Patent Publication Number: US-2015085902-A1

Title: RFDAC Transmitter Using Multiphase Image Select FIR DAC and Delta Sigma Modulator with Multiple Rx Band NTF Zeros

Description:
BACKGROUND 
     The present disclosure relates to electronic circuits, and more particularly to a transmitter used in such circuits. 
     A wireless communication device, such as a cellular phone, includes a transmitter for transmitting signals and a receiver for receiving signals. The receiver often downconverts an analog radio frequency (RF) signal to an analog baseband signal or analog intermediate frequency (IF) signal which is filtered, amplified, and converted to a digital baseband signal in an analog to digital converter (ADC). Likewise, the transmitter converts a baseband digital signal to an analog signal, which is filtered and upconverted to an RF signal before being transmitted. In many wireless communication devices one or more receivers and one or more transmitters operate concurrently on different frequency bands. This means that the transmitters must control their spurious emissions into the receive bands so as not to degrade the performance of the concurrently operating receivers. The transmit spurious emissions into the receive band of a concurrently operating receiver can be called receive band noise. 
     In a transmitter, the receive band noise and transmit signal linearity need to be concurrently met while maintaining optimal power consumption and increasingly wider signal bandwidth. As transceiver design moves to smaller geometries and processing nodes, the relatively high cost of integrating such components as baseband digital-to-analog converters (DAC), analog filters, upconverters, and the like, on the same semiconductor substrate is posing a challenge. Furthermore, the images (also referred to as aliases or harmonics) associated with Nyquist sampling of the transmit signal, as well as the quantization noise in concurrently operated receive bands need to be properly handled in order to meet emission requirements and receiver sensitivity. 
     BRIEF SUMMARY 
     A communication device, in accordance with one embodiment of the present invention, includes a transmitter that in turn includes, in part, a delta-sigma modulator receiving an RF signal and characterized by a noise transfer function having a multitude of zeroes positioned substantially near frequency bands of a concurrently received signals, and a multi-phase digital-to-analog (DAC) converter configured to convert the output signal of the delta-sigma modulator to an analog signal. The DAC is characterized by a transfer function that passes a selected desired Nyquist image of a sampled signal to its output (i.e., the desired signal), while attenuating a multitude of the undesired images of the sampled signal. 
     In one embodiment, the communication device further includes, in part, a digital modulator configured to upconvert the transmit signal from a baseband frequency signal to the RF signal. In one embodiment, the RF signal received by the delta-sigma modulator is a digital RF signal. 
     In one embodiment, the DAC includes a multitude of stages each of which is associated with a gain coefficient (tap weight) of a finite impulse response filter (FIR). In one embodiment, the communication device is configured to transmit at a frequency defined by an odd multiple of a fraction of the sampling frequency. In one embodiment, the communication device is configured to transmit at odd multiples of one-fourth of the sampling frequency. 
     In one embodiment, the baseband signal includes an in-band signal component and a quadrature-phase signal component. In one embodiment, the DAC attenuates the third, fifth, and seventh harmonics of the sampled signal. In one embodiment, the DAC is a current steering DAC each stage of which includes a current source providing a current whose value is defined by a tap weight associated with that stage. 
     In one embodiment, the fraction of the sampling frequency used to transmit the signal defines the number of phases of the sampling clock signal received by the DAC. In one embodiment, the DAC&#39;s output is applied to a load, the output of which is applied to an amplifier. In one embodiment, the delta-sigma modulator includes a multitude of stages each of which comprises a forward path section and a feedback path section. The forward path section is associated with a different one of the zeroes and the feedback path section is associated, along with the feedback path sections of the rest of the stages, with the poles of the signal and noise transfer functions. In one embodiment, each stage of the delta-sigma modulator receives up to three tap coefficients. 
     In one embodiment, the communication device further includes a receiver configured to receive at a frequency defined by an odd multiple of a fraction of the sampling clock signal frequency. In one embodiment, the fraction is ¼. In one embodiment, the communication device further includes, in part, a local oscillator shared by the transmitter and the receiver. The shared LO has a frequency that is a multiple of the receive frequency. In one embodiment, the subset of the plurality of images being attenuated is defined by odd multiples of a fraction of the sampling clock signal frequency. In one embodiment, such fraction is defined by a ratio of the transmit frequency to the receive frequency. 
