Abstract:
There is provided an apparatus for the linearization of a digitally controlled oscillator. The apparatus includes a first filter outputting only a low frequency band signal of an input signal to the digitally controlled oscillator; a negative feedback loop causing the signal of an input port of the digitally controlled oscillator to pass through a frequency table and a frequency-to-digital code mapper in sequence and correcting an input of the digitally controlled oscillator by performing negative feedback to an input port of the first filter; and a frequency table generator storing a frequency value of an output signal of the digitally controlled oscillator in the frequency table.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priorities of Korean Patent Application Nos. 10-2008-0122269 filed on Dec. 4, 2008 and 10-2009-0055583 filed on Jun. 22, 2009, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an apparatus for the linearization of a digitally controlled oscillator (DCO), and more particularly, to an apparatus for the linearization of a DCO enabling the achievement of linearity within the entirety of the DCO by correcting an input directly applied to the DCO with the addition of a negative feedback loop, including a frequency table and a frequency-to-digital code mapper, to an input port of the DCO. 
         [0004]    2. Description of the Related Art 
         [0005]    Currently, mobile communication terminals need to support multiple modes and multiple bands, and the development period of products has also become shorter, and thus there is a need for a change in a traditional design methodology for a radio frequency (RF) front-end. As the minimum linewidth of a complementary metal-oxide semiconductor (CMOS) process is reduced, the implementation of RF blocks is undergoing a process of change from an existing analog method to a digital method in order that RF signals can be processed in the form of digital or discrete signals. 
         [0006]      FIG. 1  is a block diagram illustrating a general digitally controlled oscillator (DCO). 
         [0007]    Referring to  FIG. 1 , a DCO  100  includes a digital-to-analog converter (DAC)  101  and an analog voltage controlled oscillator (VCO)  102 . 
         [0008]    The DAC  101  converts a digital input value of an oscillator control word into an analog voltage form and outputs the converted voltage to the analog VCO  102 . The analog VCO  102  receives the output voltage of the DAC  101  and outputs a continuous signal having an oscillation frequency corresponding to that voltage. 
         [0009]    A DCO, which is a core block of a digital frequency synthesizer, receives an input of an oscillator control word and outputs a continuous signal having an output frequency corresponding to that oscillator control word. However, due to external environments such as process variation, voltage, and temperature, it is impossible to output an exact frequency corresponding to the oscillator control word without a tuning process. 
         [0010]    A code gain of the DCO is defined as a variation in the output frequency of the DCO with respect to a variation in a Least Significant Bit (LSB) of the oscillator control word. In general, the code gain of the DCO is not consistent in an entirety of an oscillator control word area. When the code gain is not consistent, the characteristics of the digital frequency synthesizer may be variable according to each oscillator control word, so variations in performance may be undesirably large. 
         [0011]    Also, when the digital frequency synthesizer is generally designed, other blocks are designed with an assumption that the code gain is consistent, so it may be desirable to make the code gain consistent. Particularly, when a transmitter is designed by using a direct modulation method, the code gain of the DCO is required to be consistent. This is because when the code gain is not consistent, distortion of transmitted data may occur. 
       SUMMARY OF THE INVENTION 
       [0012]    An aspect of the present invention provides an apparatus for the linearization of a digitally controlled oscillator (DCO) enabling the achievement of linearity within the entirety of the DCO by correcting an input directly applied to the DCO with the addition of a negative feedback loop, including a digital loop filter, a frequency table and a frequency-to-digital code mapper, to an input port of a DCO having non-linearity. 
         [0013]    According to an aspect of the present invention, there is provided an apparatus for linearization of a digitally controlled oscillator, the apparatus including: a first filter outputting only a low frequency band signal of an input signal to the digitally controlled oscillator; a negative feedback loop causing the signal of an input port of the digitally controlled oscillator to pass through a frequency table and a frequency-to-digital code mapper in sequence and correcting an input of the digitally controlled oscillator by performing negative feedback to an input port of the first filter; and a frequency table generator storing a frequency value of an output signal of the digitally controlled oscillator in the frequency table. 
         [0014]    The negative feedback loop may include the frequency table storing a frequency value of an output signal of the digitally controlled oscillator corresponding to each digital code input by using digital codes as address values, and the frequency-to-digital code mapper outputting a digital code corresponding to an output frequency value of the frequency table by using the output frequency value of the frequency table as an input. 
         [0015]    The address values of the frequency table may be a 2 m  number of digital codes corresponding to decimal numbers from zero to 2 M −1, respectively. 
         [0016]    The frequency-to-digital code mapper may determine and output a digital code corresponding to a frequency value according to a preset standard. 
