Patent Publication Number: US-8120394-B2

Title: Automatic frequency calibration circuit and automatic frequency calibration method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 99107142, filed on Mar. 11, 2010. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present disclosure relates an automatic frequency calibration circuit and an automatic frequency calibration method. 
     2. Description of Related Art 
     In many electronic systems, a frequency synthesizer is required to provide a stable and specific frequency. The frequency synthesizer can dynamically adjust an output frequency according to an actual requirement.  FIG. 1  is a circuit schematic diagram illustrating a voltage-controlled oscillator (VCO)  100  of a conventional frequency synthesizer. A control voltage VTUNE can change a capacitance of a varactor  110  in a resonant cavity of the VCO  100 . By adjusting the capacitance of the varactor  110 , an output frequency Fosc of the VCO  100  can be changed. 
       FIG. 2A  is a circuit schematic diagram illustrating the varactor  110  of the VCO  100 . For a wideband system, a relatively large varactor is required to be used in the resonant cavity of the VCO  100 , so as to achieve all of the target frequencies Fosc. However, the large varactor corresponds to an increased oscillator gain.  FIG. 2B  is a schematic diagram illustrating a relationship curve of the control voltages VTUNE and the output frequencies Fosc in the large varactor  110  of  FIG. 2A . Utilization of the large varactor  110  can increase a slope K VCO  (i.e. the oscillator gain) of the relationship curve of  FIG. 2B , which may increase a phase noise of the VCO  100 . 
       FIG. 3A  is a circuit schematic diagram illustrating another varactor  110  of the VCO  100 . To satisfy a requirement of the wideband system, and meanwhile reduce the slope K VCO  (i.e. the oscillator gain), when the wideband VCO  100  is designed, the varactor  110  of  FIG. 3A  can use a small varactor  111  in collaboration with a switched-capacitor bank  112 , so as to achieve an enough bandwidth without increasing the oscillator gain. Namely, the VCO  100  has a plurality of capacitor configurations. By changing connection states of one or a plurality of switches in the switched-capacitor bank  112 , various possible values of the capacitance can be achieved. 
       FIG. 3B  is a schematic diagram illustrating relationship curves of the control voltages VTUNE and the output frequencies Fosc in the varactor  110  of  FIG. 3A . As shown in  FIG. 3B , a single tuning curve with a large slope can be converted into a plurality of curves with small slopes. Each of the curves in  FIG. 3B  corresponds to one of the aforementioned capacitor configurations. Namely, when the VCO  100  is set to one of the capacitor configurations, the VCO  100  is operated according to one of the curves of  FIG. 3B . An automatic frequency calibration circuit is used for finding the tuning curve (i.e. finding the corresponding capacitor configuration) covering a target frequency before the frequency synthesizer is locked at the target frequency. 
     When the VCO  100  is applied to an integer-N frequency synthesizer, since a division ratio of a frequency divider is a fixed value, the automatic frequency calibration circuit can directly compare a reference frequency REF and an output frequency DIV of the frequency divider according to such division ratio, so as to find a suitable tuning curve. However, when the VCO  100  is applied to a fractional-N frequency synthesizer, since the division ratio of the frequency divider is constantly varied to achieve that an average thereof is in line with a precise fraction, when the reference frequency REF and the output frequency DIV of the frequency divider is compared, the conventional automatic frequency calibration circuit may have errors in the comparison. 
     Most of the currently known automatic frequency calibration circuits are designed in allusion to the integer-N frequency synthesizer, though if such conventional automatic frequency calibration circuit is applied to the fractional-N frequency synthesizer, a problem is probably occurred. Assuming an integer part and a fractional part of a target division ratio in the frequency synthesizer are respectively K and M, and the frequency found by the automatic frequency calibration circuit according to the target division ratio is F afc , since when the conventional automatic frequency calibration circuit finds the most suitable tuning curve, only the integer part is considered, the found F afc  is approximately K times of the reference frequency REF. However, after the calibration is completed, the locked frequency of the frequency synthesizer is K.M times of the reference frequency REF, so that there is 0.M times of error there between. Such error is probably as much as the reference frequency REF, which may lead to a result that the VCO  100  is locked at a control voltage with worse phase noise, or even cannot be locked. 