     A method of wireless communication, in accordance with one embodiment of the present invention, includes, in part, modulating an RF signal to generate a multitude of zeroes positioned substantially near the frequency band of a receive signal or a multitude of frequency bands of concurrently received signals, attenuating a multitude of odd-harmonically spaced Nyquist images of a sampled signal, converting the modulated RF signal to an analog signal, and transmitting the analog signal. 
     In one embodiment, the method further includes upconverting a baseband signal to generate the RF signal, which may be a digital RF signal. In one embodiment, the RF signal is transmitted at a frequency defined by an odd multiple of a fraction of the sampling clock signal frequency. In one embodiment, the RF signal is transmitted at a frequency defined by an odd multiple of one-fourth of the sampling clock signal frequency. 
     In one embodiment, the modulated RF signal is converted to the analog signal using a current steering DAC. In one embodiment, the current steering DAC includes a number of stages that is one higher than twice the number of the undesired Nyquist images of the sampled signal being attenuated. In one embodiment, each stage of the current steering DAC includes a current source providing a current whose value is defined by a tap weight associated with that stage. 
     In one embodiment, the fraction of the sampling frequency used to transmit the signal defines the number of phases of the sampling clock signal received by the DAC. In one embodiment, the method further includes applying the output of the DAC to a load, and applying the output of the load to an amplifier. In one embodiment, the method further includes modulating the RF signal via a multitude of stages each of which is associated with a different one of the zeroes. In one embodiment, the method further includes applying up to three tap coefficients to each of the stages. 
     In one embodiment, the method further includes, in part, receiving a second RF signal at a frequency defined by an odd multiple of a fraction of a sampling clock signal frequency used to sample the baseband transmit signal. In one embodiment, the fraction is ¼. In one embodiment, the method further includes sharing a local oscillator between a transmitter transmitting the RF signal and a receiver receiving the second RF signal. In one embodiment, the shared LO has a frequency that is a multiple of the receive frequency. In one embodiment, the subset of the plurality of images being attenuated is defined by odd multiples of a fraction of the sampling clock signal frequency. In one embodiment, such a fraction is defined by a ratio of the transmit frequency to the receive frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the disclosure are illustrated by way of example. In the accompanying figures, like reference numbers indicate similar elements, and: 
         FIG. 1  is a block diagram of a wireless communication device, in accordance with one embodiment of the present invention. 
         FIG. 2  is a block diagram of a Delta-Sigma modulator disposed in the wireless communication device of  FIG. 1 , in accordance with one embodiment of the present invention. 
         FIG. 3  shows the noise spectrum of the Delta-Sigma modulator of  FIG. 2  when configured to having zero pairs at select receiver frequency bands, in accordance with one embodiment of the present invention. 
         FIG. 4  is a block diagram of a finite impulse response (FIR) digital-to-analog (DAC) converter, disposed in the wireless communication device of  FIG. 1 , in accordance with one embodiment of the present invention. 
         FIG. 5  is a simplified schematic diagram of one of the stages of the FIR DAC of  FIG. 4 , in accordance with one embodiment of the present invention. 
         FIG. 6A  is the signal transfer function of the FIR DAC of  FIG. 5  when configured to suppress the 3 rd , 5 th  and 7 th  harmonics of the sampled signal, in accordance with one embodiment of the present invention. 
         FIG. 6B  shows the relationship between phases φ 1 , φ 2 , φ 3 , φ 4  of the clock signals applied to various delay stages of the FIR DAC of  FIG. 5  when configured to suppress the 3 rd , 5 th  and 7 th  harmonics of the sampled signal, in accordance with one embodiment of the present invention. 
         FIG. 7A  is the signal transfer function of the FIR DAC of  FIG. 5  when configured to suppress the 1st, 5 th  and 7 th  harmonics of the sampled signal, in accordance with one embodiment of the present invention. 
         FIG. 7B  shows the relationship between phases φ 1 , φ 2 , φ 3 , φ 4  of the clock signals applied to various delay stages of the FIR DAC of  FIG. 5  when configured to suppress the 1 st , 5 th  and 7 th  harmonics of the sampled signal, in accordance with one embodiment of the present invention. 
