Abstract:
An electronic circuit, such as a transmitter, for receiving a modulating signal including an in-phase component (I) and a quadrature component (Q). The electronic circuit has a first digital-to-RF-amplitude convertor (DRAC) receiving the in-phase component and a second digital-to-RF-amplitude convertor (DRAC) receiving the quadrature component. The first digital-to-RF-amplitude convertor is operative in a first duty cycle that is different from 50% and the second digital-to-RF-amplitude convertor is operative in a second duty cycle that is different from 50% and substantially the same in value as said first duty cycle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority under 35 U.S.C. §119 to Dutch Patent Application No. 2003880 (filed on Nov. 30, 2009), which is hereby incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    Embodiments of the invention relate to an electronic circuit, such as a digital power amplifier as part of a transmitter, for receiving a modulating signal including an in-phase component (I) and a quadrature component (Q), the electronic circuit comprising a first digital-to-RF-amplitude convertor (DRAC) receiving the in-phase component and a second digital-to-RF-amplitude convertor (DRAC) receiving the quadrature component. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    A. Analog-Intensive RF Transmitters. Until around mid-1990&#39;s, virtually all monolithic radio frequency (RF) transmitters (TX) have been analog intensive and based on an architecture similar to that shown in the block diagram of  FIG. 1 . At the transmitter back-end, the baseband user information symbols get pulse-shaped to obtain two orthogonal components of complex-number digital samples, i.e., in-phase (I) and quadrature (Q), that are constrained to the allocated frequency channel, using digital baseband block  2  (e.g. a DSP). They are then converted into analog continuous time domain through a set of digital-to-analog converters  3  (DAC) with a typical zero-order-hold (ZOH) function. The low-pass filter  4  (LPF) following the DAC  3  then filters out the switching harmonics. Thus obtained analog baseband signal gets then up converted (frequency translated) into RF through an image reject single-sideband (SSB) modulator. The SSB modulator typically comprises two multipliers or mixers  6 , and uses a local oscillator  5  signal, which is 90° shifted for the I- and Q-phase using a phase shifter  7 , and an adder  8  adding the two up converted signals. A following (typically external) power amplifier  9  (PA) increases the RF power level at an antenna  11  to that required by the wireless standard, which could be as high as 2 W for a GSM handset. The frequency synthesizer-based local oscillator  5  (LO) performs the frequency translation. It is typically realized as a charge-pump PLL with ΣΔ dithering of the divider modulus to realize the fractional-N frequency division ratio. 
         [0004]    The complete architecture and monolithic circuit design techniques of the conventional transmitter of  FIG. 1  have been well described in numerous literature and text books, see in particular B. Razavi,  RF Microelectronics , Upper Saddle River, N.J.: Prentice Hall, (1998), and T. H. Lee,  The Design of CMOS Radio - Frequency Integrated Circuits  (2003). These architectures have been successfully used in integrated CMOS transmitters [ 3 ] for over a decade since late 1990&#39;s. See, A. A. Abidi, “RF CMOS comes of age,”  IEEE Journal of Solid - State Circuits,  vol. 39, iss. 4, pp. 549-561 (April 2004). Unfortunately, its useful lifetime is slowly coming to an end in favor of analog polar and more digitally-intensive architectures as described in the following. 
         [0005]    Alternative to the I/Q topology of the  FIG. 1  transmitter is a polar realization, in which the two orthogonal components are amplitude A=|S| and phase φ=angle(S), or S=Ae jφ , where S is a complex-envelope signal. The phase modulation could be performed by a direct or indirect frequency modulation of a phase-locked loop (PLL). The amplitude modulation could be performed by a low dropout (LDO) voltage regulator modulating the V dd  supply of a high-efficiency switched-mode PA, such as class-E PA. The analog polar TX architecture is a more recent development to address the inherent power efficiency and noise issues of the I/Q architecture. 
