Abstract:
Apparatus and methods for improving the performance of a modulator, typically in an I/Q modulation system, are described. Input I and Q current modulation signals may be processed to generate a sign signal and a magnitude signal, with the magnitude signal selectively applied to the inputs of a mixer based on the sign signal so as to generate respective I and Q modulation signal components. These may then be combined to generate a composite modulation signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/012,005, entitled LOW POWER I/Q MODULATOR, filed on Dec. 6, 2007. This application is also related to U.S. Utility patent application Ser. No. 11/369,897, entitled LINEAR WIDEBAND PHASE MODULATION SYSTEM, filed Mar. 6, 2006. The content of each of these applications is hereby incorporated by reference herein in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to radio transmitters using I/Q modulation. More particularly but not exclusively, the present invention relates to apparatus and methods for improving the efficiency of radio transmitters using I/Q modulation. 
     BACKGROUND 
     Radio transmitters generate the modulated signals required for wireless communications using modulation techniques such as QPSK, 8-PSK, 16-QAM, 64-QAM, and OFDM. These techniques vary the amplitude, phase, and/or frequency of the transmitter&#39;s RF carrier. 
     An I/Q modulator provides a convenient and straightforward method to realize the modulation. It uses quadrature signals to drive double-balanced mixers that combine to form the transmit signal. Unfortunately, the mixers generate noise and operate inefficiently. This causes problems in low-noise, low-power applications. It would therefore be advantageous to reduce the mixer&#39;s current consumption while lowering its output noise level. 
     SUMMARY 
     In one or more embodiments of the present invention, apparatus and methods for lowering the current consumption and reducing the noise of an I/Q modulator are described. 
     In one aspect, the present invention relates to an apparatus for use in a modulator comprising a first sign circuit configured to generate a first sign signal representing the sign of a first current signal component of a modulation signal provided to said modulator, a first magnitude circuit configured to generate a first magnitude signal representing the magnitude of the first current signal component, and a first input selection circuit including a first switching circuit configured to selectively apply the first magnitude signal to one of a first input and a second input of a first mixer, wherein said first input and said second input are selected based on said first sign signal. 
     In another aspect, the present invention relates to a modulator comprising a first mixer, a second mixer, an in-phase (I) processing circuit coupled to the first mixer comprising a first sign circuit configured to generate a first sign signal representing the sign of an I current signal component provided to said modulator, a first magnitude circuit configured to generate a first magnitude signal representing the magnitude of the I current signal component, and a first input selection circuit including a first switching circuit configured to selectively apply the first magnitude signal to one of a first input and a second input of the first mixer, wherein said first input and said second input are selected based on said first sign signal, and a quadrature (Q) processing circuit coupled to the second mixer comprising a second sign circuit configured to generate a second sign signal representing the sign of a Q current signal component provided to said modulator, a second magnitude circuit configured to generate a second magnitude signal representing the magnitude of the Q current signal component, and a second input selection circuit including a first switching circuit configured to selectively apply the second magnitude signal to one of a first input and a second input of the second mixer, wherein said first input and said second input are selected based on said second sign signal. 
     In yet another aspect, the present invention relates to a method for enhancing the performance of a modulator comprising generating a first sign signal representing the sign of a first current signal component of a modulation signal provided to the modulator, generating a first magnitude signal representing the magnitude of the first current signal component, and selectively applying the first magnitude signal to one of a first input and a second input of a first mixer, wherein said first input and said second input are selected based on said first sign signal. 
     Additional aspects are described below with respect to the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following is a brief description of the drawings wherein: 
         FIG. 1  shows a diagram of an I/Q modulator; 
         FIG. 2  shows a diagram of the double-balanced mixer generally used in the I/Q modulator; 
         FIG. 3  shows the signal trajectory for a complex transmit signal; 
         FIG. 4(   a ) shows an embodiment of an extended mixer, in accordance with aspects of the present invention; 
         FIG. 4(   b ) shows an alternate embodiment of an extended mixer, in accordance with aspects of the present invention; 
         FIG. 5  shows an embodiment of signal conditioning applied to an input before it is applied to an extended mixer, in accordance with aspects of the present invention; 
         FIG. 6  shows an embodiment of an extended mixer with offsets added to keep both differential pairs active, in accordance with aspects of the present invention; 
         FIG. 7  shows a diagram of an embodiment of a low-power I/Q modulator based on an extended mixer, in accordance with aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     An I/Q (In Phase/Quadrature) modulator is shown in  FIG. 1 . It uses a pair of mixers driven by orthogonal or quadrature signals to generate the amplitude, phase, and/or frequency modulated transmit signal. 
     In typical implementations, the mixers in the I/Q modulator rely on a double-balanced structure as shown in  FIG. 2 . The circuit is configured to feed a complementary current representing the input signal to the differential pairs formed by transistors N 1 -N 4 . The input signal may be formed by a D/A converter, current mirror, or transconductance amplifier. The LO signal drives these differential pairs, creating a commutating action that translates the signal current to the RF carrier frequency. 
     The input I and Q signals applied to the mixers in the I/Q modulator must be bipolar to realize any arbitrary phase. This is because these signals represent the complex modulation translated to I and Q coordinates as shown in  FIG. 3 . Since the mixer processes only positive signals, each bipolar signal maps to
 