         FIG. 8  is a block diagram of a wireless communication device where local oscillator can be shared by the transmitter and the receiver, in accordance with one embodiment of the present invention 
         FIG. 9  shows a flowchart for transmitting an RF signal, in accordance with one embodiment of the present invention. 
         FIG. 10  is a block diagram of another finite impulse response (FIR) digital-to-analog (DAC) converter, disposed in the wireless communication device of  FIG. 1 , in accordance with one embodiment of the present invention. 
         FIG. 11  shows a block diagram of a combined Fs/4 and 3F s /4 mode clock phase generator and 1 st  tap data generator, in accordance with one embodiment of the present invention. 
         FIG. 12  shows the relationship between fs2x, fs2xb, φ a , φ b , and phases φ 1  and φ 2  of the clock signals applied to various delay stages of the FIR DAC of  FIG. 10 , in accordance with one embodiment of the present invention. 
         FIG. 13  shows tap clock and data path of a register delay block, in accordance with one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Several illustrative embodiments will now be described with respect to the accompanying drawings, which form a part hereof. While particular embodiments, in which one or more aspects of the disclosure may be implemented, are described below, other embodiments may be used and various modifications may be made without departing from the scope of the disclosure. 
       FIG. 1  is a simplified block diagram of a wireless communication device  50 , in accordance with one embodiment of the present invention. Device  50  may be a cellular phone, a personal digital assistant (PDA), a modem, a handheld device, a laptop computer, and the like. Device  50  may communicate with one or more base stations on the downlink (DL) and/or uplink (UL) at any given time. The downlink (or forward link) refers to the communication link from a base station to the device. The uplink (or reverse link) refers to the communication link from the device to the base station. 
     Device  50  may be a multiple-access system capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such systems include code division multiple access (CDMA) systems, wide-band CDMA (WCDMA), frequency division duplex long term evolution (LTE), time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, orthogonal frequency division multiple access (OFDMA) systems, spatial division multiple access (SDMA) systems, and the like. 
     Device  50  is shown as including, in part, digital modulator  10 , DAC  20 , load  30 , antenna  45 , and oscillator  55 . Device  50  is also shown as including optional drive amplifier  35  and power-amplifier  40  and RF filter  90 . Oscillator  55  is configured to generate a sampling clock signal F s  whose frequency is defined by the frequency of the transmit clock signal F TX . The following description of device  50  is made with reference to a sampling clock signal F s  having a frequency that is (4/n) times the frequency of the transmit clock signal F TX , where n is an odd integer ranging from 1 to 7 corresponding to the harmonics of the Nyquist images of the sampled signal T X RF being attenuated. It is understood, however, that embodiments of the present invention apply to any other relationship between clock signals F s  and F TX . It is also understood that in other embodiments of the present invention n may be any other odd integer, such as 9, 11, etc. 
     Digital modulator  10  is configured to upconvert the I/Q baseband transmit signals TxBB_I and TXBB_Q to an upsampled digital RF signal TxRF which is delivered to DAC  20 . DAC  20  is shown as including a Delta-Sigma modulator  100  and a multi-phase harmonic attenuator  200 , also referred to herein as Finite Impulse Response (FIR) DAC. As described in detail below, Delta-Sigma modulator  100  is configured to attenuate the noise generated by the transmitter at the frequencies where the received signal is present. As is also described in detail below, DAC  200  is configured to attenuate the odd-harmonically spaced Nyquist images (also referred to herein as images or aliases) generated as a result of the sampling operation performed by digital modulator  10 . In response to the output signal of Delta-Sigma modulator  100 , DAC  200  drives a passive load  30  that may be an LC tank resonating at the RF frequency. The LC tank may be formed by connecting one or more inductors in parallel with one or more capacitors. The output of load  30  is applied to antenna  45  via optional driver amplifier (DA)  30  and optional power amplifier (PA)  40  and optional RF Filter  90 . In common applications RF Filter  50  may be a surface acoustic wave (SAW) filter or a duplexer. 
     Delta-Sigma modulator  100  is adapted to generate quantization noise transfer function zero pairs at frequencies substantially near the concurrently operating receive frequency bands. Delta-Sigma modulator  20  has a z-domain quantization noise transfer function H NTF (z) defined as follows: 
     
