Abstract:
A frequency calibration method for calibrating an output frequency of a voltage-controlled oscillator is provided. The voltage-controlled oscillator includes a first capacitor bank, a second capacitor bank, and a third capacitor bank. The first capacitor bank and the third capacitor bank are initially disabled and the second capacitor bank is initially enabled. The method includes, when the initial output frequency is lower than a reference frequency, adjusting the capacitance of the second capacitor bank until the calibrated output frequency is greater than the reference frequency, and when the initial output frequency is greater than the reference frequency, enabling the first capacitor bank and gradually increasing the capacitance of the first capacitor bank until the calibrated output frequency is lower than the reference frequency.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a voltage-controlled oscillator, and more particularly to a frequency calibration method for the voltage-controlled oscillator. 
     Description of the Related Art 
     Electronic circuits often use clock signals to regulate and control their operation. Events in the electronic circuits are timed by rising and/or falling edges of the clock signals. Clock signals may be generated by a number of oscillating circuits, such as an LC voltage-controlled oscillator (VCO). In this type of oscillating circuit, an electrical charge is alternately accumulated and discharged to form the basis of the clock signal. The charge accumulates in inductors and capacitors of an LC tank circuit in the VCO, and the time needed for the charge to accumulate and discharge is based on the inductance and capacitance values. The frequency of the clock signal in one exemplary VCO may thus be controlled by varying the capacitance of the LC tank circuit in the VCO. For example, the VCO may include a voltage-controlled variable capacitor so that the voltage of a control signal may be used to control the output frequency. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of a frequency calibration method for calibrating the output frequency of a voltage-controlled oscillator is provided. The voltage-controlled oscillator comprises a first capacitor bank, a second capacitor bank, and a third capacitor bank. The capacitance of one capacitor unit in the second capacitor bank is lower than the capacitance of one capacitor unit in the first capacitor bank. The capacitance of one capacitor unit in the second capacitor bank is greater than the capacitance of one capacitor unit in the third capacitor bank. The method comprises turning off the first capacitor bank and the third capacitor bank; turning on the second capacitor bank; and adjusting the output frequency of the voltage-controlled oscillator by controlling the number of enabled unit capacitors in the second capacitor bank. When the output frequency is lower than a reference frequency, the method comprises disabling one capacitor unit in the second capacitor bank and adjusting the output frequency by controlling the number of enabled unit capacitors in the second capacitor bank. When the output frequency is greater than the reference frequency, the method comprises enabling at least one capacitor unit in the first capacitor bank until the output frequency is lower than the reference frequency. 
     An embodiment of a frequency calibration method for calibrating an output frequency of a voltage-controlled oscillator is provided. The voltage-controlled oscillator comprises a first capacitor bank, a second capacitor bank, and a third capacitor bank. The capacitance of one capacitor unit in the second capacitor bank is lower than the capacitance of one capacitor unit in the first capacitor bank. The capacitance of one capacitor unit in the second capacitor bank is greater than the capacitance of one capacitor unit in the third capacitor bank. The method comprises setting the capacitance of the first capacitor bank to be the minimum capacitance of the first capacitor bank; setting the capacitance of the third capacitor bank to be the minimum capacitance of the third capacitor bank; setting the capacitance of the second capacitor bank to be the maximum capacitance of the second capacitor bank; and determining whether the output frequency is greater than a reference frequency. When the output frequency is lower than the reference frequency, the method includes disabling one capacitor unit in the second capacitor bank and adjusting the output frequency by gradually increasing the capacitance of the second capacitor bank. When the output frequency is greater than the reference frequency, the method includes gradually increasing the capacitance of the first capacitor bank until the output frequency is lower than the reference frequency. 
     A frequency calibration method for calibrating an output frequency of a voltage-controlled oscillator is provided. The voltage-controlled oscillator comprises a first capacitor bank, a second capacitor bank, and a third capacitor bank. The capacitance of one capacitor unit in the second capacitor bank is lower than the capacitance of one capacitor unit in the first capacitor bank. The capacitance of one capacitor unit in the second capacitor bank is greater than the capacitance of one capacitor unit in the third capacitor bank. The method comprises disabling the first capacitor bank and the third capacitor bank; enabling the second capacitor bank; and determining whether an initial output frequency is greater than a reference frequency. When the initial output frequency is lower than the reference frequency, the method entails adjusting the capacitance of the second capacitor bank until the calibrated output frequency is greater than the reference frequency. When the initial output frequency is greater than the reference frequency, enabling the first capacitor bank and gradually increasing the capacitance of the first capacitor bank until the calibrated output frequency is lower than the reference frequency. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of a voltage-controlled oscillator according to an embodiment of the invention. 
         FIG. 2  is a schematic diagram of a capacitor bank. 
         FIG. 3  is a frequency calibration method according to an embodiment of the invention. 