         [0006]    In the past several years, such analog-intensive polar transmitters (both small-signal, i.e. at the transceiver IC level, and large signal, i.e. encompassing PA) have been touted for their reconfigurability, implementational and performance benefits over their traditional Cartesian counterparts that are based on an I/Q upconversion mixer. A number of recent publications have demonstrated their superiority through highly-integrated silicon realizations P. Nagle, P. Burton Cambridge, UK: Cambridge University Press, 1998. E. Heaney, F. McGrath, “A wide-band linear amplitude modulator for polar transmitters based on the concept of interleaving delta modulation,”  IEEE J. Solid - State Circuits,  vol. 37, no. 12, pp. 1748-1756 (Dec. 2002). E. McCune and W. Sander, “EDGE transmitter alternative using nonlinear polar modulation,”  Proc. IEEE Int. Symp. Circuits and Systems  ( ISCAS ), pp. 439-445 (May  2003 ). W. B. Sander, S. V. Schell, B. L. Sander, “Polar modulator for multimode cell phones,” Proc. 2003 IEEE Custom Integrated Circuits Conf., pp. 439-445 (Sept. 2003). T. Sowlati, D. Rozenblit, R. Pullela, M. Damgaard, E. McCarthy, D. Koh, D. Ripley, F. Balteanu, and I. Gheorghe, “Quad-band GSM/GPRS/EDGE polar loop transmitter,” IEEE J. Solid-State Circuits, vol. 39, no. 12, pp. 2179-2189 (Dec. 2004). M. R. Elliott, T. Montalvo, B. P. Jeffries, F. Murden, J. Strange, A. Hill, S. Nandipaku, and J. Harrebek, “A polar modulator transmitter for GSM/EDGE,” IEEE J. Solid-State Circuits, vol. 39, pp. 2190-2199 (Dec. 2004). P. Reynaert, M. Steyaert, “A 1.75-GHz polar modulated RF power amplifier for GSM/EDGE,” IEEE J. Solid-State Circuits, vol. 39, no. 12, pp. 2598-2608 (December 2005). A. W. Hietala, “A quad-band 8PSK/GMSK polar transceiver,” IEEE J. Solid-State Circuits, vol. 41, no. 5, pp. 1133-1141 (May 2006). But these are only for the narrowband (200 kHz allocated bandwidth) modulation standard, EDGE, of the Second Generation or 2G cellular. The polar architecture migration to wideband modulation standards, such as Third Generation or 3G (WCDMA—allocated bandwidth of 5 MHz), Fourth Generation or 4G (3GPP LTE, WiMAX—allocated bandwidth of up to 20 MHz) and other evolving wideband wireless standards (e.g., 802.11n) remains a daunting task and, so far, there has been no silicon demonstrations in the open literature. The effort, however, appears to continue with so far one theoretical proposal of the polar topology for WCDMA (Y. Huang, J. H. Mikkelsen, and T. Larsen, “Investigation of polar transmitters for WCDMA handset applications,” Proc. Norchip Conference, pp. 155-158 (November 2006)) and one 90-nm CMOS demonstration of the RF front-end block components of the WCDMA digital polar transmitter (S. Akhtar, P. Litmanen, M. Ipek, J. (H.-C.) Lin , S. Pennisi, F.-J. Huang, and R. B. Staszewski, “Analog path for triple band WCDMA polar modulated transmitter in 90 nm CMOS,” Proc. 2007 IEEE Radio Frequency Integrated Circuits (RFIC) Symp., pp. 185-188 (June 2007)). 
         [0007]    B. Digitally-Intensive RF Transmitters. As mentioned above, the digital approach to designing RF circuits and architectures is taking over in industry. The main reasons behind this sea-like transformation are the ever improving cost advantages and processing capabilities of the CMOS technology, which have been happening at regular intervals with the pace according to the so-called Moore&#39;s Law. Basically, with every CMOS process technology advancement node (i.e., from 130-nm to 90-nm, then to 65-nm, and then to 40-nm, and so on) happening every 18-24 months, the digital gate density, being a measure of the digital processing capability, doubles (i.e., gate area scaling factor of 0.5×). At the same time, the basic gate delay, being a measure of the digital processing speed, improves linearly (i.e., gate delay scaling factor of 0.7×). Likewise, the cost of fabricated silicon per unit area remains roughly the same at its high-volume production maturity stage. Indeed, over the last decade, the cost of silicon charged by integrated circuit (IC) fabs has remained constant at around US$ 0.10-0.25/mm 2 , depending on the wafer volume and targeted gross profit margin (GPM). The main implication of this is that a cost of a given digital function, such as a GSM detector or an MP3 decoder, can be cut in half every 18-24 months when transitioned to a newer CMOS technology. At the same time, the circuits consume proportionately less power and are faster. 
         [0008]    Unfortunately, these wonderful benefits of the digital scaling are not shared by the traditional RF circuits. What&#39;s more, the strict application of earlier described architectures to the advanced CMOS process node might actually result in a larger silicon area, poorer RF performance and higher consumed power. The constant scaling of the CMOS technology has had an unfortunate effect on the linear capabilities of analog transistors. To maintain reliability of scaled-down MOS devices, the Vdd supply voltage keeps on going down, while the threshold voltage Vt remains roughly constant (to maintain the low level of leakage current). This has a negative effect on the available voltage margin when the transistors are intended to operate as current sources. What&#39;s more, the implant pockets added for the benefit of digital operation, have drastically degraded the MOS channel dynamic resistance r ds , thus severely reducing the quality of MOS current sources and the maximum available voltage self-gain g m ·r ds  (g m  is the transconductance gain of a transistor). Furthermore, due to the thin gate dielectric becoming ever thinner, large high-density capacitors realized as MOS switches are becoming unacceptably leaky. This prevents an efficient implementation of low-frequency baseband filters and charge-pump PLL loop filters. 
         [0009]    C. New Paradigm of RF Design in Nanometer-Scale CMOS. An early attempt at designing RF circuits in advanced CMOS revealed a new paradigm. See, R. B. Staszewski and P. T. Balsara, “ All - Digital Frequency Synthesizer in Deep - Submicron CMOS ,” New Jersey: John Wiley &amp; Sons, Inc. (September 2006). 
         [0010]    In a deep-submicron CMOS process, time-domain resolution of a digital signal edge transition is superior to voltage resolution of analog signals. 
