Abstract:
One embodiment of the present invention provides a transmitter for wireless communication. The transmitter includes a wideband tunable modulator, a number of power amplifiers with a particular power amplifier associated with a particular frequency range, and a wideband power amplifier (PA) driver. The PA driver is configured to receive an output signal from the wideband tunable modulator, amplify the output signal, and send the amplified output signal to at least one of the power amplifiers.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/363,056, entitled “WIDEBAND TRANSMITTER FRONT-END,” by inventors Shih Hsiung Mo, Yan Cui, and Chung-Hsing Chang, filed 31 Jan. 2012, which claims the benefit of U.S. Provisional Application No. 61/526,853, entitled “Wideband Transmitter Front-End,” by inventors Shih Hsiung Mo, Yan Cui, and Chung-Hsing Chang, filed 24 Aug. 2011. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure relates generally to the transmitter front-end of a wireless communication system. More specifically, the present disclosure relates to a novel low-power wideband transmitter front-end. 
     2. Related Art 
     Traditional wireless communication systems are usually designed for a specific standard, such as GSM (Global System for Mobile Communications) or Wideband Code Division Multiple Access (W-CDMA), each requiring different carrier frequencies. For example, the carrier frequency of the GSM signals varies from 800 MHz to 1 Ghz, while the carrier frequency of the W-CDMA signals varies between 2-3 GHz. Current demand for the convergence of wireless services, in which users can access different standards from the same wireless device, is driving the development of multi-standard and multi-band transceivers, which are capable of transmitting/receiving radio signals in the entire wireless communication spectrum (from 300 MHz to 3 GHz). 
     To meet multi-standard and multi-band requirements, the RF front-end (which includes circuitry between the antenna and the first intermediate frequency (IF) stage) needs to operate over a frequency range covering the entire wireless communication spectrum. 
     SUMMARY 
     One embodiment of the present invention provides a transmitter for wireless communication. The transmitter includes a wideband tunable modulator, a number of power amplifiers with a particular power amplifier associated with a particular frequency range, and a wideband power amplifier (PA) driver. The PA driver is configured to receive an output signal from the wideband tunable modulator, amplify the output signal, and send the amplified output signal to at least one of the power amplifiers. 
     In a variation on this embodiment, the transmitter further includes a single-pole multiple-throw switch situated between the wideband PA driver and the power amplifiers. The switch is configured to switch the amplified output signal to one of the power amplifiers based on a carrier frequency of the output signal. 
     In a variation on this embodiment, the wideband tunable modulator and the power amplifier are located on a same integrated circuit (IC) chip. 
     In a variation on this embodiment, the wideband tunable modulator is a quadrature modulator. 
     In a variation on this embodiment, the wideband tunable modulator has a tuning range between 300 MHz and 3.6 GHz. 
     In a further variation, the wideband tunable modulator includes an amplifier. The bandwidth of the amplifier is compensated for by using an inductive peaking technique. 
     In a variation on this embodiment, the wideband PA driver includes a cascode amplifier. 
     In a further embodiment, the cascode amplifier has two poles. 
     In a further embodiment, the cascode amplifier includes an inductor that is serially coupled to a parasitic capacitor. 
     In a further embodiment, the inductor has an inductance between 0.5 nH and 10 nH. 
     In a variation on this embodiment, the wideband PA driver has a 10 dB bandwidth that is up to 3.6 GHz. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  presents a diagram illustrating the architecture of a conventional wireless transmitter (prior art). 
         FIG. 2  presents a diagram illustrating the architecture of a wireless transmitter, in accordance with an embodiment of the present invention. 
         FIG. 3A  presents a schematic of a conventional mixer (prior art). 
         FIG. 3B  presents a schematic of a mixer, in accordance with an embodiment of the present invention. 
         FIG. 4A  presents a schematic of a conventional power amplifier (PA) driver (prior art). 
         FIG. 4B  presents a diagram illustrating an exemplary frequency response of the return loss for a conventional PA driver (prior art). 