     SUMMARY 
     Accordingly, the embodiments of the present disclosure are directed to an automatic frequency calibration circuit and an automatic frequency calibration method for a fractional-N frequency synthesizer, in which a fractional part of a target division ratio is considered during frequency calibration. 
     The exemplary embodiment of the present disclosure provides an automatic frequency calibration circuit for a fractional-N frequency synthesizer, the frequency synthesizer has a voltage-controlled oscillator (VCO) and a frequency dividing unit, wherein the VCO has a plurality of capacitor configurations. The automatic frequency calibration circuit includes a first frequency detecting unit, a second frequency detecting unit, a comparator, a judging interval unit and a state machine. The first frequency detecting unit detects a reference frequency. The second frequency detecting unit detects an output frequency of the frequency dividing unit. The comparator receives and compares outputs of the first and the second frequency detecting units, and outputs a comparison result. The judging interval unit defines at least one judging period in a total comparison time, and enables the comparator to output the comparison result during the judging period. In a calibration mode, the state machine selects one of the capacitor configurations to set the VCO, and modify a fractional part from a target division ratio for obtaining a modified fraction according to a precision, and combines the modified fraction with an integer part of the target division ratio to set a division ratio of the frequency dividing unit. Wherein, the state machine selects another one of the capacitor configurations to set the VCO when the comparison result shows that the reference frequency does not match the output frequency of the frequency dividing unit. 
     The exemplary embodiment of the present disclosure provides an automatic frequency calibration method for a fractional-N frequency synthesizer. The automatic frequency calibration method can be described as follows. In a calibration mode, one of capacitor configurations is selected to set a VCO. In the calibration mode, a fractional part from a target division ratio is modified for obtaining a modified fraction according to a precision, and a division ratio of a frequency dividing unit is set according to the combination of the modified fraction and an integer part of the target division ratio. At least one judging period is defined in a total comparison time. In the calibration mode, a reference frequency and an output frequency of the frequency dividing unit are detected and compared during the judging period, so as to obtain a comparison result. In the calibration mode, another one of the capacitor configurations is selected to set the VCO when the comparison result shows that the reference frequency does not match the output frequency of the frequency dividing unit. 
     According to the above descriptions, the exemplary embodiment of the present disclosure provides an automatic frequency calibration circuit/method for a fractional-N frequency synthesizer. According to the automatic frequency calibration circuit/method, the fractional part of the target division ratio can be considered during the frequency calibration, so that during the comparison time, the VCO curve suitable for locking the frequency synthesizer can be quickly and effectively found. 
     In order to make the aforementioned and other features and advantages of the present disclosure comprehensible, several exemplary embodiments accompanied with figures are described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure. 
         FIG. 1  is a circuit schematic diagram illustrating a voltage-controlled oscillator (VCO) of a conventional frequency synthesizer. 
         FIG. 2A  is a circuit schematic diagram illustrating a varactor of a conventional VCO. 
         FIG. 2B  is a schematic diagram illustrating a relationship curve of control voltages VTUNE and output frequencies Fosc in a large varactor of  FIG. 2A . 
         FIG. 3A  is a circuit schematic diagram illustrating another varactor of a conventional VCO. 
         FIG. 3B  is a schematic diagram illustrating relationship curves of control voltages VTUNE and output frequencies Fosc in a varactor of  FIG. 3A . 
         FIG. 4  is a functional block diagram illustrating a frequency synthesizer according to an exemplary embodiment of the present disclosure. 
         FIG. 5  is a diagram illustrating an exemplary embodiment of an automatic frequency calibration circuit of  FIG. 4 . 
         FIG. 6  is a diagram illustrating relationship curves of control voltages VTUNE and output frequencies Fosc of a VCO of  FIG. 5 . 
