Abstract:
A mobile radio terminal includes a system clock having a controller, a frequency generator for outputting a clock signal and a temperature sensor for detecting an operating temperature of the frequency generator. The controller adjusts a frequency of the clock signal by inputting a frequency generator control value associated with the detected operating temperature to the frequency generator. A control circuit that manages overall operation of the mobile radio terminal is configured to place the mobile radio in one of plural operational modes and the clock signal is used to clock an electrical component activated in connection with the one of the operational modes.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates generally to electronic equipment, such as a mobile radio terminal for engaging in voice communications. More particularly, the invention relates to a method and system for maintaining the accuracy of a system clock for the mobile radio terminal when the system clock functions autonomously (e.g., the system clock is not synchronized with a communications network). 
   DESCRIPTION OF THE RELATED ART 
   Mobile and/or wireless electronic devices are becoming increasingly popular. For example, mobile telephones and portable media players are now in wide-spread use. In addition, the features associated with certain types of electronic devices have become increasingly diverse. To name a few examples, many electronic devices have cameras, text messaging capability, Internet browsing functionality, electronic mail capability, video playback capability, audio playback capability, image display capability and hands-free headset interfaces. 
   Mobile devices that communicate with a communications network typically have a system clock that is synchronized with a reference clock of the communications network. Clock synchronization also may be referred to as frequency synchronization. Clock accuracy plays a role in the optimal functioning of various subsystems of many mobile devices, including placing and a receiving calls, interfacing with positioning systems (e.g., assisted GPS or A-GPS), interfacing with wireless local area networks (WLANs) and receiving mobile radio downstreams or mobile television downstreams such as under the digital video broadcasting—handheld (DVB-H) protocol. 
   In some situations, the mobile device may not be able to synchronize its system clock with the communications network&#39;s reference clock. One situation where synchronization may not be possible is when the mobile device is outside the coverage of the communications network. For example, if the mobile device were a cellular telephone, the system clock may not be synchronized with a cellular system reference clock if the phone were not in the cellular coverage area. In other situations, the mobile device may be placed in a “flight mode” (e.g., for use when aboard an aircraft), a “TV mode” or other mode where call functionality is not active and, hence, clock synchronization is not available. 
   When synchronization is not possible, the system clock of the mobile device may not be as accurate as desired. For instance, system clock behavior as a function of temperature and/or age may cause the clock to output a clock signal that has a less than optimum frequency. 
   SUMMARY 
   In view of the above, there exists a need in the art for a method and system to maintain the accuracy of a system clock for a mobile radio terminal when the system clock cannot be synchronized against a communications network clock or when it is preferable for the system clock to operate in an autonomous mode. Aspects of the present invention are directed to a method and system for maintaining the accuracy of a system clock for the mobile radio terminal in these situations. 
   According to one aspect of the invention, a mobile radio terminal includes a system clock having a controller, a frequency generator for outputting a clock signal and a temperature sensor for detecting an operating temperature of the frequency generator, wherein the controller adjusts a frequency of the clock signal by inputting a frequency generator control value associated with the detected operating temperature to the frequency generator; and a control circuit that manages overall operation of the mobile radio terminal, the control circuit configured to place the mobile radio in one of plural operational modes and the clock signal is used to clock an electrical component activated in connection with the one of the operational modes. 
   In one embodiment of the mobile radio terminal, the frequency generator is a voltage controlled crystal oscillator (VCXO). 
   In one embodiment of the mobile radio terminal, the frequency generator control value is input to a voltage controller of the frequency generator as a digital control signal and the voltage controller applies a corresponding analog voltage to a voltage control node of an oscillator of the frequency generator. 
   In one embodiment of the mobile radio terminal, the frequency generator does not include a phase locked loop (PLL) circuit. 
   In one embodiment of the mobile radio terminal, the mobile radio terminal further includes a radio circuit clocked with the clock signal, the radio circuit configured to establish communications with a communications network. 
   In one embodiment of the mobile radio terminal, the operational modes include at least one of a mobile television mode, a wireless network interface mode, a location determining mode or a mobile telephone flight mode. 
   In one embodiment of the mobile radio terminal, the mobile radio terminal further includes a look-up table that stores frequency generator control values for plural operating temperatures. 