       
         
           
             
               
                 
                   
                     
                       H 
                       NTF 
                     
                      
                     
                       ( 
                       z 
                       ) 
                     
                   
                   = 
                   
                     
                       ∏ 
                       
                         K 
                         = 
                         1 
                       
                       
                         N 
                         / 
                         2 
                       
                     
                      
                     
                         
                     
                      
                     
                       
                         1 
                         - 
                         
                           2 
                            
                           cos 
                            
                           
                               
                           
                            
                           2 
                            
                           π 
                            
                           
                             
                               f 
                               Rxk 
                             
                             
                               F 
                               S 
                             
                           
                            
                           
                             z 
                             
                               - 
                               1 
                             
                           
                         
                         + 
                         
                           z 
                           
                             - 
                             2 
                           
                         
                       
                       
                         1 
                         - 
                         
                           2 
                            
                           
                             r 
                             k 
                           
                            
                           cos 
                            
                           
                               
                           
                            
                           
                             ϕ 
                             k 
                           
                            
                           
                             z 
                             
                               - 
                               1 
                             
                           
                         
                         + 
                         
                           
                             r 
                             k 
                             2 
                           
                            
                           
                             z 
                             
                               - 
                               2 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In the above expression (1), F s  represents the sampling frequency used by digital modulator  10 , r k  represents the pole magnitudes, φ k  represents the angular frequency of poles geometrically distributed around π/2, f Rxk  represents the multiple receive frequency bands with k being an index varying from 1 to N/2, and N represents the number of zero pairs being generated at the receive frequency bands. In one example, r k  may vary from 0.25 to 0.5. As described above, the quantization noise transfer function H NTF (z) is selected to have zero pairs at frequencies substantially near the concurrently operating receive frequency bands. 
       FIG. 2  is an exemplary block diagram of a Delta-Sigma modulator with an H NTF (z) characterized by expression (1), and in which N is 6. The Delta-Sigma modulator is shown as including 3 stages  120 ,  150  and  180 , each of which is a second order stage adapted to generate a pair of zeros. Consequently, the Delta-Sigma modulator shown in  FIG. 2  is adapted to generate 3 pairs of zeros at frequencies substantially near the concurrently operating receive frequency band. As is well known, each z −1  block represents a delay stage implemented by a register. Quantizer block  185  receives the output of stage  180  and quantizes output to the desired bit-width. Quantizer block  185  can be modeled as a summation block that receives the output of stage  180  as well as the quantization noise E. It is understood that H NTF (z) represents the transfer function of the noise source generating the quantization noise E created by quantization block  185 . In an embodiment quantization block  185  could take a 16-bit input and quantize it to 4 bits. 
     Tap filter values are set in accordance with coefficients α, β 1  and β 2 . Coefficients α are selected to define the zero pairs of the noise transfer function at multiple receive band frequencies f Rxk  and may be computed in accordance with the expression below: 
     
       
         
           
             
               α 
               k 
             
             = 
             
               
                 - 
                 2 
               
                
               cos 
                
               
                   
               
                
               2 
                
               π 
                
               
                 
                   f 
                   Rxk 
                 
                 
                   F 
                   S 
                 
               
             
           
         
       
     
     Coefficients β 1  and β 2  are selected to define the poles of the noise transfer function and thus determine the stability of the Delta-Sigma modulator. An algorithm for determining β 1  and β 2  of stage  120  is shown below. It is understood that a similar algorithm may be used to determine coefficients β 1  and β 2  of the stages  150 ,  180 , as well as similar coefficients of any higher order stage (not shown) of a Delta-Sigma modulator. For stage  120 , k is 1, therefore: 
     
       
         
           
             
               
                 
                   
                     
                       α 
                       1 
                     
                     = 
                     
                       
                         - 
                         2 
                       
                        
                       cos 
                        
                       
                           
                       
                        
                       2 
                        
                       π 
                        
                       
                         
                           f 
                           
                             Rx 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                         
                           F 
                           S 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         H 
                         NTF 
                       
                        
                       
                         ( 
                         z 
                         ) 
                       
                     
                     = 
                     
                       
                         1 
                         - 
                         
                           
                             α 
                             1 
                           
                            
                           
                             z 
                             
                               - 
                               1 
                             
                           
                         
                         + 
                         
                           z 
                           
                             - 
                             2 
                           
                         
                       
                       
                         1 
                         - 
                         
                           2 
                            
                           
                             r 
                             1 
                           
                            
                           cos 
                            
                           
                               
                           
                            
                           
                             ϕ 
                             1 
                           
                            
                           
                             z 
                             
                               - 
                               1 
                             
                           
                         
                         + 
                         
                           
                             r 
                             1 
                             2 
                           
                            
                           
                             z 
                             
                               - 
                               2 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Using well known rules for deriving transfer functions from their associated signal flow graphs, it is seen that H SFG  may be defined as follows: 
     
       
         
           
             
               
                 
                   
                     
                       H 
                       SFG 
                     
                     = 
                     
                       
                         Δ 
                          
                         
                           ∑ 
                           Mout 
                         
                       
                       E 
                     
                   
                 
               
               
                 
                   
                     = 
                     
                       1 
                       
                         1 
                         - 
                         
                           
                             B 
                             2 
                           
                            
                           
                             
                               z 
                               
                                 - 
                                 1 
                               
                             
                             
                               1 
                               - 
                               
                                 
                                   α 
                                   1 
                                 
                                  
                                 
                                   z 
                                   
                                     - 
                                     1 
                                   
                                 
                               
                               + 
                               
                                 z 
                                 
                                   - 
                                   2 
                                 
                               
                             
                           
                         
                         - 
                         
                           
                             β 
                             1 
                           
                            
                           
                             z 
                             
                               - 
                               1 
                             
                           
                            
                           
                             
                               z 
                               
                                 - 
                                 1 
                               
                             
                             
                               1 
                               - 
                               
                                 
                                   α 
                                   1 
                                 
                                  
                                 
                                   z 
                                   
                                     - 
                                     1 
                                   
                                 
                               
                               + 
                               
                                 z 
                                 
                                   - 
                                   2 
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
             
               
           
         
       
