Abstract:
A method and apparatus for suppressing a first replica of an image spectra and reinforcing a second replica of the image spectra are disclosed. A Hilbert transformer is applied to a first digital signal in a first path. The Hilbert-transformed signal in the first path is converted to an analog signal in the first path. The analog signal in the first path is delayed by an additional 90 degrees to produce a first phase-shifted analog signal. A phase delay is applied to the first digital signal in the second path and the phase-delayed signal in the second path is converted to analog to produce a second phase-shifted analog signal. A combiner combines the first and second phase-delayed analog signals to produce an analog output signal with at least one replica of the image spectra that is substantially suppressed.

Description:
TECHNICAL FIELD 
     The present invention relates to wireless communication, and in particular to a method and system for image spectra suppression in a digital signal. 
     BACKGROUND 
     Digital processors are used in modern communication devices to perform complex signal processing while adhering to reasonable power and size constraints. For example, in order to transfer information from one radio to another radio, digital signals are converted into analog transmission signals. This conversion process is performed by a digital-to-analog converter (DAC). 
     The frequency representation of a digital signal consists of an infinite number of replicas of the equivalent baseband analog signal. The replicas, also referred to as image spectra, are separated in the frequency domain by f s , which is the frequency of the digital sampling clock. All but one replica must be suppressed at the output of a digital-to-analog converter (DAC). Otherwise, due to the close frequency spacing of the replicas, non-linear action in an up-conversion mixer or power amplifier will result in inter-modulation distortion in the pass band. 
     In the frequency representation of the signal, the signal energy that lies between zero and f s /2 is said to be in the first Nyquist zone. The signal energy that lies between f s /2 and f s  is said to be in the second Nyquist zone. The third Nyquist zone is between f s  and 3f s /2, etc. Conventionally the signal energy in the first Nyquist zone is the desired replica. The higher frequency replicas are suppressed at the DAC output by an analog low-pass filter and the sample-and-hold function of the DAC. However, a low-pass filter used at the output of the conventional DAC does not integrate well onto integrated circuit chips due to the large physical area consumed by the filter and the precision issues related to passive devices. 
     In an alternative approach, the digital-to-analog conversion and frequency up-conversion can be achieved by using a higher frequency replica at the DAC output instead of the replica in the first Nyquist zone. This approach uses a band pass filter at the DAC output to suppress the other replicas. The band pass filter could be centered at 3f s  with a pass band bandwidth equal to f s . This technique is useful when the desired transmit signal is centered at a multiple of the sampling frequency. Also, a sample-and-hold function of the DAC must not suppress the desired higher-frequency replica (like the return-to-zero, or 2-phase return-to-zero). 
     However, transmitters may need to operate in multiple frequency bands. The digital clock frequency within these transmitters cannot easily be changed to ensure that a high-frequency replica lands exactly at the desired transmit frequency. Therefore, the baseband signal is shifted from DC to some digital IF frequency (f IF ) to offset the replicas away from multiples of the clock frequency. The band pass filter at the DAC output must choose only one of the high frequency replicas. If two replicas are closely spaced in the frequency domain then there is a small region for the filter to transition from pass band to stop band. A short transition region requires a highly selective filter, which means the filter must be physically large and complex to design. 
     Therefore, what is needed is a method and system for performing digital-to-analog conversion that suppresses unwanted image spectra, does not require highly selective filtering, and scalable to other frequencies. 
     SUMMARY 
     The present invention advantageously provides a method and system for suppressing a first replica of an image spectra and reinforcing a second replica of the image spectra, the second replica being adjacent to the first replica. According to one aspect, the invention provides a device for converting a digital input signal to an analog signal. The device includes a first path and a second path. The first path has a transformer and a first digital-to-analog converter (DAC). The transformer phase-shifts replicas of the image spectra of the input signal that fall in even Nyquist zones by substantially a first predetermined amount and phase-shifts replicas of the image spectra that fall in odd Nyquist zones by substantially the negative of the first predetermined amount. The first DAC converts the transformed input signal in the first path to a first analog signal in the first path. The second path has a coarse delay compensator and a second DAC. The coarse delay compensator delays the digital input signal in the second path by an integer number of clock cycles. The second DAC converts the coarse-delayed signal to a second analog signal in the second path. The device also includes a combiner that combines the first analog signal with the second analog signal to produce an output signal with at least one replica of the image spectra being substantially suppressed. 
