BACKGROUND 
     The invention generally relates to integrating an FM transmitter into a cellular telephone and more particularly relates to an FM transmitter that has features that facilitate integration of the transmitter into a cellular telephone. 
     A modern cellular telephone may have the capability of playing digital music files (MP3 files, for example). Due to the limited capability of the cellular telephone&#39;s speaker system, the telephone may contain a low power frequency modulation (FM) transmitter for purposes of communicating the digital music file content over an FM channel to a nearby stereo system. However, a potential challenge with incorporating an FM transmitter into a cellular telephone is that out of band spectral energy that is generated by the FM transmitter may encroach into the receive channels of the telephone, thereby potentially impairing the telephone&#39;s ability to receive an incoming signal. 
     As a more specific example, for the Global System for Mobile communications (GSM) standard, the RF signal that is received by the cellular telephone may have a relatively small magnitude, such as about −108 dBm. Any spectral energy (such as out of band spectral energy that is generated by the FM transmitter, for example) that appears in the GSM receive channel must be smaller than the noise floor of the receive channel, which may be approximately −117 dBm. Therefore, stringent requirements may be placed on the out of band spectral energy that is transmitted by the FM transmitter. 
     Thus, there is a continuing need for better ways to integrate an FM transmitter and a cellular telephone. 
     SUMMARY 
     In an embodiment of the invention, a transmitter includes at least one mixer, a filter and an antenna tuning network. The mixer(s) translate a frequency of an input signal to generate a second signal. The filter is separate from the antenna tuning network and removes spectral energy from the second signal to generate a third signal. The antenna tuning network produces a fourth signal to drive an antenna in response to the third signal. 
     In another embodiment of the invention, a system includes a cellular radio and an FM transmitter that are both fabricated in the same semiconductor die. The FM transmitter includes at least one mixer, a filter and an antenna tuning network. The mixer(s) translate an intermediate carrier frequency of an input signal to generate a second signal that has an FM carrier frequency. The filter removes spectral energy from the second signal to generate a third signal. The antenna tuning network is separate from the filter and produces a fourth signal to drive an antenna in response to the third signal. 
     In yet another embodiment of the invention, a method includes translating a frequency of an input signal to generate a second signal. The second signal is communicated through a filter to remove spectral energy from the second signal to generate a third signal. The third signal is communicated to an antenna tuning network, which is separate from the filter to produce a fourth signal to drive an antenna. 
     Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic diagram of a wireless communication system according to an embodiment of the invention. 
         FIG. 2  is a schematic diagram of a signal processing path of the FM transmitter of  FIG. 1  according to an embodiment of the invention. 
         FIG. 3  is a schematic diagram of a filter of the FM transmitter according to an embodiment of the invention. 
         FIG. 4  is a schematic diagram of an IF amplifier of the transmitter according to an embodiment of the invention. 
         FIG. 5  is a schematic diagram of a mixer of the transmitter according to an embodiment of the invention. 
         FIGS. 6 ,  7  and  8  are schematic diagrams of signal processing paths according to different embodiments of the invention. 
         FIG. 9  is a schematic diagram of a low pass filter and associated control circuitry of the signal processing path of  FIG. 8  according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , in accordance with an embodiment of the invention, a cellular radio  24  and a frequency modulation (FM) transmitter  35  are part of a cellular telephone  10 . Thus, the cellular radio  10  and FM transmitter  35  are located inside the same telephone housing. The cellular radio  24  and FM transmitter  35  may be fabricated on separate semiconductor dies or may be fabricated on the same semiconductor die, depending on the particular embodiment of the invention. Furthermore, the cellular radio  24  and FM transmitter  35  may be part of the same semiconductor packet, or “chip,” or may be part of different semiconductor packages, depending on the particular embodiment of the invention. Additionally, in some embodiments of the invention, the FM transmitter  35  may be part of a semiconductor package that includes FM and AM receive paths (which may be disabled). Thus, many variations are possible and are within the scope of the appended claims. 
     The FM transmitter  35  may be constructed to transmit (via its FM antenna  14 ) an FM signal over a low power FM link  48  to an FM receiver  50 . In this regard, the FM receiver  50  may receive an RF FM signal (via its antenna  51 ) from the low power FM link  48  and produce audio content (encoded in the RF signal) over its speaker system  54  in response to the received FM signal. As an example, in accordance with some embodiments of the invention, the cellular telephone  10  may function as an MP3 player, which is capable of downloading, storing and playing (via the FM receiver  50  and speaker system  54 ) MP3-based digital music files. 