   In one embodiment of the mobile radio terminal, the controller updates the look-up table in conjunction with a synchronization of the system clock with a reference signal from a communications network, the updating including changing a stored frequency generator control value for the operating temperature of the frequency generator at the time of synchronization to a control value applied to the frequency generator to establish synchronization of the system clock. 
   According to another aspect of the invention, a method of adjusting a frequency of a clock signal generated by a system clock of a mobile radio terminal where the system clock has a frequency generator includes determining an operating temperature of the frequency generator; identifying a frequency generator control value associated with the determined temperature; applying the control value to the frequency generator to control the frequency generator to output the clock signal with a target frequency; and placing the mobile radio terminal in one of plural operational modes and clocking an electrical component activated in connection with the one of the operational modes with the clock signal. 
   In one embodiment of the method, the frequency generator is a voltage controlled crystal oscillator (VCXO). 
   In one embodiment of the method, the frequency generator control value is input to a voltage controller of the frequency generator as a digital control signal and the voltage controller applies a corresponding analog voltage to a voltage control node of an oscillator of the frequency generator. 
   In one embodiment of the method, the frequency generator does not include a phase locked loop (PLL) circuit. 
   In one embodiment of the method, the method further includes clocking at least one of a radio circuit or a control circuit of the mobile telephone with the clock signal. 
   In one embodiment of the method, the operational modes include at least one of a mobile television mode, a wireless network interface mode, a location determining mode or a mobile telephone flight mode. 
   In one embodiment of the method, identifying the frequency generator control value associated with the determined temperature includes retrieving the frequency generator control value from a look-up table that stores frequency generator control values for plural operating temperatures. 
   In one embodiment of the method, the method further includes synchronizing the system clock with a reference signal from a communications network; and in conjunction with the synchronizing, updating the look-up table by changing a stored frequency generator control value for the operating temperature of the frequency generator at the time of synchronization to a control value applied to the frequency generator to establish synchronization of the system clock. 
   According to yet another aspect of the invention, a method of adjusting a frequency of a clock signal generated by a system clock of a mobile radio terminal where the system clock has a frequency generator includes determining if a synchronization function to synchronize the system clock with a reference signal provided by a communications network is available and, if so, synchronizing the system clock with the reference signal from the communications network; and updating a look-up table that stores frequency generator control values for plural operating temperatures of the frequency generator by changing a stored frequency generator control value for the operating temperature of the frequency generator at the time of synchronization to a control value applied to the frequency generator to establish synchronization of the system clock. If the synchronization function is not available, then the method controls the system clock by determining an operating temperature of the frequency generator; retrieving a previously stored frequency generator control value associated with the determined temperature from the look-up table; and applying the control value to the frequency generator to control the frequency generator to output the clock signal with a target frequency. 
   In one embodiment of the method, the frequency generator is a voltage controlled crystal oscillator (VCXO). 
   In one embodiment of the method, the frequency generator control value is input to a voltage controller of the frequency generator as a digital control signal and the voltage controller applies a corresponding analog voltage to a voltage control node of an oscillator of the frequency generator. 
   In one embodiment of the method, the method further includes placing the mobile radio terminal in one of plural operational modes and clocking an electrical component activated in connection with the one of the operational modes with the clock signal. 
   These and further features of the present invention will be apparent with reference to the following description and attached drawings. In the description and drawings, particular embodiments of the invention have been disclosed in detail as being indicative of some of the ways in which the principles of the invention may be employed, but it is understood that the invention is not limited correspondingly in scope. Rather, the invention includes all changes, modifications and equivalents coming within the spirit and terms of the claims appended hereto. 
   Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments. 
   It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a mobile telephone as an exemplary electronic equipment in accordance with an embodiment of the present invention; 
       FIG. 2  is a schematic block diagram of the relevant portions of the mobile telephone of  FIG. 1  in accordance with an embodiment of the present invention; 
       FIG. 3  is a schematic diagram of a communications system in which the mobile telephone of  FIG. 1  may operate; 
       FIG. 4  is a block diagram of a system clock for the mobile telephone in accordance with an embodiment of the present invention; and 
       FIG. 5  is flow chart of an exemplary method of controlling the system clock in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   The present invention will now be described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. It will be understood that the figures are not necessarily to scale. 
