example, are implemented in an RF transmitter associated with a portable transceiver  100 . The portable transceiver  100  illustrated in  FIG. 1  is intended to be a simplified example and to illustrate one of many possible applications in which the LO 2LO upconverter for an I/Q modulator can be implemented. One having ordinary skill in the art will understand the operation of a portable transceiver. The portable transceiver  100  includes a transmitter  110 , a receiver  120 , a baseband subsystem  130 , a digital-to-analog converter (DAC)  160  and an analog-to-digital converter (ADC)  170 . The transmitter includes a modulator  116  and an upconverter  200 . In an embodiment, the upconverter  200  can be a subsystem of the modulator  116 . In alternative embodiments, the upconverter  200  can be a separate circuit block or circuit element. The upconverter  200  implements embodiments of the LO 2LO upconverter for an I/Q modulator as described herein. 
     The transmitter also includes any other functional elements that modulate and upconvert a baseband signal. The receiver  120  includes filter circuitry and downconverter circuitry that enable the recovery of the information signal from the received RF signal. The portable transceiver  100  also includes a power amplifier  140 . The output of the transmitter  110  is provided over connection  112  to the power amplifier  140 . Depending on the communication methodology, the portable transceiver may also include a power amplifier control element (not shown). 
     The receiver  120  and the power amplifier  140  are connected to a front end module  144 . The front end module  144  can be a duplexer, a diplexer, or any element that separates the transmit signal from the receive signal. The front end module  144  is connected to an antenna  138  over connection  142 . 
     In transmit mode, the output of the power amplifier  140  is provided to the front end module  144  over connection  114 . In receive mode, the front end module  144  provides a receive signal to the receiver  120  over connection  146 . 
     If portions of the LO 2LO upconverter for an I/Q modulator are implemented in software, then the baseband subsystem  130  also includes LO 2LO upconverter software  155  that can be executed by a microprocessor  135 , or by another processor, to control the operation of the LO 2LO upconverter for an I/Q modulator to be described below. 
     When transmitting, the baseband transmit signal is provided from the baseband subsystem  130  over connection  132  to the DAC  160 . The DAC  160  converts the digital baseband transmit signal to an analog signal that is supplied to the transmitter  110  over connection  134 . The modulator  116  and the upconverter  200  modulate and upconvert the analog transmit signal according to the modulation format prescribed by the system in which the portable transceiver  100  is operating. The modulated and upconverted transmit signal is then supplied to the power amplifier  140  over connection  112 . 
     When receiving, the filtered and downconverted receive signal is supplied from the receiver  120  to the ADC  170  over connection  136 . The ADC digitizes the analog receive signal and provides the analog baseband receive signal to the baseband subsystem  130  over connection  138 . The baseband subsystem  130  recovers the transmitted information. 
       FIG. 2  is a simplified schematic diagram illustrating an embodiment of an LO 2LO upconverter for an I/Q modulator. The upconverter  200  includes an oscillator  202  configured to generate an LO signal on connection  204  that is twice the frequency of the desired LO signal. For example, if the desired LO frequency is a nominal 100 MHz, the signal on connection  204  is nominally 200 MHz. The upconverter  200  also includes a mixer core  212  and a mixer core  214 . The mixer cores  212  and  214  are arranged to operate on the quadrature signals I and Q. In an example, the in-phase signal, I_in, is supplied over connection  206  to the mixer core  212  and the quadrature-phase input signal, Q_in, is supplied over connection  208  to the mixer core  214 . 
     The 2LO signal on connection  204  is supplied to the mixer cores  212  and  214 , and is also supplied to a divider  222 . In an embodiment, the divider  222  is a quadrature divider. The divider  222  divides the 2LO signal on connection  204  to a nominal value of LO on connections  216  and  218 . In this example, an LO_I signal is supplied to the mixer core  212  over connection  216  and an LO_Q signal is supplied to the mixer core  214  over connection  218 . 
     As will be described in greater detail below, the mixer cores  212  and  214  each receive the LO signal and the 2LO signal. The mixer core  212  upconverts the I_in signal and the mixer core  214  upconverts the Q_in signal with minimal noise and impairments. The upconverted I_in signal is supplied to a combining element  228  over connection  224  and the upconverted Q_in signal is supplied to the combining element  228  over connection  226 . The output of the combining element  228  on connection  232  is the output signal that is supplied to the power amplifier  140  ( FIG. 1 ). Either the in-phase signal or the quadrature-phase signal can be chosen either by changing the final combining element operation to addition (or subtraction), or by interchanging the I and Q LO signals without changing the final operation. 
