Abstract:
An RF transmitter capable of transmitting over a wide range of frequencies includes a mixer, a wideband high-Q balun, a first driver amplifier and a second driver amplifier. The balun has a single primary winding and two secondary windings. A differential output of the mixer is coupled to the primary winding. A first of the two secondary windings is coupled to drive the first driver amplifier. A second of the two secondary windings is coupled to drive the second driver amplifier. One driver amplifier is used when transmitting at lower frequencies whereas the other driver amplifier is used when transmitting at higher frequencies. By appropriate sizing of the inductances of the secondary windings and by switching out one of the secondary windings at certain times, the balun is tunable to operate over the wide frequency range while having a high quality factor Q, thereby facilitating reduced power consumption while simultaneously meeting performance requirements.

Description:
BACKGROUND INFORMATION 
     1. Technical Field 
     The present disclosure relates to wideband transmitters, and more particularly to mixers and baluns used in wideband transmitters. 
     2. Background Information 
     Many types of RF (Radio Frequency) transceivers, including cellular telephone transceivers such as in multi-band cellular telephones, must work over a wide frequency range. In one example, a cellular telephone may be required to operate in a first frequency band referred to here as a “cell band” or a “low band”. The same cellular telephone may also be required to operate in a second frequency band referred to here as a “PCS band” or a “mid band”.  FIG. 1  (Prior Art) is a diagram that shows frequency along the horizontal axis. Low band  1  in this example extends from 824 MHz to 915 MHz and mid band  2  extends from 1710 MHz to 1980 MHz. 
       FIG. 2  (Prior Art) is a circuit diagram that illustrates a first way that a transmitter of a multi-band cellular telephone transceiver can be made to operate over the wide frequency range of  FIG. 1 . The transmitter includes a transmit baseband filter  3 , a mixer  4 , a balun  5 , a driver amplifier  6 , a power amplifier  7 , a duplexer  8 , and an antenna  9 . The dashed box  10  indicates the portion of the transmitter that is realized on an RF transceiver integrated circuit. Balun  5  includes one primary winding  11  and one secondary winding  12 . A first programmable capacitor  13  is coupled in parallel with the primary winding and a second programmable capacitor  14  is coupled in parallel with the secondary winding. To make the transmitter operable over the wide frequency range, the capacitors  13  and  14  are made to be large and tunable capacitors. Such a large and tunable capacitor typically involves banks of capacitors and associated transistor switches. The transistors are used to switch the capacitors in and out of the overall structure to increase or decrease the overall capacitance. Unfortunately, making the first and second capacitors large and programmable in this way reduces the quality factor (the “Q”) of the balun. Due in part to this low quality factor, the transceiver when transmitting in the low band may emit an undesirable amount of receive band noise in a nearby receive band even though the transmitter is tuned to transmit in a transmit band. The transmit and receive bands are typically quite narrow and are located quite close to one another within the wider low band or the wider mid band. 
       FIG. 3  (Prior Art) illustrates a transmit band  15  and a receive band  16  that may, for example, exist side by side in the low band  1  of  FIG. 1 . When the circuit of  FIG. 2  is used to transmit in transmit band  15 , an unwanted amount of energy is also transmitted into receive band  16  due to the low Q of balun  5  of  FIG. 2 . 