     
     The above expression may further be simplified as: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             H 
                             SFG 
                           
                           = 
                           
                             
                               1 
                               - 
                               
                                 
                                   α 
                                   1 
                                 
                                  
                                 
                                   z 
                                   
                                     - 
                                     1 
                                   
                                 
                               
                               + 
                               
                                 z 
                                 
                                   - 
                                   2 
                                 
                               
                             
                             
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     
                                       α 
                                       1 
                                     
                                      
                                     
                                       z 
                                       
                                         - 
                                         1 
                                       
                                     
                                   
                                   + 
                                   
                                     z 
                                     
                                       - 
                                       2 
                                     
                                   
                                 
                                 ) 
                               
                               - 
                               
                                 
                                   β 
                                   2 
                                 
                                  
                                 
                                   z 
                                   
                                     - 
                                     1 
                                   
                                 
                               
                               - 
                               
                                 
                                   β 
                                   1 
                                 
                                  
                                 
                                   z 
                                   
                                     - 
                                     2 
                                   
                                 
                               
                             
                           
                         
                       
                     
                     
                       
                         
                           = 
                           
                             
                               1 
                               - 
                               
                                 
                                   α 
                                   1 
                                 
                                  
                                 
                                   z 
                                   
                                     - 
                                     1 
                                   
                                 
                               
                               + 
                               
                                 z 
                                 
                                   - 
                                   2 
                                 
                               
                             
                             
                               
                                 
                                   ( 
                                   
                                     1 
                                     - 
                                     
                                       α 
                                       1 
                                     
                                     + 
                                     
                                       β 
                                       2 
                                     
                                   
                                   ) 
                                 
                                  
                                 
                                   z 
                                   
                                     - 
                                     1 
                                   
                                 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     1 
                                     - 
                                     
                                       β 
                                       2 
                                     
                                   
                                   ) 
                                 
                                  
                                 
                                   z 
                                   
                                     - 
                                     2 
                                   
                                 
                               
                             
                           
                         
                       
                     
                   
                     
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Given that the numerators of expression (2) and (3) are equal, solving for coefficients β 1  and β 2  results in the following: 
       β 1 =1− r   1   2  
 
