Abstract:
A system includes first unit, a processor and a transmitter. The processor is located in the first unit to generate data to be communicated to a second unit that is physically separated from the first unit. The transmitter includes an antenna, a first circuit and a push-pull amplifier. The first circuit generates signals that are indicative of the data, and the push-pull amplifier drives the antenna with a signal that is indicative of the data in response to the signals generated by the first circuit. The amplifier also establishes an output power from the amplifier to the antenna to less than about one hundred milliwatts.

Description:
BACKGROUND  
         [0001]    The invention generally relates to a technique and apparatus for low power wireless communication.  
           [0002]    In a typical computer system, a main computer unit (a desktop or laptop computer, for example) communicates with various peripheral devices, such as a printer, a keyboard, a mouse, etc. Traditionally, this communication has been made possible by various cables that connect the computer unit to the peripheral devices. However, these cables place restraints on the positions of the peripheral devices relative to the computer unit, as well as contribute to excessive clutter near the computer unit.  
           [0003]    For purposes of eliminating the cable connections between the peripheral devices and the main computer unit, short-range wireless links may be used. As an example, communication over these wireless links may be defined by a standard, such as the Institute of Electrical and Electronics Engineers (IEEE) 802.11b standard, IEEE Std. 802.11, published in 1999, and IEEE Std. 802.11b, published as a supplement to the IEEE Std. 802.11 in 1999. As another example of another standard, communication over the wireless links may be defined by what is called the Bluetooth standard, set forth by the Bluetooth Special Interest Group (SIG) in a specification entitled, “Specification of the Bluetooth System, Volume 1: Core and Volume 2: Profiles,” version 1.1, February 2001. As an example of the implementation of one of these standards, a computer system  10  may have several wireless devices that communicate over short range microwave links. For example, a main computer unit  12  may communicate over wireless links with various peripheral devices, such as a printer  14 , a keyboard  16 , a mouse  18 , a local area network (LAN) access point  22  and a personal digital assistant (PDA)  20 . The main computer unit  12  may also communicate via wireless links with one or more other computer systems, such as a computer system  29  that is depicted in FIG. 1.  
           [0004]    A conventional wireless device that implements the IEEE 802.11b standard may include a transceiver semiconductor package, or “chip,” to demodulate signals received from an antenna of the device and modulate signals to be transmitted from the antenna. For purposes of transmitting, this transceiver chip generates a pair of balanced, modulated carrier signals. In this context, “balanced signals” refers to a pair of signals in which a current that is associated with one of the balanced signals is equal in magnitude but one hundred eighty degrees (180°) out of phase with the current that is associated with the other balanced signal. Balanced signals are ideally symmetrical with respect to ground. Thus, due to this balanced arrangement, all of the current that is communicated from one balanced output terminal of the transceiver chip returns to the other balanced output terminal of the transceiver chip without returning through ground.  
           [0005]    In addition to the transceiver chip, the transmitter circuitry of a conventional wireless device typically includes a single-ended amplifier to generate a signal to drive the antenna in response to the balanced signals that are provided by the transceiver chip. For purposes of interfacing the amplifier to the transceiver chip, the transmitter circuitry includes a balun that transforms the balanced signals provided by the transceiver chip into a single-ended signal that is received and amplified by the amplifier. Besides transforming the balanced signals into a single-ended signal, this circuitry also attempts to match the output impedance of the transceiver to the input impedance of the amplifier. Additional impedance matching circuitry may be used between the amplifier and the antenna for purposes of attempting to match the output impedance of the amplifier to the impedance of the antenna. Unfortunately, the input and output impedances of the amplifier are typically complex and small, a condition that presents challenges in designing the impedance matching circuitry and may cause excessive power dissipation by the transmitter circuitry.  
           [0006]    Thus, there is a continuing need for an arrangement that addresses one or more of the problems states above. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0007]    [0007]FIG. 1 is a schematic diagram of a wireless communication system of the prior art.  