   The term “electronic equipment” includes portable radio communication equipment. The term “portable radio communication equipment,” which herein after is referred to as a “mobile radio terminal,” includes all equipment such as mobile telephones, pagers, communicators, electronic organizers, personal digital assistants (PDAs), smartphones, portable communication apparatus or the like. 
   In the present application, the invention is described primarily in the context of a mobile telephone. However, it will be appreciated that the invention is not intended to be limited to a mobile telephone and can be any type of electronic equipment. Also, embodiments of the invention are described primarily in the context of maintaining system clock accuracy when synchronization of the system clock to a reference clock from a communications network is not available. It will be appreciated that the frequency of the system clock may be maintained in the manner described herein for certain operational modes of the mobile telephone, even when synchronization is available. Those operational modes may include, for example, a mobile radio mode or mobile television mode (e.g., to receive and process a time-sliced data downstream, such as a DVB-H broadcast), a networking mode (e.g., to interface to a WLAN), a location determination mode (e.g., a GPS mode), a flight mode, and so forth. 
   Referring initially to  FIG. 1 , an electronic equipment  10  is shown in accordance with the present invention. The electronic equipment  10  includes a tunable system clock that provides a clock signal. The clock signal may be used by the electronic equipment to operate according to a particular communication standard, such as global system for mobile communications (GSM), universal mobile telecommunications system (UMTS), code divisional multiple access (CDMA), advanced versions of these standards, or any other appropriate standard. The clock signal may also serve as a clock source for various components of the electronic equipment  10 , such as a processor, an application specific integrated circuit (ASIC), etc. 
   The electronic equipment shown in the illustrated embodiments is a mobile telephone and will be referred to as the mobile telephone  10 . The mobile telephone  10  is shown as having a “brick” or “block” form factor housing  12 , but it will be appreciated that other type housings, such as a clamshell housing or a slide-type housing, may be utilized. 
   The mobile telephone  10  includes a display  14  and keypad  16 . As is conventional, the display  14  may be used to display information to a user such as operating state, time, telephone numbers, contact information, various navigational menus, etc., which enable the user to utilize the various feature of the mobile telephone  10 . The display  14  may also be used to visually display content received by the mobile telephone  10  and/or retrieved from a memory  18  ( FIG. 2 ) of the mobile telephone  10 . 
   Similarly, the keypad  16  may be conventional in that it provides for a variety of user input operations. For example, the keypad  16  typically includes alphanumeric keys  20  for allowing entry of alphanumeric information such as telephone numbers, phone lists, contact information, notes, messages, etc. In addition, the keypad  16  typically includes special function keys such as a “call send” key for initiating or answering a call, and a “call end” key for ending or “hanging up” a call. Special function keys may also include menu navigation keys, for example, for navigating through a menu displayed on the display  14  to select different telephone functions, profiles, settings, etc., as is conventional. Other keys associated with the mobile telephone may include a volume key, an audio mute key, an on/off power key, a web browser launch key, a camera key, etc. Keys or key-like functionality may also be embodied as a touch screen associated with the display  14 . 
   The mobile telephone  10  includes conventional call circuitry that enables the mobile telephone  10  to establish a call and/or exchange signals with a called/calling device, typically another mobile telephone or landline telephone. However, the called/calling device need not be another telephone, but may be some other device such as an Internet web server, content providing server, etc. 
     FIG. 2  represents a functional block diagram of the mobile telephone  10 . For the sake of brevity, generally conventional features of the mobile telephone  10  will not be described in great detail herein. The mobile telephone  10  includes a primary control circuit  24  that is configured to carry out overall control of the functions and operations of the mobile telephone  10 . The control circuit  24  may include a processing device  26 , such as a CPU, microcontroller or microprocessor. The processing device  26  executes logical instructions to perform various functions of the mobile telephone  10 . The logical instructions may be stored in the form of code in a memory (not shown) within the control circuit  24  and/or in a separate memory, such as memory  18 . The memory  18  may be, for example, one or more of a buffer, a flash memory, a hard drive, a removable media, a volatile memory, a non-volatile memory or other suitable device. It will be apparent to a person having ordinary skill in the art of computer programming, and specifically in applications programming for mobile telephones or other electronic devices, how to program a mobile telephone  10  to operate and carry out the various functions described herein. Accordingly, details as to specific programming code have been left out for the sake of brevity. Also, while various functions may be executed by the processing device  26  in accordance with the preferred embodiment of the invention, such functionality could also be carried out via dedicated hardware, firmware, software, or combinations thereof. 
