Abstract:
Modern portable communications units, and in particular cellular telephones, can contain several frequency bands for receiving and several frequency bands for transmitting signals. Typically these units contain a baseband unit and a frequency synthesizer unit, which may be embodied as VLSI integrated circuits. The baseband unit commonly contains the user interfaces and control signals for controlling other portions of the circuitry. The second unit is sometimes called a frequency synthesizer unit. The second unit is dedicated to producing frequencies that are used by the communications system to create RF signals for broadcast and also to take RF signals and extract the modulated signal from them for decoding. As personal communications units have begun using an increasing number of bands it is often necessary to configure different filters to receive or broadcast the different bands. Typically, the baseband Integrated Circuit or separate circuitry does this filter configuration management. The data for filter switching, however, can be decoded from the data that is communicated across the serial bus to the frequency synthesizer integrated circuit. By allowing the frequency synthesizer Integrated Circuit to control the filtering as well as the frequency synthesizer functions, integrated circuit pins can be eliminated from the baseband integrated circuit. In addition, timing and latency problems involved with commanding the frequency change over a serial bus and switching filters directly are eliminated.

Description:
FIELD OF THE INVENTION 
     The present invention relates, generally, to systems, processes and devices which use frequency synthesizers and filters and, in particular embodiments, to processes, systems and devices in which the architecture of frequency synthesizing and filtering stages of communication transceivers is improved. 
     DESCRIPTION OF THE RELATED ART 
     Portable electronic devices have become part of many aspects of personal, business, and recreational activities and tasks. As the popularity of various personal communication devices, such as portable phones, portable televisions and personal pagers increases, the demand for smaller, lighter, more powerful, and more power efficient electronics, which comprises these devices, has also continued to increase. 
     The demand for smaller, lighter, more powerful, and more power efficient electronics provides motivation for ever increasing levels of circuit integration, in order to minimize the number of integrated circuits and improve the functioning of circuits which compose such systems. As the levels of integration increase and the actual number of integrated circuits within a system decrease, each integrated circuit may need to perform an increased portion of the functions of the overall system. Accordingly, the integration level of integrated circuits continues to increase as the number of integrated circuits within such systems continues to shrink. As more functions are integrated into fewer and fewer integrated circuit packages the number of pins on integrated circuits, i.e. input and output connections, has risen. As the levels of integration, of integrated circuits, increase, circuit packaging and input output pin count become critical design considerations. 
     Integrated circuits for communication systems must also be concerned with interoperability, that is integrated circuits from one manufacturer must be able to work with a variety of other manufacturers&#39; integrated circuits. The more integrated circuits that a manufacturer&#39;s product is compatible with the more integrated circuits that that manufacturer may sell. Because of the desire for interoperability, various manufacturers often develop similar interfaces between different integrated circuits. The need for common interfaces is increasingly important as higher density integrated circuits integrate more functions. As more functions are integrated, there is a need for more input output connections to connect the ever-increasing number of functions, within an integrated circuit, to the outside world. To meet the increasing input/output needs of increasingly complex integrated circuits, manufacturers have turned to multiplexed input and output pins, serial and parallel busses to convey information between parts. 
     Some of the buses, such as the serial I 2 C bus, are standardized and well defined. Others may adopt similar physical connections, so that the interconnections between parts become de facto standards and only the data communicated is changed, depending on which manufacturer&#39;s devices are being used. One such de facto standard is the serial bus used by the baseband electronics in communications circuits to communicate with the synthesizer portion of the circuitry. The baseband portion of the communications system circuitry is a portion of the circuitry that is used for controlling the system. It commonly includes logic circuitry to control other subsystems, and may control the receiving and processing of commands from the user of the system as well as displays. The synthesizer portion of the circuitry is the portion that commonly controls the synthesis of frequencies for modulating and demodulating signals. Examples of frequency synthesizers controlled by serial busses are the MB15E07SL integrated circuit produced by Fujitsu, the LMX2326 produced by National Semiconductor, and the MC145202 produced by Motorola. 