     According to another aspect, the invention provides a method of suppressing image spectra of a digital input signal. The method includes splitting the digital input signal into a first path and a second path. The first path has a Hilbert transformer, a first DAC and an analog phase delay device. The Hilbert transformer phase-shifts replicas of the image spectra that fall within even Nyquist zones by substantially 90 degrees and phase-shifts replicas of the image spectra that fall within odd Nyquist zones by substantially −90 degrees, to produce a first digital signal. The DAC converts the first digital signal to a first analog signal. The analog phase delay device delays the first analog signal by an additional 90 degrees. The second path has a compensation device and a second DAC. The compensation device compensates for a phase delay introduced by the Hilbert transformer and the first DAC of the first path to produce a second digital signal. The second DAC converts the second digital signal to a second analog signal. The method further includes combining the first and second analog signals from the two paths to produce an output analog signal with at least one replica of the image spectra suppressed. 
     According to another aspect, the invention provides a radio frequency (RF) device having a first path and a second path. The first path has a transformer arranged to phase shift replicas of an image spectra of a digital input signal. The first path also has a first DAC that converts the output of the transformer to a first analog signal. The second path has a compensator that compensates for a group delay response of at least the transformer. The second path also has a second DAC to convert an output of the compensator to produce a second analog signal. A combiner combines the first and second analog signals to produce an analog output signal having at least one substantially suppressed replica of the image spectra. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a block diagram of an exemplary transmitter circuit constructed in accordance with principles of the present invention; 
         FIG. 2  is a block diagram of an exemplary alternative transmitter circuit constructed in accordance with principles of the present invention; 
         FIG. 3  is a block diagram of an exemplary alternative transmitter circuit constructed in accordance with principles of the present invention; 
         FIG. 4  is a block diagram of an exemplary alternative transmitter circuit constructed in accordance with principles of the present invention; 
         FIG. 5  is a block diagram of an exemplary alternative transmitter circuit constructed in accordance with principles of the present invention; 
         FIG. 6  is a block diagram of an exemplary alternative transmitter circuit constructed in accordance with principles of the present invention; and 
         FIG. 7  is a flow chart of an exemplary embodiment for producing a filtered analog signal for transmission. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing in detail exemplary embodiments that are in accordance with the present invention, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to image suppression in a radio frequency (RF) signal of a wireless communication system. Accordingly, the system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. 
     In embodiments described herein, analog outputs from two paths are combined in a manner to attenuate a replica of the image spectra of a signal in one of the Nyquist zones. Prior to combining the analog outputs from two paths, at least one of the paths does not exhibit both an anti-symmetric phase response and a symmetric magnitude response about the multiple of the digital clock frequency that borders the Nyquist zone containing the replica to be attenuated. If the undesirable replica is located at Nf s +f IF  or Nf s −f IF , then the frequency to which the symmetry conditions refer is Nf s . Here, N is an integer and f IF  is the up-conversion frequency in the digital domain. 
     Referring now to the drawing figures, in which like reference designators denote like elements, there is shown in  FIG. 1  a diagram of an exemplary transmitter circuit constructed in accordance with principles of the present invention, and generally denoted as system “10.” System  10  may include a quadrature modulator  12  that receives in-phase (I) signals and quadrature-phase (Q) signals and mixes them with a cosine wave and sine wave, respectively, to perform intermediate frequency (IF) conversion in the digital domain. The cosine wave and sine wave are obtained from a phase shifter  14  that receives a digital sine wave from a numerically controlled oscillator  16 . 
     The output of the modulator  12  is split into two paths. The first path includes a Hilbert transformer  18 , an optional interpolator  20 , a digital-to-analog converter (DAC)  22  and a delay line  24 . The second path includes a coarse delay compensator  26 , a real tap filter  28 , an optional interpolator  30 , and a DAC  32 . The outputs of each path are combined by an adder  34  to produce an analog RF output with a desired replica suppressed, as will be explained below. 