         [0017]    The frequency table generator may include an M-bit counter generating a 2 M  number of digital codes in sequence from zero and providing the generated codes to the digitally controlled oscillator and the frequency table as an input and an address value with which to store a frequency, respectively; a frequency meter measuring frequency values of output signals of the digitally controlled oscillator and outputting the measured frequency values to a second filter; and the second filter receiving an output of the frequency meter, averaging the received frequency values, and outputting the averaged frequency values to the frequency table. The frequency table may store a frequency value corresponding to each address value provided by the M-bit counter. 
         [0018]    The second filter may be a digital low pass filter allowing only a low frequency band signal to pass therethrough. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0020]      FIG. 1  illustrates a configuration of an existing digitally controlled oscillator (DCO); 
           [0021]      FIG. 2  illustrates an apparatus for the linearization of a DCO using a negative feedback loop according to an exemplary embodiment of the present invention; 
           [0022]      FIG. 3  illustrates a configuration of an apparatus for storing the frequency values of output signals of a DCO in a frequency table of the DCO; 
           [0023]      FIG. 4  illustrates a configuration of an apparatus for storing errors, which the frequency values of output signals of a DCO have, in a frequency table of the DCO; 
           [0024]      FIG. 5  illustrates a configuration of an apparatus for the linearization of a DCO, including an apparatus for storing the frequency values of output signals of the DCO in a frequency table of the DCO; and 
           [0025]      FIG. 6  illustrates a configuration of an apparatus for the linearization of a DCO, including an apparatus for storing errors, which the frequency values of output signals of the DCO have, in a frequency table of the DCO. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, detailed descriptions related to well-known functions or configurations will be ruled out in order not to unnecessarily obscure subject matters of the present invention. Throughout the drawings, the same reference numerals will be used to designate the same or like parts. 
         [0027]    In addition, throughout the specification, it will be understood that when an element is referred to as being “connected to” another element, it can be both directly connected to the other element or intervening elements may be present. Also, unless explicitly described to the contrary, the word “include” and variations such as “includes” or “including,” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. 
         [0028]      FIG. 2  illustrates an apparatus for the linearization of a digitally controlled oscillator (DCO) according to an exemplary embodiment of the present invention. 
         [0029]    Referring to  FIG. 2 , the apparatus for the linearization of the DCO according to this embodiment includes a subtracter  230 , a digital loop filter  200 , a frequency table  220 , a frequency-to-digital code mapper  210 , and a DCO  100 . 
         [0030]    The subtracter  230 , the frequency table  220 , and the frequency-to-digital code mapper  210  form a negative feedback loop. 
         [0031]    When a loop gain of a negative feedback circuit is considerably larger than one and the negative feedback loop is stable, a frequency control word and an output digital code value of the frequency-to-digital code mapper  210  become equal due to negative feedback mechanism. 
         [0032]    The frequency-to-digital code mapper  210  implements mapping in a manner that converts a frequency value into a corresponding digital code according to a preset standard, so it has linearity. Therefore, an input frequency value of the frequency-to-digital code mapper  210 , i.e., an output frequency value of the frequency table  220  may become an exact frequency value corresponding to the frequency control word. 
         [0033]    Meanwhile, since the frequency table  220  stores a frequency value of an output signal of the DCO  100  corresponding to each input, it has the same input/output transfer function as the DCO  100 . Accordingly, the frequency value of the output signal of the DCO  100  is equal to the output frequency value of the frequency table  220 . Also, as described above, since the output frequency value of the frequency table  220  is the exact frequency value corresponding to the frequency control word, the frequency value of the output signal of the DCO  100  becomes the exact frequency value corresponding to the frequency control word. 
         [0034]    That is, when a frequency control word is applied as an input to the apparatus according to this embodiment, an oscillator control word, applied as a direct input to the DCO  100 , is corrected in order that the frequency value of an output signal of the DCO  100  becomes equal to the exact frequency value corresponding to the frequency control word due to the negative feedback mechanism. 
         [0035]      FIG. 3  illustrates a frequency table generator storing the frequency values of output signals of the DCO  100  in the frequency table  220 . 
         [0036]    Referring to  FIG. 3 , an apparatus for storing the frequency values of output signals of the DCO  100  in the frequency table  220  includes an M-bit counter  300 , a frequency meter  320 , a digital low pass filter  330 , and the frequency table  220 . 
         [0037]    The M-bit counter  300  generates a 2 M  number of digital codes in the range of zero to 2 m −1 and provides them as an input to the DCO  100  and as an address value to the frequency table  220 . 
         [0038]    The DCO  100  receives an output of the M-bit counter  300  and outputs a signal having a specific frequency. At this time, due to non-linearity of the DCO  100 , the frequency value of the output signal has a slight error, as compared to an exact frequency value which is applied as an input. 
