Abstract:
The present application discloses a voltage-controlled oscillator device and a method of correcting the voltage-controlled oscillator. The voltage-controlled oscillator device comprises predistortion module, configured to predistort an input voltage to obtain a predistorted voltage; and a voltage-controlled oscillator, configured to generate an output signal with a corresponding oscillation frequency according to the predistorted voltage, wherein the predistortion module corrects a non-linear characteristic of the voltage-controlled oscillator, so that there is a linear relationship between the input voltage and the oscillation frequency of the output signal. The voltage-controlled oscillator device may be applied to a phase-locked circuit in a communication system.

Description:
TECHNICAL FIELD 
       [0001]    The disclosure relates to a voltage-controlled oscillator (VCO) device and a method of correcting the VCO. In particular, the disclosure relates to a VCO device with a high linearity and a large linear range. 
       BACKGROUND 
       [0002]    A voltage-controlled oscillator (VCO) is one of important basic components of a phase-locked loop (PLL) circuit in a modern communication system. A performance of the modern communication system may greatly depend on the performance of the PLL circuit, and a performance of the PLL circuit may greatly depend on a performance of the VCO. Therefore, the performance of the modern communication system may depend on the performance of the VCO to a great extent. 
         [0003]    With continuous advances in the integrated circuit technique, feature sizes of devices and metal wirings are continuously reduced. Consequently, the devices have a limited linearity and a limited linear range. 
         [0004]      FIG. 1  schematically illustrates a block diagram of a conventional VCO  11 . In the conventional VCO  11 , an input voltage vin may be directly applied to an input of the VCO  11 , and an output signal with an oscillation frequency f may be output from an output of the VCO  11 . 
         [0000]        f=f 0 +k*vin   (1)
 
         [0005]    wherein, 
         [0006]    f is an oscillation frequency of the output signal when a voltage value of a control signal at a VCO input is vin; 
         [0007]    f 0  is the oscillation frequency of the output signal when the voltage value of the control signal at the VCO input is 0; 
         [0008]    vin is the voltage value of the control signal as the VCO input; and 
         [0009]    k is a control factor of the oscillation frequency of the output signal of the yea 
         [0010]      FIG. 2  illustrates an ideal curve showing an input voltage vs. an oscillation frequency of the conventional VCO  11  as shown in  FIG. 1 . The linear range of the conventional VCO  11  is −vin max ≦vin≦+vin max , wherein k is a constant only depending on electric characteristics of the VCO  11   
         [0011]    However, the linear range of the VCO  11  is only an approximation, which ignores a high-order effect of the VCO  11 , but not really an ideal linear range, Even in the linear range of −vin max ≦vin≦+vin max , k is not really a constant, as shown in a dashed line in  FIG. 3 . 
         [0012]    To improve the linearity and increase the linear range of the VCO, k should not be seen as a constant. 
       SUMMARY OF THE DISCLOSURE 
       [0013]    One object of the present disclosure is to provide a voltage-controlled oscillator device with a high linearity and a large linear range, and a method for correcting the voltage-controlled oscillator. 
         [0014]    According to one aspect of the present disclosure, a voltage-controlled oscillator device is provided. The voltage-controlled oscillator comprises: a predistortion module, configured to predistort an input voltage to obtain a predistorted voltage; and a voltage-controlled oscillator, configured to generate an output signal with a corresponding oscillation frequency according to the predistorted voltage, wherein the predistortion module corrects a non-linear characteristic of the voltage-controlled oscillator, so that there is a linear relationship between the input voltage and the oscillation frequency of the output signal. 
         [0015]    According to another aspect of the present disclosure, a method of correcting a voltage-controlled oscillator is provided. The method comprises: measuring a predistortion function of the voltage-controlled oscillator; storing the predistortion function in a predistortion module; performing predistortion processing on an input voltage by the predistortion module to obtain a predistorted voltage; providing the predistorted voltage to the voltage-controlled oscillator; and generating an output signal with a corresponding oscillation frequency by the voltage-controlled oscillator according to the predistorted voltage, wherein the predistortion module corrects a non-linear characteristic of the voltage-controlled oscillator, so that there is a linear relationship between the input voltage and the oscillation frequency of the output signal. 