         [0008]    [0008]FIG. 2 is a schematic diagram of a wireless communication device according to an embodiment of the invention.  
         [0009]    [0009]FIG. 3 is a schematic diagram of transmission circuitry of the wireless communication device of FIG. 2 according to an embodiment of the invention.  
         [0010]    [0010]FIG. 4 is a more detailed schematic diagram of the transmission circuitry of FIG. 4 according to an embodiment of the invention.  
         [0011]    [0011]FIG. 5 is a schematic diagram of a computer system in accordance with an embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0012]    Referring to FIG. 2, an embodiment  30  of a wireless communication device in accordance with the invention includes a host  40  and a transceiver unit  50 . The transceiver unit  50  includes a transmitter  52  that is enabled to transmit a short range, low power microwave communication signal to nearby wireless devices, and the transceiver unit  50  also includes a receiver  51  that is enabled to receive such signals from one or more adjacent wireless devices. Thus, the transceiver unit  50  establishes short-range, low power microwave communication between the host  40  and another wireless device.  
         [0013]    As an example, the wireless communication device  30  may be a main computer unit or a peripheral device of a computer system. In this manner, in some embodiments of the invention, the wireless communication device  30  may be a desktop or laptop computer. In these embodiments of the invention, the transceiver unit  50  may be an expansion card (for example) that is inserted into a slot connector of the computer and communicates with the rest of the computer (i.e., the host  40 ) via a bus  48 . In other embodiments of the invention, the wireless communication device may be a personal digital assistant (PDA), a mouse, a keyboard, a printer, a local area network (LAN) access point, another computer system, etc.  
         [0014]    More specifically, in some embodiments of the invention, the transceiver unit  50  may comply with a short range wireless standard, such as the IEEE 802.11b standard or Bluetooth standard. In some embodiments of the invention, for purposes of transmitting, the transmitter  52  drives an antenna  70  of the wireless device  30  with a modulated carrier signal that has a microwave frequency. The signal level at which the transmitter  52  drives the antenna is sufficiently small to limit the output power from an amplifier of the transmitter  52  to the antenna  70  to between approximately 30 to 100 milliwatts (mW). Such an output power establishes a transmission range less than about 100 meters.  
         [0015]    For purposes of improving the efficiency of transmissions from the transceiver unit  50 , in some embodiments of the invention, the transceiver unit  50  includes a push-pull amplifier  60 , a component of the transmitter  52  of the transceiver unit  50 . In contrast to the single-ended amplifiers used in conventional transmitters for low power and short range microwave transmissions, the use of the push-pull amplifier  60  reduces the overall power consumption of the transmitter  52 . This power reduction is attributable to the reduction in the power consumed in a push-pull amplifier versus a single-ended amplifier, as well as the reduction in power loss in the less complicated output matching circuitry to connect the amplifier  60  to the antenna  70 , as discussed below.  
         [0016]    There are two figures of merits to characterize the amplifier&#39;s efficiency.  
         [0017]    One of these figures of merit, the power added efficiency (called “KPA”), is defined as follows:  
                 K   PA     =         Po                 Pi       VSupply   *   ISupply       *   100      %       ,           Equation                 1                               
 
         [0018]    where “Pi” represents the power input to the amplifier  60 , “Po” represents the power output from the amplifier, “Psupply” represents the supply voltage to the amplifier  60  and “ISupply” represents the supply current to the amplifier  60 .  
         [0019]    The other figure of merit, the total efficiency (called “η tot”), is defined as follows:  
               η   tot     =       Po     VSupply   *   ISupply       *   100        %   .               Equation                 2                               
 
         [0020]    As described below, unlike the single-ended amplifier used in conventional, short range microwave transmitters, the push-pull amplifier  60  provides relatively high efficiency and high output power, while presenting low input and output impedances. In this manner, the conventional amplifier used in conventional short range microwave transmitters of the prior art often present complex input and output impedances, rendering matching task tough. Furthermore, because the high power transistors in a conventional amplifier has very low input and output impedances, designers are challenged to match combined devices to load. In contrast, the push-pull amplifier  60  presents higher input and output impedances that are easier to match. Therefore the amplifier  60  minimize losses through better impedance matching.  