       FIG. 4  (Prior Art) is a diagram of a second way that a transmitter of a transceiver integrated circuit  32  of a multi-band cellular telephone can be made to operate over a wide frequency range such as the wide frequency range illustrated in  FIG. 1 . Because the balun tuning range is a function of both inductance and capacitance, if the tuning range of the balun capacitances is limited as in  FIG. 2  due to quality factor issues then an amount of inductance tuning is provided by providing two higher-Q baluns having different winding inductances. Accordingly, one transmit baseband filter  17  is provided, but the remainder of the transmitter is duplicated. A low band circuit path  18  involves mixer  19 , balun  20 , driver amplifier  21 , power amplifier  22 , and a duplexer  23 . This low band circuit path  18  is optimized for operation in the low band of  FIG. 1 . A mid band circuit path  24  involves mixer  25 , balun  26 , driver amplifier  27 , power amplifier  28 , and a duplexer  29 . This mid band circuit path is optimized for operation in the mid band of  FIG. 1 . The two mixers  19  and  25  are both driven by the same transmit local oscillator signal TX LO. An antenna switch  30  couples the appropriate one of the two circuit paths to antenna  31 . If the transmitter is to transmit in the low band, then signal EN DA 1  is asserted to enable driver amplifier  21  and signal EN DA 2  is not asserted such that driver amplifier  27  is disabled. Conversely, if the transmitter is to transmit in the mid band, then signal EN DA 2  is asserted to enable driver amplifier  27  and signal EN DA 1  is not asserted such that driver amplifier  21  is disabled. 
     The two-path transmitter circuit of  FIG. 4  does not have the low Q balun problem of the transmitter circuit of  FIG. 2 , but the two-path transmitter circuit of  FIG. 4  is undesirably large as implemented due to the redundant circuitry. The two-path transmitter also consumes an undesirably large amount of power. Interconnections between the divider circuitry that generates the transmit local oscillator signal TX LO and the mixers can be long when there are two mixers  19  and  25 . Such long interconnections often result in increased current consumption. 
     SUMMARY 
     An RF transmitter capable of transmitting over a wide range of frequencies includes a mixer, a wideband high-Q balun, a first driver amplifier and a second driver amplifier. The wideband high-Q balun has a single primary winding and two secondary windings. A differential output of the mixer is coupled to the primary winding. A first of the two secondary windings is coupled to drive the first driver amplifier in single-ended fashion. A second of the two secondary windings is coupled to drive the second driver amplifier in single-ended fashion. One driver amplifier is used when transmitting at lower frequencies whereas the other driver amplifier is used when transmitting at higher frequencies. By appropriate sizing of the inductances of the secondary windings and by switching out one of the secondary windings at certain times, the balun is tunable to operate over the wide range of frequencies while having a high quality factor Q, thereby facilitating reduced power consumption in the mixer/balun circuit while simultaneously meeting performance requirements. 
     In one specific example, the mixer/balun circuit is “wideband” in the sense that it is operable over a frequency range that has an upper bound at an upper frequency and that has a lower bound at a lower frequency where the upper frequency is at least twice the lower frequency. The balun of the mixer/balun circuit has a quality factor (Q) of at least 6.0 over this entire wideband frequency range. The mixer/balun circuit provides at least one milliwatt of signal power to the appropriate driver amplifier while consuming no more than twenty-seven milliwatts, and performs this way for any frequency in the wideband frequency range. 
     The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and does not purport to be limiting in any way. Other aspects, inventive features, and advantages of the devices and/or processes described herein, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  (Prior Art) is a diagram that shows a wideband frequency range that extends from a lower bound of a low band to an upper bound of a mid band. 
         FIG. 2  (Prior Art) is circuit diagram that illustrates a first way that a transmitter can be made to operate over the wideband frequency range of  FIG. 1 . 
         FIG. 3  (Prior Art) is a diagram that shows a transmit band and a receive band. 
         FIG. 4  (Prior Art) is a circuit diagram that illustrates a second way that a transmitter can be made to operate over the wideband frequency range of  FIG. 1 . 
         FIG. 5  is a diagram of a mobile communication device that includes a mixer/balun circuit in accordance with one novel aspect. 
         FIG. 6  is a more detailed diagram of the transceiver and antenna parts of the mobile communication device of  FIG. 5 . 
         FIG. 7  is a more detailed diagram of certain parts of the RF transceiver integrated circuit of  FIG. 6 . 
         FIG. 8  is a circuit diagram that shows the single primary dual secondary balun of  FIG. 7  in further detail. 
         FIG. 9  is a more detailed diagram of the active mixer of  FIG. 8 . 