       β 2 =2 r   k  cos φ k −α 1  
 
     The Delta-Sigma modulators in  FIGS. 1 and 2  are shown as being 16-bits wide and having 4-bit outputs. It is understood however, that a Delta-Sigma modulator, in accordance with the present invention, may have an output that has fewer or more than 4 bits. The bit-width of the Delta-Sigma modulator defines the number of quantization levels and corresponding quantization noise power spectral density (PSD), in turn affecting the peak noise PSD outside the receive band spectrum. As a consequence of this peak noise PSD, the bit-width of the Delta-Sigma modulator is also defined, in part, by the transmitter spectral emission requirements with which device  50  is required to comply.  FIG. 3  shows the noise spectrum of the Delta-Sigma modulator of  FIG. 2  when configured to have 2 zero pairs at receiver frequencies 2170 MHz (identified as point b), 2 zero pairs in the receiver frequency band 1810-1875 MHz (identified as point c), and 2 zero pairs at receive frequency 1575 MHz (identified as point d). 
     FIR DAC  200  is adapted to suppress a number of Nyquist images of the sampled signal TxRF. For example, if the transmit frequency of TxRF is ¼ of F s , FIR DAC  200  may be configured to eliminate the 3 rd , 5 th  and 7 th  odd-harmonically spaced images of TxRF. Likewise, if the transmit frequency is ¾ of F s , FIR DAC  200  may be configured to eliminate, the 1 st , 5 th  and 7 th  images. It is understood that in other embodiments multiple integers of a fraction other than ¼ of the sampling clock F s  frequency may be used for transmission. 
     The following description is made with reference to a FIR DAC configured to suppress  3  of the images of the TxRF. It is understood however that, in accordance with the present invention, any number of images of the TxRF, such as four (e.g.,  3   rd , 5 th ,  7   th  and 9 th  harmonics) may be suppressed. It is further assumed below that the transmit signal TxRF has a frequency F Tx  that is (n*¼) of the sampling clock F s  frequency, where n is a member of the set {1, 3, 5, 7}. To achieve this, the FIR DAC is configured to have a signal transfer function that has a defined gain at the desired frequency-representative of a desired image at multiple F s /4 frequencies indexed by any one of {1, 3, 5, 7}—and a zero at each of the undesired harmonics to be suppressed—representing undesired images indexed by {α1, α2, α3} where α1, α2, α3 may take on any of the values of {1, 3, 5, 7} except for the value selected for n. For example, if the desired image, i.e., n is selected to be the first harmonic, α1, α2, α3 may have values of 3, 5, 7 representing the undesired harmonics. To suppress three of the harmonics of signal F s , the FIR DAC is selected to have 7 taps. 
       FIG. 4  is a schematic block diagram of a 7-tap, 16-level FIR DAC  200  (e.g., first embodiment) in accordance with one exemplary embodiment of the present invention. FIR DAC  200  is shown as including a thermo decoder  202  receiving the output signal of the Delta-Sigma modulator, and 7 delay stages  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216 . The outputs of the delay stages are shown as being applied to current steering DAC stages  220 ,  222 ,  224 ,  226 ,  228 ,  230 ,  232  which respectively receive tap weights of 1, h 1 , h 2 , h 3 , h 2 , h 1 , and 1. The currents generated by the 7 stages, namely currents I out0 , I out1 , I out2 , I out3 , I out4 , I out5  and I out6  are summed by summing network  250  and subsequently converted to an analog voltage forming the output of FIR DAC  200 . As DAC stages  220 ,  222 ,  224 ,  226 ,  228 ,  230  and  232  can be current steering DAC stages, summing network  250  can be as simple as a pair of wires connecting the differential DAC stage outputs. 
     Thermo decoder  202  is well known and is adapted to convert the 4-bit output of the Delta-Sigma modulator to 15-bit data corresponding to 16 distinct DAC output levels that are delivered to each of the delay stages  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216  (z −1/4 ) each of which is shown as including a register having a clock signal that receives a different one of four different clock phases φ 1 , φ 2 , φ 3 , φ 4 . As described above, the output of each delay stage is received by an associated DAC stage. For example, the output of delay stage  204  is received by associated DAC stage  220 . Likewise, the output of delay stage  208  is received by associated DAC stage  224 ; and the output of delay stag  216  is received by associated DAC stage  232 . 
     Each of DAC stages  220 ,  222 ,  224 ,  226 ,  228 ,  230 ,  233  is adapted to generate a current defined by the data it receives from its associated delay stage and its selected tap weight. For example, the output of DAC stage  220  is defined by the 15-bit data it receives from its associated delay stage  204  and its tap weight which is selected to be 1. Likewise the output of DAC stage  222  is defined by the 15-bit data it receives from its associated delay stage  206  and its selected tap weight h 1 ; and the output of DAC stage  224  is defined by the 15-bit data it receives from its associated delay stage  208  and its selected tap weight h 2 . 
       FIG. 5  is a simplified schematic diagram of DAC stage  226 . DAC stage  226  is shown as including 15 parallel input stages each receiving a different one of differential data bits q3&lt;n&gt;, nq3&lt;n&gt; (n is an integer varying from 1 to 15 in this exemplary embodiment) and generating a pair of differential currents I out3   +  and I out3   −  in response. The current source  302  disposed in DAC stage  226  has a value defined by h 3 *I ref , where I ref  is a reference current and h 3  is the tap weight selected for stage  226 . While not shown, it is understood that each of the remaining DAC stages  220 ,  222 ,  224 ,  228 ,  230 ,  232  has a current source defined by a product of I ref  and the stage&#39;s selected tap weight. For example, the current sources in DAC stage  222  and  228  have a value defined by h 1 *I ref ; likewise the current sources in DAC stage  220  and  230  have a value defined by 1*I ref , i.e., I ref . Furthermore, although not shown, it is understood that each of the remaining DAC stages  220 ,  222 ,  224 ,  228 ,  230 ,  232  includes 15 parallel input stages each receiving the differential data bits q3&lt;n&gt;, nq3&lt;n&gt; and generating a current in a manner similar to that shown for DAC stage  226 . As described above, the currents generated by the DAC stages are added together by summing block  250  and converted to an analog signal representing the FIR DAC&#39;s output voltage. 
     FIR DAC  200  shown in  FIGS. 1 and 4  has the following signal transfer function h(z): 
     
       
         
           
             