         [0021]    More particularly, referring to FIG. 3, an embodiment  52  of the transmitter of the transceiver unit  50  includes a transceiver circuit  150 . The transceiver circuit  150  includes two output terminals  150   a  and  150   b  that, in response to the transmitter  150  being enabled, furnish two respective balanced and modulated carrier signals having frequencies (frequencies between 2.4 to 2.5 GHz, for example) in the microwave range. In this context of this application, “balanced signals” means that the signals are substantially symmetrical with respect to ground. Thus, due to this balanced arrangement, substantially all of the current that is communicated from one of the output terminals  150   a  and  150   b  returns to the other output terminal  150   a,    150   b  without returning through a ground path.  
         [0022]    The output terminal  150   a  is coupled to an impedance transformation and input matching circuit  152   a  that substantially matches the output impedance at the output terminal  150   a  to the input impedance of a transistor  154   a , one of the two amplifying components of the amplifier  60 . The circuit  152   a  also transforms the balanced signal that is provided by the output terminal  150   a  into a single-ended signal that is amplified by the transistor  154   a . The output terminal  150   b  of the transmitter  150  is coupled to an impedance transformation and input matching circuit  152   b  that substantially matches the output impedance at the output terminal  150   b  to the input impedance of a transistor  154   b , the other amplifying component of the amplifier  60 . The circuit  152   b  also transforms the balanced signal that is provided by the output terminal  150   b  into a single-ended signal that is amplified by the transistor  154   b.    
         [0023]    As an example, in some embodiments of the invention, the signal that is provided by the output terminal  150   a  may be an in-phase signal (relative to the signal present at the output terminal  150   b ), and the transistor  154   a  provides an amplified version of this signal at its output terminal. In these embodiments, the signal that is provided by the output terminal  150   b  is one hundred eighty degrees out of phase with the signal that appears at the output terminal  150   a , and the transistor  154   b  provides an amplified version of the signal on the output terminal  150   b  at the output terminal of the transistor  154   b.    
         [0024]    The output terminal of the transistor  154   a  is coupled to an impedance transformation and output matching circuit  156   a  that matches the output impedance of the transistor  154   a  to the input resistance of a push-pull combiner  160 . The output terminal of the transistor  154   b  is coupled to an impedance transformation and output matching circuit  156   b  that matches the output impedance of the transistor  154   b  to the input resistance of the push-pull combiner  160 . The circuits  156   a  and  156   b  provide two balanced output signals to the push-pull combiner  160  that, in turn, drives an antenna  70 . In some embodiments of the invention, a filter  162  is interposed between the output terminals of the push-pull combiner  160  and the antenna  70 .  
         [0025]    [0025]FIG. 4 depicts a more detailed schematic of an embodiment of the transmission circuitry  52 . Among the features of the circuitry  52 , the transformation and impedance matching circuits  152   a,    152   b ,  156   a  and  156   b  include balun transformers, in some embodiments of the invention. A balun transformer transforms a balanced system that is symmetrical (with respect to ground) into an unbalanced system with one side grounded. As an example, balun transformers can also reduce a 4:1 impedance ratio, for example, to 6.25 ohms or less. A design will offer many balun transformers advantages: the balun transformers and associated matching circuits have greater bandwidth, lower losses and reduced even harmonic levels.  