         [0011]    On a pragmatic level, this means that a successful design approach in this environment would exploit this paradigm by emphasizing the following: 
         [0012]    fast switching characteristics or high ƒ r  (20 ps and 250 GHz in 40-nm CMOS process, respectively) of MOS transistors: high-speed clocks and/or fine control of timing transitions; 
         [0013]    high density of digital logic (1 Mgates/mm 2 ) and SRAM memory (4 Mb/mm 2 ) makes digital functions and assistant software extremely inexpensive; 
         [0014]    ultra-low equivalent power-dissipation capacitance C pd  of digital gates leading to both low switching power consumption (P r =ƒ·Cpd·V dd   2 ) as well as potentially low coupling power into sensitive analog blocks; 
         [0015]    small device geometries and precise device matching made possible by the fine lithography in order to create high-quality analog data converters; 
         [0016]    Conversely, on a pragmatic level, it means a successful design approach in this environment would exploit this paradigm by avoiding the following: 
         [0017]    biasing currents that are commonly used in analog designs; 
         [0018]    reliance on voltage resolution with ever decreasing supply voltages and increasing noise and interferer levels; 
         [0019]    nonstandard devices that are not needed for memory and digital circuits, which constitute majority of the silicon die area. 
         [0020]    Despite the early misconceptions that the digitalization of RF would somehow produce more phase noise, spurs and distortion, the resulting digitally-intensive architecture is likely to be overall more robust by actually producing lower phase noise and spurious degradation of the transmitter chain and lower noise figure of the receiver chain in face of millions of active logic gates on the same silicon die, as repeatedly proven in subsequent publications. See, R. B. Staszewski, K. Muhammad, D. Leipold, C.-M. Hung, Y.-C. Ho, J. L. Wallberg, C. Fernando, K. Maggio, R. Staszewski, T. Jung, J. Koh, S. John, I. Y. Deng, V. Sarda, O. Moreira-Tamayo, V. Mayega, R. Katz, O. Friedman, O. E. Eliezer, E. de-Obaldia, and P. T. Balsara, “All-digital TX frequency synthesizer and discrete-time receiver for Bluetooth radio in 130-nm CMOS,” IEEE Journal of Solid-State Circuits, vol. 39, iss. 12, pp. 2278-2291 (December 2004). R. B. Staszewski, J. Wallberg, S. Rezeq, C.-M. Hung, O. Eliezer, S. Vemulapalli, C. Fernando, K. Maggio, R. Staszewski, N. Barton, M.-C. Lee, P. Cruise, M. Entezari, K. Muhammad, and D. Leipold, “All-digital PLL and transmitter for mobile phones,” IEEE Journal of Solid-State Circuits, vol. 40, iss. 12, pp. 2469-2482 (December 2005). R. B. Staszewski, D. Leipold, O. Eliezer, M. Entezari, K. Muhammad, I. Bashir, C.-M. Hung, J. Wallberg, R. Staszewski, P. Cruise, S. Rezeq, S. Vemulapalli, K. Waheed, N. Barton, M.-C. Lee, C. Fernando, K. Maggio, T. Jung, I. Elahi, S. Larson, T. Murphy, G. Feygin, I. Deng, T. Mayhugh, Y.-C. Ho, K.-M. Low, C. Lin, J. Jaehnig, J. Kerr, J. Mehta, S. Glock, T. Almholt, S. Bhatara, “A 24mm2 quad-band single-chip GSM radio with transmitter calibration in 90 nm digital CMOS,” Proc. of IEEE Solid-State Circuits Conf., sec. 10.5, pp. 208-209, 607 (February 2008). J. Mehta, R. B. Staszewski, O. Eliezer, S. Rezeq, K. Waheed, M. Entezari, G. Feygin, S. Vemulapalli, V. Zoicas, C.-M. Hung, N. Barton, I. Bashir, K. Maggio, M. Frechette, M.-C. Lee, J. Walberg, P. Cruise, N. Yanduru, “A 0.8 mm2 all-digital SAW-less polar transmitter in 65 nm EDGE SoC,” Proc. of IEEE Solid-State Circuits Conf., sec. 3.2, pp. 58-59, San Francisco, Calif., USA (February 2010). Additionally, the new architecture would be highly reconfigurable with analog blocks that are controlled by software to guarantee the best achievable performance and parametric yield. Another benefit of the new architecture would be an easy migration from one process node to the next without significant rework. 
         [0021]    D. All-Digital Polar Transmitter. The transmitter architecture that is amenable to the digital nanoscale CMOS technology is shown in the schematic diagram of  FIG. 2   a . See, R. B. Staszewski, K. Muhammad, D. Leipold, C.-M. Hung, Y.-C. Ho, J. L. Wallberg, C. Fernando, K. Maggio, R. Staszewski, T. Jung, J. Koh, S. John, I. Y. Deng, V. Sarda, O. Moreira-Tamayo, V. Mayega, R. Katz, O. Friedman, O. E. Eliezer, E. de-Obaldia, and P. T. Balsara, “All-digital TX frequency synthesizer and discrete-time receiver for Bluetooth radio in 130-nm CMOS,” IEEE Journal of Solid-State Circuits, vol. 39, iss. 12, pp. 2278-2291 (December 2004). R. B. Staszewski, J. Wallberg, S. Rezeq, C.-M. Hung, O. Eliezer, S. Vemulapalli, C. Fernando, K. Maggio, R. Staszewski, N. Barton, M.-C. Lee, P. Cruise, M. Entezari, K. Muhammad, and D. Leipold, “All-digital PLL and transmitter for mobile phones,” IEEE Journal of Solid-State Circuits, vol. 40, iss. 12, pp. 2469-2482 (December 2005). It has been deployed in mass production since around mid-2000&#39;s. Similar to the embodiments described above, a digital baseband block  2  provides I- and Q-signals, now to an I/Q-to-polar converter  12 , providing polar signals (ρ, θ). The LO  5  is realized as an all-digital PLL (ADPLL) that produces a phase/frequency-modulated digital clock carrier using the two-point digital modulation scheme. The clock is fed into a digital-to-RF-amplitude converter  15  (DRAC) that produces an RF output whose envelope is substantially proportional to the amplitude control word (ACW) or ρ. Hence, the architecture is termed a digital polar TX. Again, a power amplifier  9  could be used to provide the signal to the antenna  11 . 