     The FM transmitter  35  has features that minimize distortion that otherwise may be introduced into the receive channels of the cellular telephone  10  due to the transmissions by the transmitter  35 ; and the FM transmitter  35  has a design that minimizes its die area. As described further below, the FM transmitter  35  includes an analog upconversion path  36 , which includes a low pass filter (LPF)  76  that significantly filters out spectral energy from the transmitted FM signal, which would otherwise fall outside of the FM band. An antenna tuning network  38  of the FM transmitter  35  drives the antenna  14  in response to the filtered signal that is provided by the LPF  76 . Due to the filtering of the out of band spectral energy by the LPF  76 , the Q factor of the antenna tuning network  38  may be kept relatively low (a Q factor of approximately 10, for example), while still minimizing the spectral energy that the FM transmitter  35  introduces into the receive channels of the cellular telephone  10 . 
     The cellular telephone  10  may have numerous different designs, one of which is depicted for purposes of example in  FIG. 1 . The cellular radio  24  is constructed to communicate with a cellular network via at least one antenna  12 . The FM transmitter  35  includes a digital signal processor (DSP)  40 , which may, for example, convert a digital signal that is provided by the cellular radio  24  into an intermediate frequency (IF) signal. The DSP  40  provides the IF signal to digital-to-analog converters (DACs)  60  and  70 , which provide analog signals to the analog upconversion path  36  that frequency translates the received IF signals into an RF signal. More specifically, the RF signal that is generated by the analog upconversion path  36  is communicated through an antenna tuning network  36  (of the FM transmitter  35 ) to the antenna  14 . 
     A microcontroller unit (MCU)  42  of the FM transmitter  35  generally controls the overall operation of the transmitter  35 . More specifically, the MCU  34  may control signal gains of the FM transmitter  35 , as further described below. 
     Among its other features, the cellular telephone  10  may include a speaker system  18 , a microphone  19 , a keypad  15  and a display  17 . 
       FIG. 2  depicts an exemplary embodiment of a signal processing path  58  of the FM transmitter  35  in accordance with some embodiments of the invention. In some embodiments of the invention, the DACs  60  and  70  receive in-phase (called “I”) and quadrature (called “Q”) baseband signals, respectively, from the cellular radio  24  (see  FIG. 1 ). The DACs  60  and  70  include digital input terminals  65  and  63 , respectively. 
     The analog conversion path  36  translates the IF I and Q signals from IF to RF. More specifically, in accordance with some embodiments of the invention, the FM transmitter  35  includes an in-phase signal processing path  81  and a quadrature signal processing path  83 . The in-phase signal processing path  81  includes the DAC  60  and an IF transconductor  62  that amplifies the analog signal that is provided by the DAC  60  and provides the amplified signal to a mixer  64 . In a similar manner, the quadrature signal processing path  83  includes the DAC  70  and an IF transconductor  72  that amplifies the analog signal that is provided by the DAC  70  and provides the amplified signal to a mixer  74 . In accordance with some embodiments of the invention, the IF transconductors  62  and  72  have the same design, which is discussed further below. 
     In accordance with some embodiments of the invention, the mixers  64  and  74  have the same design and are each square wave mixers that receive square wave mixing signals (provided by a frequency divider  66 , for example) for purposes of translating the IF frequencies into RF frequencies. The square wave mixing signals that are provided to the mixers  64  and  74  are offset by 90°, pursuant to the in-phase/quadrature mixing. 
     As a result of the square wave mixing, the harmonics of the mixed signals may contain a considerable amount of out of band spectral energy. Thus, the FM signal that is provided by an adder  68  (that combines the mixed signals that are provided by the mixers  64  and  74 ) may contain a significant amount of out of band spectral energy. 
     More specifically, in accordance with some embodiments of the invention, the LPF  76  receives the FM signal from the mixers  64  and  74  and produces an RF signal that has a significantly reduced spectral content outside of the FM band. An RF transconductor  78  receives the filtered signal from the LPF  76  to produce an amplified RF signal that is communicated to a current driver  100 . 
     The current driver  100  forms a transition between the high power side (containing the antenna tuning network  38 ) and the lower power side (the portion of the signal processing path  58  upstream of the current driver  100 ) of the FM transmitter  35 . As depicted in  FIG. 2 , in some embodiments of the invention, the current driver  100  may include a transimpedance amplifier that is formed from a current mirror. A resistor  120  that is coupled to an output terminal  121  of the current mirror converts the output current from the current mirror into a voltage that drives the antenna tuning network  38 . 