 i   I(in)   →i   I(in)   +I   bias    i   Q(in)   →i   Q(in)   +I   bias  
 
in the current domain, where I bias  represents the dc offset needed to keep i I(in)  and i Q(in)  positive. As such, the bias is typically set to equal the maximum peak value of either input signal i I(in)  or i Q(in) . Unfortunately, this dc level mixes with the LO signal and effectively leaks the LO signal to the mixer&#39;s output. To avoid this problem, a differential signal may be used where
 
 i   I(in)   =Δi   I(in) →(i I(in)+   I   bias )−( i   I(in)−   +I   bias )= i   I(in)+   −i   I(in)− 
 
     Without an input signal, the two differential pairs operate at the same current level. Since their outputs are cross-coupled, the two outputs cancel and the net output ideally equals zero. 
     The differential input signal results in a constant current level since
 
 i   I(in)+   +i   I(in)−   =i   total =2 I   bias  
 
This means that the current consumption remains fixed. Moreover, the output level then varies with the peak-to-average ratio of the input signal. In other words, the output power falls as the peak-to-average ratio increases.
 
     In accordance with aspects of the present invention, it is possible to realize the sign inversion without using the traditional approach based on differential signals. An embodiment of this approach is illustrated in  FIG. 4(   a ). Here, the circuit is configured to invert the input signal. Ideally, only one of the differential pairs operates at any given time depending on the polarity on the input signal. Since the outputs of the two differential pairs are cross-coupled, their outputs are inherently opposite. As a result, switching the active signal between the two differential pairs inverts the input signal and consequently produces the desired output signal. An alternative approach is illustrated in  FIG. 4(   b ). 
     An embodiment of signal conditioning associated with the extended mixer is illustrated in  FIG. 5 . As shown, the input I and Q signals may each be processed to extract a magnitude signal and a sign signal. The sign signal may then be used to selectively apply the magnitude signal to one of the mixer differential inputs such as is shown in  FIGS. 4(   a ) and  4 ( b ), with selection based on whether the sign is positive or negative. 
     As a result of this processing, the applied input now equals the absolute value of the signal according to
 
 i   I(in)   =|i   I(in) |
 
As a result, the current consumption of the mixer tracks the amplitude of the input signal and thereby varies with its peak-to-average. Consequently, the current consumption may decrease, in some cases dramatically. Furthermore, the noise may also drop since it also tracks the operating current.
 
     In some embodiments, better performance results may be obtained with a small offset added to both the positive and negative input signals, with the net output equal to zero. The offset currents I offset  keep both differential pairs biased and active. One embodiment of this approach is illustrated in  FIG. 6 . 
     In an exemplary embodiment, a low-power I/Q modulator includes signal conditioning circuitry for both I and Q signal inputs, and uses identical extended mixers as shown in  FIG. 7 . This implementation may advantageously provide the same functionality as a traditional I/Q modulator, while lowering RMS current and the noise floor. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously, many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, they thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following claims and their equivalents define the scope of the invention.