         FIG. 10  is a top-down diagram of a layout of the balun of  FIG. 8 . 
         FIG. 11  is a table that that sets forth various parameters of the mixer/balun circuit of  FIG. 8  including the inductances of the three windings and including the tuning ranges of the three programmable variable capacitors. 
         FIG. 12  is a table that sets forth how the balun digital control values P[4:0], SLB[5:0], SMB[6:0], and SW ON/OFF are set depending on the frequency range in which the transmitter is operating. 
         FIG. 13  is a graph that illustrates how current consumption of the mixer/balun circuit and how the quality factor of the balun vary as the operating frequency of the mixer/balun circuit varies throughout the wideband frequency range from 824 MHz to 1980 MHz. 
         FIG. 14  is a simplified flowchart of a method  200  in accordance with one novel aspect. 
         FIG. 15  is a simplified flowchart of a method  300  in accordance with another novel aspect. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 5  is a diagram of a mobile communication device  51  that includes a mixer/balun circuit in accordance with one novel aspect. In this example, mobile communication device  51  is a multi-band cellular telephone handset. Device  51  includes (among other parts not illustrated) an antenna  52  usable for receiving and transmitting cellular telephone communications, an RF (Radio Frequency) transceiver integrated circuit  53 , and a digital baseband processor integrated circuit  54 . In some examples, the transceiver circuitry and the digital baseband circuitry are implemented on the same integrated circuit, but a two integrated circuit implementation is set forth here for illustration purposes. 
     Digital baseband integrated circuit  54  includes a processor  55  that executes a program  56  of processor-executable instructions. Program  56  is stored in a processor-readable medium  57  that in this case is a semiconductor memory. Processor  55  accesses memory  57  via local bus  58 . Processor  55  interacts with and controls the RF transceiver integrated circuit  53  by sending control information to integrated circuit  53  via serial bus interface  59 , serial bus  60 , serial bus interface  61 , and groups of control conductors  62  and  63 . Information to be transmitted is converted into digital form on digital baseband processor integrated circuit  54  by a Digital-to-Analog Converter (DAC)  64  and is communicated across conductors  65  to the transmitter portion of transceiver integrated circuit  53 . Data received by the receiver portion of transceiver integrated circuit  53  is communicated in the opposite direction across conductors  66  from RF transceiver integrated circuit  53  to digital baseband processor integrated circuit  54  and is converted into digital form by an Analog-to-Digital Converter (ADC)  67 . 
       FIG. 6  is a more detailed diagram of the transceiver and antenna parts of the cellular telephone of  FIG. 5 . In one very simplified explanation of the operation of the cellular telephone, if the cellular telephone of  FIG. 5  is being used to receive information as part of a cellular telephone call, then an incoming transmission  68  is received on antenna  52 . The incoming signal passes through an antenna switch  69 , and then passes through one of two receive paths of a receiver portion  81  of the RF transceiver integrated circuit  53 . In one path, the incoming signal passes through duplexer  70 , a matching network  71 , terminals  72 , a Low Noise Amplifier (LNA)  73 , a mixer  74 , a baseband filter  75 , and conductors  66  to the ADC  67  within digital baseband processor integrated circuit  54 . In another path, the incoming signal passes through antenna switch  69 , duplexer  76 , matching network  77 , terminals  78 , LNA  79 , mixer  80 , baseband filter  75 , and conductors  66  to the ADC  67  of the digital baseband processor integrated circuit  54 . A local oscillator  82  (also referred to as a frequency synthesizer) supplies a receive local oscillator signal RX LO to the mixers  74  and  80 . How the receiver downconverts is controlled by changing the frequency of the local oscillator signal RX LO and by selecting the appropriate receive path. One of the receive paths is used to receive signals in a first frequency band whereas the other of the receive paths is used to receive signals in a second frequency band. 