         FIG. 7  is a flowchart illustrating an automatic frequency calibration method according to an exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS 
       FIG. 4  is a functional block diagram illustrating a frequency synthesizer according to an exemplary embodiment of the present disclosure. The frequency synthesizer  400  includes a phase frequency detector (PFD)  10 , a charge pump (CP)  20 , a loop filter  30 , a voltage-controlled oscillator  100 , a frequency dividing unit  40  and an automatic frequency calibration circuit  500 . The automatic frequency calibration circuit  500  can control a connecting state/connecting states of one or a plurality of switches of a switched-capacitor bank  112  within the VCO  100 . Namely, the automatic frequency calibration circuit  500  can select one of capacitor configurations of the VCO  100  to set the VCO  100 . The VCO  100  correspondingly generates an output frequency Fosc in response to a control voltage VTUNE. In the present exemplary embodiment, a first switch SW 1  is disposed between the CP  20  and the loop filter  30 , and a second switch SW 2  and a third switch SW 3  are disposed at two ends of the first switch SW 1  as that shown in  FIG. 4 , though the third switch SW 3  can be omitted according to an actual design requirement. 
     In a normal operation mode, the first switch SW 1  is turned on, and the second switch SW 2  and the third switch SW 3  are turned off. Therefore, the frequency synthesizer  400  forms a loop to carry on frequency locking. The automatic frequency calibration circuit  500  can transmit a fraction part M and an integer part K of an external target division ratio to the frequency dividing unit  40 , so as to set a division ratio of the frequency dividing unit  40  as K.M. The frequency dividing unit  40  performs a frequency dividing operation to the output frequency Fosc according to the target division ratio K.M provided by the automatic frequency calibration circuit  500 , and then transmits a frequency-divided output frequency DIV to the PFD  10 . In the present exemplary embodiment, the frequency dividing unit  40  includes a frequency divider  41  and a modulus controller  42 . The modulus controller  42  can be implemented by any approach, for example, an integrator can be used to implement the modulus controller  42 , or a delta-sigma modulator (ΔΣ modulator) can be used to implement the modulus controller  42 . The modulus controller  42  can correspondingly adjust an integer division ratio of the frequency divider  41  according to the target division ratio K.M. By constantly adjusting the integer division ratio of the frequency divider  41 , a fractional average of the output frequency DIV of the frequency divider  41  can be precisely adjusted to the target division ratio K.M. Implementations of the PFD  10 , the CP  20 , the loop filter  30 , the VCO  100 , the frequency divider  41  and the modulus controller  32  are well known by those skilled in the art, so that detailed descriptions thereof are not repeated. 
     In a calibration mode, the first switch SW 1  is turned off, and the second switch SW 2  and the third switch SW 3  are turned on. Therefore, the frequency synthesizer  400  forms an open loop under the calibration mode, and a reference voltage Vref (a fixed voltage) is provided to the VCO  100  to serve as the control voltage VTUNE through the second switch SW 2  and the loop filter  30 . Namely, the control voltage VTUNE is fixed to a predetermined level (for example, a level of the reference voltage Vref). The level of the reference voltage Vref can be arbitrarily determined according to an actual design requirement. For example, the level of the reference voltage Vref can be set to a half of a system voltage VDD, wherein the system voltage VDD is the supply voltage in this frequency synthesizer  400 . 
     In the calibration mode, the automatic frequency calibration circuit  500  can transmit the fraction part M and the integer part K of the target division ratio to the frequency dividing unit  40 , so as to set the division ratio of the frequency dividing unit  40  as K.M. The frequency dividing unit  40  performs the frequency dividing operation to the output frequency Fosc according to the target division ratio K.M provided by the automatic frequency calibration circuit  500 , and then transmits the frequency-divided output frequency DIV to the automatic frequency calibration circuit  500 . The automatic frequency calibration circuit  500  can select one of the capacitor configurations of the VCO  100 , and can determine whether a currently selected capacitor configuration is correct by comparing a frequency difference of frequency signals REF and DIV. If the frequency difference of the frequency signals REF and DIV exceeds a predetermined range, the automatic frequency calibration circuit  500  selects another one of the capacitor configurations to set the VCO  100  until the frequency difference of the frequency signals REF and DIV falls in the predetermined range. The other one of the capacitor configurations can be selected according to a linear search rule or a binary search rule. If the correct capacitor configuration is found, the calibration mode is ended, and the normal operation mode is resumed. 