         [0022]    The DRAC  15  can be realized as an all-digital RF power generation circuit. P. Cruise, C.-M. Hung, R. B. Staszewski, O. Eliezer, S. Rezeq, D. Leipold, and K. Maggio, “A digital-to-RF-amplitude converter for GSM/GPRS/EDGE in 90-nm digital CMOS,” Proc. of 2005 IEEE Radio Frequency Integrated Circuits (RFIC) Symp., sec. RMO1A-4, pp. 21-24 (June 2005). Sometimes it is also referred to in the literature as a digitally-controlled power amplifier (DPA), although this would appear somewhat of a misnomer. The input signal is an internal digital clock, so measuring the amplification gain of the DPA seems a little problematic. The DRAC  15  has proven its use for the power ramp as well as amplitude modulation in more advanced modulation schemes, such as the extended data rate (EDR) mode of Bluetooth, EDGE and WCDMA. 
         [0023]    The DRAC  15  operates as a near-class-E RF PA and is driven by the square wave output of the digitally-controlled oscillator (DCO) in the ADPLL. The rise-/fall-time of digital signals, including clocks, is typically 20-40 ps, which makes the trapezoidal shape quite close to a square one for cellular band signals. A large number of core NMOS transistors are used as on/off switches, each with a certain conductance 1/R, and are followed by a matching network that interfaces with an antenna  11  or an external PA  9 . The number of active switches, and thus the total conductance Σ(1/R), is controlled digitally and establishes the instantaneous amplitude of the output RF envelope. The RF output power is created by coherently moving the resonating energy through an LC tank between the load and the switches. The supply V dd  replenishes the energy lost to the load and internally. The RF amplitude is based on the relationship between the total switch conductance and the conductance of the matching network. 
         [0024]    The class-E PA operation is attempted to be satisfied at the maximum output power (all the switches are active), where the highest achievable efficiency matters the most. Note that there could be different types of conversions from digital to RF amplitude as well as classes of operation. For example, class-D PA would use two switching devices operating complementary and connected to the supply/ground and the matching network. 
         [0025]    Fine amplitude resolution is achieved through high-speed ΣΔ transistor switch dithering. The timing diagram in  FIG. 2   b  (P. Cruise, C.-M. Hung, R. B. Staszewski, O. Eliezer, S. Rezeq, D. Leipold, and K. Maggio, “A digital-to-RF-amplitude converter for GSM/GPRS/EDGE in 90-nm digital CMOS,” Proc. of 2005 IEEE Radio Frequency Integrated Circuits (RFIC) Symp., sec. RMO1A-4, pp. 21-24 (June 2005) assumes that the data changes every RF clock cycle, which is reasonable given the high-speed dithering. In practice, the integer ACW signals would change every certain number of DCO cycles. Despite the high speed of digital logic operation, the overall power consumption of the transmitter architecture is lower than that of architectures to date. 
         [0026]    E. All-Digital I/Q Transmitter. As mentioned above, due to the bandwidth expansion of ρand θ, which could be as much as 10× of the original I/Q signal bandwidth, it might be difficult to apply the digital polar TX architecture to the wideband modulation, especially the most recent 3GPP LTE cellular and 802.11n wireless connectivity standards. The required bandwidths would be on the order of hundreds of MHz. Consequently, the digital I/Q architecture was introduced to maintain the digital RF approach while addressing the bandwidth expansion problem of the polar topology. See, A. Jerng and C. G. Sodini, “A wideband ΔΣ digital-RF modulator for high data rate transmitters,” IEEE J. Solid-State Circuits, vol. 42, no. 8, pp. 1710-1722 (August 2007). P. Eloranta, P. Seppinen, S. Kallioinen, T. Saarela, and A. Pärssinen, “A multimode transmitter in 0.13_m CMOS using direct-digital RF modulator,” IEEE J. Solid-State Circuits, vol. 42, no. 12, pp. 2774-2784 (December 2007). 
         [0027]    A typical architecture is shown in  FIG. 3 . A new optional circuit, sample-rate converter  16  (SRC), is added to convert the lower-rate baseband I/Q signal (processed at integer multiple of the symbol rate) into a much higher rate necessary to spread the quantization noise, thus lowering its spectral density. Note that this optional SRC circuit  16  could also be used in the  FIG. 2   a  digital polar TX as part of the I/Q-to-polar converter  12 . 