         [0026]    In the simplest balun transformer that uses a coaxial transmission line, the grounded outer conductor make an unbalanced termination, and the floating end makes a balanced termination. Alternatively, a particular balun transformer may be an air core device that is formed from, for example, a microstrip, an arrangement that avoids the inherent connection problem of semi-rigid coaxial cable for microwave circuits. A potential problem in the implementation of microstrip or stripline on FR4 printed circuit board (PCB) material is the signal loss associated with the capacitance of the FR4 material. The availability of easy bending and forming-type coaxial cable eliminate the connection problems that are associated with balun transformer designs that are implemented with semi-rigid coaxial cable. The balun transformers that are implemented with easy bending and forming-type coaxial cable reduces the power loss caused by the PCB material FR4 in the input and output matching circuitry.  
         [0027]    If broadband performance is desired, push-pull configuration makes the impedance matching easier to implement to a 50-ohm interface due to initially higher device impedance levels. The impedance transformation required in push-pull configuration is simpler and easier because of the higher impedance offered by the devices. Another advantage with the push-pull design is that the power levels of two amplification devices are automatically combined for higher power output levels, which allows the use of electrically smaller individual devices. An obvious advantage with push-pull transistors is the close proximity of the two transistors.  
         [0028]    The base-to-base impedance in a class C-biased push-pull transistor pair is approximately four times that of the base-to-ground impedance of one transistor. However when the push-pull transistors are biased to class A, the base-to-base impedance of the push-pull transistor pair approaches two times the base-to-ground impedance primarily because of the heavily forward biased base-emitter junctions. Since in a push-pull arrangement, the output voltage swing is twice that of the voltage swing of a single-ended device, the impedance will be four times higher. Up to frequencies at which the output impedance on the Smith chart plot becomes inductive, Equations 1 and 2 recited above give accurate results for all practical purposes, but the output capacitance may have to be taken into account especially in narrow band circuits.  
         [0029]    When designing an amplifier for a frequency of operation between approximately 2.4 to 2.5 GHz, the total efficiency is the first goal. For the case where the transistors  154   a  and  154   b  are bipolar junction transistors (BJT)s, the overall efficiency is the combination of good collector efficiency and high gain. The purposes of achieving sufficient collector efficiency, in some embodiments of the invention, the transistors  154   a  and  154   b  are biased to operate as class C amplifiers. Also to achieve sufficient collector efficiency, the circuits  156   a  and  156   b  ensure that the load impedances seen by the amplifiers significantly matches the transistors output impedances at the operation power level.  
         [0030]    Class C amplifiers are non-linear devices. Thus, it is possible that the harmonic content of the output signal of this type of amplifier can be very high, thereby possibly wasting efficiency and reducing the efficiency of the amplifier. However, due to the use of balanced amplification by the transistors  154   a  and  154   b , all the even harmonics are largely suppressed to minimize the dissipation of power. The push-pull arrangement has the additional advantage that the input and output impedances of the two transistors  154   a  and  154   b  are effectively coupled together. These larger impedances significantly simplify the design of the circuits  152   a,    152   b ,  156   a  and  156   b  for purposes of impedance matching. Because the impedance transformation by each of these circuits is lower, the power losses are smaller and the gain and efficiency are higher than non push-pull-type amplifiers.  
         [0031]    Another important consideration in the design of an amplifier that operates near 2.4 to 2.5 GHz amplification is the ruggedness of the amplifier. The amplifier should be designed to withstand at least a voltage standing wave ratio (VSWR) of about  3 : 1 . The amplifier  60  satisfies this criteria as the collector voltage swing (assuming the transistors are BJTs) can be close to three times the collector supply voltage, in some embodiments of the invention.  
         [0032]    Since two transistors are sharing the overhead between themselves, the thermal management is made somewhat easier, as compared to a single-ended design. In contrast to the push-pull design, single ended amplifiers require multiple amplification stages to achieve a 100 milliwatt output power. Furthermore, the push-pull design improves the isolation against reverse intermodulation distortion, as compared to a single-ended amplifier design.  
