Patent Publication Number: US-7715493-B2

Title: Digital transmitter and methods of generating radio-frequency signals using time-domain outphasing

Description:
TECHNICAL FIELD 
   Some embodiments of the present invention pertain to wireless communications. Some embodiments relate to radio-frequency (RF) transmitters. Some embodiments relate to the transmission of multicarrier waveforms, such as orthogonal frequency division multiplexed (OFDM) waveforms. Some embodiments of the present invention relate to time-domain outphasing transmitters. Some embodiments of the present invention relate to polar transmitters. 
   BACKGROUND 
   Multicarrier waveforms, such as orthogonal frequency division multiplexed (OFDM) waveforms, have amplitude and phase information modulated thereon. These waveforms are conventionally amplified for transmission using linear or near-linear power amplifiers to reduce distortion of this information. The inherently low efficiency of these linear and near-linear power amplifiers results in increased power consumption and/or increased heat generation. Furthermore, the significant peak-to-average power ratios (i.e., 10-15 dB) of some multicarrier waveforms may further reduce the average efficiency of linear or near-linear power amplifiers. Increased power consumption and increased heat generation are undesirable characteristics, particularly for portable, mobile and handheld wireless devices that rely on batteries. Non-linear power amplifiers are generally more efficient than linear and near-linear power amplifiers, however direct amplification of a multicarrier waveform by non-linear power amplifiers may distort the amplitude and phase information, making non-linear power amplifiers unsuitable for use in conventional OFDM transmitters. 
   Many conventional transmitters use analog circuitry, which is more sensitive to process, voltage and temperature (PVT) variations than digital circuitry. Analog circuitry also utilizes large inductors that may occupy a larger die area and are less compatible with some semiconductor processes, such as complementary metal-oxide semiconductor (CMOS) processes, than digital circuitry. Analog circuitry may also require greater voltages than digital circuitry making it less compatible with low-voltage semiconductor processes. 
   Thus, there are general needs for multicarrier transmitters that are more efficient, consume less power, and/or are less sensitive to PVT variations. There are also general needs for multicarrier transmitters that are compatible with low-voltage semiconductor processes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a functional block diagram of a multicarrier transmitter in accordance with some embodiments of the present invention; 
       FIG. 2  illustrates mathematically the generation of signals from outphased components in accordance with some embodiments of the present invention; 
       FIG. 3  illustrates a delay vernier delay-line suitable for use with some embodiments of the present invention; 
       FIG. 4  illustrates a coupled oscillator delay-line suitable for use with some embodiments of the present invention; 
       FIGS. 5A and 5B  illustrate a delay-locked loop (DLL) delay-line suitable for use with some embodiments of the present invention; 
       FIGS. 6A-6E  illustrate a multi-stage digital delay-line with a single output port in accordance with some embodiments of the present invention; 
       FIG. 7  is a procedure for generating RF signals for transmission in accordance with some embodiments of the present invention; 
       FIG. 8  illustrates a phase filter suitable for use with some embodiments of the present invention; and 
       FIG. 9  is a functional block diagram of a polar transmitter in accordance with some embodiments of the present invention. 
   

   DETAILED DESCRIPTION 
   The following description and the drawings sufficiently illustrate specific embodiments of the invention to enable those skilled in the art to practice them. Other embodiments may incorporate structural, logical, electrical, process, and other changes. Examples merely typify possible variations. Individual components and functions are optional unless explicitly required, and the sequence of operations may vary. Portions and features of some embodiments may be included in, or substituted for, those of other embodiments. Embodiments of the invention set forth in the claims encompass all available equivalents of those claims. Embodiments of the invention may be referred to herein, individually or collectively, by the term “invention” merely for convenience and without intending to limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. 
     FIG. 1  is a functional block diagram of a multicarrier transmitter in accordance with some embodiments of the present invention. Multicarrier transmitter  100  generates RF signals for transmission using non-linear switching power amplifiers (PAs)  118 A &amp;  118 B to amplify outphased switching waveforms  117 A &amp;  117 B allowing multicarrier transmitter  100  to operate more efficiently than some conventional multicarrier transmitters. In these embodiments, the modulating signals are generated as 1 bit delta-signal signals and only need to switch on and off switching power amplifiers  118 A &amp;  118 B. 
   In accordance with some embodiments, digital baseband signals  109 A &amp;  109 B are decomposed to generate phase-selection signals  115 A &amp;  115 B, and tapped delay-lines  106 A &amp;  106 B provide a plurality of phases  107  of square-wave signal  105 . Square-wave signal  105  may be at the transmit frequency. In some embodiments, phase multiplexers (MUX)  116 A &amp;  116 B select one of phases  107  based on phase-selection signals  115 A &amp;  115 B to provide outphased switching waveforms  117 A &amp;  117 B to switching power amplifiers  118 A &amp;  118 B. After amplification, outphased switching waveforms  117 A &amp;  117 B are combined to generate RF signals  121  for transmission by antenna  122 . 
   In some embodiments, tapped delay-lines  106 A &amp;  106 B may be stabilized tapped delay-lines. These embodiments are described in more detail below. In some embodiments, the plurality of phases  107  provided by tapped delay-lines  106 A &amp;  106 B may comprise delayed versions of square-wave signal  105 . 