     More specifically, the current driver  100  may include a current sink  104  that is coupled to an input node  90  of the driver  100 . The input node  90  is coupled to the output terminal of the RF transconductor  78  and is also coupled to the drain terminal of a p-channel metal-oxide-semiconductor field-effect-transistor (PMOSFET)  106 . The drain and the gate terminals of the PMOSFET  106  are coupled together, and the source terminal of the PMOSFET  106  is coupled to a voltage supply (called “V DD ”). Another PMOSFET  108  has its gate terminal connected to the gate terminal of the PMOSFET  106 . The source terminal of the PMOSFET  108  of the current driver  100  is coupled to the V DD  supply voltage, and the drain terminal of the PMOSFET  108  is coupled to the output terminal  121 . 
     As shown in  FIG. 2 , in accordance with some embodiments of the invention, the antenna tuning network  38  may be formed from a parallel combination of an inductor  124  and a capacitor  126  that are coupled between an output terminal  39  of the output tuning network  38  and ground. The output terminal  39  is coupled to the antenna  14 . The resistor  120  may be coupled in parallel with the inductor  124  and capacitor  126 . 
     Referring to  FIG. 3 , in accordance with some embodiments of the invention, the LPF  76  may be a passive RC ladder-type filter that receives and provides differential signals. In this regard, the LPF  76  may be formed from capacitors  130  and resistors  134  that are arranged in a low pass filter arrangement between input terminals  69  and  71  of the LPF  765  and the LPF&#39;s output terminals  81  and  79 , respectively. It is noted that in other embodiments of the invention, other topologies may be used for the LPF  76 , and these other topologies may include active filters, in accordance with embodiments of the invention. 
     Referring to  FIG. 4 , in accordance with some embodiments of the invention, the IF transconductor  62 ,  72  may be a transconductance amplifier that receives a differential input signal across its input terminals  163  and  177  and provides a corresponding differential output signal at its output terminals  151  and  189 . In accordance with some embodiments of the invention, the transconductor  62 ,  72  may have a general symmetrical design, which includes a PMOSFET  174 , which has a gate terminal that serves as the input terminal  177 . The source terminal of the PMOSFET  174  is coupled to a node  175 , and the drain terminal of the PMOSFET  174  is coupled to a node  179 . A current source  176  is coupled between the V DD  supply voltage and the node  175 , and a current sink  178  is coupled between the node  179  and ground. A low pass filter formed from a resistor  184  and a capacitor  186  is coupled between the node  179  and the gate terminal of the an n-channel metal-oxide-semiconductor field-effect-transistor (NMOSFET)  188 . The NMOSFET  188  converts the voltage that is present on the node  179  into the current that flows into the output terminal  189 . 
     In its symmetrical design, the IF transconductor  62 ,  72  also includes an input PMOSFET  160  that has its gate terminal that serves as the other input terminal  163 . The source terminal of the PMOSFET  160  is coupled to a node  161 , and the drain terminal of the PMOSFET  160  is coupled to a node  156 . A current source  162  is coupled between the V DD  supply voltage and the node  161 , and a current sink  158  is coupled between the node  156  and ground. A low pass filter formed from a resistor  154  and a capacitor  152  is coupled between the node  156  and the gate terminal of an NMOSFET  150 . The source terminal of the NMOSFET  150  is coupled to ground, and the drain terminal of the NMOSFET  150  is connected to receive current from the other output terminal  151 . 
     For purposes of biasing, an NMOSFET  164  has its drain terminal coupled between the node  161  and the drain terminal of an NMOSFET  166 . Similarly, an NMOSFET  170  has its drain terminal coupled to the node  175 , and the source terminal of the NMOSFET  170  is coupled to the drain terminal of an NMOSFET  172 . The source terminals of the NMOSFETs  166  and  172  are coupled to ground. Additionally, the gate terminals of the NMOSFETs  164  and  170  receive a bias voltage (called “V BIAS ”), and the gate terminals of the NMOSFETs  166  and  172  are coupled to the nodes  156  and  179 , respectively. 
     Feedback between the above-described halves of the IF transconductor  62 ,  72  is provided by a resistor  168  that is coupled between the nodes  161  and  175 . In accordance with some embodiments of the invention, the gain of the IF transconductor  62 ,  72  may be regulated by the MCU  42  (see  FIG. 1 ) by adjusting the resistance of the resistor  168 . Thus, the resistor  168  may be formed from a set of resistors that are selectively coupled in parallel (to form the overall resistance for the resistor  168 ) via switches that are controlled by the MCU  42 . 