   Continuing to refer to  FIGS. 1 and 2 , the mobile telephone  10  includes an antenna  28  coupled to a radio circuit  30 . The radio circuit  30  includes a radio frequency transmitter and receiver for transmitting and receiving signals via the antenna  28  as is conventional. The radio circuit  30  may be configured to operate in a mobile communications system, as well as to receive data and/or audiovisual content. For example, the receiver may be an IP datacast compatible receiver compatible with a hybrid network structure providing mobile communications and digital broadcast services, such as DVB-H mobile television and/or mobile radio. Other receivers for interaction with a mobile radio network or broadcasting network are possible and include, for example, GSM, CDMA, WCDMA, MBMS, WiFi, WiMax, ISDB-T, etc. 
   The mobile telephone  10  further includes a sound signal processing circuit  32  for processing audio signals transmitted by/received from the radio circuit  30 . Coupled to the sound processing circuit  32  are a speaker  34  and a microphone  36  that enable a user to listen and speak via the mobile telephone  10  as is conventional. The radio circuit  30  and sound processing circuit  32  are each coupled to the control circuit  24  so as to carry out overall operation. Audio data may be passed from the control circuit  24  to the sound signal processing circuit  32  for playback to the user. The audio data may include, for example, audio data from an audio file stored by the memory  18  and retrieved by the control circuit  24 . The sound processing circuit  32  may include any appropriate buffers, decoders, amplifiers and so forth. 
   The mobile telephone  10  also includes the aforementioned display  14  and keypad  16  coupled to the control circuit  24 . The display  14  may be coupled to the control circuit  24  by a video decoder  38  that converts video data to a video signal used to drive the display  14 . The video data may be generated by the control circuit  24 , retrieved from a video file that is stored in the memory  18 , derived from an incoming video data stream received by the radio circuit  30  or obtained by any other suitable method. Prior to being fed to the decoder  38 , the video data may be buffered in a buffer  40 . 
   The mobile telephone  10  further includes one or more I/O interface(s)  42 . The I/O interface(s)  42  may be in the form of typical mobile telephone I/O interfaces and may include one or more electrical connectors. As is typical, the I/O interface(s)  42  may be used to couple the mobile telephone  10  to a battery charger to charge a battery of a power supply unit (PSU)  44  within the mobile telephone  10 . In addition, or in the alternative, the I/O interface(s)  42  may serve to connect the mobile telephone  10  to a wired personal hands-free adaptor (not shown), such as a headset (sometimes referred to as an earset) to audibly output sound signals output by the sound processing circuit  32  to the user. Further, the I/O interface(s)  42  may serve to connect the mobile telephone  10  to a personal computer or other device via a data cable. The mobile telephone  10  may receive operating power via the I/O interface(s)  42  when connected to a vehicle power adapter or an electricity outlet power adapter. 
   The mobile telephone  10  may include a camera  46  for taking digital pictures and/or movies. Image and/or video files corresponding to the pictures and/or movies may be stored in the memory  18 . The mobile telephone  10  also may include a position data receiver  48 , such as a global positioning system (GPS) receiver, Galileo satellite system receiver or the like. The mobile telephone  10  also may include a local wireless interface  50 , such as an infrared transceiver and/or an RF adaptor (e.g., a Bluetooth adapter), for establishing communication with an accessory, a hands-free adaptor (e.g., a headset that may audibly output sounds corresponding to audio data transferred from the mobile telephone  10  to the adapter), another mobile radio terminal, a computer or another device. 
   The mobile telephone  10  may be configured to transmit, receive and process data, such as text messages (e.g., colloquially referred to by some as “an SMS”), electronic mail messages, multimedia messages (e.g., colloquially referred to by some as “an MMS”), image files, video files, audio files, ring tones, streaming audio, streaming video, data feeds (including podcasts) and so forth. Processing such data may include storing the data in the memory  18 , executing applications to allow user interaction with data, displaying video and/or image content associated with the data, outputting audio sounds associated with the data and so forth. 