   In some embodiments, outphased switching waveforms  117 A &amp;  117 B comprise substantially constant-envelope signals representing two outphased components of the RF signal for transmission. The two outphased components may be referred to as s 1  and s 2 , although the scope of the invention is not limited in this respect. Substantially constant-envelope signals may refer to signals having a non-varying or substantially constant amplitude. In other words, information is not encoded on the amplitude component. 
   In some embodiments, phase multiplexers  116 A &amp;  116 B may comprise first phase multiplexer  116 A and second phase multiplexer  116 B. First phase multiplexer  116 A may select an output based on first phase-selection signal  115 A comprising a first outphased component of the decomposed baseband signals. Second phase multiplexer  116 B may select an output based on second phase-selection signal  115 B comprising another outphased component of the decomposed baseband signals. 
   In some embodiments, tapped delay-lines  106 A &amp;  106 B may comprise first tapped delay-line  106 A and second tapped delay-line  106 B. First tapped delay-line  106 A may provide a first plurality of phases  107  of square-wave signal  105  to first phase multiplexer  116 A. Second tapped delay-line  106 B may provide a second plurality of phases  107  of square-wave signal  105  to second phase multiplexer  116 B. In some alternate embodiments, instead of two tapped delay-lines  106 A &amp;  106 B, a single tapped delay-line may be used coupled to both phase multiplexers  116 A and  116 B, although the scope of the invention is not limited in this respect. 
   In some embodiments, switching power amplifiers  118 A &amp;  118 B may operate in a non-linear range to separately amplify outphased switching waveforms  117 A &amp;  117 B. In these embodiments, outphased switching waveforms  117 A &amp;  117 B may comprise substantially constant-envelope square-wave signals, although the scope of the invention is not limited in this respect. When outphased switching waveforms  117 A &amp;  117 B are constant-envelope square-wave signals, switching power amplifiers  118 A &amp;  118 B may operate in a highly efficient non-linear range. In some embodiments, switching power amplifiers  118 A &amp;  118 B may comprise class-D, class-E, or class-F non-linear power amplifiers, although the scope of the invention is not limited in this respect. In some embodiments, switching power amplifiers  118 A &amp;  118 B may comprise linear power amplifiers biased in deep class-AB or class-B, although the scope of the invention is not limited in this respect. In some of these embodiments, two or more switching power amplifiers may be used for power amplifier  118 A, and two or more switching power amplifiers may be used for power amplifier  118 B. In some alternate embodiments, 1-bit power digital-to-analog converters (DAC) may be used for power amplifiers  118 A and  118 B, although the scope of the invention is not limited in this respect. 
   In some of these embodiments, each of power amplifiers  118 A &amp;  118 B may only need to deliver half the power as compared to some conventional transmitters that use a single power amplifier. In these embodiments, higher efficiency may be achieved in non-linear operation while avoiding break-down voltage issues and reducing hot-carrier effects sometimes associated with higher power levels. 
   In some embodiments, combiner  120  may be used to combine the separately amplified outphased switching waveforms  117 A &amp;  117 B to generate RF signal  121  for transmission by antenna  122 . In some embodiments, combiner  120  may be a microwave signal or power combiner, although the scope of the invention is not limited in this respect. In some embodiments, combiner  120  may also be a current or voltage combiner, although the scope of the invention is not limited in this respect. 
   In accordance with embodiments, power amplifier  118 A may amplify outphased  117 A, and power amplifier  118 B may amplify outphased  117 B. The separate amplification of outphased switching waveforms  117 A &amp;  117 B allows these constant-envelope signals to be amplified with switching power amplifiers achieving high efficiency while preserving the phase information. By combining the separately amplified outphased switching waveforms  117 A &amp;  117 B, a multicarrier signal, such as an OFDM signal, may be provided at the output of combiner  120  with both amplitude and phase information. 
   In some embodiments, multicarrier transmitter  100  includes coordinate rotation circuitry  110  to decompose digital baseband signals  109 A &amp;  109 B. Coordinate rotation circuitry  110  may decompose an in-phase (I) component and a quadrature-phase (Q) component of digital baseband signals  109 A &amp;  109 B into theta component  111 A and phi component  111 B. Theta component  111 A and phi component  111 B may be polar representations of the in-phase and quadrature-phase components of digital baseband signals  109 A &amp;  109 B. The in-phase and quadrature-phase components, on the other hand, may be viewed as Cartesian representations. In these embodiments, the amplitude and phase information of the in-phase and quadrature-phase components are converted to phase information by coordinate rotation circuitry  110  and are preserved in theta component  111 A and phi component  111 B. In some embodiments, theta component  111 A and phi component  111 B may be phasors and may contain the amplitude and phase information present in digital baseband signals  109 A &amp;  109 B, although the scope of the invention is not limited in this respect. 
   In these embodiments, outphased switching waveform  117 A may comprise square-wave signal  105  delayed by the sum of theta component  111 A and phi component  111 B. In these embodiments, outphased switching waveform  117 B may comprise square-wave signal  105  delayed by the difference between theta component  111 A and phi component  111 B. 
   In some embodiments, coordinate rotation circuitry  110  may comprise processing circuitry to perform a “COordinate Rotation DIgital Calculation” (CORDIC) algorithm to generate theta component  111 A and phi component  111 B, although the scope of the invention is not limited in this respect. In some embodiments, coordinate rotation circuitry  110  may generate theta component  111 A and phi component  111 B by performing digital arithmetic operations including a vector rotation on the in-phase and quadrature-phase components of digital baseband signals  109 A &amp;  109 B. In some embodiments, the operations performed by coordinate rotation circuitry  110  may be performed without the use of multipliers other than for output scaling, although the scope of the invention is not limited in this respect. 
