Abstract:
Methods and apparatus are presented for performing coarse frequency tuning in a voltage controlled oscillator. The methods and apparatus are directed towards the use of a new voltage controlled oscillator comprising both a binary coding module and a thermometer coding module. The combination of the binary coding module and the thermometer coding module control a capacitance corresponding to a resonant tank which is used to coarse tune the frequency of the voltage controlled oscillator.

Description:
BACKGROUND 
   I. Field 
   The disclosed embodiments relate to the field of voltage controlled oscillators. 
   II. Background 
   A cellular telephone or other wireless communication device transmits and receives signals at specific frequencies. One or more voltage controlled oscillators, commonly referred to as VCOs, are typically used to set or establish desired transmit and/or receive frequencies. One basic type of VCO design is that of an inductor-capacitor (LC) resonant tank VCO. 
   In an LC resonant tank VCO, one common way by which the frequency is set, entails utilizing a set of metal-insulator-metal capacitors (MIMcaps) which can be switched on and off. By selectively switching MIMcaps, the center frequency of the VCO can be coarse tuned. Furthermore, MIMcaps confer improved compensation for process variation in the fabrication of VCOs. MIMcaps also provide a wider VCO frequency tuning range than would typically be available when a VCO is only implemented with traditional voltage variable capacitors (i.e., varactors). Moreover, using MIMcaps enables the VCO to have a lower tuning sensitivity, also called Kv, than is typically available when a VCO is only implemented with traditional varactors because the varactor elements can be smaller for the same VCO frequency tuning range. 
   When utilizing MIMcaps for tuning the frequency of an LC resonant tank VCO, there are typically two different MIMcaps configurations. One common configuration entails implementing the LC resonant tank VCO with a set of binary weighted MIMcaps. Specifically, the center frequency of the VCO can be coarse tuned or adjusted over a relatively wide range of frequencies simply by selectively controlling the binary weighting of the MIMcaps. Although this binary weighting scheme offers flexibility, versatility, adaptability, and scalability, it suffers in that it commonly results in suboptimal coverage over the entire VCO tuning frequency range. For example, with a binary weighted VCO implemented with a varactor (which is common practice), there are typically either gaps in the tuning frequency range where the varactor must cover a wider range of frequencies than desired or, conversely, there are overlaps whereby adjacent digital coarse frequency tuning settings are crowded too close together and the varactor is underutilized. In many instances, a binary weighted VCO exhibits both undesired gaps as well as overlaps across its respective frequency range. This disadvantage is virtually impossible to eliminate, given the realistic analog parasitics associated with the binary weighted scheme. 
   The other common MIMcaps configuration for tuning the frequency of an LC resonant tank VCO entails implementing the VCO with a thermometer coded MIMcaps tuning bank. Specifically, the center frequency of the VCO can be coarse tuned over a relatively wide range of frequencies simply by activating a specific number of MIMcap units, wherein each unit includes a similar amount of capacitance. Consequently, by using the thermometer coded scheme in VCOs, a more optimal spacing of digital coarse tuning in frequency can be achieved. However, when extended to cover larger numbers of bits to support a wider frequency range, a thermometer coded MIMcap tuning bank requires a relatively large section of silicon. In other words, the thermometer coded MIMcap tuning bank may grow to consume a large area of a chip&#39;s limited silicon die area. This is highly disadvantageous because either the chip must be made larger or other functionalities must be compromised. In addition, the unavoidable parasitics resulting from the larger and larger biasing circuitry associated with implementing smaller thermometer MIMcap units would also grow correspondingly. Furthermore, the smaller thermometer MIMcap units also tend to result in net worse Q factor; the Q factor represents the quality factor of the LC resonant tank of the VCO. A lower Q factor directly translates into a degradation of the VCO phase noise and power consumption. 