   As indicated, the mobile telephone  10  may include a system clock  52 . The system clock  52  may be configured to output a clock signal that is used by the radio circuit  30 , the control circuit  24  and/or other electronic components of the mobile telephone  10 . The system clock  52  may include any appropriate frequency generator, such as a phase lock oscillator (PLO), a voltage controlled oscillator (VCO) and/or a voltage controlled crystal oscillator (VCXO) with or without a phase lock loop (PLL) circuit. In the embodiments described herein, the frequency generator is a VCXO without a PLL. A VCXO is a crystal controlled oscillator that varies the frequency of the output signal in direct proportion to an applied control voltage. A suitable VCXO is described in commonly assigned U.S. Pat. No. 7,009,460, which is incorporated herein by reference in its entirety. While the illustrated embodiments are described in terms of a VCXO, those skilled in the art will appreciate that the principles described herein may be applied to other frequency generators. 
   With additional reference to  FIG. 3 , the mobile telephone  10  may be configured to operate as part of a communications system  54 . The system  54  may include a communications network  56  having a server  58  (or servers) for managing calls placed by and destined to the mobile telephone  10 , transmitting data to the mobile telephone  10  and carrying out any other support functions. The server communicates with the mobile telephone  10  via a transmission medium. The transmission medium may be any appropriate device or assembly, including, for example, a communications tower, another mobile telephone, a wireless access point, a satellite, etc. Portions of the network may include wireless transmission pathways. The network  56  may support the communications activity of multiple mobile telephones  10 , although only one mobile telephone  10  is shown in the illustration of  FIG. 3 . 
   In one embodiment, the server  58  may operate in stand alone configuration relative to other servers of the network  52  or may be configured to carry out multiple communications network  58  functions. As will be appreciated, the server  58  may be configured as a typical computer system used to carry out server functions and may include a processor configured to execute software containing logical instructions that embody the functions of the server  58 . 
     FIG. 4  illustrates a block diagram of an exemplary system clock  52 . The exemplary system clock  52  includes a VCXO  60 , a VCXO controller  62 , a temperature sensor  64  and a look-up table  66 . The VCXO  60  may include a voltage controller  68  and an oscillator  70 . The voltage controller  68  applies a variable voltage to an oscillator input node  72  so that the oscillator  70  may generate the clock signal (or oscillator output signal Vo(f)) that oscillates at a desired frequency. The controller  62  inputs a digital control signal to the voltage controller  68 , which converts the control signal to the voltage input signal applied to the oscillator  70 . As will be described in greater detail below, at certain times the control signal output by the controller  62  may be based on a sensed temperature as determined by the temperature sensor  64  and a value extracted from a look-up table  66 . In one embodiment, the look-up table  66  may be stored by the memory  18 , which may be accessed by the VCXO controller  62  via the control circuit  24 . In one embodiment, some or all of the functions of the VCXO controller  62  may be carried out by the control circuit  24 , in which case the controller  62  may be implemented by the control circuit  24 . The VCXO  60  may further include a start-up controller (not illustrated) that assists in bringing the oscillator  70  to a steady state so as to activate the various electronic components of the mobile telephone  10  when the mobile telephone  10  is started-up and before an attempt to synchronize the oscillator  70  output with a communications network  56  reference clock has been made. 
   The VCXO  60  shown in  FIG. 4  is a representative implementation, and the system clock  52  may vary from that which is depicted. In this exemplary embodiment, the oscillator  70  includes a tank network  74  and an active oscillator core  76 . The tank network  74  includes a variable capacitor network, including varactor diode  78  and a crystal resonator  80  that sets the output frequency generated by oscillator  70  based on the magnitude of the voltage input signal applied to the varactor diode  78  at node  72 . In general, the tank network  78  is a feedback circuit for the active oscillator core  76  that shifts the output frequency generated by oscillator  70  in response to the sour voltage input signal. 