     Commonly included with a baseband electronics section and a synthesizer electronics section is a filtering section. A filtering section is typically separate from a synthesizer portion of the circuitry and contains discrete filtering elements. If only one transmit band and one receive frequency band is used, within a communications unit, the associated bandpass filters used with those frequency bands may be hardwired into the circuit, as they would never need to be changed. More and more modern communications devices, however, are required to operate in several communications bands and have the ability to be switched between the bands. An example of a communication system being required to support more than one band is the Japanese Personal Digital Cellular System (PDC). An additional allocation of bandwidth for the Japanese Personal Digital Cellular (PDC) system has required handset radios to support communication channels in three separate receive and transmit frequency bands. Existing SAW (Surface Acoustic Wave) filters, such as the PDC800 produced by Fujitsu, which are commonly used in such applications, can be only used to support one or two of the bands at a time. Because the SAW filters can be used to support only one or two of the bands at a time methods for selecting correct filters for a given communication channel need to be devised. 
     It is common practice to have an embedded processor control such subsystems. The straightforward approach to solving the filter/band selection problem is to add, to the embedded processor subsystem, digital logic and control signals to switch bandpass filters. The control signals needed may be supplied to circuitry external to the embedded processor subsystem. Embodiments of the present disclosure dispense with the straightforward solution of adding digital logic and control signals to the embedded processor subsystem. 
     SUMMARY OF THE DISCLOSURE 
     Accordingly, preferred embodiments of the present invention are directed to frequency synthesis and filter selection circuitry, and systems employing the same. Embodiments described herein relate to methods and apparatus for improving control over the selection of bandpass filters, in systems where more than one filter is present. External circuitry can then direct signals through the appropriate filters. Embodiments of the present invention instead use the data which is used to program the frequency synthesizer circuits to control filter selection. By decoding the data from the signals used to program the frequency synthesizer, logic circuitry can determine which frequency is being synthesized, and hence which filter is required. By decoding the data used to program the frequency synthesizer and using it to control the selection of filters the need for additional logic and control signals to select filters can thereby be eliminated. 
     The elimination of additional control signals to select filters, results in a reduction of the number of connections between the filter selection circuits and the embedded processor subsystem. This reduction saves space, makes for easier circuit board layout, and reduces test time of the baseband processor Integrated Circuit by reducing the amount of I/O pins which must be tested. In addition, designs are simpler because software, logic circuits and other circuitry, which might have been required to drive the additional control circuits, are no longer needed. 
     An illustrative embodiment of the present invention includes apparatus for producing and detecting radio frequencies. This illustrative embodiment includes a first unit that generates serial bus data, and a serial bus, coupled to the first unit that receives the serial bus data that is provided to it. This illustrative embodiment also includes a second unit, coupled to the serial bus, for accepting the serial bus data and creating second unit control signals, from the serial bus data. A frequency signal generating mechanism is included within the second unit. The second unit has inputs for accepting control signals and generating a frequency signal based on those control signals. The second unit also activates one of the filters and deactivates the remaining filters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram representing a system environment according to an example embodiment of the present invention. 
     FIG. 2 is a more detailed block diagram representation of transmit and receive functions as may be found in the exemplary portable communication system in FIG.  1 . 
     FIG. 3 is block diagrams representing a system interconnect arrangement, such as may be found in a portable telephone containing two transmit and two receive filters. 
     FIG. 4 is an enhanced detail block diagram of a Base Band Electronics unit, such as the unit shown in block  317  of FIG.  3 . 
     FIG. 5 is a functional block diagram of a frequency synthesizer, such as the frequency synthesizer shown in block  313  of FIG.  3 . 
     FIG. 6 a  is a block diagram of a preferred embodiment of the invention. 
     FIG. 6 b  is block diagram of an exemplary filter selection mechanism, according to an embodiment of the invention. 
     FIG. 7 is a chart showing the relationship of the data lines logic values to the frequency of the synthesizer and to the filter used. 
     FIG. 8 is a block diagram of a preferred embodiment of the invention. 
     FIG. 9 is a chart showing the relationship of data line value to the synthesizer frequency and bandpass filter selected, for example, in the embodiment depicted in FIG.  8 . 
     FIG. 10 is a block diagram of an embodiment of the invention incorporating the filter selection circuits and mixers into the frequency synthesizer unit. 
     FIG. 11 is a flowchart showing a method for controlling filter selection, according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following description of preferred embodiments, reference is made to the accompanying drawings, which form a part thereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. It should be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present disclosure. 