   SUMMARY 
   Methods and apparatus are presented herein for performing coarse frequency tuning in a voltage controlled oscillator. The methods and apparatus are directed towards the use of a new voltage controlled oscillator comprising both a binary coding module and a thermometer coding module. The combination of the binary coding module and the thermometer coding module control a capacitance corresponding to a resonant tank which is used to coarse tune the frequency of the voltage controlled oscillator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an embodiment directed towards a voltage controlled oscillator. 
       FIG. 2  is a schematic of an embodiment directed towards a resonant tank circuit. 
       FIG. 3  is a schematic of an embodiment directed towards an electrical modeling circuit of a capacitor coding module of  FIG. 2 . 
       FIG. 4  is a schematic of another embodiment directed towards an electrical modeling circuit of a capacitor coding module of  FIG. 2 . 
       FIG. 5  is a flowchart of an embodiment directed towards a method for coarse frequency tuning. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. It is understood that these specific embodiments are not intended to be limiting. In the following detailed description of the embodiments, numerous specific details are set forth in order to provide a thorough understanding of the embodiments. However, it will be evident to one of ordinary skill in the art that the embodiments may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the embodiments. 
   The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs. 
     FIG. 1  is a block diagram of an exemplary voltage controlled oscillator  100  in accordance with embodiments. The voltage controlled oscillator  100  can be utilized in a wide variety of ways. For example, the voltage controlled oscillator  100  can be utilized as part of a transmitter, a receiver, and/or a transceiver of a wireless communication device, but is not limited to such. Specifically, in one embodiment the voltage controlled oscillator  100  can be implemented as part of a system that sets or establishes a transmit and/or receive frequency for wireless communication. The voltage controlled oscillator  100  can be implemented to include an inductor capacitor (LC) resonant tank  102  and a digital tuner  104 , along with other circuitry not shown. The digital tuner  104  and the LC resonant tank  102  are coupled together such that the digital tuner  104  can transmit or issue control signals (e.g., a digital word) that adjust the capacitance of the LC resonant tank  102 . Therefore, by adjusting the capacitance of the LC resonant tank  102 , the digital tuner  104  is able to coarse tune the frequency of the voltage controlled oscillator  100 . 
   It is noted that the coupling between the digital tuner  104  and the LC resonant tank  102  can be implemented as one or more bit lines or as one or more communication buses, but is not limited to such. It is appreciated that the digital tuner  104  can be implemented in a wide variety of ways. For example, the digital tuner  104  can be implemented with, but is not limited to, electronic hardware, software, or any combination thereof. 
     FIG. 2  is a schematic of an exemplary LC resonant tank circuit  200  of a voltage controlled oscillator (e.g.,  100 ) in accordance with embodiments. It is understood that the LC resonant tank circuit  200  can be used as an exemplary implementation for the LC resonant tank  102  of  FIG. 1 . In the present embodiment, the LC resonant tank circuit  200  utilizes binary coding and thermometer coding in order to digitally coarse tune the frequency of the VCO (e.g.,  100 ). Specifically, the binary coding portion can include capacitor tuning elements that increment in size by an approximate factor of 2 and can be addressed by the digital tuner  104  using binary weighted code. The thermometer coding portion can include capacitor tuning elements that are nominally each of substantially equal unit size and can be addressed by the digital tuner  104  using a thermometer code. Within the present embodiment of the resonant tank  200 , the binary coding can make up the least significant bits (LSBs) of the digital coarse tuning while the thermometer coding can make up the most significant bits (MSBs) of the digital coarse tuning, but are not limited to such. 