   In some embodiments, multicarrier transmitter  100  may also include signal generator  102  and buffer  104 . Signal generator  102  may generate RF signals  103  at the transmit frequency and buffer  104  may generate square-wave signal  105  at the transmit frequency from RF signals  103  for receipt by tapped delay-lines  106 A &amp;  106 B. In some embodiments, signal generator  102  may be a voltage controlled oscillator (VCO), although other stable and/or low-phase noise sources for generating RF signals may also be suitable. In some embodiments, an on-board synthesizer or clock source may be used for signal generator  102 , although the scope of the invention is not limited in this respect. 
   In some embodiments, the RF signals that are provided by signal generator  102  at the outputs of phase multiplexers  116 A &amp;  116 B (e.g., outphased switching waveforms  117 A and  117 B) are referred to as “outphased” waveforms because they are delayed respectively by the amounts of theta plus phi (i.e., a sum component) and theta minus phi (i.e., a difference component). 
   In some embodiments, multicarrier transmitter  100  may include circuitry to generate phase-selection signals  115 A &amp;  115 B. The circuitry to generate phase-selection signals  115 A &amp;  115 B may include digital adder elements  112  to generate sum component  113 A and difference component  113 B. Sum component  113 A may comprise a sum of theta component  111 A and phi component  111 B. Difference component  113 B may comprise a difference between theta component  111 A and phi component  111 B. In some of these embodiments, outphased switching waveform  117 A may comprise square-wave signal  105  delayed by sum component  113 A, and outphased switching waveform  117 B may comprise square-wave signal  105  delayed by difference component  113 B. 
   In some embodiments, the circuitry to generate phase-selection signals  115 A &amp;  115 B may also include delta-sigma (Δ-Σ) modulators  114 A &amp;  114 B comprising first delta-sigma modulator  114 A and second delta-sigma modulator  114 B. First delta-sigma modulator  114 A may generate first phase-selection signal  115 A from sum component  113 A, and second delta-sigma modulator  114 B may generate second phase-selection signal  115 B from difference component  113 B. In some embodiments, the delta-sigma modulators  114 A and  114 B may generate fractional output values by rapidly switching between two integer values based on their input, although the scope of the invention is not limited in this respect. Fractional output values may allow the selection of one of phases  107  by phase multiplexers  116 A &amp;  116 B. For example, to produce an output fractional value of 5.5, delta-sigma modulator  114 A may switch rapidly between five and six so that on the average, the output is about five half the time and about six the other half of the time. In these embodiments, the fractional output values may correspond to phase-selection signals  115 A &amp;  115 B. In some embodiments, delta-sigma modulators  114 A and  114 B may allow the selection of an appropriate one of phases  107  in a time-varying manner to produce outphased switching waveforms  117 A &amp;  117 B with desired phases. The use of delta-sigma modulators  114 A and  114 B may allow for a finer average phase to be generated at the output of phase multiplexers  116 A &amp;  116 B. 
   In some embodiments, multicarrier transmitter  100  may also include a digital baseband processor  108  to generate digital baseband signals  109 A &amp;  109 B from input bit stream  101 . Digital baseband processor  108  may include, among other things, circuitry to modulate bits of bit stream  101  to generate symbols in the frequency domain. Digital baseband processor  108  may also include circuitry to perform an inverse discrete Fourier transform (IDFT), such as an inverse fast Fourier transform (IFFT), on the frequency-domain symbols to generate time-domain digital waveforms corresponding to digital baseband signals  109 A &amp;  109 B. 
   In some multicarrier and OFDM embodiments, digital baseband processor  108  may generate quadrature-amplitude modulated (QAM) symbols for each of a plurality of subcarriers to provide a plurality of frequency-domain symbol modulated subcarriers. An IDFT may be performed on the frequency-domain symbol modulated subcarriers to generate the time-domain digital waveforms. In these embodiments, digital baseband signals  109 A &amp;  109 B may comprise multicarrier time-domain baseband signals, although the scope of the invention is not limited in this respect. 
   In some embodiments, tapped delay-lines  106 A &amp;  106 B comprise a plurality of delay elements  126  having intrinsic delays substantially less than a period of the square-wave signal  105 , although the scope of the invention is not limited in this respect. In some embodiments, each delay element  126  may provide one of phases  107 , although the scope of the invention is not limited in this respect as other configurations for tapped delay-lines  106 A &amp;  106 B may also be suitable. In some embodiments, each delay element  126  may synthesize a different one of phases  107 . In some embodiments, square-wave signal  105  may comprise a clock waveform having a clock period, and delay elements  126  may have intrinsic delays substantially less than the clock period, although the scope of the invention is not limited in this respect. 
   In some embodiments, the amount of delay between phases  107  (e.g., a step size) generated by each tapped delay-line  106 A &amp;  106 B may be relatively small (e.g., 10 pico-seconds of phase shift) while the oversampling ratio with respect to the period of square-wave signal  105  may be large (e.g., a few hundred pico-seconds). In some embodiments, the quantization noise produced by tapped delay-lines  106 A &amp;  106 B may be low both in-band and out-of-band, allowing for relaxed filtering requirements. These embodiments may be less sensitive to process, voltage and temperature (PVT) variations, causing less degradation in the modulation quality (i.e., a low the error vector magnitude (EVM)), although the scope of the invention is not limited in this respect. In some embodiments, conventional tapped delay-lines may have adequate resolution for use as tapped delay-lines  106 A &amp;  106 B. In some alternate embodiments, finer resolution tapped delay-lines may be used to provide even less sensitivity to PVT variations and/or cause less degradation in the modulation quality, although the scope of the invention is not limited in this respect. Some examples of tapped delay-lines suitable for use as tapped delay-lines  106 A &amp;  106 B are described in more detail below. 