         [0033]    Turning now to a more specific design for a particular embodiment of the invention depicted in FIG. 4, the circuit  152   a,    152   b  may include a balun transfomer  180 , a balun transformer that may be represented by two inductors  182  and  184 . The inductor  182  has one terminal that is coupled to the output terminal  150   a  of the transceiver unit  55  and another terminal of the inductor  182  forms a node  183  that is coupled to a terminal of the inductor  184 . The other terminal of the inductor  184  is coupled to ground. An inductor  186  of the circuit  152   a,    152   b  is coupled between the node  183  and the base terminal of the bipolar junction transistor (BJT)  154   a ,  154   b.    
         [0034]    For the transistor  154   a , the BJT is an NPN transistor. For the transistor  154   b , the BJT is an NPN transistor. The emitter terminal of the transistor  154   a ,  154   b  is coupled to ground, and the collector terminal of the transistor  154   a ,  154   b  is coupled to the output transformation and impedance matching circuit  156   a,    156   b.    
         [0035]    In some embodiments of the invention, the output transformation and impedance matching circuit  156   a,    156   b  includes an inductor  191  that has one terminal that is coupled to the collector terminal of the transistor  154 . The other terminal of the inductor  191  forms a node  193 . A terminal of a capacitor  190  is coupled between the node  193  and ground. A balun  196  of the circuit  156  is coupled between the node  193  and the push-pull combiner  210 . The balun  196  includes two inductors  198  and  199  that are coupled in series between the nodes  193  of both baluns  196 .  
         [0036]    The filter  162  may be coupled between the adjoining terminals of the two inductors  198  and ground. The filter  162  is also coupled to a positive supply voltage (called “V dd ”) for purposes of biasing the output side of the transistors  154   a  and  154   b . The base terminals of the transistors  154   a  are biased by circuitry (not shown) to place the transistors  154   a  and  154   b  in the class AB mode of operation.  
         [0037]    The combiner  210  may include, for example, a balun transformer  212  that transforms the balanced signals that are provided by the transistors  154   a  and  154   b  into single-ended signal to drive the antenna  20 . This balun  212  is coupled to an output terminal  220  of the transceiver unit  60 .  
         [0038]    Referring to FIG. 5, in some embodiments of the invention, the wireless communication device  30  may be a computer system. In this manner, the transceiver unit  50  may be a Peripheral Component Interconnect (PCI) card that is inserted into a PCI bus slot connector that couples the transceiver unit  50  to the rest of the computer system serving with the host  40 . The PCI Specification is available from The PCI Special Interest Group, Portland, Oreg. 97214. In some embodiments of the invention, it includes a processor  100  (a microprocessor, for example) that is coupled to a host bus  102 . A north bridge, or memory controller hub  104 , may be coupled to the host bus  102  and establish interfaces to a memory bus  106 , an Accelerated Graphics Port (AGP) bus  110  and a hub link  120 . The AGP standard is described in detail in the Accelerated Graphics Port Interface Specification, revision 1.0, published on Jul. 31, 1996, by Intel Corporation of Santa Clara, Calif. A display driver  112  may be an AGP device that is coupled to the AGP bus  110  to drive signals for a display  114 . The memory bus  106  is coupled to a system memory  108 .  
         [0039]    The hub link  120  establishes communications between the memory controller hub  104  and a south bridge, or input/output (V/O) controller hub  122 . In some embodiments of the invention, the I/O controller hub  122  may provide an interface to the PCI bus  48  (that establishes communication between the transceiver unit  50  and host  40 ), as well as to an I/O expansion bus  124 .  
         [0040]    An I/O controller hub  130  may be coupled to the I/O expansion bus  124  and receive data from a mouse  136  and a keyboard  134 , as well as control operations of a floppy disk drive  132 . The I/O controller hub  122  may, for example, control operations of a hard disk drive  126  and a CD-ROM drive  138 . The PCI bus  48  may, for example, be coupled to a network interface card (NIC)  49  as well as be coupled to the transceiver unit  50  through slot connectors, for example.  
         [0041]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.