   The resonant tank  200  can be implemented to include any number of digital bits of binary coding and any number of digital bits of thermometer coding. For example, in the present embodiment the resonant tank  200  is implemented to include 64 digital course tuning codes with 3 digital bits of binary coding (corresponding to 8 settings) and 3 digital bits of thermometer coding (corresponding to 8 settings). It is appreciated that in one embodiment, the desired number of digital bits for the binary coding can be the same number of binary coding capacitor modules (e.g.,  270 ,  272 , and  274 ) that can be included as part of the resonant tank  200 . Therefore, 3 digital bits of binary coding can equal 3 binary coding capacitor modules  270 ,  272 , and  274 . Additionally, in one embodiment, if “n” is the desired number of bits for the thermometer coding, then there can be 2 n −1 thermometer coding capacitor modules (e.g.,  276  and  278 ) included as part of the resonant tank  200 . Therefore, 3 digital bits of thermometer coding can equal 7 thermometer coding capacitor modules  276  and  278  (where 5 thermometer modules are not shown). 
   Within  FIG. 2 , the “M” can represent the multiplicity factor related to capacitance. For example, a capacitor  232  having M=2 can represent two times the capacitance of a capacitor  222  having M=1. Additionally, a capacitor  242  having M=4 can represent two times the capacitance of the capacitor  232  having M=2. Therefore, in one embodiment when the digital tuner  104  activates the binary coding capacitor module  272 , its resultant capacitance can be two times that produced when the digital tuner  104  activates the binary coding capacitor module  270 . It is noted that within the present embodiment, the capacitance of each of the binary coding capacitor modules  270 ,  272 , and  274  is incremented by a factor of 2. Additionally, the capacitance of each of the thermometer coding capacitor modules (e.g.,  276  and  278 ) is substantially equal in size when activated by the digital tuner  104 . 
   Within the resonant tank  200 , note that the capacitance of the thermometer coding module  276  can be sized such that it is a factor of 2 larger than the capacitance of the binary coding module  274 . As such, the capacitance of the thermometer coding module  276  can be equivalent to the next capacitance value associated with the binary coding of the binary coding modules  270 ,  272 , and  274 . In this manner, there can be a smooth transition throughout all of the 64 different capacitance settings of the resonant tank  200  which can be each associated with a unique digital word output by the digital tuner  104 . 
   Specifically, Bit 0 , Bit 1 , and Bit 2  of the present embodiment refer to binary coding bits of a digital word output by the digital tuner  104  and received by resonant tank  200 . Furthermore, TBit 0  through TBit 6  refer to thermometer coding bits of the digital word output by the digital tuner  104  and received by resonant tank  200 . The Bit 0 , Bit 1 , and Bit 2  can be the three LSBs while TBit 0  through TBit 6  are the MSBs. It is understood that the binary coded modules  270 ,  272 , and  274  can be directly addressed with a digital word transmitted by the digital tuner  104 . However, the digital tuner  104  activates each of the thermometer coding modules (e.g.,  276  and  278 ) in sequence via transmission of the digital word. Within the present embodiment, each bit of the thermometer code is equivalent to 8 times the capacitance of the binary coding module  270 . For example, Table 1 below illustrates a sampling of decimal values from 0–63 along with their conversion into binary (e.g., that is separated into MSBs and LSBs) and also their conversion into VCO resonant tank code that includes both a binary code portion (e.g., as the LSBs) and a thermometer code portion (e.g., as the MSBs) which can be output by the digital tuner  104 . Note that the combination of the binary code portion and the thermometer code portion of the VCO resonant tank code can be referred to as a digital word. 
   