   In some alternate embodiments, phase-selection signals  115 A &amp;  115 B may comprise control words for use in selecting one of phases  107  by phase multiplexers  116 A &amp;  116 B. In some of these embodiments, tapped delay-lines  106 A &amp;  106 B may comprise digital delay-lines, discussed in more detail below in reference to  FIGS. 6A-6E . 
   Antenna  122  may comprise one or more directional or omnidirectional antennas, including, for example, dipole antennas, monopole antennas, patch antennas, loop antennas, microstrip antennas or other types of antennas suitable for transmission of RF signals. In some multiple-input, multiple-output (MIMO) embodiments, two or more antennas may be used. In some of these multiple-antenna embodiments, instead of two or more antennas, a single antenna with multiple apertures may be used. In these multiple-antenna embodiments, each aperture may be considered a separate antenna. In some multiple-antenna embodiments, each antenna may be effectively separated to take advantage of spatial diversity and the different channel characteristics that may result between each of the antennas and another wireless communication device. In some multiple-antenna embodiments, the antennas may be separated by up to 1/10 of a wavelength or more. 
   In some embodiments, multicarrier transmitter  100  may be part of a multi-mode communication device. In these embodiments, multicarrier transmitter  100  may be configured to communicate in accordance with one or more communication standards and/or techniques through the switching in and out of logic gates and variation of frequencies. 
   In some embodiments, delta-sigma modulators  114 A &amp;  114 B may improve time resolution to a required accuracy by shaping the time quantization noise out-of-band. Since the quantization steps may be smaller than the voltage or current quantization steps achievable on chip, out-of-band filtering requirements may be relaxed. In some conventional outphasing transmitters, the limiting action of power amplifiers may result in significant spectral growth. Multicarrier transmitter  100 , on the other hand, may reduce and/or eliminate significant spectral growth by encoding outphased switching waveforms  117 A &amp;  117 B with very fine time delays. This is accomplished by driving power amplifiers  118 A and  118 B with phase-encoded constant-envelope signals, rather than with signals having an amplitude component. 
   In accordance with some embodiments, mismatches between the two paths that generate outphased switching waveforms  117 A and  117 B may be digitally calibrated and/or corrected. In some of these embodiments, mismatches between the two paths may be measured at the output of combiner  120  and corrected by adjusting the component outputs of coordinate rotation circuitry  110 . In some embodiments, an adaptive filter or adaptive equalizer may be used, although the scope of the invention is not limited in this respect. In some embodiments, the calibration may reduce and/or cancel spectrum spreading to prevent disruption of adjacent frequency channels. Because multicarrier transmitter  100  uses phase information from digital baseband signals  109 A &amp;  109 B, calibration may be easier. 
   In some embodiments, multicarrier transmitter  100  may include optional phase filters  128 A and  128 B to help filter out-of-band components from outphased switching waveforms  117 A &amp;  117 B prior to respective amplification by switching power amplifiers  118 A &amp;  118 B. These embodiments may further reduce spectral growth of RF signals  121 , although the scope of the invention is not limited in this respect. 
   Although multicarrier transmitter  100  is illustrated as having several separate functional elements, one or more of the functional elements may be combined and may be implemented by combinations of software-configured elements, such as processing elements including digital signal processors (DSPs), and/or other hardware elements. For example, some elements may comprise one or more microprocessors, DSPs, application specific integrated circuits (ASICs), and combinations of various hardware and logic circuitry for performing at least the functions described herein. In some embodiments, the functional elements of multicarrier transmitter  100  may refer to one or more processes operating on one or more processing elements. In some digital embodiments, the functional elements of multicarrier transmitter  100  illustrated in  FIG. 1  may all be implemented digitally (except for antenna  122  and/or combiner  120 ) on a single semiconductor die, although the scope of the invention is not limited in this respect. The use of phase information (e.g., time resolution), rather than amplitude information (e.g., voltage resolution) may allow circuitry to be fabricated using low-voltage semiconductor processes, such as low-voltage CMOS processes, although the scope of the invention is not limited in this respect. 
     FIG. 2  illustrates mathematically the generation of signals from outphased components in accordance with some embodiments of the present invention. Dashed circles  200  illustrate constant-envelope signals y 1 (t) and y 2 (t) corresponding respectively to outphased switching waveforms  117 A and  117 B ( FIG. 1 ) representing the two outphased components of RF signal  121  ( FIG. 1 ) for transmission, illustrated as y 0 (t). Equations  209 A and  209 B describe the generation of the combined signal y 0 (t) from phi component φ(t) and theta component θ(t). Equation  209 B is a simplification of equation  209 A. 