     In addition, the present disclosure is illustrated by use of examples referenced to portable communication unit such as a portable phone. It is to be understood that, although the present invention is useful in the portable telephone art, the present invention is applicable to a variety of systems that employ frequency synthesis and filtering mechanisms. Illustrative embodiments presented are by way of example and are not to be construed as limiting the usefulness or applicability of the present invention. 
     To realize a cost-efficient design, portable telephone manufacturers may attempt to minimize size, weight, complexity, and power consumption. Embodiments of the present invention therefore relate to portable communication transceivers in which bandpass filters may be switched using frequency synthesizer data. It should be noted, however, that frequency synthesizers and filtering stages that accompany them, according to embodiments of the present invention, are not unique to portable communications. Frequency synthesizers and filtering stages may be employed in a variety of electronics, including both wireless transmission systems as well as wired systems. For purposes of simplifying the present disclosure, however, preferred embodiments of the present invention are described in relation to personal wireless communications systems, including, but not limited to digital mobile telephones, digital cordless telephones and the like. Such personal communications systems typically include one or more portable receiver/transmitter units. 
     A generalized representation of a system environment, according to an embodiment of the present invention, is shown in FIG.  1 . In FIG. 1 a portable communications system  101  communicates with a base station receiving and broadcast unit  103 . The portable communications system  101 , communicates with the base station  103 , receiving information across a communications channel  109  and sending information across a second communications channel  107 . The receive and send channels,  107  and  109 , may be the same channel, different channels or even channels which change with time. The base station  103  couples the communications of the portable communications system  101  into a node  105 , which is an access point into the land based phone system. Portable communications system  101 , includes both transmit and receive sections as generally illustrated in FIG.  2 . 
     FIG. 2 represents, in detail, transmit and the receive functions as may be found in a portable communication system, such as, for example, the portable communications system  101 . In the present illustrative embodiment, the signal source may include, for example, a microphone  201  for converting sound waves into electronic signals representative of the sound waves. The electronic signals representative of the sound waves are then coupled to a transmitter  203 . The transmitter unit  203  receives, from the microphone  201 , the electronic signals, which represents the sound waves, and modulates a carrier signal with a representation of them. The transmitter,  203 , amplifies the modulated signal and otherwise prepares the electronic signals, for transmission. The signals, which have been prepared for transmission, are provided to a duplexer  205 , which then further couples the signals into an antenna  211 , for the purpose of broadcasting the signal via radio transmission. In other embodiments, the signal source may include any suitable device for producing data signals for communication over channel  107  such as, but not limited to, a keyboard, a digital voice encoder, a mouse or other user input device, a sensor, monitor, testing apparatus, or the like. 
     A portable communication system  101  may also include a receiving antenna  211  for the reception of signals. In the portion of the example portable communication system shown in FIG. 2 the antenna  211  is coupled to a duplexer  205 , which then provides the received signals to the receiver unit  207 . The receiver unit  207  demodulates, amplifies and otherwise processes the received signal and provides it to a speaker  209 . The speaker transforms provided signal into sound waves, which may then be perceived by a user. In other embodiments the signal received may represent other information other than voice, such as, but not limited to data input to a computer, remote telemetry, fax data, or the like. 
     FIG. 3 is a detailed block diagram of the illustrative portable communications system, of FIG.  2 . For illustration purposes, it may be assumed that the portable communications system, is a Japanese Personal Digital Cellular (PDC) telephone. This example is chosen because it will be particularly illustrative, as a real world example, when describing the disclosure herein. Those skilled in the art will recognize that the disclosure, while useful in this application, is not limited to this application, and that the disclosure may be applied in a variety of different systems in a variety of different ways. A microphone  301 , is used to accept the user&#39;s voice and convert it into an electrical signal, representative of the user&#39;s voice. The electrical signal representing the user&#39;s voice is then provided to the input circuitry  303 , where it is amplified and digitized. The digitized voice signal is then provided to a modulator  305  where it is used to modulate a carrier signal. The modulated signal is then provided to a mixer  307 , where it is mixed with a signal provided to the mixer  307  by a frequency synthesizer unit  313 . The mixer  307  translates the frequency of the modulated signal to the proper frequency for broadcast. The broadcast signal is then provided to either filter  309  or filter  311  as controlled by the baseband electronics  317 . The baseband electronics uses solid state switch  337 , or solid state switch  339  to select which filter the broadcast signal will be coupled to. The selected filter then attenuates any undesired frequency components, and passes the desired components. The filtered transmission signal is then coupled into the amplifier and duplexer block  319 , where it is amplified, by the amplifier, and coupled, via the duplexer, into antenna  321 , for broadcast via radio transmission. 