   The voltage controller  68  includes a digital-to-analog converter (DAC)  82  connected to an impedance network  84  defined in the illustrated example by resistors R 1  and R 2  and capacitor C 1 . The DAC  82  converts the digital control signal received from the controller  62  into an analog DC voltage. The digital control signal may be, for example, a hexadecimal command word between 000 and 3FF, where 000 corresponds to a minimum source voltage, e.g., 0.3 V, and 3FF corresponds to a maximum source voltage, e.g., 2.6 V. Thus, the digital control signal drives the voltage input signal so as to enable the oscillator  70  to generate a clock signal having a particular frequency. 
   With additional reference to  FIG. 5 , illustrated is a flow chart of logical blocks that make up a system clock control function executed by the controller  62 . The flow chart may be thought of as depicting steps of a method of maintaining accuracy of the system clock  52  even when synchronization of the system clock with the communications network  56  cannot be preformed and/or a method of maintaining accuracy of the system clock  52  to support an operational mode of the mobile telephone  10 . 
   Although  FIG. 5  shows a specific order of executing functional logic blocks, the order of execution of the blocks may be changed relative to the order shown. Also, two or more blocks shown in succession may be executed concurrently or with partial concurrence. Certain blocks also may be omitted. In addition, any number of commands, state variables, semaphores or messages may be added to the logical flow for purposes of enhanced utility, accounting, performance, measurement, troubleshooting, and the like. It is understood that all such variations are within the scope of the present invention. 
   The method may be carried out at any appropriate time during operation of the mobile telephone  10 . Example times include at mobile telephone  10  start up, on a periodic basis as determined by a schedule or upon the occurrence of an event (e.g., a change in communications network  56  connectivity), upon entering an operational mode of the mobile telephone  10  (e.g., a mobile radio mode or mobile television mode, a wireless network interfacing mode, a location determination mode) and so on. 
   The method may begin in block  86  where a determination of whether the mobile telephone  10  is in a calling mode is made. The term calling mode is used to describe any mode or state of the mobile telephone  10  that allows communication with the communications network  56  by which synchronization of the system clock may be made. For instance, in a cellular-based communications network  56 , the calling mode may be referred to as a cellular mode. 
   If a positive determination is made in block  86 , the logical flow may proceed to block  88  where a determination of whether the system clock  52  may be synchronized with the communications network  56  is made. Techniques for system clock synchronization with the communications network  56  will be generally known to those of ordinary skill in the art and, for the sake of brevity, will not be described in detail herein. In one embodiment, synchronization is of the system clock  52  is made using a virtual channed link (VCL) between the mobile telephone  10  and the communications network  56 . Through the VCL, the communications network  56  may monitor the system clock signal of the mobile telephone  10  for a frequency error relative to a communications network  56  reference clock. Based on the frequency error, the communications network  56  may coordinate with the controller  62  to adjust the digital control signal input to the voltage controller  68  to adjust the output frequency of the oscillator  70 . In other embodiments, a clock reference signal that is contained in a baseband signal from the communications network  56  may be acquired by the mobile telephone  10  and the acquired signal may be used by the mobile telephone  10  to determine and correct any frequency error. Typically, the synchronization is conducted in conjunction with a base station (not shown) of the communications network  56  that is currently servicing the mobile telephone  10 . Therefore, an example of when the clock synchronization operation is unavailable may be when the mobile telephone  10  is out of range of an appropriate base station. 
   If a positive determination is made in block  88 , meaning that clock synchronization is available, the logical flow may proceed to block  90 . In some embodiments, a negative determination may be made in block  88  even if synchronization is available. For instance, if the user of the mobile telephone  10  takes action to place the mobile telephone  10  in a certain operational mode where control of the system clock  52  without synchronization is desired, a negative determination may be made in block  88   
   In block  90 , the system clock  52  of the mobile telephone  10  is synchronized with the communications network  56  reference clock. Synchronization may be carried out in any suitable manner, including the techniques described above. 
   Proceeding to block  92 , a determination may be made as to whether the look-up table  66  should be updated. Updating of the look-up table  66  may be made on a periodic basis, such as each time synchronization of the system clock  52  with the communications network  56  is made, based on a time schedule (e.g., after a specified time has elapsed since the last look-up table update), or based on an event schedule (e.g., after a specified number of mobile telephone  10  start-ups have been made). If a negative determination is made in block  92 , the logical flow may end. 