   Referring to  FIGS. 1 and 2  together, equation  211 A illustrates the generation of theta component θ(t) from the in-phase (I) and quadrature-phase (Q) components of digital baseband signals  109 A &amp;  109 B. Equation  211 B illustrates the generation of phi component φ(t) from the in-phase and quadrature-phase components of digital baseband signals  109 A &amp;  109 B. In these equations, the in-phase component of digital baseband signals  109 A &amp;  109 B is illustrated as I(t), and the quadrature-phase component of digital baseband signals  109 A &amp;  109 B is illustrated as Q(t). In some embodiments, processing circuitry of coordinate rotation circuitry  110  may implement equations  211 A and  211 B to generate theta component  111 A and phi component  111 B, respectively, although the scope of the invention is not limited in this respect. 
     FIG. 3  illustrates a delay vernier delay-line in accordance with some embodiments of the present invention. In these embodiments, delay vernier delay-line  300  may be suitable for use as tapped delay-line  106 A and for tapped delay-line  106 B. Delay vernier delay-line  300  may include a plurality of delay elements (DE)  304  to delay square-wave signal  305 , and a plurality of latches  302  to sample the delayed signals. Some of delay elements  304  may be arranged to form first delay-line  311  to provide the input signals to latches  302 , and some of delay elements  304  may be arranged to form second delay-line  313  to provide the clock (CLK) inputs to latches  302  as illustrated. Delay vernier delay-line  300  may provide a plurality of phases  307 , which may comprise differences between delay-lines  311  and  313 , although the scope of the invention is not limited in this respect. 
   In these embodiments, phases  307  may correspond to phases  107  ( FIG. 1 ), and square-wave signal  305  may correspond to square-wave signal  105  ( FIG. 1 ). In some embodiments, latches  302  may comprise flip-flop circuitry, and delay elements  304  may comprise either buffers or inverters, although the scope of the invention is not limited in this respect. 
     FIG. 4  illustrates a coupled oscillator delay-line in accordance with some embodiments of the present invention. In these embodiments, coupled oscillator delay-line  400  may be suitable for use as tapped delay-line  106 A and for tapped delay-line  106 B. Coupled oscillator delay-line  400  may comprise a plurality of delay elements (DE)  404  configured as illustrated to generate a plurality of phases  407  of square-wave signal  405 . In some embodiments, coupled oscillator delay-line  400  may also include wire-mapping circuitry  406  which may reconfigure the connections between inputs  409  and outputs  411  to allow finer differences between phases  407  to be provided. In some embodiments, delay elements  404  may comprise either buffers or inverters. In these embodiments, one row of phases  407  may correspond to phases  107  ( FIG. 1 ), and square-wave signal  405  may correspond to square-wave signal  105  ( FIG. 1 ), although the scope of the invention is not limited in this respect. 
     FIGS. 5A and 5B  illustrate delay-locked loop (DLL) delay-lines in accordance with some embodiments of the present invention.  FIG. 5A  illustrates DLL delay-line  500 , and  FIG. 5B  illustrates DLL delay-line  550 . In these embodiments, either DLL delay-line  500  or DLL delay-line  550  may be suitable for use as tapped delay-line  106 A and for tapped delay-line  106 B. DLL delay-lines  500  and  550  may comprise a plurality of delay elements  526  to provide a plurality of phases  507  from square-wave signal  505 . In these embodiments, phases  507  may correspond to phases  107  ( FIG. 1 ), and square-wave signal  505  may correspond to square-wave signal  105  ( FIG. 1 ), although the scope of the invention is not limited in this respect. 
   DLL delay-lines  500  and  550  may comprise phase detector (DET) and charge pump (CP)  502  to generate an output based on a difference between input signals  501  and  503  for controlling the delay provided by delay elements  526 . In these embodiments, phase detector and charge pump  502  may drive the phase difference between input signals  501  and  503  to zero. DLL delay-lines  500  and  550  may include loop filters  504  to suppress switching noise and/or noise that may be generated by delta-sigma modulators  114 A &amp;  114 B ( FIG. 1 ). The use of loop filters  504  may help suppress out-of-band components that may be generated by power amplifiers  118 A &amp;  118 B, although the scope of the invention is not limited in this respect. In some embodiments, delay elements  526  may comprise either buffers or inverters. 
   DLL delay-line  500  may also include tunable capacitances  528  which may be used to adjust the delay provided by each of delay elements  526 . In some embodiments, continuous tuning by capacitances  528  may help stabilize delay elements  526 . In some embodiments, capacitances  528  may be controlled with analog signals, while in other embodiments, capacitances  528  may be controlled digitally. In some embodiments, capacitances  528  may be gate capacitances of field-effect transistors (FETs), although the scope of the invention is not limited in this respect. 
     FIGS. 6A-6E  illustrate a multi-stage digital delay-line with single output port in accordance with some embodiments of the present invention. Digital delay-line  600 , illustrated in  FIG. 6A , may be suitable for use as both tapped delay-line  106 A ( FIG. 1 ) and phase multiplexer  116 A ( FIG. 1 ). Digital delay-line  600  may also be suitable for use as both tapped delay-line  106 B ( FIG. 1 ) and phase multiplexer  116 B ( FIG. 1 ). In these embodiments, the function of phase multiplexers  116 A &amp;  116 B ( FIG. 1 ) may be included within digital delay-line  600 . 
   Digital delay-line  600  comprises a plurality of stages  602  responsive to control words  605  to in combination provide outphased switching waveform  617  based on square-wave signal  625 . In these embodiments, digital delay-line  600  may include decoding logic  604  to provide control words  605  to the plurality of stages  602  based on phase-selection signal  615 . In these embodiments, each of the stages  602  may be controllable to either pass an input signal or loop back the input signal to generate, in combination with other stages  602 , outphased switching waveform  617  from phase-selection signal  615 . 