     The architecture of the upconverter  200  suppresses the noise contribution of the frequency divider that is used to generate the quadrature LO signals, LO_I and LO_Q, and therefore, minimizes transmitter noise and sideband generation. Further, as will be described below, the architecture of the upconverter  200  provides a high level of input isolation between the I and Q inputs for a passive mixer implementation. 
       FIG. 3A  is a schematic diagram illustrating an embodiment of the LO 2LO upconverter implemented in an active mixer architecture. A mixer core  300  is configured to receive a differential input signal over connections  302  and  304 . For simplicity, only one of the two quadrature phases is shown in  FIG. 3A . Either the in-phase (I) signal or the quadrature-phase (Q) signal is supplied to the mixer core  300 . 
     The mixer core  300  includes a set of switches  310  and a set of switches  320 . The set of switches  310  includes switches  312 ,  314 ,  316  and  318 . The switches  312 ,  314 ,  316  and  318  are illustrated as simple single-pole, single-throw switches to illustrate that any type of switch can be used. For example, any type of semiconductor switch, such as a bipolar junction transistor (BJT), a field effect transistor (FET), any variants thereof, or any other switch architecture can be implemented in the description to follow. 
     The set of switches  310  are configured to receive the 2LO signal. For example, a differential input signal supplied over connection  302  is supplied to the switches  312  and  314 . The switch  312  receives the 2LO signal while the switch  314  receives the inverse of the 2LO signal, also referred to as  2LO , or 2LO bar. The switches  316  and  318  receive the opposite differential signal over connection  304 . The switch  316  receives the  2LO  signal and the switch  318  receives the 2LO signal. 
     The set of switches  320  includes switches  322 ,  324 ,  326  and  328 . The set of switches  320  are configured to receive the LO signal. The switch  322  receives the LO signal, the switches  324  and  326  receive the inverse of the LO signal, also referred to as  LO , or LO bar, and the switch  328  receives the LO signal. 
     The output of the switch  312  is supplied via connection  319  to the switches  322  and  324 . The output of the switch  318  is supplied via connection  321  to the switches  326  and  328 . The output of the switch  314  is provided to terminal  336  and the output of switch  316  is provided to terminal  338 . The output of switches  314  and  316  is discarded when the switches  314  and  316  are active. The output of the mixer core  300  is taken over connection  332  and  334 . 
     The mixer core  300  includes two levels of switches,  310  and  320  as shown above, i.e., LO and 2LO levels. In an active mixer, such as shown in  FIG. 3A , if low-flicker noise is desired the 2LO switches  312 ,  314 ,  316  and  318  level should be located before the LO switches  322 ,  324 ,  326  and  328 . 
       FIG. 3B  is a schematic diagram illustrating an embodiment of the LO 2LO upconverter implemented in a passive mixer arrangement. A mixer core  350  is configured to receive a differential input signal over connections  352  and  354 . For simplicity, only one of the two phases is shown in  FIG. 3B . Either the in-phase signal or the quadrature-phase signal will be supplied to the mixer core  350 . 
     The mixer core  350  includes a set of switches  360  and a set of switches  370 . The set of switches  360  includes switches  362 ,  364 ,  366  and  368 . The set of switches  360  are configured to receive the LO signal. For example, a differential input signal supplied over connection  352  is supplied to the switches  362  and  364 . The switch  362  receives the LO signal while the switch  364  receives the inverse of the LO signal, also referred to as  LO , or LO bar. The switches  366  and  368  receive the opposite differential signal over connection  354 . The switch  366  receives the  LO  signal and the switch  368  receives the LO signal. 
     The set of switches  370  includes switches  372 ,  374 ,  376  and  378 . The set of switches  370  are configured to receive the 2LO signal. The switch  372  receives the 2LO signal, the switches  374  and  376  receive the inverse of the 2LO signal, also referred to as  2LO , or 2LO bar, and the switch  378  receives the 2LO signal. 
     The output of the switch  362  is supplied via connection  369  to the switches  372  and  374 . The output of the switch  368  is supplied via connection  371  to the switches  376  and  378 . The output of the switch  364  is supplied to connection  371  and the output of the switch  366  is supplied to connection  369 . The output of the mixer core  350  is taken over connection  382  and  384 . The output of the switch  374  is provided to terminal  386  and the output of switch  376  is provided to terminal  388 . The output of switches  374  and  376  is discarded when the switches  374  and  376  are active. 