     
       
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
           
           
             
                 
                 
             
             
                 
               Binary 
               VCO Resonant Tank Code 
             
           
        
         
             
               Decimal 
               MSBs 
               LSBs 
               MSB Thermometer Code 
               LSB Binary Code 
             
             
                 
             
             
                0 
               000 
               000 
               0000000 
               000 
             
             
                7 
               000 
               111 
               0000000 
               111 
             
             
                8 
               001 
               000 
               0000001 
               000 
             
             
               10 
               001 
               010 
               0000001 
               010 
             
             
               16 
               010 
               000 
               0000011 
               000 
             
             
               22 
               010 
               110 
               0000011 
               110 
             
             
               29 
               011 
               101 
               0000111 
               101 
             
             
               35 
               100 
               011 
               0001111 
               011 
             
             
               47 
               101 
               111 
               0011111 
               111 
             
             
               49 
               110 
               001 
               0111111 
               001 
             
             
               63 
               111 
               111 
               1111111 
               111 
             
             
                 
             
           
        
       
     
   
   In the present embodiment, the LSB of the VCO resonant tank code corresponds to Bit 0  while the MSB corresponds to TBit 6 . For example, as shown in Table 1, when the digital tuner  104  outputs a VCO resonant tank code “0000001 010” for the decimal number 10, Bit 0  and Bit 2  are set at a low voltage value (e.g., logic “0”) while Bit 1  is set at a high voltage value (e.g., logic “1”). Additionally, TBit 0  is set at a high voltage (e.g., logic “1”) while TBit 1  through TBit 6  are set at a low voltage (e.g., logic “0”). In response to receiving the VCO resonant tank code “0000001 010”, the binary coding module  272  and the thermometer coding module  276  are active while the binary coding modules  270  and  274  along with the remaining thermometer coding modules (e.g.,  278  and those not shown) are inactive. 
   Within  FIG. 2 , the binary coding capacitor module  270  can include resistors  224  and  226 , capacitors  222  and  228 , along with a transistor  220 . When Bit 0  is set at a high voltage (e.g., logic “1”), then Bit 0 -bar is set at a low voltage (e.g., logic “0”). Therefore, the gate of the transistor  220  is set at a high voltage (e.g., logic “1”) and the voltage at its drain and source is set at a low voltage (e.g., logic “0”). As such, the transistor  220  is switched on (or activated) so that it is conducting current.  FIG. 3  is a schematic of an exemplary electrical modeling circuit  300  when the binary coding capacitor module  270  is activated in accordance with one embodiment. Specifically, when the binary coding module  270  is activated, the resultant electrical modeling circuit  300  includes capacitors  222  and  228  coupled in series with a resistance  302  that is associated with the transistor  220  (not shown) when it is conducting. Since the capacitance of each of the capacitors  222  and  228  is substantially the same, it is understood that the total capacitance of modeling circuit  300  is substantially equal to the capacitance of capacitor  222  (or capacitor  228 ) divided by 2. 
   Within  FIG. 3 , a first terminal of the capacitor  222  and a first terminal of the capacitor  228  are each coupled as shown in the resonant tank circuit  200 . However, a second terminal of the capacitor  222  is effectively coupled with a first terminal of the transistor resistance  302 . Additionally, a second terminal of the capacitor  228  is effectively coupled with a second terminal of the transistor resistance  302 . 