     In the lower path of  FIG. 1 , the Hilbert transformer  18  causes a minus 90 degrees phase shift to the first Nyquist zone of the signal, a plus 90 degrees phase shift to the second Nyquist zone, a minus 90 degrees phase shift to the third Nyquist zone, etc, such that each odd Nyquist zone undergoes a minus 90 degrees phase shift and each even Nyquist zone undergoes a plus 90 degrees phase shift. The delay line  24  introduces nearly a plus 90 degrees phase shift or a minus 90 degrees phase shift to one of the replicas so that the chosen replica is attenuated while a neighboring replica is reinforced. The reinforced replica is the desired analog RF output. The remaining replicas that are not suppressed are sufficiently separated from the reinforced replica, so that they may be easily filtered with a smaller, lower order filter. 
     The interpolators  20  and  30  are optional and may be used to increase the spacing between replicas within even numbered Nyquist zones, and between replicas within odd Nyquist zones. This has the effect of relaxing a filter selectivity requirement. The DAC  22  performs a digital-to-analog conversion of the signal and is constructed so that the sample-and-hold frequency response of the DAC  22  does not excessively attenuate the desired replica. 
     In the upper path of  FIG. 1 , the coarse delay compensator  26  introduces an integer number of delays to compensate for the group delay of the Hilbert transformer  18 . The real tap filter  28  adjusts the phase and fine-tunes the group delay at the IF frequency f IF  to compensate for the phase response of the delay line at Nf s ±f IF , which may deviate from exactly ±90°. 
     The real tap filter  28  can be an all-pass filter in order to keep the magnitude responses of the two paths equal. Consequently, a transfer function of a second order real tap filter  28  has real coefficients with poles at α and α* and zeros at 1/α and 1/α*, where * denotes the complex conjugate. The variable α is a real number less than 1, and is chosen based on a desired group delay. The optional interpolator  30  and the DAC  32  perform the same functions as for interpolator  20  and DAC  22 , respectively. 
     The outputs of the two paths are combined in a combiner  34 , which may be an adder or a subtractor, depending upon which replica is to be attenuated and which replica is to be reinforced. Thus, one embodiment produces an analog output signal with at least one of the replicas of the image spectra of the digital input signal suppressed and with at least one of the replicas of the image spectra of the digital input signal reinforced. 
       FIG. 2  is a block diagram of an exemplary alternative transmitter circuit  36  constructed in accordance with principles of the present invention. In  FIG. 2 , the real tap filter  28  of  FIG. 1  is replaced with a complex filter  38  prior to quadrature modulation by quadrature modulators  12   a  and  12   b . The complex filter  38  processes complex data and can have complex-valued coefficients. A role of the complex filter  38  is to compensate for any mismatches between the actual and desired outputs of the upper path of the embodiment of  FIG. 2 . The complex filter  38  provides a phase response that compensates for a deviation from plus or minus 90 degrees of the delay line  24  in the second (lower) path in the band of the desired replica. 
       FIG. 3  is a block diagram of an exemplary alternative transmitter circuit constructed in accordance with principles of the present invention. In  FIG. 3 , the complex data (I and Q) is rotated by a fixed angle by a rotation-and-quadrature modulator  13 , rather than being complex-filtered as in the embodiment of  FIG. 2 . The rotation is used to compensate for the phase error that arises from the delay line  24  not being exactly plus or minus 90 degrees at Nf s ±f IF . This rotation causes maximum attenuation at the center frequency of the undesired replica, but the group delay is not matched as it is with the embodiments of  FIGS. 1 and 2 . Matching the phase but not the group delay causes a narrower attenuation bandwidth of the replica. Although  FIG. 3  shows delay line  24  as the analog element that provides phase shift, it is understood that any element that provides a phase shift in one path can be used. 