               h 
                
               
                 ( 
                 z 
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     The transfer function h(z) has 7 terms signifying the fact that FIR DAC  200  is configured to suppress three odd-harmonically spaced images of the TxRF. Consequently, if the desired signal is centered at the first odd-harmonic image (i.e., n=1), the FIR DAC may be configured to suppress the 3 rd , 5 th  and 7 th  harmonic images of the TxRF. Likewise, if the desired signal is centered around the third harmonic image (i.e., n=3), the FIR DAC may be configured to suppress the 1 st , 5 th  and 7 th  harmonic images of the TxRF. 
       FIG. 6A  is the signal transfer function of FIR DAC  200  when n is set to 1. As is seen, the signal centered at the first harmonic (the desired image) has a relatively higher amplitude whereas the signals at 3 rd , 5 th  and 7 th  harmonic images are substantially attenuated.  FIG. 6B  shows the relationship between phases φ 1 , φ 2 , φ 3 , φ 4  applied to various delay stages of FIR DAC  200  when the first harmonic is selected as the desired frequency, and the 3 rd , 5 th  and 7 th  harmonic images are being attenuated.  FIG. 7A  is the signal transfer function of FIR DAC  200  when n is set to 3. As is seen, the signal centered at the third harmonic (the desired image) has a relatively higher amplitude whereas the signals at 1 st , 5 th  and 7 th  harmonics are substantially attenuated.  FIG. 7B  shows the relationship between phases φ 1 , φ 2 , φ 3 , φ 4  applied to various delay stages of FIR DAC  200  when the third harmonic is selected as the desired frequency, and the 1 st , 5 th  and 7 th  harmonic images are being attenuated. 
       FIG. 8  is a simplified block diagram of a wireless communication device  170 , in accordance with another exemplary embodiment of the present invention. The transmit path of wireless communication device  170  is shown as including, in part, a digital modulator  10 , a delta-sigma modulator  100 , a FIR DAC  200 , load  30 , antenna  45 , drive amplifier  35 , power-amplifier  40 , and RF filter  90 . The receive path of wireless communication device  170  is shown as including, in part, a low-noise amplifier (LNA)  70 , a frequency downconverter  72 , and baseband circuitry  74 . Wireless communication device  170  is further shown as including a single common local oscillator  76  that generates an oscillating signal OSC used for the receive signal downconversion. Signal OSC is also used to generate sampling clock signal F s  that samples the transmit signal. 
     The common local oscillator  76  has a frequency defined by k*F Rx , where k is an integer, and F Rx  is the receive frequency. In the exemplary embodiment of  FIG. 8 , k is assumed to have a value defined by the set {2, 4}, however, it is understood that k may have any other integer values. Wireless communication device  170  is also shown as including a fractional frequency divider  78  that receives signal OSC and, in response, generates the sampling clock signal F s , applied to digital modulator  10 , delta-sigma modulator  100 , and FIR DAC  200 . Fractional frequency divider  78  is configured to divide the frequency of signal OSC by the ratio n/q, where n is the index of the desired (selected for passing to the output) odd-harmonically spaced sampled signal, as described above, and q is an integer defined by the set {1, 2} in this exemplary embodiment. Divider  82  is shown as dividing the frequency of the oscillating signal OSC by k. 
     In wireless communication device  170 , the Nyquist images of the sampled transmit signal are positioned at n*(F Tx /F Rx )*F s . This is in contrast to the Nyquist images for wireless communication device  50  that are positioned at n*F s /4, as described above. Therefore, in order to suppress the undesired images of the transmit signal, the FIR DAC  200  of wireless communication device  170  has a signal transfer function h(z) defined as following: 
     
       
         
           
             
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     with the coefficients as shown below: 
     
       
         
           
             
                 
             