   Conversely, within  FIG. 2 , when Bit 0  is set at a low voltage (e.g., logic “0”), then Bit 0 -bar is set at a high voltage (e.g., logic “1”). Therefore, the gate of the transistor  220  is set at a low voltage (e.g., logic “0”) and the voltage at its drain and source is set at a high voltage (e.g., logic “1”) in order to minimize the parasitic capacitance associated with transistor  220 . As such, the transistor  220  is switched off (or deactivated) so that it is effectively not conducting current.  FIG. 4  is a schematic of an exemplary electrical modeling circuit  400  when the binary coding capacitor module  270  is deactivated in accordance with one embodiment. Specifically, when the binary coding module  270  is deactivated, the resultant electrical modeling circuit  400  includes capacitors  222  and  228  coupled in series with gate and junction parasitic capacitance (C par )  402  and  404  and with ground  406 . Note that the gate and junction parasitic capacitance  402  is associated with the source of the transistor  220  (not shown) while the gate and junction parasitic capacitance  404  is associated with the drain of the transistor  220 . Since the capacitance of each of the capacitors  222  and  228  is substantially the same and that capacitance is so much greater than the capacitance of each of the parasitic capacitance  402  and  404 , the effective result is a total capacitance much smaller than the total capacitance of the modeling circuit  300  ( FIG. 3 ). 
   Within  FIG. 4 , the first terminals of the capacitors  222  and  228  can be coupled as described herein. The second terminal of the capacitor  222  is effectively coupled with a first terminal of a resistor  224  and a first terminal of the parasitic capacitance  402  of the transistor  220  (not shown). A second terminal of the parasitic capacitance  402  is effectively coupled with the ground  406  and a first terminal of the parasitic capacitance  404  of the transistor  220 . A second terminal of the parasitic capacitance  404  is effectively coupled with the second terminal of the capacitor  228  and a second terminal of the resistor  226 . A second terminal of the resistor  224  is coupled with the first terminal of the resistor  226 . 
   It is understood that each of the binary coding capacitor modules  272  and  274  along with each of the thermometer coding capacitor modules  276  through  278  can operate in a manner similar to that described herein with reference to the binary coding capacitor module  270 . 
   Within  FIG. 2 , by splitting the coarse tuning capacitors into a thermometer coded MSB bank (e.g., modules  276  through  278 ) and a binary coded LSB bank (e.g., modules  270 – 274 ), the LC resonant tank circuit  200  can achieve a desirable tradeoff between silicon area usage, design complexity, Q factor, and optimal tuning frequency coverage for the VCO  100 . Note that the Q factor is the quality factor of the LC resonant tank  200 . By implementing the thermometer coded modules (e.g.,  276  through  278 ) as the MSBs, each MSB thermometer module capacitor size can be individually adjusted (or trimmed) away from its nominal unit value in order to provide a desired shift in the VCO  100  frequency after taking into account layout parasitics. This can result in easier optimization of the VCO  100  tuning frequency range and coverage of the thermometer coded tuning while retaining the area and Q factor qualities of the binary weighted tuning. Additionally, it can enhance the ease of designing the actual VCO (e.g.,  100 ). 
   Specifically, using thermometer coded MSBs (e.g., modules  276  through  278 ) within the resonant tank  200  allows each thermometer coded unit element to be individually adjusted during the design phase to take into account systematic layout parasitics which cause non linearity in the capacitance vs. tuning code relationship. The optimization for systematic layout parasitics is easier to perform in the thermometer coded capacitors (e.g.,  252 ,  258 ,  262 , and  268 ) because each capacitor controls one capacitance step, unlike in the binary coded capacitors (e.g.,  222 ,  228 ,  232 ,  238 ,  242 , and  248 ) where the effective capacitance step size can be set by the systematic mismatch between the sum of all previous LSBs and the next LSB. In the binary coded capacitors, adjusting one capacitance step can also potentially change all other capacitance steps making optimization difficult. 
   Within  FIG. 2 , using the combination of binary and thermometer coded capacitors (e.g., modules  270 – 278 ) as part of the resonant tank  200  can allow maximum linearity of the capacitance step size vs. coarse tuning code. Within the resonant tank  200 , this means that the size of the continuously variable analog capacitance, also known as a varactor  269 , can be minimized. This minimizes the analog tuning sensitivity of the VCO  100 , which is beneficial for VCO  100  phase noise performance. 