         [0028]    The operation of the digital I/Q modulator is as follows. The I and Q digital samples drive their respective DRAC converters  15  that produce two RF signal components, whose amplitude is ideally proportional to the respective I/Q digital input. The frequency and phase of the RF signal follows the respected clock input. As such, the phase of the DRAC Q output is 90° delayed with respect to that of the I output. The two amplitude-modulated RF components are then added together (using adder 8) to produce the desired composite RF output. The output is then transmitted to the antenna  11  directly or through a high-power PA  9 . 
         [0029]    This digital I/Q architecture does not appear to compare favorably with the polar topology of  FIG. 2   a . The digital I/Q architecture seems more complex with extra circuitry adding noise and creating signal distortion. The frequency modulation of the LO  5  could be satisfactorily accomplished through the well-known two-point modulation scheme of the ADPLL. The phase component is thus under the closed-loop feedback, which reduces the noise and distortion content falling within the ADPLL loop bandwidth. Closing the loop around the I/Q modulator RF output is typically much more difficult. The traditional issues of the timing misalignment in the analog polar architecture is not a problem anymore with the digital approach. The digital discrete-time operation is by construction clock-cycle accurate while the modern technology can easily support sampling rates even at the GHz range, as well as ultra-fast settling of the DPA conversion circuits (with the speed governed by ƒ T ). Consequently, the circuits of digital architectures can ensure fine time accuracy, which is constant and not subject to process and environmental changes. 
         [0030]    The DRAC  15  could be implemented as a digitally-controlled RF-modulated current source. See, P. Eloranta, P. Seppinen, S. Kallioinen, T. Saarela, and A. Pärssinen, “A multimode transmitter in 0.13_m CMOS using direct-digital RF modulator,” IEEE J. Solid-State Circuits, vol. 42, no. 12, pp. 2774-2784 (December 2007). This way, the addition of the two DRAC output components could be as simple as connecting them electrically. For the I/Q signal orthogonality to hold, these two current sources need to be ideal such that one signal path output does not affect the operation of the other. This means the need to resort to current source impedance boost techniques, such as cascoding. Unfortunately, stacking of the MOS transistors in a cascode structure is difficult in the modern low-voltage technologies and further produces leakage and excessive amount of noise. In addition, the quadrature phases are needed, which might complicate the LO clock  5  generation and distribution. 
       SUMMARY OF THE INVENTION 
       [0031]    Embodiments of the invention seeks to provide a solution which fulfils a need for a digital TX architecture that is capable of supporting advanced wideband wireless modulation standards, but which avoids the intrinsic bandwidth expansion issues of the polar topology and the severe noise issues of the conventional digital I/Q architectures. 
         [0032]    In accordance with embodiments of the invention, an electronic circuit according to the preamble defined above is provided, wherein the first digital-to-RF-amplitude convertor is operative in a first duty cycle that is different from 50% and the second digital-to-RF-amplitude convertor is operative in a second duty cycle that is different from 50% and substantially the same in value as said first duty cycle. In an embodiment, the first duty cycle is 25% and the second duty cycle is 25%. In this manner an improved digital transmitter architecture is provided in the area of complexity, noise, distortion and linearity issues prior art systems. 
         [0033]    In accordance with embodiments, the first DRAC receives an I-clock signal and the second DRAC receives a Q-clock signal, the duty cycle of the I-clock signal being equal to the first duty cycle and the duty cycle of the Q-clock signal being equal to the second duty cycle, the first duty cycle and second duty cycle being subsequent to each other in time, or in other words adjacent or neighboring. The first and second duty cycle may be partially overlapping, e.g. in the case of non-zero rise and fall time of the clock signals. 
         [0034]    The in-phase component and the quadrature component comprise a respective amplitude control word, each of the first and second DRAC&#39;s comprising a controllable switch array having an array of AND gates, each AND gate receiving the respective clock signal and one of the bits of the respective ACW, the output of each AND gates being connected to an associated one of a plurality of MOS transistor switches, the outputs of each of the plurality of MOS transistors being connected to a summation node. Such a controllable switch array may be easily implemented in e.g. MOS techniques. In accordance with embodiments, the switch arrays of the first and second DRAC&#39;s are implemented in the electronic circuit with an interleaved lay-out. This improves the ease of manufacturing, and also enhances the regularity of the DRAC&#39;s so formed. 
         [0035]    In accordance with embodiments, each of the first and second DRAC includes an output impedance connected in series between the output of each MOS transistor and the summation node. This allows to increase the resolution and dynamic range of the RF envelope control (when the impedance accuracy and range are better than those of the transistor switches). 
         [0036]    In accordance with embodiments, the first DRAC and the second DRAC form one (single) circuit. This also enhances the ease of developing and manufacturing the electronic circuit, e.g. in MOS technique. 
         [0037]    In accordance with embodiments, the electronic circuit further includes a matching network for driving an analog RF part, the matching network being connected to a summation node of the first and second DAC, and including an LC resonant circuit which combines subsequent I and Q contributions in a single clock cycle. The LC resonant circuit provides a memory type of operation, allowing to add the subsequent I and Q contributions in each clock cycle. E.g. the matching network operates with a third duty cycle which is equal to the sum of the first duty cycle and the second duty cycle. 