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       FIG. 9  shows a flowchart  900  for transmitting an RF signal, in accordance with one embodiment of the present invention. Before transmission, the RF signal is modulated 902 to generate a multitude of quantization noise transfer function zero pairs positioned substantially near the frequency band of the receive signal. A multitude of the harmonics of the sampled signal are then suppressed  904  in the modulated signal. The desired modulated signal is converted  906  to an analog voltage and or current and subsequently delivered to an antenna for transmission. 
       FIG. 10  illustrates a schematic diagram for an alternate 7-tap, 16-level FIR DAC using a combined clock phase generator. The difference between the FIR DAC illustrated in  FIG. 10  (e.g., second embodiment) and the FIR DAC illustrated in  FIG. 4  (e.g., first embodiment) is that the second embodiment uses a combined F s /4 and 3F s /4 clock phase generator  1003  to generate clocks for both the F x /4 and 3F s /4 modes of operation. In addition, the second embodiment places thermo decoders between outputs of the register delays and the DAC stages, followed by another set of register delays for re-timing to the clock phases. As a result, the FIR DAC in the second embodiment achieves better timing accuracy and consumes less power compared to the FIR DAC in the first embodiment. The FIR DAC in the second embodiment enables a single circuit design to implement both F s /4 and 3F s /4 frequency plans (e.g., both  FIGS. 6A and 7A ) with a programming option. 
     As illustrated in  FIG. 10 , the FIR DAC  1000  includes a combined clock phase generator  1003 , delay stages  1004 ,  1006 ,  1008 ,  1010 ,  1012 ,  1014 , and  1016 , current steering DAC stages  1020 ,  1022 ,  1024 ,  1026 ,  1028 ,  1030 , and  1032 , and a summing network  1050 . The outputs of the delay stages  1004 ,  1006 ,  1008 ,  1010 ,  1012 ,  1014 , and  1016  are shown as being applied to current steering DAC stages  1020 ,  1022 ,  1024 ,  1026 ,  1028 ,  1030  and  1032  which respectively receive tap weights of 1, h 1 , h 2 , h 3 , h 2 , h 1 , and 1. The currents generated by the seven stages, namely currents I out0 , I out1 , I out2 , I out3 , I out4 , I out5  and I out6  are summed by summing network  1050  and subsequently converted to an analog voltage forming the output of the FIR DAC  1000 . As with summing network  250 , summing network  1050  can be as simple as a wired connection of differential DAC stages  1020 ,  1022 ,  1024 ,  1026 ,  1028 ,  1030  and  1032  outputs. 
       FIG. 11  illustrates the combined clock phase generator  1003  for F s /4 and 3F s /4 modes (as illustrated in  FIG. 10 ), in accordance with one embodiment of the present disclosure. As illustrated, the combined clock phase generator  1003  may include registers  1104 ,  1108 ,  1112 ,  1118  and  1120 , XOR  1110 , gain units  1114  and  1116 , and the like. The combined clock phase generator  1003  receives the output of the Delta-Sigma modulator and the Fs2x signal as inputs and generates a 4-bit first tap data and two phases of the clock signal (e.g., φ 1  and φ 2 ) as outputs. As illustrated, φ 1  and φ 2  are generated by passing the fs2x and its invert signal (e.g., fs2xb) through gain units  1114  and  1116 , respectively. It should be noted that the en — ¾fs is passed through a logical AND operation with the φ a  signal before being input to the XOR gate  1110  (not shown). 
       FIG. 12  shows the 3F s /4 waveforms that are generated in the combined clock phase generator  1003  when the en — ¾fs signal  1102  is asserted. In this Figure, waveforms for the fs2x (e.g., which has a frequency equal to 2×F s ), fs2xb, internal signals φ 1  and φ 2  and the output clock signals φ 1  and φ 2  are illustrated. 
     The combined clock phase generator  1003  converts the 4-bit output of the Delta-Sigma modulator to 4-bit first tap data that are delivered to each of the register delay stages  1004 ,  1006 ,  1008 ,  1010 ,  1012 ,  1014 ,  1016  (e.g., z −1/4 ), each of which is shown as including a register having a clock signal that receives a different one of two different clock phases φ 1 , φ 2 . As described above, the output of each delay stage is received by an associated DAC stage. For example, the output of delay stage  1004  is received by associated DAC stage  1020 . Likewise, the output of delay stage  1008  is received by associated DAC stage  1024 ; and the output of delay stag  1016  is received by associated DAC stage  1032 . 
     Similar to the first embodiment, each of DAC stages  1020 ,  1022 ,  1024 ,  1026 ,  1028 ,  1030 , and  1032  in the second embodiment is adapted to generate a current defined by the data it receives from its associated delay stage and its selected tap weight. For example, the output of DAC stage  1020  is defined by the 15-bit data it receives from its associated delay stage  1004  and its tap weight which is selected to be 1. Likewise the output of DAC stage  1022  is defined by the 15-bit data it receives from its associated delay stage  1006  and its selected tap weight h 1 ; and the output of DAC stage  1024  is defined by the 15-bit data it receives from its associated delay stage  1008  and its selected tap weight h 2 . 
     As shown in  FIG. 10 , the z −1/4  tap delays are clocked at 2×F s  by either φ 1  or φ 2  as in the original 3F s /4 mode, but the data are modulated per the en — ¾fs signal  1102 .  FIG. 13  shows tap clock and data path of one of the z −1/4  tap delays (e.g., the tap delay  1010 ) in  FIG. 10  in more detail. As illustrated, the tap delay  1010  may include registers  1302 ,  1306  and  1310 , a FIR coefficient polarity control unit  1304  and a thermo decoder  1308 . As described earlier, the thermo decoder  1308  is placed inside the register delay  1010 , followed by another set of register delays for re-timing the clock phases. 
     The second embodiment shown in  FIG. 10  consumes less power compared to the first embodiment shown in  FIG. 4 , because placing the 4-to-15 thermo decoders after the tap delays allows the tap data path to be 4-bits wide, compared to 15-bits used in the embodiment shown in  FIG. 4 . 
     The above embodiments of the present invention are illustrative and not limitative. The embodiments of the present invention are not limited by the noise transfer function of the Delta-Sigma modulator, by the number of stages (number of zero pairs) of the Delta-Sigma modulator, or by the bit-width of the modulator. The above embodiments of the present invention are not limited by the signal transfer functions or the number of harmonics that the DAC may be configured to suppress. The above embodiments of the present invention are not limited by any particular relationship between the transmit signal frequency and the sampling clock frequency.