   Furthermore, using the combination of the binary and thermometer coded capacitors (e.g., modules  270 – 278 ) as part of the resonant tank  200  can confer better overall Q while maintaining good linearity. The easier optimization that results from using both binary and thermometer coded capacitors (e.g., modules  270 – 278 ) can also reduce the number of tuning bits utilized for the resonant tank  200 . 
   The resonant tank circuit  200  of  FIG. 2  can include the varactor  269  and inductors  204  and  206 . The varactor  269  can be referred to as the continuously variable analog capacitance of the resonant tank  200 . Within the present embodiment, the varactor  269  can include resistors  208  and  217 , diodes  214  and  216 , along with a tuning ground  210  and a voltage tuning input  211 . A regulated voltage supply (V DD )  202  can be coupled in the middle of the inductors  204  and  206  (which have substantially the same inductance). However, the regulated voltage supply  202  can be coupled in other ways. It is understood that the functionality of the varactor  269  is well know by those of ordinary skill in the art. Furthermore, the varactor  269  can be implemented in a wide variety of ways. For example, the varactor  269  can be implemented to include the diodes  214  and  216  while not including the resistors  208  and  217 , the capacitors  212  and  218 , and the tuning ground  210 . Additionally, in this embodiment, the voltage tuning input  211  can be coupled between the input terminals of the diodes  214  and  216  while the output terminals of the diodes  214  and  216  can be coupled with the inductors  204  and  206 , respectively. 
   Within the resonant tank  200 , the resistors  224 ,  226 ,  234 ,  236 ,  244 ,  246 ,  254 ,  256 ,  264 , and  266  can be implemented in a wide variety of ways. For example, note that the resistance (Z) of each of the resistors  244  and  246  of the binary coding module  274  can be implemented substantially the same. As such, the resistance of each of the resistors  254  and  256  of the thermometer coding module  276  can be substantially equal to Z/2. It is appreciated that the resistance of each of the resistors (e.g.,  264  and  266 ) of the remaining thermometer coding modules (e.g.,  278 ) can be substantially equal to Z/2. However, the resistance of each of the resistors  234  and  236  of the binary coding module  272  can be substantially equal to 2 times Z. Furthermore, the resistance of each of the resistors  224  and  226  of the binary coding module  270  can be substantially equal to 4 times Z. It is understood that the resistors  224 ,  226 ,  234 ,  236 ,  244 ,  246 ,  254 ,  256 ,  264 , and  266  are not limited to the resistance values and relationships described herein. 
   It is noted that other voltage controlled oscillator topologies (or circuitry) can be utilized in combination with the binary coding circuitry (e.g.,  270 ,  272 , and/or  274 ) and the thermometer coding circuitry (e.g.,  276  and/or  278 ) of the LC resonant tank circuit  200  of  FIG. 2 . 
   The binary coding capacitor module  272  can include resistors  234  and  236 , capacitors  232  and  238 , along with a transistor  230 . Additionally, the binary coding capacitor module  274  can include resistors  244  and  246 , capacitors  242  and  248 , along with a transistor  240 . The thermometer coding capacitor module  276  can include resistors  254  and  256 , capacitors  252  and  258 , along with a transistor  250 . Furthermore, the thermometer coding capacitor module  278  can include resistors  264  and  266 , capacitors  262  and  268 , along with a transistor  260 . It is understood that the binary coding capacitor modules  270 – 274  and the thermometer coding capacitor modules  276  and  278  can each be implemented in a wide variety of ways and is not limited to those embodiments described herein. 
   Within  FIG. 2 , the regulated voltage supply  202  can be coupled with the inductors  204  and  206 . Specifically, a first terminal of the inductor  204  can be coupled with the regulated voltage supply  202  while a second terminal of the inductor  204  can be coupled with a first terminal of the capacitors  212 ,  222 ,  232 ,  242 , 252 , and  262 . Additionally, a first terminal of the inductor  206  can be coupled with the regulated voltage supply  202  while a second terminal of the inductor  206  is coupled with a first terminal of the capacitors  218 ,  228 ,  238 ,  248 ,  258 , and  268 . A second terminal of the capacitor  212  can be coupled with a first terminal of the resistor  208  and an output terminal of the diode  214 . A second terminal of the resistor  208  can be coupled with a voltage tuning input  211  and a first terminal of the resistor  217  while the second terminal of the resistor  217  can be coupled to an output terminal of the diode  216  and a second terminal of the capacitor  218 . An input terminal of the diode  214  can be coupled with the tuning ground  210  and can be also coupled with an input terminal of the diode  216 . 