     In the present embodiment two filters, a first filter  309  and a second filter  311 , are present. These filters commonly may be SAW (Surface Acoustic Wave) filters, which are very effective in this type of application. Current SAW filters do have a limitation, however, that they can filter only frequency bands that are about 5% of their nominal center frequencies. For this reason there are two filters, i.e. to cover the necessary frequency bands. Japanese Personal Digital Cellular (PDC) telephones, as illustrated in the present embodiment, utilize three separate frequency bands to transmit and to receive signals. Because one SAW filter can only cover two of the bands, a second filter is needed. The filters are then commonly switched as the broadcast frequency is changed. A straightforward approach to the switching of filters is to allow a baseband electronics unit, such as  317 , to control the switching of the filters as just described. The baseband electronics unit may typically be the overall controller for the telephone. The baseband electronics unit may commonly accept input commands from the user and then control the other subsystems within the telephone. 
     The serial bus  315  interconnection, for controlling the frequency synthesizer, has become somewhat of a de-facto standard in the industry, typically containing data, clock, and latch enable lines. The serial bus  315 , couples programming data to the frequency synthesizer  313 , so that the frequency synthesizer,  313  synthesizes and delivers the correct frequency to the mixer  307 , thereby assuring that the proper frequency band for broadcast is chosen. 
     The signal received by the antenna  321  is coupled into the duplexer and Amplifier block  319 . The received and amplified signal is then coupled into either filter  329 , or filter  331 , depending on which frequency band is being received. The frequency band being received, and hence the selection of filter  329  or filter  331 , is commonly controlled by a baseband electronics unit in a manner similar to the selection of the selection of filter  309  or filter  311 , in the broadcast portion of the system. The baseband electronics uses solid state switch  333 , or solid state switch  335  to select which filter the received signal will be coupled to. The serial bus can be used to control the frequency synthesizer  313  for the receive side of the phone. The frequency synthesizer  313 , provides the correct frequency to the receive mixer  327 , to translate the received signal to the correct frequency for the demodulator  325 . The demodulator  325  demodulates the signal received and couples it to the output circuitry  323 . The output circuitry then processes the demodulated signal suitably for presentation to the speaker  321 , where the original voice signal is reproduced for perception by the user. 
     It will be apparent to those skilled in the art that the foregoing description is one of example only. There are many modifications and variations of ways to interconnect the circuitry to implement the electronic functions illustratively described, and still preserve the essential functioning of the personal communications unit. 
     FIG. 4 is a block diagram illustrating, with increased detail, the functioning of the baseband electronics unit  317 . FIG. 4 illustrates an approach to controlling the frequency synthesizer and selecting bandpass filters. In FIG. 4 a baseband electronics unit  401 , may be typically contained within one integrated circuit package. A baseband processor  421 , is the controller for the baseband electronics unit  401 . The baseband processor  421 , receives user input  423  in the form of keystroke commands. The baseband processor  421 , may also prompt the user with audio and visual cues (not shown). The baseband processor  421 , processes the user input  423 , and controls the baseband unit  401 , to select the correct sending and receiving bands, initiate a call, and control the other functions within the phone. 
     The baseband processor  421 , provides a signal to a receive filter control unit  407 , for selecting the appropriate filter to match the broadcast band that has been chosen. The receive filter control unit  407  accepts the signal from the baseband processor  421  and stores the filter selection in a register that provides a filter select signal  403 . The filter select signal  403  signal controls filter select circuitry  405 , which activates the appropriate filter. Filter circuitry may be embodied in solid state switch circuits, for example,  333  and  335 . 
     The baseband processor  421 , also provides a signal to the transmit filter control unit  425 , for selecting the appropriate filter to match the transmit band that has been chosen. The transmit filter control unit  425  accepts the signal from the baseband processor  421  and then stores the filter selection in an output register that provides a filter select signal  427 . The filter select signal  427  controls filter select circuitry  429  that then activates the appropriate filter. Filter circuitry may be embodied in solid state switch circuits, for example,  337  and  339 . 