     The mixer core  350  includes two levels of switches,  360  and  370  as shown above, i.e., LO and 2LO levels. In a passive mixer, such as shown in  FIG. 3B , the LO switches  362 ,  364 ,  366  and  368  can be located either before or after the 2LO switches  372 ,  374 ,  376  and  378 . In an embodiment, the LO switches  362 ,  364 ,  366  and  368  are located before the 2LO switches  372 ,  374 ,  376  and  378 . 
     Both of the embodiments shown in  FIGS. 3A and 3B  include an additional level of switches which are driven by 2 times the LO frequency, 2LO. The signal 2LO is readily available if I and Q phases of the LO signal are generated by using a frequency divider, such as the divider  222  of  FIG. 2 , which is a common way of I and Q LO signal generation. Alternatively, other ways of generating the 2LO signal as possible. 
       FIG. 4  is a graphical illustration of the LO and 2LO signals used in the mixer cores of  FIGS. 3A and 3B . The diagram  400  includes a signal trace  402  that represents the 2LO signal, a signal trace  404  that represents the LO signal, and a signal trace  406  that represents the effective LO (eLO) signal. The positive and negative cycles of the LO signal  404  are associated with “1” and “−1” respectively, which is a direct result of the switch arrangement. However, the positive and negative cycles of the 2LO signal  402  are associated with “1” and “0” since on the negative cycle the input signal is discarded. Discarding half of the signal, and hence, half of the noise, improves the noise performance of the upconverter due to second-order effects. The effective LO signal  406  illustrates the switching of the mixer core ( 300  or  350 ) only when the 2LO signal  402  switches. In this manner, the LO signal  404  is always settled and stable and is not at a switching transition when the 2LO switches are active. This is illustrated in  FIG. 4  at  410 ,  412 ,  414  and  416 . In this manner, the arrangement of the LO and 2LO phases allows the mixer core to achieve optimum performance. In order to suppress any non-ideality in the LO signals, such as phase noise or imbalance between the I and Q signals, the switching instant of the LO signal  404  occurs during a “0” cycle of the 2LO signal  402 . As shown in  FIG. 4 , this ensures that the effective LO signal  406  does not depend on the transitions of the LO signal, but only depends on the transitions of the 2LO signal  402 . 
       FIG. 5  is a schematic diagram illustrating a differential active mixer core constructed in accordance with an embodiment of the LO 2LO upconverter. The mixer core  500  includes an in-phase mixer core  505  and a quadrature-phase mixer core  555 . The in-phase mixer core  505  receives differential in-phase (I) input signals over connections  502  and  504 . The signals on connections  502  and  504  are supplied to a transconductance amplifier, also referred to as a Gm stage,  506 . The output of the Gm stage  506  is supplied over connections  508  and  509 . 
     The in-phase mixer core  505  includes a first set of switches  510  and a second set of switches  520 . The first set of switches  510  is configured to receive the 2LO signal and the second set of switches  520  is configured to receive the LO signal. 
     The first set of switches  510  includes switches  512 ,  514 ,  516  and  518 . The output of the Gm stage  506  on connection  508  is supplied to the switches  512  and  514 . The output of the Gm stage  506  on connection  509  is supplied to the switches  516  and  518 . The 2LO signal controls the switch  512 , the  2LO  signal controls the switches  514  and  516 , and the 2LO signal controls to switch  518 . 
     The second set of switches  520  includes switches  522 ,  524 ,  526  and  528 . The second set of switches  520  is configured to receive the LO signal. The switch  522  is controlled by the in-phase LO input signal, LO_I, the switches  524  and  526  are controlled by the opposite in-phase LO input signal,  LO_I  signal and the switch  528  is controlled by the LO_I signal. The output of the switch  512  is supplied via connection  511  to the switches  522  and  524 . The output of the switch  518  is supplied via connection  519  to the switches  526  and  528 . The output of the switch  522  is supplied over connection  532 , the output of the switch  524  is supplied over connection  534 , the output of switch  526  is supplied over connection  532  and the output of the switch  528  is supplied over connection  534 . The output of the switch  514  is provided to terminal  515  and the output of switch  516  is provided to terminal  517 . The output of switches  514  and  516  is discarded when the switches  514  and  516  are active. 
     The quadrature-phase mixer core  555  receives differential quadrature-phase (Q) input signals over connections  552  and  554 . The signals on connections  552  and  554  are supplied to a transconductance amplifier, also referred to as a Gm stage,  556 . The output of the Gm stage  556  is supplied over connections  558  and  559 . 