     Assuming the fractional part M of the target division ratio has N bits, generally, if the modulus controller  42  is implemented by the delta-sigma modulator, 2 N+1  periods are required to generate a correct equivalent fractional division ratio, so that a time for the automatic frequency calibration circuit  500  comparing the frequency signals REF and DIV has to be an integer multiple of 2 N+1  periods, so as to avoid generating an error fraction. To reduce a calibration time of the calibration mode, the automatic frequency calibration circuit  500  can obtain the first n bits of the fractional part M of the target division ratio according to the rounding rule, and omit the remaining bits to obtain an approximate fractional part L. In the binary system, 0 is omitted and 1 is counted according to the rounding rule. For example, assuming the first 2 bits of the fractional part M are fetched to generate the approximate fractional part L, if the fractional part M is 10001, the approximate fractional part L is then 10000, and if the fractional part M is 10101, the approximate fractional part L is then 11000. In the calibration mode, the automatic frequency calibration circuit  500  can transmit the approximate fractional part L to the modulus controller  42  to generate a control signal to the frequency divider  41 . 
     After the automatic frequency calibration circuit  500  uses the rounded division ratio K.L to accomplish selecting the correct tuning curve to end the calibration mode, the division ratio is set back to the original target value K.M, and afterwards a locked state detecting unit  410  is used to determine whether the control voltage VTUNE correctly falls in a tolerable range V TR , and then the automatic frequency calibration circuit  500  adjusts the capacitor configuration of the VCO  100  according to a determination result of the locked state detecting unit  410 . If the control voltage VTUNE is greater than the tolerable range V TR , it represents that the currently selected capacitor configuration of the VCO  100  provides exceeded capacitance (for example, a relationship curve Bank# 2  of  FIG. 6 ), so that the automatic frequency calibration circuit  500  changes to select another configuration that leads to less capacitance (for example, a relationship curve Bank# 3  of  FIG. 6 ). If the control voltage VTUNE is smaller than the tolerable range V TR , it represents that the currently selected capacitor configuration of the VCO  100  provides insufficient capacitance (for example, a relationship curve Bank# 4  of  FIG. 6 ), so that the automatic frequency calibration circuit  500  changes to select another configuration that results in more capacitance (for example, a relationship curve Bank# 3  of  FIG. 6 ). The modification operation of the locked state detecting unit  410  and the automatic frequency calibration circuit  500  is continuously carried on under the normal operation mode to detect if any sudden change in VTUNE or terminated after VTUNE is located within the tolerable range V TR . 
       FIG. 5  is a diagram illustrating an exemplary embodiment of the automatic frequency calibration circuit  500  of  FIG. 4 . The automatic frequency calibration circuit  500  includes a first frequency detecting unit  510 , a second frequency detecting unit  520 , a judging interval unit  530 , a comparator  540  and a state machine  550 . In the normal operation mode, the state machine  550  sets the division ratio of the frequency dividing unit as K.M according to the fractional part M and the integer part K of the target division ratio provided from external. 
     In the calibration mode, the frequency synthesizer  400  forms the open loop, and the control voltage VTUNE is set to the reference voltage Vref (the fixed voltage). The first frequency detecting unit  510  detects the reference frequency REF. The second frequency detecting unit  520  detects the output frequency DIV of the frequency dividing unit  40 . In the present exemplary embodiment, the first frequency detecting unit  510  includes a first counter, and the second frequency detecting unit  520  includes a second counter. The first counter and the second counter can respectively count a pulse number N 1  of the reference frequency REF and a pulse number N 2  of the output frequency DIV during a unit time, so that the first and the second frequency detecting units  510  and  520  can obtain frequency values of the reference frequency REF and the output frequency DIV. 