     If, on the other hand, cellular telephone  51  is being used to transmit information as part of a cellular telephone call, then the information to be transmitted is converted into analog form by DAC  64  in digital baseband processor integrated circuit  54 . The analog information is supplied to a baseband filter  83  of a transmit chain  84  portion of a transmitter portion  85  of the RF transceiver integrated circuit  53 . After filtering by the baseband filter, the signal is upconverted in frequency by a novel mixer block  86  as explained in further detail below. The upconverted signal passes through one of two paths to antenna  52 . In a first path, the signal passes through driver amplifier  87 , terminal  88 , power amplifier  89 , matching network  90 , duplexer  70 , antenna switch  69 , and to antenna  52  for transmission as transmission  139 . In a second path, the signal passes through driver amplifier  91 , terminal  92 , power amplifier  93 , matching network  94 , duplexer  76 , antenna switch  69 , and to antenna  52  for transmission as transmission  139 . Which of the two paths is used depends on whether the signal is to be transmitted in a first frequency band or in a second frequency band. How mixer block  86  upconverts is controlled by changing the frequency of the local oscillator signal TX LO generated by local oscillator  95  (also referred to as a frequency synthesizer) and by selecting the appropriate transmit path. 
       FIG. 7  is a more detailed diagram of certain parts of the RF transceiver integrated circuit  53  of  FIG. 5 . Mixer block  86  is a mixer/balun circuit that includes an active mixer  96  and a balun  97 . The balun is referred to here as a “single primary dual secondary balun” because it includes only one primary winding  98  but includes a first secondary winding  99  and a second secondary winding  100 . The balun converts the differential signal output of the mixer  96  into single-ended signals that drive the driver amplifiers  87  and  91 . The primary winding  98  is electromagnetically coupled to the two secondary windings  99  and  100  so that the three windings together constitute a transformer. A first programmable variable capacitor  101  is coupled in parallel with primary winding  98  as illustrated. A center tap on primary winding  98  is coupled to a supply voltage conductor  102 . When the mixer/balun circuit operates, a supply current  140  flows from the supply voltage conductor  102  and into the circuit via the center tap connection. A second programmable variable capacitor  103  is coupled in parallel with the first secondary winding  99 . An N-channel field effect transistor switch  104  can be open or closed as explained in further detail below. If switch  104  is closed, then one lead  105  of capacitor  103  is coupled to a terminal  106  of the first secondary winding  99  such that the capacitor  103  is coupled in parallel with the first secondary winding  99 . If switch  104  is open, then lead  105  of capacitor  103  is not coupled to terminal  106  and the capacitor  103  is not coupled in parallel with first secondary winding  99 . Conductor  107  communicates a signal from the first secondary winding  99  to an input lead  108  of first driver amplifier  87 . A third programmable variable capacitor  109  is coupled in parallel with the second secondary winding  100  as illustrated. Conductor  110  communicates a signal from the second secondary winding  100  to an input lead  111  of second driver amplifier  91 . 
     Complex mutual inductance interactions between the three windings  98 ,  99  and  100  allow the primary winding to be tuned to resonate over an adequate tuning range (to resonate at the low band frequencies or at the mid band frequencies) without having to provide a large variable capacitor in parallel with the primary winding. When switch  104  is open and the circuit is operating at mid band frequencies there is no current flow in the first secondary winding  99  and impact of the first secondary winding  99  on primary winding resonance and overall balun resonance is reduced. The mutual inductance effect on primary winding resonance and overall balun resonance is largely due to the relatively smaller inductance of the second secondary winding  100 . When switch  104  is closed and the circuit is operating at low band frequencies, the primary winding  98  and the first secondary winding  99  of larger inductance interact strongly whereas the second secondary winding  100  of smaller inductance has only a weak influence on primary resonance and overall balun resonance. The quality factor of the tuned balun for any frequency in the wideband frequency range from 824 MHz to 1980 MHz is 6.0 or greater. 