     The quadrature-phase mixer core  555  includes a first set of switches  560  and a second set of switches  570 . The first set of switches  560  is configured to receive the 2LO signal and the second set of switches  570  is configured to receive the LO signal. 
     The first set of switches  560  includes switches  562 ,  564 ,  566  and  568 . The output of the Gm stage  556  on connection  558  is supplied to the switches  562  and  564 . The output of the Gm stage  556  on connection  559  is supplied to the switches  566  and  568 . The  2LO  signal controls the switch  562 , the 2LO signal controls the switches  564  and  566 , and the  2LO  signal controls to switch  568 . 
     The second set of switches  570  includes switches  572 ,  574 ,  576  and  578 . The second set of switches  570  is configured to receive the LO signal. The switch  572  is controlled by the quadrature-phase LO input signal, LO_Q, the switches  574  and  576  are controlled by the opposite quadrature-phase LO input signal,  LO_Q  and the switch  578  is controlled by the LO_Q signal. The output of the switch  562  is supplied via connection  561  to the switches  572  and  574 . The output of the switch  568  is supplied via connection  569  to the switches  576  and  578 . The output of the switch  572  is supplied over connection  532 , the output of the switch  574  is supplied over connection  534 , the output of switch  576  is supplied over connection  532  and the output of the switch  578  is supplied over connection  534 . The output of the switch  564  is provided to terminal  565  and the output of switch  566  is provided to terminal  567 . The output of switches  564  and  566  is discarded when the switches  564  and  566  are active. The output of the mixer core  500  is taken over connections  532  and  534 . 
     In the configuration shown in  FIG. 5 , the I and Q inputs, shown as I+, I−, Q+ and Q−; I and Q LO signals, shown as LO_I and LO_Q, and the polarity of the output connections can be interchanged. The resulting upconverter will provide the benefits described above, regardless of whether the lower frequency sideband or higher frequency sideband is selected. 
     As described above with respect to  FIG. 4 , the switching instants are arranged appropriately such that the upper-level switches ( 520  and  570 ) switch while the current input generated by the corresponding Gm stage is being discarded by the lower-level switches ( 510  and  560 ). 
     The use of the LO 2LO upconverter architecture with an active mixer improves the noise performance by suppressing the noise of the LO generation circuitry and reducing flicker noise during signal upconversion. The LO 2LO upconverter architecture also improves sideband rejection since sideband rejection depends on the positive and negative edges of the 2LO signal, rather than the phase matching of the LO_I and LO_Q signals. Given these advantages, significant current reduction and area reduction are possible. 
       FIG. 6  is a schematic diagram illustrating a differential passive mixer core constructed in accordance with an embodiment of the LO 2LO upconverter. The mixer core  600  includes an in-phase mixer core  605  and a quadrature-phase mixer core  655 . The in-phase mixer core  605  receives differential in-phase (I) input signals from voltage sources  602  and  604 . 
     The in-phase mixer core  605  includes a first set of switches and  610  and a second set of switches  620 . The first set of switches  610  is controlled by the in-phase LO signal and the second set of switches  620  is controlled by the 2LO signal. 
     The first set of switches  610  includes switches  612  and  614 ,  616  and  618 . The output of the current source  602  on connection  606  is supplied to the switches  612  and  614 . The output of the voltage source  604  on connection  608  is supplied to the switches  616  and  618 . 
     The switch  612  is controlled by the in-phase LO signal, LO_I, the switches  614  and  616  are controlled by the opposite in-phase signal,  LO_I , and the switch  618  is controlled by the in-phase LO signal, LO_I. 
     The output of the switch  612  is supplied over connection  611  to the switch  622 . The output of the switch  614  is supplied over connection  615 , the output of the switch  616  is supplied over connection  617 , and the output of the switch  618  is supplied over connection  619  to the switch  628 . 
     The switch  622  is controlled by the 2LO signal, the switches  624  and  626  are controlled by the  2LO  signal, and the switch  628  is controlled by the 2LO signal. The output of the switch  622  is supplied over connection  632 , the output of the switch  628  is supplied over connection  634 . The output of the switch  624  is provided to terminal  635  and the output of switch  626  is provided to terminal  637 . The output of switches  624  and  626  is discarded when the switches  624  and  626  are active. 