         [0016]    The voltage-controlled oscillator device of the present disclosure corrects the non-linear characteristic of the voltage-controlled oscillator using the predistortion module, so that the linearity of the voltage-controlled oscillator may be improved, and/or the linear range of the voltage-controlled oscillator may be extended. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  schematically illustrates a block diagram of a conventional VCO; 
           [0018]      FIG. 2  illustrates an ideal curve showing an input voltage vs. an oscillation frequency of the conventional VCO as shown in  FIG. 1 ; 
           [0019]      FIG. 3  illustrates an actual curve showing an input voltage vs. an oscillation frequency of the conventional VCO as shown in  FIG. 1 ; 
           [0020]      FIG. 4  schematically illustrates a block diagram of a measurement circuit for obtaining a predistortion function of a VCO according to the present disclosure; 
           [0021]      FIG. 5  schematically illustrates a block diagram of a VCO device according to the present disclosure; 
           [0022]      FIG. 6  illustrates an actual curve showing an input voltage vs. an oscillation frequency of the VCO device as shown in  FIG. 5 ; 
           [0023]      FIG. 7  schematically illustrates a block diagram of a first embodiment of a predistortion module used in the VCO device as shown in  FIG. 5 ; and 
           [0024]      FIG. 8  schematically illustrates a block diagram of a second embodiment of a predistortion module used in the VCO device as shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The inventor recognizes that an input voltage of a VCO  11  may be predistorted to compensate variation of the control factor k and provide a linear output of the VCO  11 . 
         [0026]    In order to perform the predistortion processing, a predistortion function v predistortion (vin) of the VCO  11  needs to be firstly obtained. The predistortion function V predistortion (vin) represents a relationship between the input voltage vin and a predistorted voltage vin′. Although the control factor k may be varied, there is still a linear relationship between the input voltage vin and an oscillation frequency f of the VCO  11 . 
         [0027]    Hereinafter, the present disclosure will be described in detail with reference to the drawings. 
         [0028]      FIG. 4  schematically illustrates a block diagram of a measurement circuit  10  for obtaining the predistortion function of the VCO  11  according to the present disclosure. An input port of the VCO  11  may be connected to a controllable voltage source  12  for supplying a variable input voltage vin to the input port of the VCO  11 . The range of the input voltage vin is a designed linear range, and may be larger than a linear range, e. g. −vin′ max ≦vin≦+vin′ max , of characteristics of the VCO itself, wherein vin′ max &gt;vin max . An output port of the VCO  11  may be connected to a signal frequency measurement unit  13  for measuring an oscillation frequency f of an output signal of the VCO  11 . 
         [0029]    The VCO  11  may be an RC VCO, an LC VCO, a crystal VCO or an active VCO. 
         [0030]    The input voltage vin may be applied for a selected sampling point. The corresponding oscillation frequency f of the output signal may be calculated according to Equation (1). Then, an actual oscillation frequency of the output signal of the VCO  11  may be measured by the signal frequency measurement unit  13  while adjusting the input voltage of the controllable voltage source  12 . When the actual oscillation frequency of the output signal of the VCO  11  is the calculated oscillation frequency f of the output signal, the predistorted voltage vin′ may be determined from an actual output voltage of the controllable voltage source  12 . Therefore, the relationship between the input voltage vin and the predistorted voltage vin′ may be obtained for the selected sampling point. 
         [0031]    The above adjusting and measuring steps may be repeated for all of the selected sampling points within the range of −vin′ max ≦vin≦+vin′ max . Thus, the relationship between the input voltage vin and the predistorted voltage vin′ may be obtained for a plurality of sampling points in the designed operating range, which is taken as the predistortion function of the VCO  11 . 
         [0032]    Uniform sampling may be used. That is, the relationship between the input voltage vin and the predistorted voltage vin′ may be obtained at uniform voltage intervals. Alternatively, non-uniform sampling may also be used, That is, sampling points may be selected according to the linear characteristic of the VCO  11 . The smaller absolute value vin has, the lower density of sampling points is. When vin has an absolute value approaching vin′ max  , the density of sampling points is higher . . . 
         [0033]    The predistortion function v predistortion (v) may be implemented as a digital look-up table, in which vin and corresponding vin′ are stored. That is, a mapping relationship of vin-vin′ is built. 
         [0034]      FIG. 5  schematically illustrates a block diagram of a VCO device  20  according to the present disclosure. Compared to the conventional VCO  11 , the VCO device  20  of the present disclosure adds a predistortion module  14  between the input voltage vin and the VCO  11 . The predistortion module  14  performs voltage conversion by predistorting the input voltage vin according to the predistortion function v predistortion (v) as discussed above, so as to generate and provide the predistorted voltage vin′ to the VCO  11 . 
         [0035]    When the predistortion module  14  performs the predistortion processing, it may be firstly determined whether the input voltage vin is out of the designed linear range, i. e. −vin′ max ≦vin≦+vin′ max . If so, the predistortion module  14  does not perform the predistortion processing on the input voltage vin. 
         [0036]      FIG. 6  illustrates an actual curve (in a solid line) showing the input voltage vin (i. e. the input voltage) vs. the oscillation frequency f of the VCO device  20  as shown in  FIG. 5 . Also,  FIG. 6  illustrates an actual curve (in a dashed line) showing the actual input voltage vin′ (i. e. the predistorted voltage) vs. the oscillation frequency f of the VCO  11 . 