         [0038]    In accordance with embodiments of the invention, the matching network is adjustable. Even more, the matching network may be dynamically controlled. This adjustable matching network can be used to adjust for a new frequency channel, or to adjust the matching network impedance to a changing envelope level in order to maximize the power added efficiency (PAE). 
         [0039]    In accordance with embodiments, the electronic circuit includes a first pair of DRAC&#39;s receiving positive components of the I and Q signals, and a second pair of DRAC&#39;s receiving negative components of the I and Q signals, the output nodes of the first pair and second pair being connected to inputs of the matching network. This allows a class-D type of operation of the electronic circuit in a transmitter. This class-D embodiment, also allows to have each DRAC to comprise complementary NMOS and PMOS transistors. In order to provide a single-ended output, the matching network includes a transformer circuit with a balun component. 
         [0040]    In accordance with embodiments, each DRAC includes an additional clock input, which additional clock input is a pulse width modulated version of the respective clock input. This enhances the amplitude resolution of the electronic circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    Embodiments of the invention will be discussed in more detail below, with reference to the attached drawings, in which: 
           [0042]      FIG. 1  illustrates a block diagram of a traditional RF transmitter. 
           [0043]      FIG. 2   a  illustrates a block diagram of an all-digital polar transmitter. 
           [0044]      FIG. 2   b  illustrates a timing diagram of a digital-to-RF-amplitude converter. 
           [0045]      FIG. 3  illustrates a block diagram of a digital I/Q transmitter. 
           [0046]      FIG. 4   a  illustrates a schematic diagram of an electronic circuit, in accordance with embodiments of the invention. 
           [0047]      FIG. 4   b  illustrates a timing diagram of relevant signals in the circuit of  FIG. 4   a.    
           [0048]      FIG. 5  illustrates a flow chart of the timing of the digital I/Q transmitter, in accordance with embodiments of the invention. 
           [0049]      FIG. 6   a  illustrates a symbol representing an embodiment of the invention with related signals. 
           [0050]      FIG. 6   b  illustrates a timing diagram for the signals presented in  FIG. 6   a.    
           [0051]      FIG. 7  illustrates a schematic diagram of a further differential class-D architecture, in accordance with embodiments of the invention. 
           [0052]      FIG. 8  illustrates a schematic diagram of a matching network applied in an E-class architecture, in accordance with embodiments of the invention. 
           [0053]      FIG. 9  illustrates a schematic diagram of a matching network applied in a D-class architecture, in accordance with embodiments of the invention. 
           [0054]      FIG. 10  illustrates a schematic diagram with additional optional elements, in accordance with embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0055]    Embodiments of the invention relates to a method and system for (digitally) generating a radio-frequency (RF) signal to be transmitted. In accordance with embodiments of the invention, this is accomplished without any intermediate stages or without resorting to polar topology. 
         [0056]    Polar transmitter (TX) topology is a known solution that involves power amplifiers (PA). However, this technique is only proven so far for narrowband systems, and it cannot be readily used for wideband modulation schemes, such as those in WiMAX and 3GPP LTE. 
         [0057]    Embodiments of the invention can be applied in all RF transmission systems, e.g. wireless connectivity, cell phones and base stations. Cell phones or basestations can be achieved using the present invention that feature higher integration levels, better power efficiency (hence longer battery lifetime), and more sophisticated reconfigurability. They can be also advantageously used with antenna arrays, where advantages in the cost, size, power consumption are particulary sought. 
         [0058]    In general terms, embodiments of the invention is based on the building blocks of digital power amplifiers (DPA) based on a large array of switching elements at RF rate (i.e., multi-GHz). It combines in a novel manner two DPA&#39;s and a matching network. 
         [0059]    In accordance with embodiments, this is accompisihed by direct coupling of I and Q digital PA states with a matching network. The two power amplifiers operate at 25% duty cycle, and the matching network is designed for 50% duty cycle, in accordance with embodiments. 
         [0060]    Use of embodiments of the invention yields a solution which shows simplicity. The solution avoids intermediate stages of adding I and Q signals, which reduce power efficiency and increase noise. Furthermore, power efficiency is improved. 
         [0061]    Further refinements are implemented by utilizing adaptive pulse width modulation (PWM) and an adaptive duty cycle of the switching transistors, Further improvements can be made by using an adaptive matching network. 
         [0062]    Completed circuit level simulations have shown the benefits, specifically for a 65 nm CMOS process. 
         [0063]    In the following, reference is made to the Raab&#39;s formula. The person skilled in the art will know this formula and other knowledge on digital amplifier techniques, e.g., from the following articles, which are incorporated herein by reference: F. H. Raab, “Idealized Operation of the Class E Tuned Power Amplifier”,  IEEE Transactions on Circuits and Systems,  Vol. CAS-24, No. 12, pp. 725-735, December 1977. F. H. Raab, “Class-E, class-C, and class-F power amplifiers based upon a finite number of harmonics,”  IEEE Trans. Microwave Theory Tech.,  vol. 49, pp. 1462-1468, August 2001. F. H. Raab, P. Asbeck, S. C. Cripps, P. B. Kenington, Z. B. Popovic, N. Pothecary, J. F. Sevic, and N. O. Sokal, “Power amplifiers and transmitters for RF and microwave,”  IEEE Trans. Microw. Theory Tech.,  vol. 50, no. 3, pp. 814-826, March 2002. N. O. Sokal and A. D. Sokal, “Class E—A new class of high efficiency tuned single-ended switching power amplifiers,”  IEEE J. Solid - State Circuits , vol. SSC-10, pp. 168-176, June 1975. 