   If a positive determination is made in block  92 , the logical flow may proceed to block  94  where the look-up table  66  is updated. The look-up table  66  may contain controller  62  digital control signal values for a variety of operating temperatures. In other embodiments, the look-up table  66  may contain tuning voltages for application to the oscillator input node  72  versus temperature. 
   To update the look-up table  66 , the temperature at the time of synchronization may be detected by the temperature sensor  64 . The temperature may be detected in physical association with the VCXO  60  so that the detected temperature is or is a close approximation to the operating temperature of the VCXO  60 . The operating temperature of the VCXO  60  may differ from the ambient temperature of the environment in which the mobile telephone  10  operates due to heat generated by various electrical components of the mobile telephone  10 , or due to transfer of heat from a user in situations where the user has been holding the mobile telephone  10  or had the mobile telephone  10  in a pocket. In another embodiment, the temperature sensor  64  may detect the ambient temperature of the environment in which the mobile telephone  10  operates as an approximation of the operating temperature of the VCXO  60 . Once the temperature is detected, the digital control signal value in the look-up table  66  that is associated with the detected temperature can be updated to be the digital control signal value used to control the VCXO to generate the synchronized clock signal. Once the look-up table  66  is updated in block  94 , the logical flow may end. 
   It will be appreciated that updating the look-up table  66  on a periodic basis may function to compensate for aging of the VCXO  60 . In particular, the look-up table  66  is updated over time with controller  62  output values that have recently been used in conjunction with the synchronization process to generate a system clock signal of a desired frequency. Thus, the updated controller  62  output values stored in the look-up table  66  automatically compensate for aging of the VCXO  60 . 
   If a negative determination is made in either of blocks  86  or  88  (e.g., meaning that system clock synchronization is not available by either an operational mode setting or actual network synchronization unavailability), the logical flow may proceed to block  96 . The operations of block  96  are configured to establish an accurate system clock signal frequency, even if synchronization of the system clock  52  with the communications network  52  is not available. In block  96 , the controller  62  obtains a temperature measurement from the temperature sensor  64 . Using the temperature from the temperature sensor  64 , the controller  62  obtains a corresponding output value from the look-up table  66 . The obtained value from the look-up table  66  may be used as the output of the controller  62  that is input to the voltage controller  68  for frequency control of the oscillator  70 . Following block  96 , the logical flow may end. 
   As described above, an adaptive look-up table  66  is periodically updated when the system clock  52  is synchronized against a communications network  56 . The look-up table  66  may contain system clock  52  tuning values (e.g., controller  62  output values or tuning voltage values) versus temperature values. The data from the look-up table may be used to adjust the output signal frequency of the system clock  52  when the system clock  52  is operated autonomously from the communications network  56 . Updating the look-up table on a regular basis compensates for aging characteristics of the system clock  52 . As indicated, the above-described method and apparatus may be used to generate a clock signal having an accurate frequency when the system clock  52  may not be synchronized with the communications network  56  or when a software application is enable that places the mobile telephone  10  in a particular operation mode, such as a mobile radio or mobile television mode, a wireless network interface mode, a location determining mode, a flight mode, etc. 
   It will be appreciated that the look-up table  66  may be replaced by a different type of database structure or by computational algorithms so that a controller  62  output value may be obtained or derived as a function of temperature and/or age of the VCXO  60 . In these alternative embodiments, the updating of block  94  and the VCXO control of block  96  may be adapted in accordance with the alternative database structure or algorithmic approach. 
   It will be appreciated that the mobile telephone  10  may include multiple frequency generators. For example, baseband electronics may include a secondary frequency generator (not shown), such as a digital PLO, that maintains the clock signal while the mobile telephone  10  is in a sleep mode. When mobile telephone  10  wakes up from the sleep mode, the secondary frequency generator is disabled to enable the system clock  52  to function as a primary frequency generator and take over the responsibility of providing the desired frequency clock signal to the various components of the mobile telephone  10 . 
   Although the invention has been shown and described with respect to certain preferred embodiments, it is understood that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications, and is limited only by the scope of the following claims.