   In these embodiments, square-wave signal  625  may correspond to square-wave signal  105  ( FIG. 1 ), phase-selection signal  615  may correspond to either phase-selection signal  115 A or phase-selection signal  115 B ( FIG. 1 ), and outphased switching waveform  617  may correspond to either outphased switching waveform  117 A or outphased switching waveform  117 B. 
   In these embodiments, the delay of digital delay-line  600  may depend on the number of stages  602  configured as through-sections before a loop-back occurs. In these embodiments, outphased switching waveform  617  may be available at the same port irrespective of the control word. 
   In some of these embodiments, digital delay-line  600  may also include calibration circuitry  606  and combining element  608 . Calibration circuitry  606  may provide calibration signals for combining with phase-selection signal  615 . Calibration circuitry  606  may also provide calibration signal  609  to baseband processor  108  ( FIG. 1 ). To calibrate the delay of digital delay-line  600 , a coarse calibration may first be performed; and then a fine calibration may be performed. The coarse calibration and fine calibration may be performed by calibration circuitry  606 . 
     FIG. 6B  illustrates the operation of stage  602 B with a control word of ‘00’ which may cause stage  602 B to loop back the input signal in one direction.  FIG. 6C  illustrates the operation of stage  602 C with a control word of ‘01’ which may cause stage  602 C to feed through signals.  FIG. 6D  illustrates the operation of stage  602 D with a control word of ‘11’ which may cause stage  602 D to loop back the input signal in the opposite direction as in  FIG. 6B .  FIG. 6E  illustrates the operation of stage  602 E with a control word of ‘01’ which may cause stage  602 E to latch a signal. Stages  602 B,  602 C,  602 D and  602 E may be suitable for use as any of stages  602  in  FIG. 6A . 
   As part of the coarse calibration discussed above, a loop-back at a first stage (minimum delay setting) may be set and the loop-back position may be incremented until a digital phase comparator detects a lock indicating that the return signal is in-phase with a reference signal. The delay per stage may be estimated from the number of stages required to achieve this delay since the RF frequency of square-wave signal  625  is known. Any residual fractional delay error may impact the EVM performance and may be estimated. In some embodiments, calibration circuitry  606  may use a coarse calibration integer as the starting point. After an initial reference phase is fed to digital delay-line  600 , the loop may be closed around the current integer stages and the feedback phase may be sampled by a reference clock. If the feedback phase leads, the number of stages within the feedback loop may be increased, and the converse holds if the phase lags. This process may be repeated over a large number of clock cycles and an oversampled output sequence of integers may be decimated to improve the resolution of the delay estimation. In some embodiments, digital baseband processor  108  ( FIG. 1 ) may use a resulting fixed point real number from the delay calibration and the frequency of the reference signal (e.g., from signal generator  102  ( FIG. 1 )) to compute the stage delay, although the scope of the invention is not limited in this respect. 
   Referring to  FIG. 1 , in some embodiments, multicarrier transmitter  100  may be part of a wireless communication device that may communicate orthogonal frequency division multiplexed (OFDM) communication signals over a multicarrier communication channel. The multicarrier communication channel may be within a predetermined frequency spectrum and may comprise a plurality of orthogonal subcarriers. In some embodiments, the multicarrier signals may be defined by closely spaced OFDM subcarriers. Each subcarrier may have a null at substantially a center frequency of the other subcarriers and/or each subcarrier may have an integer number of cycles within a symbol period, although the scope of the invention is not limited in this respect. In some embodiments, multicarrier transmitter  100  may communicate in accordance with a multiple-access technique, such as orthogonal frequency division multiple access (OFDMA), although the scope of the invention is not limited in this respect. In some embodiments, multicarrier transmitter  100  may be part of a wireless communication device that may communicate using spread-spectrum signals, although the scope of the invention is not limited in this respect. 
   In some embodiments, multicarrier transmitter  100  may be part of a communication station, such as wireless local area network (WLAN) communication station including a Wireless Fidelity (WiFi) communication station, an access point (AP) or a mobile station (MS). In some other embodiments, transmitter  100  may be part of a broadband wireless access (BWA) network communication station, such as a Worldwide Interoperability for Microwave Access (WiMax) communication station, although the scope of the invention is not limited in this respect as transmitter  100  may be part of almost any wireless communication device. 
   In some embodiments, multicarrier transmitter  100  may be part of a portable wireless communication device, such as a personal digital assistant (PDA), a laptop or portable computer with wireless communication capability, a web tablet, a wireless telephone, a wireless headset, a pager, an instant messaging device, a digital camera, an access point, a television, a medical device (e.g., a heart rate monitor, a blood pressure monitor, etc.), or other device that may receive and/or transmit information wirelessly. 
   In some embodiments, the frequency spectrums for the communication signals transmitted by multicarrier transmitter  100  may comprise either a 5 gigahertz (GHz) frequency spectrum or a 2.4 GHz frequency spectrum. In these embodiments, the 5 GHz frequency spectrum may include frequencies ranging from approximately 4.9 to 5.9 GHz, and the 2.4 GHz spectrum may include frequencies ranging from approximately 2.3 to 2.5 GHz, although the scope of the invention is not limited in this respect, as other frequency spectrums are also equally suitable. In some BWA network embodiments, the frequency spectrum for the communication signals may comprise frequencies between 2 and 11 GHz, although the scope of the invention is not limited in this respect. 