         [0037]    As shown by the dashed line in  FIG. 6 , if the actual input voltage of the VCO  11  is out of the linear range of the VCO  11 , k is not a constant. The linear range of the conventional VCO  11  is only within −vin′ max ≦vin≦+vin′ max . Moreover, this linear range is still affected by the high order effect of the VCO  11 . Thus, an ideal linear characteristic cannot be achieved. 
         [0038]    However, as indicated by a horizontal dot line in  FIG. 6 , the predistortion module  14  generates the predistorted voltage vin′ at the input voltage vin, according to the predistortion function. Since the actual input voltage of the VCO  11  is the predistorted voltage vin′, the oscillation frequency f of the output signal from the VCO  11  may be f=f 0 +k 0 *vin. That is, there is still a linear relationship between the input voltage vin and the oscillation frequency f of the output signal. 
         [0039]    Since the predistortion module  14  compensates the non-linear characteristic of the VCO  11 , there is a linear relationship between the input voltage vin and the oscillation frequency f of the output signal of the VCO device  20  within the range of −vin′ max ≦vin≦+vin′ max . The linear range of the VCO device  20  is larger than that of the VCO  11 , and also the linearity of the VCO device  20  is higher than that of the VCO  11 . 
         [0040]    In the VCO device  20  according to the present disclosure, the predistortion module  14  may be established according to the vin-vin′ mapping relationship. 
         [0041]      FIG. 7  schematically illustrates a block diagram of a first embodiment of the predistortion module  14  used in the VCO device  20  as shown in  FIG. 5 . Although the input voltage vin of the predistortion module  14  and the predistorted voltage vin′ (an output voltage) of the predistortion module  14  will be described below, it should be understood that the predistorted voltage vin′ is the actual input voltage of the VCO  11  in the VCO device  20 . 
         [0042]    As shown in  FIG. 7 , the predistortion module  14  may include an analog-to-digital convertor (ADC)  1401 , an address decoding circuit (DEC)  1402 , a memory (MEM)  1403  and a digital-to-analog convertor (DAC)  1404 , which are connected in series. An input voltage vin is applied to an input port of the ADC  1401 . The ADC  1401  may convert the input voltage vin into a digital signal, and transmit it to the DEC  1402 . The DEC  1402  may generate an address of the MEM  1403  for read operation from the digitalized input voltage vin, perform read operation on the MEM  1403 , and obtains a digital value of a predistorted voltage vin′ corresponding to the input voltage The DAC  1404  may convert the digital value of the predistorted voltage vin′ into an analog value, and provide it to the VCO  11  as the predistorted voltage vin′. 
         [0043]    In the predistortion module  14 , the predistortion function may be stored in the MEM  1403 , wherein the input voltage vin is the address of the MEM  1403  for read operation, the predistorted voltage vin′ is stored content in the MEM  1403 . 
         [0044]      FIG. 8  schematically illustrates a block diagram of a second embodiment of the predistortion module  14  used in the VCO device as shown in  FIG. 5 . The predistortion module  14  may include an ADC  1401 , a microcontroller unit (MCU)  1405 , and a DAC  1404 , which are connected in series. An input voltage vin is applied to an input port of the ADC  1401 . The ADC  1401  may convert the input voltage vin into a digital signal, and send it to the MCU  1405 . The MCU  1405  may search a look-up table in the cache for the digitalized input voltage vin, and obtains the digital value of the predistorted voltage vin′ corresponding to the input voltage vin. The DAC  1404  may convert the digital value of the predistorted voltage vin′ into an analog value, and provide it to the VCO  11  as the predistorted voltage vin′. 
         [0045]    In the predistortion module  14 , the predistortion function may be the look-up table in the cache of the MCU  1405 , wherein the input voltage vin and the predistorted voltage vin′ are two columns of the look-up table. 
         [0046]    In a preferred embodiment, if the stored content of the predistortion function does not include the input voltage vin, the predistortion module  14  may search the stored content of the predistortion function for a closest input voltage to obtain an output voltage as the predistorted voltage vin′ corresponding to this input voltage vin. 
         [0047]    In a preferred embodiment, the predistortion module  14  may also include an interpolation circuit. If the stored content of the predistortion function does not include the input voltage vin, and the input voltage vin satisfies −vin′ max ≦vin≦+vin′ max , the predistortion module  14  may search the stored content of the predistortion function for lower and higher adjacent input voltages vin L  and vin H  to obtain the corresponding predistorted voltages vin L ′ and vin H ′, and calculate the predistorted voltage vin′ corresponding to the input voltage vin by interpolation in the interpolation circuit according to the following equation: 
         [0000]        vin′=vin′   L +( vin′   H   −vin′   L )/( vin   H   −vin   L )*( vin−vin   L ) 
         [0048]    The present disclosure has been described above with reference to the preferred embodiments thereof. It should be understood that various modifications, alternations and additions can be made by those skilled in the art without departing from the spirits and scope of the present disclosure. Therefore, the scope of the present disclosure is not limited to the above particular embodiments but only defined by the claims as attached.