         FIG. 4  is a frequency calibration method according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 1  is a schematic diagram of a voltage-controlled oscillator according to an embodiment of the invention. The voltage-controlled oscillator comprises an inductor  11  (labeled L), a first capacitor bank  12  (labeled CB0), a second capacitor bank  13  (labeled CB1), a third capacitor bank  14  (labeled CB2), a VCO core  15 , and a calibration circuit  16 . The VCO core  15  generates an output frequency f out  according to the inductance of the inductor  11  and the capacitance of the first capacitor bank  12 , the second capacitor bank  13  and the third capacitor bank  14 . The first capacitor bank  12 , the second capacitor bank  13 , and the third capacitor bank  14  comprise a plurality of unit capacitors and are controlled by the calibration circuit  16 . In the embodiment, the capacitance of a unit capacitor in each capacitor bank is different. The capacitance of each unit capacitor in the first capacitor bank  12  is greater than the capacitance of each unit capacitor in the second capacitor bank  13 , and the capacitance of each unit capacitor in the second capacitor bank  13  is greater than the capacitance of each unit capacitor in the third capacitor bank  14 . In other words, assuming that the capacitance of a unit capacitor in the first capacitor bank  12  is C u1 , the capacitance of a unit capacitor in the second capacitor bank  13  is C u2 , and the capacitance of a unit capacitor in the third capacitor bank  14  is C u3 , the relationship between C u1 , C u2  and C u3  is as follows:
 
C u1 &gt;C u2 &gt;C u3  
 
     The calibration circuit  16  receives and compares the output frequency f out  and a reference frequency f ref  to generate a control signal S c  to control the first capacitor bank  12 , the second capacitor bank  13  and the third capacitor bank  14 . In one embodiment, the control signal S c  comprises a first control signal to adjust the capacitance of the first capacitor bank  12 , a second control signal to adjust the capacitance of the second capacitor bank  13 , and a third control signal to adjust the capacitance of the third capacitor bank  14 . If the voltage-controlled oscillator is a full digital voltage-controlled oscillator, the control signal S c  is replaced by a binary tuning word to control the capacitances of the first capacitor bank  12 , the second capacitor bank  13  and the third capacitor bank  14 . 
       FIG. 2  is a schematic diagram of a capacitor bank. The equivalent capacitance of the capacitor bank is determined based on the capacitors C 1 ˜C 3 . Assuming that only switch SW 1  is on, the equivalent capacitance of the capacitor bank is C 1 . Assuming that switches SW 1  and SW 2  are on, the equivalent capacitance of the capacitor bank is (C 1 +C 2 ). Therefore, assuming that switches SW 1 ˜SW 3  are digitally controlled, a controller may adjust the equivalent capacitance of the capacitor bank to (C 1 +C 2 ) by sending a binary tuning word [011] to the capacitor bank, for example. The first bit of the binary tuning word means the switch SW 1  is turned on, the second bit of the binary tuning word means the switch SW 2  is turned on, and the last bit of the binary tuning word means the switch SW 3  is turned off. 
     In another embodiment, the binary tuning word can be expressed by a decimal form. For example, the decimal tuning word “0” means that no switch is turned on, the decimal tuning word “1” means that switch SW 1  is turned on, and the decimal tuning word “3” means that switches SW 1  and SW 2  are turned on. When a switch is turned on, its corresponding capacitor works to change the equivalent capacitance of the capacitor bank. 
       FIG. 3  is a frequency calibration method according to an embodiment of the invention. The method in  FIG. 3  is illustrated with the voltage-controlled oscillator shown in  FIG. 1 . In this embodiment, only three capacitor banks are illustrated, but the invention is not limited thereto. When the frequency calibration starts, the first capacitor bank  12  and the third capacitor bank  14  are turned off and only the second capacitor bank  13  is turned on in step S 31 . This means that the capacitance for the oscillator is determined only by the second capacitor bank  13 . Assuming that the capacitance of the unit capacitor in the second capacitor bank  13  is C 2  and the second capacitor bank  13  comprises N number of parallel connected capacitors, the capacitance of the second capacitor bank  13  is N*C 2  when all the unit capacitors are enabled. If only (N−3) unit capacitors are enabled, the capacitance of the second capacitor bank  13  is (N−3)*C 2 . In other words, the capacitance of the second capacitor bank  13  is determined according to the number of enabled unit capacitors in the second capacitor bank  13 . The first capacitor bank  12  and the third capacitor bank  14  operate in the same manner. 
     In step S 32 , the calibration circuit  16  determines whether the output frequency is greater than the reference frequency. If the frequency is greater than the reference frequency, step S 33  is executed. In step S 33 , the calibration circuit enables one unit capacitor in the first capacitor bank  12 . If the frequency is not greater than the reference frequency, step S 35  is executed. 