     The baseband processor  421 , also interfaces with the serial data register  419 . The serial data register  419  receives the data, necessary to program the frequency synthesizer unit  313 , from the baseband processor  421 . This data may vary depending on the manufacturer of the frequency synthesizer unit  313 , because, although the serial bus connections may be standardized between manufacturers, the data used to program the units can be different. The data necessary to program the frequency synthesizer unit is then coupled through the serial data line  409 , to the frequency synthesizer unit  313 . The serial data transfer from the baseband electronics unit  401 , to the frequency synthesizer  501 , is synchronized by the clock  411 , which is provided by the baseband processor  421 . The baseband processor also provides the data latch signal  413 . The data latch signal  413 , is coupled to the frequency synthesizer unit  501 , and signals the frequency synthesizer unit  501 , when the serial data  409 , is valid. 
     FIG. 5 is a block diagram illustrating the functioning of a frequency synthesizer unit  501 . A serial bus  517 , is coupled to a serial bus data decoder  515 . The serial bus data decoder  515 , accepts and decodes the serial data provided to it by a baseband electronics unit  401 , by means of the serial bus  517 . 
     The serial bus data decoder  515 , decodes information indicating which frequency must be provided to the transmit mixer,  503 . The decoded serial bus information is coupled to the transmit frequency source select block  505 , using the Transmit frequency select lines  513 . The transmit frequency source select block  505  then selects the proper frequency, as indicated by the data from the serial bus data decoder  515 , from one of the three transmit frequency sources,  507 ,  509  and  511 . The selected transmit frequency source is then coupled into the transmit mixer  503 , where it is used to mix with the modulated signal and provide the correct frequency for broadcast. 
     The serial bus data decoder  515 , also decodes information indicating which frequency must be provided to the receive mixer  529 . The decoded serial bus information is coupled to the receive frequency source select block  527 , using the receive frequency select lines  525 . The receive frequency source select block  527  then selects the proper frequency, as indicated by the data from the serial bus data decoder  515 , from one of the three receive frequency sources  519 ,  521  and  523 . The selected receive frequency source is then coupled into the receive mixer  529  where it is used to mix with the modulated signal and provide the correct frequency for broadcast. 
     Thus the baseband electronics unit  401  selects the frequencies that the portable communications system, illustratively the PDC phone, will use to broadcast and receive. The baseband electronics unit  401 , may then select the proper filters to be used with the transmit and receive frequencies chosen. The broadcast and receive bandpass filter select signals  403  and  427  are used to control the receive and broadcast filter selection, as appropriate. The baseband electronics unit  401  also couples the information concerning which frequencies will be used to transmit and receive to the serial bus  417 , for further coupling to the frequency synthesizer unit  501 . The frequency synthesizer unit  501 , accepts the data from the serial bus  517 , and decodes it to ascertain which frequency source will be provided to the receive mixer  529  and which will be provided to the transmit mixer  503 . 
     It will be recognized by those skilled in the arts that the foregoing description is used for illustrative purposes only and that various implementations can vary considerably from the one described. For example, the frequency of the signal provided to the transmit mixer is shown as being selected from three discrete frequency sources by a frequency source select block  505 . In an actual implementation, there may not be three discrete frequency sources present. Because of cost considerations it is likely the three different frequencies will be generated by a single source, this is multiplied or divided as appropriate. Dividers may generate the frequencies, as may phase lock loops, multipliers, crystals, or other devices using schemes well known in the art. The three transmit frequency sources  507 ,  509 , and  511  and the three receive frequency sources  519 ,  521 , and  523 , are used illustratively, in order to provide an understanding of the functioning of the circuit. 