     Although not shown in the simplified diagram of  FIG. 6 , a divider  112  and a buffer  113  are disposed in the signal path of the TX LO signal to the mixer  96 . These circuits  112  and  113  are shown in  FIG. 7  being located close to mixer  96  to indicate that these circuits are located closer to the mixer or mixers than are the corresponding circuits in the two-path conventional circuit of  FIG. 4 . The TX LO signal as output from buffer  113  actually involves two differential signals TX LO_I and TX LO_Q that are in quadrature relation to one another. In-phase local oscillator signal TX LO_I is communicated to mixer  96  via two conductors  114  and  115 . Quadrature phase local oscillator signal TX LO_Q is communicated to mixer  96  via two conductors  116  and  117 . 
     Reference numeral  118  represents four terminals of integrated circuit  53  through which two differential signals I_and Q_are received. I_P and I_N constitute the differential signal I. Q_P and Q_N constitute the differential signal Q. The transmit baseband filter  83  supplies two differential filtered signals via conductors  119 - 122  to active mixer  96 . IP and IN constitute the first differential signal. QP and QN constitute the second differential signal. Digital control bits from serial bus interface  61  are communicated via some of the control conductors  62  to mixer block  86 . These control conductors  147  are shown in further detail in  FIG. 8 . 
       FIG. 8  is a circuit diagram that shows the single primary dual secondary balun  97  in further detail. Reference numerals  123  and  124  identify terminals of primary winding  98 . Reference numeral  125  identifies the center tap of primary winding  98 . The differential mixer output signal MOP and MON from mixer  96  is supplied via a corresponding pair of conductors  126  and  127  to primary winding  98 . Signal MOP is supplied from mixer output lead  142  onto terminal  123  of the primary winding. Signal MON is supplied from mixer output lead  143  onto terminal  124  of the primary winding. The capacitance of the first programmable variable capacitor  101  is controlled by the five-bit digital value P[4:0]. Reference numerals  106  and  128  identify terminals of first secondary winding  99 . The capacitance of the second programmable variable capacitor  103  is controlled by the six-bit digital value SLB[5:0]. Signal SW ON/OFF is a single digital control bit on conductor  141  that controls switch  104 . Reference numerals  129  and  130  identify terminals of the second secondary winding  100 . The capacitance of the third programmable variable capacitor  109  is controlled by the seven-bit digital value SMB[6:0]. Reference numeral  147  identifies the control conductors that communicate the control values P[4:0], SW ON/OFF, SLB[5:0], SMB[6:0], EN DA 1 , and EN DA 2 . In operation, the digital baseband processor integrated circuit  54  sends digital information  146  (see  FIG. 7 ) across serial bus  60  to RF transceiver integrated circuit  53 . This digital information  146  is received onto RF transceiver integrated circuit  53  from the serial bus  60 . The digital information  146  either contains or is used to generate the digital control values (P[4:0], SW ON/OFF, SLB[5:0], SMB[6:0], EN DA 1 , and EN DA 2 ) that control the mixer/balun circuit and the driver amplifiers so that these circuits are properly configured to operate at the desired transmitting frequency. 
       FIG. 9  is a more detailed diagram of one example of active mixer  96 . The signals TX LO_IP, TX LO_IN, TX LO_QP and TX LO_QN are current signals and together constitute the transmit local oscillator signal TX LO. Active mixer  96  includes eight N-channel field effect transistors  131 - 138  interconnected as illustrated. 
       FIG. 10  is a simplified top-down diagram of the layout of balun  97 . The balun is realized principally in one layer of metallization on integrated circuit  53 . Conductor crossovers in the balun are realized using inter-metal layer vias (not shown) and short bridging lengths of metal (not shown) in a second layer of metallization. The center tap  125  is realized using a via (not shown). Each of the programmable capacitors  101 ,  103  and  109  is realized as a bank of metal-oxide-metal RTMoM capacitors and associated transistor switches where the transistors are used to switch the capacitors in and out of the overall structure to increase or decrease the overall capacitance. The gates of the transistor switches receive the digital control value that sets the capacitance of the capacitor. 