     It is noted that many variations and topologies are possible for the IF transconductor  62 ,  72 , depending on the particular embodiment of the invention. Thus, the embodiment that is depicted in  FIG. 4  is merely for purposes of describing one possible implementation of the IF transconductor  62 ,  72 . Other implementations are possible and are within the scope of the appended claims. 
     Referring to  FIG. 5 , in accordance with some embodiments of the invention, the mixer  64 ,  74  may be a double balanced Gilbert cell. In this regard, the mixer  64 ,  74  may include an NMOSFET pair that is formed from an NMOSFET  224  and an NMOSFET  226  that have their source terminals coupled to the output terminal  189  (see  FIG. 4 ). The drain terminal of the NMOSFET  226  may be coupled to a node  227 , and the drain terminal of the NMOSFET  224  may be coupled to a node  203 . The source-drain path of a PMOSFET  228  may be coupled between the V DD  supply voltage and the node  227 , and the source-drain path of a PMOSFET  202  may be coupled between the V DD  supply voltage and the node  203 . The nodes  227  and  203  provide output currents to the input terminals  69  and  71 , respectively, of the LPF  76 , in accordance with some embodiments of the invention. 
     The gate terminal of the NMOSFET  224  is coupled to a node  222  that serves as an input terminal to receive the local oscillator mixing signal. In this regard, the node  222  is also coupled to the gate terminal of an NMOSFET  220  that is part of another pair of NMOSFETs. More specifically, an NMOSFET  200  includes a gate terminal that serves as the input terminal for the local oscillator mixing signal. The source terminals of the NMOSFETs  200  and  220  are coupled together at a node  210  that sinks is coupled to the output terminal  151  (see  FIG. 4 ). The drain terminal of the NMOSFET  220  is coupled to the node  227 , and the drain terminal of the NMOSFET  200  is coupled to the node  203 . 
     It is noted that the implementation of the mixer  64 ,  74  is one of many possible implementations of the mixer, as other embodiments are contemplated and are within the scope of the appended claims. 
     Referring to  FIG. 6 , in accordance with some embodiments of the invention, the signal processing path  58  of  FIG. 2  may be replaced by signal processing path  300 . Like reference numerals are used to denote similar elements to the signal processing path  58 , with the following differences. In particular, the signal processing path  300  is designed to accommodate signal swing charges due to integrated circuit process corners and/or the RF frequency of the transmitted FM signal. The current consumption of the RF transconductor  78  (see  FIG. 2 ) and the silicon area that is occupied by the RF transconductor  78  may be minimized if the voltage signal swing range at the input terminal of the RF transconductor  78  is minimized. 
     In the approach depicted in  FIG. 6  to minimize signal swing variations at the input terminals of the RF transconductor  78 , a peak detector  310  provides an indication of the signal strength of the output signal that is provided by the RF transconductor  78 , and this indication is used to regulate the magnitude of the input signal to the RF transconductor  78 . More specifically, in accordance with some embodiments of the invention, a switch  322  may be closed to couple a node  323  to the input terminal of the peak detector  310 . The node  323  provides a value that is indicative of the current flowing to the output tuning network  38  and may be furnished by a resistor  342  that is coupled to the drain terminal of a PMOSFET  340  that is coupled between the node  324  and the V DD  supply voltage. The gate terminal of the PMOSFET  340 . The source-drain path of the PMOSFET  340  may be coupled to the gate terminals of the PMOSFETs  108  and  106  so that the voltage across the resistor  342  is indicative of the output signal from the RF transconductor  78 . 
     The peak detector  310 , as its name implies, compares the voltage across the resistor  342  to a predetermined or programmed threshold voltage. When this threshold is exceeded, the peak detector  310  asserts a peak detect signal (called “PK_DETECT_TRIP”) on its output terminal  311  for purposes of indicating the peak condition. The MCU  42  (see also  FIG. 1 ) detects the assertion of the PK_DETECT_TRIP signal; and this detection causes the MCU  42  to undertake measures to reduce the magnitude of the signal at the RF transconductor  78  for purposes of adjusting a gain that is upstream of the RF transconductor  78 . 