         [0039]    The frequency meter  320  measures the frequency values of output signals of the DCO  100 , and the digital low pass filter  330  averages the measured frequency values. And the frequency table  220  stores the averaged frequency values according to address. 
         [0040]    Therefore, when a cycle of the M-bit counter  300  is completed, the frequency values of output signals of the DCO  100  corresponding to all possible input are stored as corresponding address values of the frequency table  220 , respectively. 
         [0041]      FIG. 4  illustrates a frequency table generator storing only errors, which the frequency values of output signals of the DCO  100  have, in the frequency table  220 . 
         [0042]    As compared to the frequency table generator of  FIG. 3 , the frequency table generator of  FIG. 4  further includes a digital code-to-frequency mapper  400  outputting frequency values corresponding to output digital codes of the M-bit counter  300 , and a subtracter  410  outputting a difference between an output of the digital low pass filter  330  and an output of the digital code-to-frequency mapper  400 . 
         [0043]    The digital code-to-frequency mapper  400  simply converts a digital code into a frequency, so it has linearity. Accordingly, its output is an exact frequency value corresponding to an input digital code. 
         [0044]    The subtracter  410  outputs a value subtracting the output of the digital code-to-frequency mapper  400  from the output of the digital low pass filter  330 . Therefore, the output of the subtracter  410  becomes an error, which the frequency value of an output signal of the DCO  100  has, and this error is stored as a corresponding address value of the frequency table  220 . 
         [0045]    As described above, by storing only the error rather than the overall frequency value, the disadvantage of the digital hardware growth may be overcome. 
         [0046]      FIG. 5  illustrates an apparatus for the linearization of a DCO including a frequency table generator. 
         [0047]    Referring to  FIG. 5 , a switch  500  connects the M-bit counter  300  or the digital loop filter  200  to the DCO  100  according to operation. 
         [0048]    When the switch  500  is connected to the M-bit counter  300 , the frequency values of output signals of the DCO  100  are stored in the frequency table  220 . 
         [0049]    Here, the detailed process of storing an output of the DCO  100  corresponding to each input in the frequency table  220  is the same as described above with reference to  FIG. 3 . 
         [0050]    On the other hand, when the switch  500  is connected to the digital loop filter  200 , this apparatus acts as a linearized DCO due to the negative feedback mechanism. At this time, the frequency meter  320  and the digital low pass filter  330  become inactive. 
         [0051]    Here, the detailed process of correcting an input directly applied to the DCO  100  in order that the DCO  100  has linearity due to the negative feedback mechanism is the same as described above with reference to  FIG. 2 . 
         [0052]      FIG. 6  is an apparatus for the linearization of a DCO including an apparatus for storing errors, which the frequency values of output signals of the DCO have, in the frequency table  220 . 
         [0053]    When the switch  500  is connected to the M-bit counter  300 , the errors, which the frequency values of the output signals of the DCO have, are stored in the frequency table  220 . 
         [0054]    Here, the detailed process of storing the errors, which the frequency values of the output signals of the DCO have, in the frequency table  220  is the same as described above with reference to  FIG. 4 . 
         [0055]    On the other hand, when the switch  500  is connected to the digital loop filter  200 , this apparatus acts as a linearized DCO due to the negative feedback mechanism. At this time, the frequency meter  320  and the digital low pass filter  330  become inactive. 
         [0056]    Here, a value stored in the frequency table  220  is not the overall frequency value of an output signal corresponding to each input of the DCO  100 , but is rather an error relative to an exact frequency value corresponding to each input. Accordingly, as compared to the configuration of  FIG. 5 , further elements are required. 
         [0057]    Specifically, a digital code-to-frequency mapper  610  outputting an exact frequency value corresponding to each input of the DCO  100  and an adder  600  adding an output of the digital code-to-frequency mapper  610  and an output of the frequency table  220  are further included. 
         [0058]    Here, the exact frequency value corresponding to the input of the DCO  100  (the output of the digital code-to-frequency mapper  610 ) and the error which an actual output signal of the DCO  100  has (the output of the frequency table  220 ) are added by the adder  600 , so the frequency value of the actual output signal of the DCO  100  becomes an input of the frequency-to-digital code mapper  210 . 
         [0059]    Therefore, with a reduction in the use of the hardware capacity of the frequency table  220 , the linearization of the DCO may be enabled. 
         [0060]    Like this, the apparatus for the linearization of the DCO may be realized by correcting input signals directly applied to the DCO using the negative feedback loop. 
         [0061]    As set forth above, according to exemplary embodiments of the invention, the apparatus for the linearization of the DCO is able to cause a DCO having non-linearity to be linearized, so errors in an output of the digital frequency synthesizer may be minimized. 
         [0062]    Also, due to the achievement of linearity, the DCO may be applicable to a direct modulation method of a mobile communication transmitter. 
         [0063]    While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.