     In the default setting, all the unit capacitors in the first capacitor bank  12  are disabled. This means that the equivalent capacitance of the first capacitor bank  12  is zero. Assuming that the capacitance of the unit capacitor in the first capacitor bank  12  is C 1 , step S 33  means that the equivalent capacitance of the first capacitor bank  12  becomes C 1 . In step S 34 . The calibration circuit  16  determines whether the output frequency is lower than the reference frequency. If the frequency is not lower than the reference frequency, a first tuning word Cap 1  is increased by 1 to enable one more unit capacitor in the first capacitor bank  12 . In other words, the current equivalent capacitance of the first capacitor bank  12  is 2*C 1 . 
     If the frequency is lower than the reference frequency in step S 34 , step S 35  is executed. In step S 35 , the calibration circuit  16  disables one unit capacitor in the second capacitor bank  13 , i.e. the second tuning word Cap 2  for the second capacitor bank  13  is decreased by 1. Then, in step S 36 , the calibration circuit  16  determines whether the output frequency is greater than the reference frequency. If the output frequency is not greater than the reference frequency, the calibration circuit  16  disables one more unit capacitor in the second capacitor bank  13 . 
     If the output frequency is greater than the reference frequency, step S 37  is executed. The calibration circuit  16  enables one unit capacitor in the third capacitor bank  14 , i.e. the third tuning word Cap 3  for the third capacitor bank  14  is increased by 1. Then, in step S 38 , the calibration circuit  16  determines whether the output frequency is lower than the reference frequency. If yes, the frequency calibration procedure is done. If not, the calibration circuit  16  enables one unit capacitor in the third capacitor bank  14 , i.e. the third tuning word Cap 3  for the third capacitor bank  14  is increased by 1. 
       FIG. 4  is a frequency calibration method according to another embodiment of the invention. The method in  FIG. 4  is illustrated with the voltage-controlled oscillator shown in  FIG. 1 . In this embodiment, only three capacitor banks are illustrated, but the invention is not limited thereto. When the frequency calibration starts, the capacitance of the first capacitor bank  12  and the third capacitor bank  14  are each set to the corresponding minimum capacitance, and the capacitance of the second capacitor bank  13  is the maximum capacitance the second capacitor bank  13  has in step S 41 . 
     In step S 42 , the calibration circuit  16  determines whether the output frequency is greater than the reference frequency. If the frequency is greater than the reference frequency, step S 43  is executed. In step S 43 , the calibration circuit enables one unit capacitor in the first capacitor bank  12 . If the frequency is not greater than the reference frequency, step S 45  is executed. 
     In step S 44 , the calibration circuit  16  determines whether the output frequency is lower than the reference frequency. If the frequency is not lower than the reference frequency, a first tuning word Cap 1  is increased by 1 to enable one more unit capacitor in the first capacitor bank  12 . 
     If the frequency is lower than the reference frequency in step S 44 , step S 45  is executed. In step S 45 , the calibration circuit  16  disables one unit capacitor in the second capacitor bank  13 , i.e. the second tuning word Cap 2  for the second capacitor bank  13  is decreased by 1. Then, in step S 46 , the calibration circuit  16  determines whether the output frequency is greater than the reference frequency. If the output frequency is not greater than the reference frequency, the calibration circuit  16  disables one more unit capacitor in the second capacitor bank  13 . 
     If the output frequency is greater than the reference frequency, step S 47  is executed. The calibration circuit  16  enables one unit capacitor in the third capacitor bank  14 , i.e. the third tuning word Cap 3  for the third capacitor bank  14  is increased by 1. Then, in step S 48 , the calibration circuit  16  determines whether the output frequency is lower than the reference frequency. If yes, the frequency calibration procedure is done. If not, the calibration circuit  16  enables one unit capacitor in the third capacitor bank  14 , i.e. the third tuning word Cap 3  for the third capacitor bank  14  is increased by 1. 
     According to above paragraphs, the proposed frequency calibration method starts by adjusting the capacitance of a first capacitor bank, wherein the first capacitor bank is not the largest or the smallest capacitor bank. When the capacitance of the selected capacitor bank is determined, the proposed frequency calibration method starts adjusting the capacitance of a second capacitor bank, wherein the unit capacitance in the second capacitor bank is greater than the unit capacitance in the first capacitor bank. When the capacitance of the second capacitor bank is determined, the proposed frequency calibration method starts adjusting a third capacitor bank, wherein the unit capacitance in the second capacitor bank is lower than the unit capacitance in the first capacitor bank. When the capacitance of the third capacitor bank is determined, the proposed frequency calibration method starts adjusting the capacitance of a fourth capacitor bank, wherein the unit capacitance in the fourth capacitor bank is greater than the unit capacitance in the second capacitor bank. According to the described calibration mechanism, the frequency calibration method stops when the capacitance of each capacitor bank is determined. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it should be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.