     The quadrature-phase mixer core  655  receives differential quadrature-phase (Q) input signals from voltage sources  652  and  654 . The quadrature-phase mixer core  655  includes a first set of switches  660  and a second set of switches  670 . The first set of switches  660  is controlled by the quadrature-phase LO signal and the second set of switches  670  is controlled by the 2LO signal. 
     The first set of switches  660  includes switches  662  and  664 ,  666  and  668 . The output of the voltage source  652  on connection  656  is supplied to the switches  662  and  664 . The output of the voltage source  654  on connection  658  is supplied to the switches  666  and  668 . 
     The switch  662  is controlled by the quadrature-phase LO signal, LO_Q, the switches  664  and  666  are controlled by the opposite quadrature-phase signal,  LO_Q , and the switch  668  is controlled by the quadrature-phase LO signal, LO_Q. 
     The output of the switch  662  is supplied over connection  661  to the switch  672 . The output of the switch  664  is supplied over connection  665 , the output of the switch  666  is supplied over connection  667 , and the output of the switch  668  is supplied over connection  669  to the switch  678 . 
     The switch  672  is controlled by the  2LO  signal, the switches  674  and  676  are controlled by the 2LO signal, and the switch  678  is controlled by the  2LO  signal. The output of the switch  672  is supplied over connection  632 , the output of the switch  678  is supplied over connection  634 . The output of the switch  674  is provided to terminal  675  and the output of switch  676  is provided to terminal  677 . The output of switches  674  and  675  is discarded when the switches  674  and  675  are active. The output of the mixer core  600  is taken over connections  632  and  634 . 
     In the configuration shown in  FIG. 6 , the I and Q inputs, shown as I+, I−, Q+ and Q−; I and Q LO signals, shown as LO_I and LO_Q, and the polarity of the output connections can be interchanged. The resulting upconverter provides the benefits described above, regardless of whether the lower frequency sideband or higher frequency sideband is selected. 
     As described above with respect to  FIG. 4 , the switching instants are arranged appropriately such that the lower-level switches ( 610  and  660 ) switch while the corresponding voltage input upconverted by the lower-level switches ( 610  and  660 ) is being discarded by the upper-level switches ( 620  and  670 ). 
     The use of the LO 2LO upconverter architecture described herein with a passive mixer improves the noise performance by suppressing the noise of the LO generation circuitry. The LO 2LO upconverter architecture also improves sideband rejection since sideband rejection depends on the positive and negative edges of 2LO rather than the phase matching of LO_I and LO_Q. Given these advantages, significant current reduction and area reduction are possible. 
     Another important benefit provided by the LO 2LO upconverter architecture is to reduce the loading effect of the I and Q inputs with respect to each other. In a conventional passive mixer, the I and Q inputs are constantly loading each other, which results in significant performance degredation. However, using the LO 2LO upconverter architecture, the switches that operate on the 2LO signal provide isolation between the I and the Q inputs. However, this effect can also be minimized if current-mode input is chosen while keeping the switches in passive mode. 
       FIG. 7  is a schematic diagram illustrating an alternative embodiment of a differential passive mixer core constructed in accordance with an embodiment of the LO 2LO upconverter. The elements in  FIG. 7  that are identical to the elements in  FIG. 6  are labeled  7 XX, where XX refers to the corresponding element in  FIG. 6 . If voltage-mode operation is chosen, the 2LO switch that is used to discard the input signal can be removed as shown in  FIG. 7 . As shown by comparing  FIG. 6  to  FIG. 7 , the switches  624 ,  626 ,  674  and  676  are omitted. 
       FIG. 8  is a schematic diagram illustrating another alternative embodiment of a differential passive mixer core constructed in accordance with an embodiment of the LO 2LO upconverter. 
     The embodiments of the above-described LO 2LO upconverter can be implemented in an alternative manner that reduces the number of switches, so that a conventional mixer architecture can be used. In this embodiment appropriate logic operations are performed with the LO and 2LO signals and the resulting waveforms are used to drive conventional I and Q upconverter mixers. 
     In  FIG. 8 , the mixer core  800  includes an in-phase mixer core  805  and a quadrature-phase mixer core  855 . The in-phase mixer core  805  includes voltage sources  802  and  804 , which provide differential in-phase (I) input signals. The in-phase mixer core  805  includes switches  812 ,  814 ,  816  and  818 . The output of the voltage source  802  over connection  806  is provided to switches  812  and  814 . The output of the voltage source  814  is provided over connection  808  to the switches  816  and  818 . 