         FIG. 4C  presents a schematic of a wideband power amplifier (PA) driver, in accordance with an embodiment of the present invention. 
         FIG. 4D  presents a diagram illustrating an exemplary frequency response of the return loss for a wideband PA driver, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     Overview 
     Embodiments of the present invention provide a solution for a low-power transmitter front-end. In one embodiment, the transmitter chip includes a wideband power-amplifier driver that is capable of amplifying RF (radio frequency) signals over a wide frequency range. 
     Wideband Transmitter Front-End 
       FIG. 1  presents a diagram illustrating the architecture of a conventional wireless transmitter (prior art). Transmitter  100  includes a baseband signal processor  102 , a digital-to-analog converter (DAC)  104 , a low-pass filter (LPF)  106 , a number of quadrature modulators (such as modulator  108 ), a number of power-amplifier (PA) drivers (such as PA driver  110 ), a number of PAs (such as PA  112 ), an n×1 switch  114 , and an antenna  116 . 
     During operation, baseband signals provided by baseband signal processor  102  are converted from the digital domain to the analog domain by DAC  104 . LPF  106  filters out any out-of-band noise. Modulators operating at different frequencies modulate the baseband signal at different frequency bands. The output of a particular modulator, thus RF signal at a particular carrier frequency, is sent to a dedicated PA driver operating at that carrier frequency. For example, the output of modulator  108  is sent to PA driver  110 , which is optimized to work at the operating frequency of modulator  108 . The output of a PA driver is then sent to a corresponding PA (often located off-chip), which is also optimized to work at the same frequency band. Note that a PA driver needs to be able to deliver a considerable power to its corresponding PA, which has a 50 Ohm input impedance. For example, the output of PA driver  110  is sent to PA  112 . Depending on the currently active standard or mode of operation, n×1 switch  114  selects the output of a desired PA, thus RF signal at the desired carrier frequency, and sends the RF signal to antenna  116  for transmission. 
     Note that, as shown in  FIG. 1 , the PA drivers (such as PA driver  110 ), the modulators (such as modulator  108 ) and LPF  106  are often integrated onto a single integrated circuit (IC) chip  118 . With a dedicated modulator and a dedicated PA driver for each frequency band, the transmitter performance can be optimized for each frequency band. However, a large amount of chip area is required to accommodate the multiple PA drivers and modulators. In addition, a large number of input/output lines will be needed, which not only increases the chip size, but also the size of a supporting printed circuit board (PCB), thus significantly increasing the size of the packaged device. 
     To reduce the size of the transmitter chip and the supporting PCB, it is desirable to use a single modulator and a single PA driver for the multiple frequency bands.  FIG. 2  presents a diagram illustrating the architecture of a wireless transmitter, in accordance with an embodiment of the present invention. In  FIG. 2 , transmitter  200  includes a baseband signal processor  202 , a DAC  204 , a low-pass filter (LPF)  206 , a tunable quadrature modulator  208 , a wideband PA driver  210 , a number of PAs (such as PA  214 ), a 1×n switch  216 , an n×1 switch  218 , and an antenna  222 . 
     LPF  206 , tunable modulator  208 , and wideband PA driver  210  form transmitter chip  212 , which has far fewer I/O lines than conventional transmitter chip  118 . A 1×n switch  216 , an n×1 switch  218 , and the multiple PAs form a PA chip  220 . 1×n switch  216  ensures that RF signals of a particular frequency band are sent to a corresponding narrowband PA, and n×1 switch  218  ensures that the output of the different PAs are sent to antenna  222  via a single input. 
     To enable multi-standard/multi-band application, tunable modulator  208  and wideband PA driver  210  need to have a bandwidth covering the entire wireless communication bandwidth, which is from 300 MHz all the way to 3.6 GHz. Hence, the tuning range of tunable modulator  208  needs to be between 300 MHz and 3.6 GHz. Such a wide bandwidth imposes a number of design challenges, especially for wideband PA drivers, which have a large dynamic range. On the other hand, the gain requirement for a mixer used inside a typical modulator is relatively low, making designing the modulator less difficult. In one embodiment, an inductive peaking technique is used to increase the bandwidth of an amplifier used in the mixer. 