         [0064]    The DRAC circuit as used in accordance with embodiments of the invention is realized as a digitally-controlled RF-modulated resistor structure. A schematic view thereof is shown as part of the RF circuit diagram of  FIG. 4   a , inside the boundary indicated by reference numeral  25 . Note that the two instances of circuit  25  are shown separately for conceptual reasons. The actual implementation does not have to be separate. In fact, it might be beneficial to make the two as part of one block or even interleave the respective switches. Each DRAC  25  has a clock input (Clock I or Clock Q) and an amplitude control word for the quadrature signal (I and Q, respectively), and includes a series of AND-gates followed by MOS transistors M i,I  and M j,Q . This could result in benefits of the simpler circuit, the lack of stacked devices and the lack of noisy current sources. The antenna  11  and optional PA  9  are represented in this drawing by a load  28  (Z L ). A supply voltage V dd  is fed to a common node X via an RF choke 29 (RFC). 
         [0065]    In the prior art I/Q TX system described above, the final signal summation operation might be difficult to accomplish since the outputs are not currents but voltages. Hence, if it is the RF waveforms at the output of their matching networks that are to be added, then a bulky microwave-type isolator would be needed. Otherwise, the RF voltage level of the I path will affect the impedance of the Q path, and vice versa. Hence, the I/Q orthogonality will not be preserved. 
         [0066]    On the other hand, attempting to connect together the drains of the I and Q switching transistor arrays (i.e., at the ‘X’ nodes of the I and Q DRAC circuits  25  as indicated in  FIG. 4   a ) would require only a single common matching network  27 , but it would also fail to preserve the orthogonality of the I/Q paths since they would depend on the each other&#39;s state. The reason for this loss of the orthogonality is the time overlap of the I/Q operations. Let us first assume that the durations of the I and Q operations are controlled by the logic high state of their respective clocks, although the actual duration is rather controlled by the clock/data overlap, i.e., their AND operation. For the conventional 50% clock duty cycle, 25% of the two clock tops coincide. During this time, the I and Q switches are simultaneously turned on, thus distorting the PA interaction of the switches with the matching network. 
         [0067]    In order to eliminate this interaction, it is, therefore, proposed to simply avoid the overlap by reducing the clock duty cycle D to 25%, as shown in the timing diagram of  FIG. 4   b . Creating the D=25% clock from the 2× frequency differential clock is quite straightforward and merely involves dynamic selection of the proper clock edges. As apparent from the lower signal line in  FIG. 4   b , the matching network  27  needs to be designed to the combined duty cycle of 2×D=50%. 
         [0068]    The solution could be thought of as a time-division duplexing (TDD), in which the linear addition of the time-shifted I and Q paths is accomplished by allocating separate time slots to enter the I/Q information into the system. The system contains the matching network  27  including, e.g., an LC resonant circuit that serves as a memory to combine the neighboring I/Q contributions. The memory operation is a good assumption here since the LC resonant circuit has an inertia. Once the I/Q switch turn-on times are non-overlapping, the circuit operation becomes orthogonal. The orthogonality can be easily proven with the assumption of the linear time-invariant model of the switched PA, which is valid when the total switch resistance is much larger than the matching network impedance. Equivalently, the RF output power is much smaller than its saturation point. Adding an incremental switch conductance will linearly increase the RF output envelope, independent from the total instantaneous conductance level. Changing the clock phase by 90° will change the RF output carrier also by 90°. Since I and Q contributions are independent and time shifted, their vector addition will be linear. 
         [0069]      FIG. 5  is a flowchart illustrating the operation, wherein in step  31 , the I data is presented to the resonant circuit (matching network  27 ) during substantially 0-25% f the period. In step  32  the Q data is presented to the resonant circuit during substantially 25-50% of the period. In step  33  the system advances to the next time period. 
         [0070]    The digital I/Q RF transmitter could be also viewed as an RF-DAC  30  with clock and I/Q inputs, as shown in  FIG. 6   a . The clock would e.g. be provided by a low-phase-noise LO. The I/Q code words could be aligned with each other, as shown in  FIG. 6   b  or time-shifted by T 0 /4 or 90° to reflect their intended timing. The clock could be of twice the frequency, such that after an internal division-by-2, its falling edges could be used to create the Q clock. 
         [0071]    As discussed above, the proposed techniques and embodiments described above can be applied to the other classes of PA operation.  FIG. 7  shows a schematic diagram of a voltage-mode class-D differential architecture. The load  128  (Z L ) represents again the antenna  11 , optionally with a PA  9 . In this embodiment four DRAC&#39;s  125  are used, one pair for the positive part and one pair for the negative part. Each DRAC  125  includes a complementary pair of PMOS and NMOS transistors, as well as an optional impedance Z. Since the unit inverter including the NMOS and PMOS transistors pulls its output to either ground or supply, the voltage supply feed V dd  to the common node ‘X’ (as shown in the embodiment of  FIG. 4 ) is not explicitly needed anymore. The matching network  127  can be based on a transformer, as shown in the schematic diagram of  FIG. 9  and discussed below. In this case, it might provide an additional benefit of a balun (balanced-to-unbalanced) to convert the output to single-ended. 