   A second terminal of the capacitor  222  can be coupled with a first terminal of the resistor  224  and the source of the transistor  220 . A second terminal of the resistor  224  can be coupled with a first terminal of the resistor  226  and can be coupled with the digital tuner  104  in order to receive the Bit 0 -bar signal. A second terminal of the resistor  226  can be coupled with the drain of the transistor  220  and a second terminal of the capacitor  228 . The gate of the transistor  220  can be coupled with the digital tuner  104  in order to receive the Bit 0  signal. 
   Within  FIG. 2 , a second terminal of the capacitor  232  can be coupled with a first terminal of the resistor  234  and the source of the transistor  230 . A second terminal of the resistor  234  can be coupled with a first terminal of the resistor  236  and can be coupled with the digital tuner  104  in order to receive the Bit 1 -bar signal. A second terminal of the resistor  236  can be coupled with the drain of the transistor  230  and a second terminal of the capacitor  238 . The gate of the transistor  230  can be coupled with the digital tuner  104  in order to receive the Bit 1  signal. 
   A second terminal of the capacitor  242  can be coupled with a first terminal of the resistor  244  and the source of the transistor  240 . A second terminal of the resistor  244  can be coupled with a first terminal of the resistor  246  and can be coupled with the digital tuner  104  in order to receive the Bit 2 -bar signal. A second terminal of the resistor  246  can be coupled with the drain of the transistor  240  and a second terminal of the capacitor  248 . The gate of the transistor  240  can be coupled with the digital tuner  104  in order to receive the Bit 2  signal. 
   Within  FIG. 2 , a second terminal of the capacitor  252  can be coupled with a first terminal of the resistor  254  and the source of the transistor  250 . A second terminal of the resistor  254  can be coupled with a first terminal of the resistor  256  and can be coupled with the digital tuner  104  in order to receive the TBit 0 -bar signal. A second terminal of the resistor  256  can be coupled with the drain of the transistor  250  and a second terminal of the capacitor  258 . The gate of the transistor  250  can be coupled with the digital tuner  104  in order to receive the TBit 0  signal. 
   A second terminal of the capacitor  262  can be coupled with a first terminal of the resistor  264  and the source of the transistor  260 . A second terminal of the resistor  264  can be coupled with a first terminal of the resistor  266  and can be coupled with the digital tuner  104  in order to receive the TBit 6 -bar signal. A second terminal of the resistor  266  can be coupled with the drain of the transistor  260  and a second terminal of the capacitor  268 . The gate of the transistor  260  can be coupled with the digital tuner  104  in order to receive the TBit 6  signal. 
   Within  FIG. 2 , note that each of the transistors  220 ,  230 ,  240 ,  250  and  260  can be implemented in a wide variety of ways. For example, each of the transistors  220 ,  230 ,  240 ,  250  and  260  can be implemented as, but is not limited to, a P-channel MOSFET (metal-oxide semiconductor field-effect transistor) which is also known as a PMOS or PFET. Furthermore, each of the transistors  220 ,  230 ,  240 ,  250  and  260  can be implemented as, but is not limited to, a N-channel MOSFET which is also known as a NMOS or NFET. It is appreciated that each of the transistors  220 ,  230 ,  240 ,  250  and  260  can be implemented as, but is not limited to, a PMOS, a NMOS, or any other type of transistor. Noted that each of the transistors  220 ,  230 ,  240 ,  250  and  260  can be referred to as a switching element. It is understood that a gate, a drain, and a source of a transistor can each be referred to as a terminal of its transistor. Additionally, the gate of a transistor can also be referred to as a control terminal of its transistor. 
   It is appreciated that the resonant tank circuit  200  may not include all of the elements illustrated by  FIG. 2 . Furthermore, the resonant tank circuit  200  can be implemented to include other elements not shown by  FIG. 2 . Moreover, the resonant tank circuit  200  may include fewer or greater number of binary coding capacitor unit cells (e.g.,  270 ,  272 , and  274 ) than those illustrated by  FIG. 2 . Additionally, the resonant tank circuit  200  may include fewer or greater number of thermometer coding capacitor unit cells (e.g.,  276 – 278 ) than those illustrated by  FIG. 2 . 