       FIG. 4  is a block diagram of an exemplary alternative transmitter circuit constructed in accordance with principles of the present invention.  FIG. 4  is similar to the configuration of  FIG. 1  except that the delay line  24  of  FIG. 1  is replaced by an analog quadrature modulator  44 , fed by a sine wave and a cosine wave. More particularly, the analog quadrature modulator  44  mixes the signal from each path by the sine wave and the cosine wave, respectively. The cosine wave and sine wave are obtained from a phase shifter  46  that receives an analog sine wave from a local oscillator  48 . A function of the analog quadrature modulator  44  is to obtain a plus or minus 90 degrees phase shift between the two paths. The fine delay compensation for minor phase deviation in the analog quadrature modulator  44  may be provided by introducing one of the filters or rotation components of  FIGS. 1 ,  2  and  3 . 
       FIG. 5  is a block diagram of an exemplary alternative transmitter circuit constructed in accordance with principles of the present invention. The embodiment of  FIG. 5  includes a transformer  52  that produces a delay of ±θ in alternating Nyquist zones, and is therefore a generalization of the Hilbert transformer  18 . In this case, the shortened delay line  54  produces a delay of ±θ so that the delay between paths at the desired frequency is 180°−2θ. This phase relationship results in a degradation of the magnitude response at the output of the combiner  35 . When the shortened delay line  54  is larger than λ/8, then the degradation of the desired replica is less than −3 dB. 
       FIG. 6  is a block diagram of an exemplary alternative transmitter circuit constructed in accordance with principles of the present invention. In the embodiment of  FIG. 6 , the delay line  24  is replaced by an analog phase shifter  58 . In an alternative embodiment, a single component can be employed to implement the analog phase shift of the analog phase shifter  58  and the combiner  34 . 
     Thus, embodiments enable suppression of one of two closely spaced replicas from a digital-to-analog converter in a manner that avoids use of a highly selective analog filter. In the embodiment of  FIG. 1 , only one quadrature modulator is used. The real tap filter  28  can be a low order all pass infinite impulse response (IIR) filter. A lower order filter advantageously results in fewer mathematical operations, resulting in lower power consumption and reduced size. Using the real tap filter results in more accurate fine tuning of the group delay, which results in better attenuation over the desired attenuation bandwidth. 
     In the embodiment of  FIG. 2 , a finite impulse response (FIR) filter can be used that is simpler to design than an IIR filter and can be constrained to exhibit a linear phase response over an entire baseband bandwidth. In the embodiment of  FIG. 3 , digital filtering is not used, but a more complex digital quadrature modulator  13  may be used. The embodiment of  FIG. 4  omits the delay line  24  which advantageously decreases the overall size of the transmitter circuit  42 . The embodiment of  FIG. 5  uses a smaller delay line than the embodiment of  FIGS. 1 ,  2  and  3 , resulting in reduced size. The embodiment of  FIG. 6  omits the delay line  24 , and enables image attenuation in more than one Nyquist zone. The performance depends upon the magnitude and phase response of the analog phase shifter  58 . 
       FIG. 7  is a flow chart of an exemplary embodiment for producing a filtered analog signal that has a suppressed first replica of an image spectra of the signal and a reinforced second replica of the image spectra of the signal. A digital signal is split into two electrically parallel paths (step S 100 ). In the first one of the two paths, a Hilbert transform or other anti-symmetric phase transform is applied to the signal (step S 102 ). In the second one of the two paths, phase compensation is applied to the signal in the second path to compensate for phase delay of the signal in the first path caused by the digital components of the first path (step S 104 ). The signals in each path are converted to an analog signal in each path (step S 106 ). The analog signal in the first path is phase-shifted by an analog phase shifter (step S 108 ). The analog signals from each path are combined (step S 110 ). 
     The present invention can be realized in hardware, or a combination of hardware and software. Any kind of computing system, or other apparatus adapted for carrying out the methods described herein, is suited to perform the functions described herein. A typical combination of hardware and software could be a specialized computer system, having one or more processing elements and a computer program stored on a storage medium that, when loaded and executed, controls the computer system such that it carries out the methods described herein. The present invention can also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods described herein, and which, when loaded in a computing system is able to carry out these methods. Storage medium refers to any volatile or non-volatile storage device. 
     Computer program or application in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following a) conversion to another language, code or notation; b) reproduction in a different material form. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.