         [0072]    The optional Z impedance connected to each switching circuit could be a resistor, capacitor, inductor or a combination thereof. If it is bypassed (i.e., Z=0), then the RF envelope control range and resolution is controlled by the NMOS and PMOS transistor resistance, just like in accordance with embodiments described with reference to  FIG. 4 . The benefits of using explicit elements of impedance Z is that it would allow to increase the resolution and dynamic range of RF envelope control in case the impedance accuracy and range are better than those of the transistor switches. 
         [0073]    The final driving stage transistors as shown in the DRAC&#39;s  125  in the embodiments of  FIG. 7  typically form an inverter. However, in order to ensure the proper TDD operation, the NMOS and PMOS transistors of the driving stage are placed in hi-Z state whenever the other associated array is used. This is achieved by pulling the gate of NMOS transistor low while pulling the gate of PMOS transistor high, using the other clock signal (Clock Q+ in the I part DRAC  125 , and Clock I+ in the Q part DRAC  125 ). This will ensure that the common node will not be driven into conflicting states. 
         [0074]      FIG. 8  shows a possible implementation of a matching network  27 , including an output inductance L o , as can be used in the class-E implementations described above.  FIG. 9  shows a schematic diagram of a possible implementation of a matching network  127  in a differential (or complementary) class-D amplifier as shown in  FIG. 7 , including a transformer. In each of the matching networks  27 ,  127 , there are two (resp. three) adjustable capacitors C L  and C D  (C D+  and C D. ) that need to be properly set to roughly satisfy the class-E (or class-D) operation for a given frequency. 
         [0075]      FIG. 8  shows the class-E digital I/Q modulator from the standpoint of its matching network  27 , whose design follows principles of the class-E PA. The matching network  27  components can be adjustable, either semi-statically to adjust for the new frequency channel, or dynamically to adjust the matching network impedance to the changing envelope level in order to maximize the power added efficiency (PAE). They also could be statically adjusted to account for the process variation of the various components. The adjustable capacitors C L  and C D  can be realized using digitally-switchable metal-in-metal (MiM) capacitors or MOS capacitors. Since one of the terminals of the drain capacitor (C D ) is grounded, it can be beneficially realized as a high-density MOS capacitor. 
         [0076]      FIG. 9  illustrates the differential class-D digital I/Q modulator from the standpoint of its matching network  127 , whose design follows principles of the class-D PA. The matching network  127  uses a transformer, which transforms the typical 50-ohm load impedance into a much lower driver impedance compatible with the low-voltage CMOS operation. The transformer&#39;s leakage inductance is also be used as part of the resonant circuit. The final driving stage of each quadrature output forms a tri-state inverter. Its output is hi-Z whenever the associated quadrature driver is enabled. For example, the I+ driver&#39;s output goes into high impedance if the Q+ driver is active. The same applies to I− and Q− drivers. This arrangement prevents driving conflicts of the two inverters whose outputs are connected (i.e., I+/Q+ as well as I−/Q−). Consequently, each inverter&#39;s output is tri-stated in three out of four quadrature time slots. It is actively driven at its allotted timeslot (one out of four time slots). In the remaining two time slots, its output is typically connected to ground (low logic state), although high logic state can also be considered as a default inactive state. Note that the driver topology of  FIG. 9  forms a H-bridge structure, known as such, which produces three states: +1, 0, −1. 
         [0077]    In accordance with embodiments of the invention, an additional enhancement is the dynamic digital control of the matching network components. The matching network  27  has a significant effect not only on the general RFDAC operation, but also on the proper operation of the TDD I/Q combining, such as maintaining the I/Q orthogonality. This is indicated in the embodiment shown in  FIG. 10  by the control arrow in the matching network  27 . The matching network  27  is thus adjustable, or can be dynamically controlled. 
         [0078]    An amplitude resolution enhancement method using incremental pulse-width modulation (PWM) was described in [22]. It was meant for a digital polar TX architecture of  FIG. 2   a . This technique adds an additional transistor of the same weight as the LSB transistors but the new transistor is subject to PWM. This way, an additional three bits of the RF-DAC resolution have been obtained. It is proposed to apply the same idea to the digital I/Q TX architecture. Consequently, instead of applying the incremental PWM to the single digitally-controlled RF switched resistor (DRAC  25 ), both such RF-switched resistor structures of  FIG. 4  will undergo the incremental PWM. In the schematic diagram shown in  FIG. 10 , this is indicated by the PWM control module  36  in the clock path of both I and Q DRAC&#39;s  25 . 
         [0079]    In summary, a novel digital I/Q transmitter architecture is presented in accordance with embodiments described above that avoids area, complexity, noise, distortion and linearity issues of the existing solutions by allocating separate time slots to the I and Q operations during which the respective digitally controlled RF-switched resistors, realized as arrays of MOS switches, are active. This way, the operational orthogonality of the I and Q paths is maintained and the I/Q switch arrays can be simply connected together. An example embodiment uses substantially 25% duty cycle I/Q clocks. 
         [0080]    Although embodiments have been described herein, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.