       FIG. 3A  presents a schematic of a conventional amplifier used for a mixer (prior art). In  FIG. 3A , amplifier  300  includes a transistor  302  and a resistor  306 . Note that a capacitor  304  in  FIG. 3A  is the parasitic capacitance. An exemplary resistance of resistor  306  is 50 Ohm, and an exemplary capacitance of parasitic capacitor  304  is 800 fF. The bandwidth of amplifier  300  is limited by the existence of parasitic capacitor  304 . More specifically, the gain curve of amplifier  300  dips at higher frequencies. 
       FIG. 3B  presents a schematic of an amplifier in accordance with an embodiment of the present invention. Amplifier  320  has a similar structure compared with amplifier  300  except that amplifier  320  includes an additional inductor  308  situated between resistor  306  and the power supply. In one embodiment, the inductance of inductor  308  is 12 nH. Note that at lower frequencies, inductor  308  has minimum impact, and at higher frequencies (such as in the GHz range), inductor  308  resonates with parasitic capacitor  304 , thus increasing the bandwidth of amplifier  320 . In one embodiment, the bandwidth of amplifier  320  expands beyond 3 GHz. 
     Due to the large dynamic range of PA driver, inductive peaking is no longer an optimum solution for increasing the bandwidth of the PA driver because the resistance load (resistor  306 ) introduces voltage drop. In one embodiment, a double-pole solution is used to increase the bandwidth of an amplifier, such as an operational amplifier. 
       FIG. 4A  presents a schematic of a conventional power amplifier (PA) driver (prior art). PA driver  400  includes a pair of transistors  402  and  404 , and an inductor  406 . Transistors  402  and  404  form a cascode amplifier. The bandwidth of PA driver  400  is limited by parasitic effects. More specifically, parasitic capacitors resonate with inductor  406 , causing the input return loss (the S 11  parameter) frequency response curve to dip at the resonance frequency (or the pole location).  FIG. 4B  presents a diagram illustrating an exemplary frequency response of the return loss for a conventional PA driver (prior art). 
       FIG. 4C  presents a schematic of a wideband power amplifier (PA) driver, in accordance with an embodiment of the present invention. Similarly to conventional PA driver  400 , wideband PA driver  420  includes a cascode amplifier (transistors  422  and  424 ), a parasitic capacitor  430 , and a choke inductor  428 . Note that parasitic capacitor  430  may include an additional parallel capacitor. In addition, wideband PA driver  420  includes a serial inductor  426 , a parallel capacitor  432 , and a DC-block capacitor  434 . Serial inductor  426  is situated between parasitic capacitor  430  and parallel capacitor  432 , as shown in  FIG. 4C . In one embodiment, parallel capacitor  432  is coupled to an output bond pad  436 , and choke inductor  428  is located off-chip. 
     The added serial LC increases the bandwidth of wideband PA driver  420  because now there are two resonance frequencies (two poles), corresponding to two dips in the return loss frequency response curve.  FIG. 4D  presents a diagram illustrating an exemplary frequency response of the return loss for a wideband PA driver in response to an embodiment of the present invention. As one can see from  FIG. 3D , the existence of the two poles (one at f 0  and another one at f 1 ) significantly increases the bandwidth of PA driver  420 . In one embodiment, the bandwidth of the two-pole amplifier is 20% more than that of the conventional amplifier. By carefully selecting the values of inductors  426  and  428 , on communication frequency range (up to 3.6 GHz). In one embodiment, the inductance of inductor  426  is between 0.5 nH and 10 nH, and the inductance of inductor  428  equals or is greater than 33 nH. 
     The wideband PA driver and wideband mixer allow the transmitter chip manufacturer to manufacture the same type of transmitter chips that can be sold to different markets where different standards are implemented, thus significantly reducing the designing and manufacturing cost. 
     The foregoing descriptions of embodiments of the present invention have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit this disclosure. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. The scope of the present invention is defined by the appended claims.