     FIG. 6 a  is an exemplary functional block diagram, of a preferred embodiment, of the invention. In FIG. 6 a , the baseband unit  621  is coupled via a serial bus  619  to the bus data decoder  617  The bus data decoder  617 , is coupled to the transmit frequency source select  609  via data lines  605 ,  607  and  649 . The data lines  605 ,  607  and  649 , control the selection of frequency, which is provided to the transmit mixer  603 , according to the Frequency-Filter Select Table shown in FIG.  7 . If the transmit frequency source to be selected is  611 , filter  641  will be activated and data values lines  605 ,  607 , and  649  will have data values “1”, “0”, “0” coupled to their respective lines from the bus data decoder  617 . If the transmit frequency to be selected is  613 , filter  641  will again be activated and data lines  605 ,  607 , and  649  will again have data values “1”, “0”, “1” coupled to the respective lines from the bus data decoder  617 . If the transmit frequency to be selected is  615 , filter  643  will be activated and data lines  605 ,  607 , and  649  will have data “0”, “1”, “X” (“X”=don&#39;t care state, i.e. either a “1” or a “0”) coupled to their respective lines from the bus data decoder,  617 . Typically filters may not have activate inputs to turn them on and off. If they do not an alternative common approach is illustrated in FIG. 6 b . FIG. 6 b  is block diagram of an exemplary filter selection mechanism, according to an aspect of the invention. In order to select filter  651 , a control signal is applied to solid state switches  655  and  653  using filter control  661 . When solid state switches  655  and  653  are activated the filter  651  is coupled to the input  657  and output  659  of the filter selection module and the filter  651  is activated. When solid state switches  655  and  653  are deactivated the filter  651  is decoupled from the input  657  and output  659  of the filter selection module and the filter  651  is deactivated. 
     Similarly the bus data decoder  617  is coupled to the receive frequency source select  631  via data lines  637 ,  635  and  651 . These three data lines  637 ,  635  and  651 , control the selection of frequency, which is provided to the receive mixer  633 , according to the Frequency-Filter Select Table shown in FIG.  7 . If the receive frequency to be selected is  625 , filter  647  will be activated and data lines  637 ,  635  and  651  will have data logic levels “1”, “0”, “0” coupled to their respective lines from the bus data decoder,  617 . If the receive frequency to be selected is  627 , filter  647 , will again be activated and data lines  637 ,  635  and  651  will have data logic levels “1”, “0”, “1” coupled to the irrespective lines from the bus data decoder,  617 . If the transmit frequency to be selected is  629 , filter  645  will be activated and data lines  637 ,  635  and  651  will have data logic levels “0”, “1”, “X” (“X”=don&#39;t care state, i.e. either a 1 or a 0) coupled to their respective lines, from the bus data decoder  617 . 
     The receive and transmit signals, so selected, are coupled to the duplexer and amplifier  623 , and thereby to the antenna  639  in the same manner as the FIG. 3, current art embodiment. 
     In the current embodiment of the disclosure, as shown in FIG. 6 a , the baseband unit  621 , does not contain the filter select signals as the approach illustrated in FIG. 3, does. Instead of controlling the filter select from the baseband unit  621 , that function is relegated to the frequency synthesizer unit  601 . This arrangement provides several advantages to the designer who is attempting to design and integrate a baseband unit with a frequency synthesizer unit into a complete system. The first of the advantages is that it saves four integrated circuit pins in a baseband integrated circuit, as compared to a conventional baseband design. The pins saved are dedicated pins, that must be maintained at a logic level at all times to properly select the filters which will be used. If the pins are not dedicated pins, i.e. are pins on which the data is valid for only certain time periods, then another data valid pin would need to be added to ensure that only valid data on the lines are used to select the filters to be used. 
     The reduction in Baseband unit pins provides several advantages. It enables the baseband unit pins, which are saved, to be used for other purposes, thereby increasing the functionality that can be contained within the baseband integrated circuit and communicated to the outside world. Another advantage is that it eliminates the testing of those pins so it can shorten the time for the testing of the baseband integrated circuit. A further advantage is that the filter control signals from the baseband integrated circuit are no longer needed, and the circuitry and circuit board layout may be simplified. In addition there is the advantage that the filter control signals and the frequency synthesis function are now within one integrated circuit (the frequency synthesizer) instead of being contained partly in the baseband integrated circuit and partially within the frequency synthesizer circuit. Combining both related control functions within the same circuit eliminates the timing problems, which can occur if the selection of the frequency to the mixer and the filter selection are not accomplished simultaneously. Commonly the filters are selected directly from circuitry within the baseband electronics portion of the circuit, or from added control circuitry. The frequencies coupled to the mixer circuits would, however, be selected as a consequence of information delivered via the serial bus. Receiving and decoding serial bus information and then coupling the correct frequency into the mixers takes time. If the selection of the filters and the coupling of the proper frequency into the mixer are not simultaneous, interference or noise may result, degrading the perceived quality of the unit. The selection of the filters and the coupling of the proper frequency into the mixer can be timed to coincide. Frequency synthesizer integrated circuits from different manufacturers, however, may perform the frequency synthesis function differently, producing different latencies between the coupling of the data to the serial bus and in the production of the frequencies to be coupled into the mixers. By consolidating the functions of selecting the filters and the coupling of the proper frequency into the mixer within one integrated circuit, the selection of the filters and the coupling of the proper frequency into the mixer can be easily synchronized, thus eliminating timing problems. Placing the control of the selection of the filters and the coupling of the proper frequency into the mixer within the frequency synthesizer integrated circuit, also creates the possibility of bringing the mixer and filtering functions into the frequency synthesizer integrated circuit. 