     In this embodiment, the switch  812  is controlled by the LO signal, 2LO*LO_I, the switches  814  and  816  are controlled by the LO signal 2LO*  LO_I , and the switch  818  is controlled by the LO signal, 2LO*LO_I. 
     The output of the switch  812  is supplied over connection  822 , the output of the switch  814  is supplied over connection  824 , the output of the switch  816  is supplied over connection  822  and the output of the switch  818  is supplied over connection  824 . 
     The quadrature-phase mixer core  855  includes voltage sources  852  and  854 , which provide differential quadrature-phase (Q) input signals. The quadrature-phase mixer core  855  includes switches  862 ,  864 ,  866  and  868 . The output of the voltage source  852  over connection  856  is provided to switches  862  and  864 . The output of the voltage source  854  is provided over connection on  858  to the switches  866  and  868 . 
     In this embodiment, the switch  862  is controlled by the LO signal,  2LO *LO_Q, the switches  864  and  866  are controlled by the LO signal,  2LO *  LO_Q  and the switch  868  is controlled by the LO signal,  2LO *LO_Q. 
     The output of the switch  862  is supplied over connection  822 , the output of the switch  864  is supplied over connection  824 , the output of the switch  866  is supplied over connection  822  and the output of the switch  868  is supplied over connection  824 . The output of the mixer core  800  is taken over connections  822  and  824 . 
       FIG. 9  is a schematic diagram illustrating an example of circuitry that can generate the LO signals described herein. Other circuit architectures can be used to generate the LO signals as described herein. The circuitry includes NAND gates  910 ,  920 ,  930  and  940 . The NAND gate  910  receives the 2LO signal over input connection  902  and receives the LO_I signal over input connection  904 . The 2LO*LO_I signal is generated on connection  909  by operation of the NAND gate  910 . 
     The NAND gate  920  receives the 2LO signal over input connection  922 . The LO_I signal is supplied over connection  904  to an inverter  926 . The output of the inverter  926  on input connection  928  is the  LO_I  signal. The 2LO*  LO_I  signal is generated on connection  929  by operation of the NAND gate  920 . 
     The NAND gate  930  receives the output of an inverter  936  at one of its input connections. The 2LO signal is supplied over connection  932  to the inverter  936 . The output of the inverter  936  on input connection  938  is the  2LO  signal. The NAND gate  930  receives the LO_Q signal on input connection  934 . The  2LO *LO_Q signal is generated on connection  939  by operation of the NAND gate  930 . 
     The NAND gate  940  receives the output of inverter  946  on input connection  947  and receives the output of inverter  948  on input connection  945 . The 2LO signal is supplied over connection  942  to the inverter  946 . The output of the inverter  946  on input connection  947  is the  2LO  signal. The LO_Q signal is supplied over connection  944  to the inverter  948 . The output of the inverter  948  on input connection  945  is the  LO_Q  signal. The  2LO *  LO_Q  signal is generated on connection  949  by operation of the NAND gate  940 . 
       FIG. 10  is a graphical illustration  1000  showing the LO signals used in the embodiment of the LO 2LO upconverter described in  FIG. 8 . The 2LO signal is shown by signal trace  1002 , the LO_I signal is shown by signal trace  1004  and the LO_Q signal is shown by signal trace  1006 . 
     The 2LO*LO_I signal is shown by signal trace  1008 , the 2LO*  LO_I  signal is shown by signal trace  1012 , the  2LO *LO_Q signal is shown by signal trace  1014  and the 2LO*  LO_Q  signal is shown by signal trace  1016 . 
     An advantage of the alternative embodiment shown in  FIG. 8 ,  FIG. 9  and  FIG. 10  is that it removes any non-ideality due to extra switches, such as the switches described above that operate based on the 2LO signal. This is particularly important for fabrication processes that may form switches that may have less than ideal operating characteristics. The drawback is the increased power consumption due to the use of low-noise logic operations, as shown in  FIG. 9 . 
       FIG. 11  is a flow chart  1100  describing the operation of an embodiment of the LO 2LO upconverter. The blocks in the flowchart can be performed in or out of the order shown by the elements described above. 
     In block  1102 , an LO signal is supplied to a mixer core. In block  1104 , a signal at twice the LO signal (2LO) frequency is supplied to the mixer core. In block  1106 , the output of the mixer core switches on the 2LO signal. 
     While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of the invention. For example, the invention is not limited to a specific type of radio transmitter or transceiver. Embodiments of the invention are applicable to different types of radio transmitters and transceivers and are applicable to any transmitter that upconverts a transmit signal.