   In some embodiments, multicarrier transmitter  100  may transmit signals in accordance with specific communication standards, such as the Institute of Electrical and Electronics Engineers (IEEE) standards including IEEE 802.11(a), 802.11(b), 802.11(g), 802.11(h) and/or 802.11(n) standards and/or proposed specifications for wireless local area networks, although the scope of the invention is not limited in this respect as they may also be suitable to transmit and/or receive communications in accordance with other techniques and standards. In some broadband wireless access network embodiments, transmitter  100  may receive signals in accordance with the IEEE 802.16-2004 and the IEEE 802.16(e) standards for wireless metropolitan area networks (WMANs) including variations and evolutions thereof, although the scope of the invention is not limited in this respect as they may also be suitable to transmit and/or receive communications in accordance with other techniques and standards. For more information with respect to the IEEE 802.11 and IEEE 802.16 standards, please refer to “IEEE Standards for Information Technology—Telecommunications and Information Exchange between Systems”—Local Area Networks—Specific Requirements—Part 11: “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY), ISO/IEC 8802-11: 1999”, and Metropolitan Area Networks—Specific Requirements—Part 16: “Air Interface for Fixed Broadband Wireless Access Systems,” May 2005 and related amendments/versions. 
   Although multicarrier transmitter  100  is described as transmitting multicarrier signals, the scope of the invention is not limited in this respect as other non-multicarrier waveforms, such as single-carrier waveforms, may be transmitted. In some of these alternate embodiments, transmitter  100  may communicate in accordance with standards such as the Pan-European mobile system standard referred to as the Global System for Mobile Communications (GSM). In some of these embodiments, transmitter  100  may also operate in accordance with packet radio services such as the General Packet Radio Service (GPRS) packet data communication service. In some embodiments, transmitter  100  may communicate in accordance with the Universal Mobile Telephone System (UMTS) for the next generation of GSM, which may, for example, implement communication techniques in accordance with 2.5G and 3G wireless standards (see 3GPP Technical Specification, Version 3.2.0, March 2000). In some of these embodiments, transmitter  100  may provide packet data services (PDS) utilizing packet data protocols (PDP). In some embodiments, transmitter  100  may communicate in accordance with other standards or other air-interfaces including interfaces compatible with the enhanced data for GSM evolution (EDGE) standards (see 3GPP Technical Specification, Version 3.2.0, March 2000), although the scope of the invention is not limited in this respect. 
   In another embodiment, transmitter  100  may communicate in accordance with a short-range wireless standard such as the Bluetooth® short-range digital communication protocol, although the scope of the invention is not limited in this respect. Bluetooth® wireless technology is a de facto standard, as well as a specification for small-form factor, low-cost, short-range radio links between mobile PCs, mobile phones and other portable devices. (Bluetooth is a registered trademark owned by Bluetooth SIG, Inc.). 
     FIG. 7  is a procedure for generating RF signals for transmission in accordance with some embodiments of the present invention. Procedure  700  may be performed by multicarrier transmitter  100 , although other transmitters may also be suitable. 
   Operation  702  comprises decomposing digital baseband signals to generate phase-selection signals. In some embodiments, operation  702  may be performed by coordinate rotation circuitry  110  ( FIG. 1 ), digital adder elements  112  ( FIG. 1 ) and delta-sigma modulators  114 A &amp;  114 B ( FIG. 1 ) to generate phase-selection signals  115 A &amp;  115 B ( FIG. 1 ), although the scope of the invention is not limited in this respect. 
   Operation  704  comprises generating a plurality of phases of a square-wave signal at a transmit frequency. In some embodiments, operation  704  may be performed by tapped delay-lines  106 A &amp;  106 B ( FIG. 1 ) to provide phases  107  ( FIG. 1 ), although the scope of the invention is not limited in this respect. 
   Operation  706  comprises selecting one of the phases of the square-wave signals based on the phase-selection signals to provide outphased switching waveforms. In some embodiments, operation  706  may be performed by phase multiplexers  116 A &amp;  116 B ( FIG. 1 ), although the scope of the invention is not limited in this respect. In some embodiments, digital delay-line  600  ( FIG. 6 ) may perform both operations  704  and  706 . 
   Operation  708  comprises separately amplifying the outphased switching waveforms with switching power amplifiers. In some embodiments, operation  708  may be performed by switching power amplifiers  118 A &amp;  118 B ( FIG. 1 ), although the scope of the invention is not limited in this respect. 
   Operation  710  comprises combining the separately amplified outphased switching waveforms to generate an RF signal for transmission. In some embodiments, operation  710  may be performed by combiner  120  ( FIG. 1 ), although the scope of the invention is not limited in this respect. 
   Although the individual operations of procedure  700  are illustrated and described as separate operations, one or more of the individual operations may be performed concurrently, and nothing requires that the operations be performed in the order illustrated. Unless specifically stated otherwise, terms such as processing, computing, calculating, determining, displaying, or the like, may refer to an action and/or process of one or more processing or computing systems or similar devices that may manipulate and transform data represented as physical (e.g., electronic) quantities within a processing system&#39;s registers and memory into other data similarly represented as physical quantities within the processing system&#39;s registers or memories, or other such information storage, transmission or display devices. Furthermore, as used herein, a computing device includes one or more processing elements coupled with computer-readable memory that may be volatile or non-volatile memory or a combination thereof. 