     FIG. 8 is a block diagram of a second preferred embodiment of the disclosure. The second preferred embodiment of the disclosure is similar to the first preferred embodiment, illustrated in FIG. 6 a . In both the first and second embodiments, the filter select function has been moved out of the baseband Unit  621  and into the frequency synthesizer unit  601 . Filter  641 , of FIG. 6 a  has been replaced by filter  801 , of FIG.  8 . Filter  801 , is of a type that is activated by a logic “0” instead of a logic “1”. filter  801 , of FIG. 8 is activated by a logic “0” on data line  605 , and Filter  643 , of FIG. 8 is activated by a logic “1” on data line  605 . In other words when filter  801 , is activated Filter  643 , is deactivated. Similarly, filter  647 , of FIG. 6 a  has been replaced by Filter  803 , of FIG.  8 . Filter  803 , of FIG. 8 is similar to filter  801 , in that filter  803 , is of a type that is activated by a logic “0” instead of a logic “1”. Filter  803 , of FIG. 8 is activated by a logic “0” on data line  637 , and filter  645 , of FIG. 8 is activated by a logic “1” on data line  635 . In other words when filter  645 , is activated filter  803  is deactivated. This situation is summarized in the table of FIG.  9 . If the transmit frequency source of  611  or  613  is to be activated then data line  607  will have a logic “0” value coupled to it which will activate filter  801 , and deactivate filter  643 . If the transmit frequency source of  615  is to be activated, then data line  607  will be a logic “1” that will deactivate filter  801 , and activate filter  643 . Similarly if the transmit frequency source of  625  or  627  is to be activated then data line  635  will have a logic “0” coupled to it which will activate filter  803 , and deactivate filter  645 . If the transmit frequency source of  629  is to be activated then data line  635  will have a logic “1” coupled to it which will deactivate filter  803  and activate filter  645 . By choosing filters with complimentary activation inputs and correctly coding the bus data decoder data lines, the bandpass filters for transmit and receive can be selected with a single external pin for the transmit stage and a single external pin for the receive stage. 
     In a third preferred embodiment in FIG. 10 the mixers and the filter selection circuits are combined into one integrated circuit  1001 . The integrated circuit components are identified as being within the dotted line of the illustration in FIG.  10 . This embodiment conserves IC pins over a prior art embodiment in which the filter selection is controlled by separate circuitry. In other words, by selecting the transmit and receive filters by using the frequency synthesizer data, control lines to the filters from the baseband unit  621  can be avoided. This can allow the transmit mixer  603 , receive mixer  633 , as well as filter selection circuits  1003  and  1009  to be integrated into one package as shown by  1015 . 
     A method for controlling filter selection is illustrated in the flowchart shown in FIG.  11 . In one embodiment of the method shown in FIG. 11, controlling filter selection comprises producing control data in a first unit, as shown at  1102 . The control data is then transmitted from the first unit to a second unit, as shown at  1104 . The control data is then accepted by the second unit, as shown at  1106 . In one embodiment, the first unit may be a baseband unit of a portable communication unit, and the second unit may comprise a frequency synthesizer. In one embodiment, the first unit may be coupled to a serial bus that receives control data as serial bus data that is provided to it. The second unit may also be coupled to the serial bus, for accepting the control data as serial bus data. 
     A frequency signal is then generated within the second unit using control data, as shown at  1108 . In one embodiment, the frequency is generated by a frequency synthesizer within the second unit. In one embodiment, the control data is also used to enable at least one filter from a plurality of filters and disable the remaining filters using the control data, as shown at  1110 . 
     Those skilled in the art will recognize that the techniques of this disclosure may be extended and modified to meet the needs of particular implementations, without departing from the spirit and the substance of the disclosure. Those skilled in the art will also recognize that the illustrative implementations in the disclosure serve as explanation only, and not as limits to the invention, which is defined by the claims appended below.