     FIG. 5  is a flowchart of a method  500  for controlling a frequency of a voltage controlled oscillator in accordance with embodiments. Although specific operations are disclosed in method  500 , such operations are exemplary. That is, method  500  may not include all of the operations illustrated by  FIG. 5 . Alternatively, method  500  may include various other operations and/or variations of the operations shown by  FIG. 5 . Likewise, the sequence of the operations of method  500  can be modified. It is noted that the operations of method  500  can each be performed by software, by firmware, by electronic hardware, or by any combination thereof. 
   Specifically, the method  500  can include receiving a digital word that includes binary code and thermometer code. Additionally, the method  500  can include controlling a capacitance of a voltage controlled oscillator, in response to the digital word. Furthermore, the method  500  can include controlling a frequency of the voltage controlled oscillator in response to the capacitance of the voltage controlled oscillator. 
   At operation  502  of  FIG. 5 , the present embodiment can involve receiving a digital word that includes binary code and thermometer code. In one embodiment, the binary code of the digital word can address a plurality of capacitor tuning elements that increment in capacitive size by an approximate factor of 2. In another embodiment, the plurality of capacitor tuning elements includes a plurality of binary coding capacitor modules (e.g.,  270 ,  272 , and  274 ). In one embodiment, the thermometer code of the digital word can address a plurality of capacitor tuning elements that are each of substantially equal size. In another embodiment, the plurality of capacitor tuning elements includes a plurality of thermometer coding capacitor modules (e.g.,  276  and  278 ). It is appreciated that the digital word at operation  502  can be implemented in a wide variety of ways. For example, the digital word can be implemented to include, but is not limited to, a combination of Bit 0 , Bit 1 , Bit 2 , TBit 0 , TBit 1 , TBit 2 , TBit 3 , TBit 4 , TBit 5 , and TBit 6 , as described herein. Note that the receiving of a digital word that includes binary code and thermometer code at operation  502  can be implemented in any manner similar to that described herein, but is not limited to such. 
   At operation  504 , the present embodiment can involve controlling a capacitance of a voltage controlled oscillator (e.g.,  100 ), in response to the digital word. In one embodiment, the controlling of the capacitance of the voltage controlled oscillator, in response to the digital word, can include activating or deactivating a capacitor tuning element (e.g., module  270  or  276 ) of a plurality of capacitor tuning elements (e.g., modules  270 – 276  and  278 ). It is appreciated that the plurality of capacitor tuning elements of operation  504  can be implemented in any manner similar to that described with reference to operation  502 , but is not limited to such. Note that the controlling of a capacitance of a voltage controlled oscillator at operation  504 , in response to the digital word can be implemented in any manner similar to that described herein, but is not limited to such. 
   At operation  506  of  FIG. 5 , the present embodiment can involve controlling a frequency of the voltage controlled oscillator (e.g.,  100 ) in response to the capacitance. It is appreciated that operation  506  can be implemented in a wide variety of ways. For example, the controlling of a frequency of the voltage controlled oscillator at operation  506  in response to the capacitance can be implemented in any manner similar to that described herein, but is not limited to such. 
   Therefore, a LC resonant tank of a VCO in accordance with embodiments can confer a desirable tradeoff between silicon area consumption, design complexity, Q factor, and optimal tuning frequency coverage of the VCO. For example, by implementing the LC resonant tank with thermometer coded modules as its MSBs, each thermometer module capacitor size can be individually adjusted (or trimmed) away from its nominal unit value in order to provide a desired shift in the VCO frequency after taking into account layout parasitics. As such, this can result in easier optimization of the VCO tuning frequency range and coverage of the thermometer coded tuning while retaining the area and Q factor qualities of binary weighted tuning. Additionally, it can enhance the ease of designing the actual VCO. 
   The foregoing descriptions of specific embodiments in accordance with the invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The invention can be construed according to the Claims and their equivalents.