     FIG. 8  illustrates a phase filter suitable for use with some embodiments of the present invention. Phase filter  800  may be suitable for use as phase filter  128 A ( FIG. 1 ) and/or phase filter  128 B ( FIG. 1 ), although other types of phase filters may also be suitable. Phase filter  800  includes phase detector  802 , loop filter  804  and continuously variable delay-line  806 . Continuously variable delay-line  806  delays square-wave signal  805  based on the output of loop filter  804 , and phase detector  802  compares the phases of input signal  817  with output signal  819  to generate a phase-difference output for loop filter  804 . 
   In some embodiments, when phase filter  800  is used for phase filter  128 A ( FIG. 1 ), input signal  817  may correspond to outphased switching waveform  117 A ( FIG. 1 ) provided by phase multiplexer  116 A ( FIG. 1 ), and output signal  819  may correspond to the outphased switching waveform provided to power amplifier  118 A ( FIG. 1 ). In these embodiments, the delay provided by continuously variable delay-line  806  may comprise a replica of the delay provided by tapped delay-line  106 A ( FIG. 1 ). 
   In some embodiments, when phase filter  800  is used for phase filter  128 B ( FIG. 1 ), input signal  817  may correspond to outphased switching waveform  117 B ( FIG. 1 ) provided by phase multiplexer  116 B ( FIG. 1 ), and output signal  819  may correspond to the outphased switching waveform provided to power amplifier  118 B ( FIG. 1 ). In these embodiments, the delay provided by continuously variable delay-line  806  may comprise a replica of the delay provided by tapped delay-line  106 B ( FIG. 1 ). 
   In these embodiments, square-wave signal  805  may correspond to square-wave signal  105  ( FIG. 1 ), and the operation of loop filter  804  and continuously variable delay-line  806  may remove out-of-band components from output signal  819 . This may result in more efficient amplification by the power amplifiers, although the scope of the invention is not limited in this respect. 
     FIG. 9  is a functional block diagram of a polar transmitter in accordance with some embodiments of the present invention. Polar transmitter  900  comprises tapped delay-line  906  to provide a plurality of phases  907  of square-wave signal  905  at the transmit frequency, and phase multiplexer (MUX)  916  to select one of phases  907  based on phase-selection signal  915  to provide switching waveform  917 . In these embodiments, phase-selection signal  915  may be generated by decomposing digital baseband signals  909 A &amp;  909 B. Switching waveform  917  may be used to generate RF signal  921  for transmission. In these embodiments, switching waveform  917  may comprise a substantially constant-envelope signal. 
   In these embodiments, coordinate rotation circuitry  910  may decompose I and Q components of digital baseband signals  909 A &amp;  909 B into phi component  911  and amplitude component  913 . Switching waveform  917  may comprise square-wave signal  905  delayed by phi component  911 . 
   In these embodiments, polar transmitter  900  may also include power digital-to-analog converter (DAC)  918  to convert switching waveform  917  to RF signal  921 , and amplitude modulator  936  to amplitude-modulate the components of RF signal  921  based on amplitude component  913  provided by coordinate rotation circuitry  910 . The output of power DAC  918  may be amplified by one or more power amplifiers prior to transmission by one or more antennas. In some alternate embodiments, one or more switching power amplifiers may be used in place of power DAC  918 , although the scope of the invention is not limited in this respect. 
   In some embodiments, polar transmitter  900  may also comprise phase filter  928  to help remove out-of-band components from switching waveform  917  prior to amplification by power DAC  918 . Phase filter  800  ( FIG. 8 ) may be suitable for use as phase filter  928 , although other phase filters and DLLs may also be suitable. 
   In polar transmitter  900 , input bit stream  901  may correspond to input bit stream  101  ( FIG. 1 ), digital baseband processor  908  may be similar to digital baseband processor  108  ( FIG. 1 ), and components of digital baseband signals  909 A &amp;  909 B may correspond respectively to components of digital baseband signals  109 A &amp;  109 B. Also in polar transmitter  900 , coordinate rotation circuitry  910  may be similar to coordinate rotation circuitry  110  ( FIG. 1 ), delta-sigma (Δ-Σ) modulator  914  may be similar to either delta-sigma (Δ-Σ) modulator  114 A or  114 B ( FIG. 1 ), tapped delay-line  906  may be similar to either tapped delay-line  106 A or  106 B ( FIG. 1 ), and phase multiplexer  916  may be similar to either phase multiplexer  116 A or  116 B ( FIG. 1 ). Also in polar transmitter  900 , signal generator  902  may be similar to signal generator  102  ( FIG. 1 ) and buffer  904  may be similar to buffer  104  ( FIG. 1 ). 
   Some embodiments of the invention may be implemented a combination of hardware, firmware and/or software. Some embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by at least one processor to perform the operations described herein. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium may include read-only memory (ROM), random-access memory (RAM), magnetic disk storage media, optical storage media, flash-memory devices, electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. 
   The Abstract is provided to comply with 37 C.F.R. Section 1.72(b) requiring an abstract that will allow the reader to ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to limit or interpret the scope or meaning of the claims. 
   In the foregoing detailed description, various features are occasionally grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the subject matter require more features than are expressly recited in each claim. Rather, as the following claims reflect, invention may lie in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate preferred embodiment.