       FIG. 11  is a table that sets forth various characteristics and parameters of the mixer/balun circuit including the inductances of the three windings  98 ,  99  and  100  and including the tuning ranges of the three programmable variable capacitors  101 ,  103  and  109 . 
       FIG. 12  is a table that sets forth how the balun digital control values P[4:0], SLB[5:0], SMB[6:0], and SW ON/OFF are set depending on the frequency range in which transmitter  85  is operating. The mixer/balun circuit is operable over the entire wideband frequency range from 824 MHz to 1980 MHz even though the transmitter in this example is only made to operate in the low band (824 MHz to 915 MHz) and in the mid band (1710 MHz to 1980 MHz). In one advantageous aspect, the current consumption of the mixer/balun circuit is 20 mA or less at 1.3 volts of supply voltage (27 mW or less) throughout this 824 MHz to 1980 MHz wideband frequency operating range while delivering at least 1.0 mW of signal power to the enabled driver amplifier. The ratio of power consumption to power supplied to the driver amplifier is therefore greater than 25/1. The supply current flowing into the mixer/balun circuit is the 1.3 volt supply current  140  (see  FIG. 8 ) that flows from the supply voltage conductor  102  and into the center tap  125  of primary winding  98 . 
       FIG. 13  is a graph that illustrates how the current consumption of the mixer/balun circuit and how the quality factor of the balun vary as the operating frequency of the mixer/balun circuit varies throughout the wideband frequency range  148  from the lower bound  144  of the wideband range at 824 MHz to the upper bound  145  of the wideband range at 1980 MHz. The current consumption of the mixer/balun circuit is below 20 mA throughout the wideband frequency range  148 . The quality factor Q of the balun is above 6.0 throughout wideband frequency range  148 . 
       FIG. 14  is a simplified flowchart of a method  200  in accordance with one novel aspect. A mixer in an RF transmitter is coupled (step  201 ) to a first driver amplifier and to a second driver amplifier using a wideband balun, where the wideband balun includes only one primary winding (coupled to the mixer) but includes a first secondary winding (coupled to an input of the first driver amplifier) and a second secondary winding (coupled to an input of the second driver amplifier). 
       FIG. 15  is a simplified flowchart of a method  300  in accordance with another novel aspect. Digital information is received (step  301 ) via a serial bus onto an integrated circuit. In one example, the digital information is information  146  and the serial bus is serial bus  60  and the integrated circuit is RF transceiver integrated circuit  53 . This digital information is then used (step  302 ) on-chip to control a single primary dual secondary balun circuit. In one example, the digital information includes or is decoded to include first digital control information, second digital control information, and third digital control information, where the first digital control information sets a capacitance of a first capacitor  101  in parallel with the single primary winding  98  of the balun, where the second digital control information sets a capacitance of a second capacitor  99  in parallel with the first secondary winding  99  of the balun, and where the third digital control information sets a capacitance of a third capacitor  109  in parallel with the second secondary winding  100  of the balun. 
     Although certain specific embodiments are described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. In some embodiments, more than two pairs of tuned secondary windings and driver amplifiers are coupled to a single tuned primary winding. In some embodiments, the two secondary windings are actually two parts of a tapped secondary where one terminal end of the secondary winding is grounded, where the tap is coupled to the input of a first driver amplifier, and where the other terminal end of the secondary winding is coupled to the input of a second driver amplifier. In some embodiments a switch is provided to detune each of the secondary winding/capacitors rather than just the first secondary winding/capacitor for the low band as set forth above. Multiple suitable different layout structures of the balun transformer are possible. The post-balun amplification need not be performed in two stages using an on-chip driver amplifier and a separate external power amplifier, but rather in some embodiments the post-balun amplification is performed in one stage using only a single amplifier. Accordingly, various modifications, adaptations, and combinations of the various features of the described specific embodiments can be practiced without departing from the scope of the claims that are set forth below.