     In this regard, as depicted in  FIG. 6 , in accordance with some embodiments of the invention, assertion of the PK_DETECT_TRIP signal causes the MCU  42  to lower the gains of digital amplifiers  302  and  304  that are located upstream of the DACs  60  and  70 . Thus, by regulating the gains of the digital amplifiers  302  and  304 , the amplitude of the signal that is present at the input terminals of the RF transconductor  78  may be controlled. 
     Therefore, the signal processing path  300  that is depicted in  FIG. 6  may be used for purposes of regulating the magnitude of the input signal to the RF transconductor  78 . 
     The peak detector  310  may be used for purposes other than regulating the input signal to the RF transconductor  78  in accordance with some embodiments of the invention. For example, a switch  320  may be closed after an FM frequency change for purposes of monitoring the input signal to the antenna tuning network  38  to maximize the output power from the FM transmitter. 
     In the approach that is depicted in  FIG. 6 , each DAC  60 ,  70  may no longer operate at full scale. This may affect the signal-to-noise (SNR) of each DAC  60 ,  70  by as much as 12 dB (as an example). Additionally, DC offsets in the DACs, IF transconductors  62  and  72  and mixers  64  and  74  may become more important because any non-zero DC offset may be a larger fraction of the signal swing. This condition may cause significant frequency spurs to appear in the RF spectrum. 
       FIG. 7  depicts an alternative signal processing path  350  in accordance with other embodiments of the invention. The FM transmitter  350  has the same general design as the signal processing path  300 , with like references being used to denote similar components. The signal processing paths  300  and  350  have the following differences. In particular, instead of controlling digital amplifiers to control the input signal swing of the RF transconductor  78 , the MCU  42  controls the gains of the IF transconductors  62  and  72  based on the output of the peak detector  310 . This approach may reduce DC offsets in the IF path. Because the output current of the IF transconductors is relatively small, however, the collective DC offset that is introduced by the IF transconductors  62  and  72  and mixers  64  and  74  may now be a relatively large fraction of the signal swing, again leading to significant spurs in the RF spectrum. 
     Therefore, in accordance with some embodiments of the invention, an alternative signal processing path  400  that is depicted in  FIG. 8  may be used. The signal processing path  400  has a similar design to the signal processing path  300 , with like reference numerals being used. The signal processing paths  300  and  400  differ as follows. In particular, instead of manipulating the gains in the digital amplifier (see  FIG. 6 ) or the IF amplifiers (see  FIG. 7 ) to control the magnitude of the input signal to the RF transconductor  78 , the signal processing path  400  controls the gain of the LPF  76 . 
     More specifically, the signal processing path  400  controls the signal gain downstream of the mixers  64  and  74  (see  FIG. 2 ) and after the IF-to-RF frequency translation. Thus, in response to the assertion of the PK_DETECT_TRIP signal by the peak detector  310 , the MCU  42  lowers the gain of the LPF  76  for purposes of adjusting the signal swing. In accordance with some embodiments of the invention, the MCU  42  adjusts the gain of the LPF  76  may be adjusted by changing a particular resistance or capacitor of the filter  76 . 
     Referring also to  FIG. 9 , in embodiments of the invention in which the LPF  76  is a passive ladder-type filter, the capacitance of the first capacitors  130   a  (see  FIG. 3 ) of the LPF  76  may be controlled by the MCU  42  to control the gain (less than unity) of the LPF  76 . More specifically, the LPF  76  may include a binary-to-thermometer code converter  452  that operates switches  464  that are coupled between capacitors  460  and ground. The other terminals of the capacitors  460  are coupled together. Thus, by selectively asserting the switches  464 , the MCU  42  may write information to the converter  452  for purposes of selecting the value of the capacitor  130   a  and thus, selecting the gain of the LPF  76 . The capacitors  460  may be logarithmically-weighted in accordance with some embodiments of the invention; and the MCU  42  may “sweep” the capacitance value upwardly from a minimum value until the peak detector  310  trips to indicate the appropriate gain for the LPF  76 . 
     An advantage of the above-described control using the gain of the LPF  76  is that the overall signal processing path  400  is not as sensitive to DC offsets in the signal path. 
     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.
A system includes a cellular radio and an FM transmitter that are fabricated in the same semiconductor. The FM transmitter includes at least one mixer, a filter and an antenna tuning network. The mixer(s) translate an intermediate carrier frequency of an input signal to generate a second signal that has an FM carrier frequency. The filter removes spectral energy from the second signal to generate a third signal. The antenna tuning network is separate from the filter and produces a fourth signal to drive an antenna in response to the third signal.