     The comparator  540  receives the outputs of the first and the second frequency detecting units  510  and  520 . The judging interval unit  530  defines one or a plurality of judging periods in a total comparison time STOP, and enables the first counter and the second counter to respectively count the pulse number of the reference frequency REF and the pulse number of the output frequency DIV during each of the judging periods. When each of the judging periods is ended, the judging interval unit  530  enables the comparator  540  to compare the outputs of the first and the second frequency detecting units  510  and  520 , and enables the comparator  540  to output a comparison result to the state machine  550 . 
     In the calibration mode, the state machine  550  selects one of the capacitor configurations of the VCO  100  to set the VCO  100 . The state machine  550  receives the fractional part M and the integer part K of the target division ratio provided by external, and modify the fractional part M from the target division ratio for obtaining a modified fraction (i.e. the approximate fractional part L) according to a required precision CYCLE, and then combines the modified fraction L with the integer part K of the target division ratio to set the division ratio of the frequency dividing unit  40 . Wherein, the state machine  550  selects another one of the capacitor configurations to set the VCO  100  when the comparison result output by the comparator  540  shows that the reference frequency REF does not match the output frequency DIV of the frequency dividing unit  40 . 
     The precision CYCLE represents a number of bits to be obtained from the fractional part M. A method of determining the precision CYCLE is as follows. Assuming the state machine  550  obtains the first n bits of the fractional part M according to the rounding rule, and omits the remaining bits to obtain the approximate fractional part L. Selection of the value n is determined according to  FIG. 6 .  FIG. 6  is a diagram illustrating relationship curves of the control voltages VTUNE and the output frequencies Fosc of the VCO  100  of  FIG. 5 . Voltages Vmin and Vmax define the tolerable range V TR  of the control voltage VTUNE for acceptable phase noise performance, and in the calibration mode, the control voltage VTUNE is fixed to a middle value Vmid of the tolerable range V TR . The relationship curves Bank# 0 , Bank# 1 , Bank# 2 , . . . , Bank# 7  of  FIG. 6  respectively correspond to one of the capacitor configurations of the VCO  100 . 
     Assuming a frequency found by the automatic frequency calibration circuit  500  according to the modified fractional division ratio K.L from the first n bits is F afc , a target frequency to be actually locked by the frequency synthesizer  400  is F out , a slope (i.e. the oscillator gain) of the relationship curve between the control voltage VTUNE and the output frequency Fosc is K VCO , the reference frequency is REF, the tolerable range of the control voltage VTUNE is V TR , and a difference between the target frequency F out  and the frequency F afc  is F err , following equations are then obtained:
 
 F   out =( K.M )×REF  (1)
 
 F   afc =( K.L )×REF  (2)
 
| F   err   |=|F   out   −F   afc   |=|[ 0. M )−(0. L )]×REF|≦(1/2 n+1 )×REF  (3)
 
     According to the equation (3), it is known that a maximum of the frequency difference F err  can be ±REF/2 n+1 . To ensure that the target frequency F out  falls in an adjustable frequency range of the control voltage VTUNE, namely, to ensure the target frequency F out  falls in a range of K VCO ×V TR , a following condition is required to be satisfied:
 
 K   VCO   ×V   TR ≧2×REF/2 n+1   (4)
 
     The slope K VCO  and the tolerable range V TR  are determined by a characteristic of the VCO  100 , and the reference frequency REF is determined according to an actual design requirement. A most suitable n value can be obtained according to the slope K VCO  of the relationship curve, the tolerable range V TR  and the reference frequency REF through the equation (4). 
       FIG. 7  is a flowchart illustrating an automatic frequency calibration method according to an exemplary embodiment of the present disclosure. Referring to  FIG. 5  and  FIG. 7 , regarding the frequency comparison of the present exemplary embodiment, the pulse numbers of the reference frequency REF and the frequency-divided output frequency DIV are compared within a specific time (the judging period), so as to determine the frequency difference. When the state machine  550  enters the calibration mode (step S 705 ), the switch SW 1  is turned off, and the switch SW 2  is turned on. The state machine  550  obtains the front n bits from the fractional part M of the target division ratio according to the precision CYCLE, and sets a division ratio of the frequency dividing unit  40  as K.L according to the front n bits (the approximate fractional part L) and the integer part K of the target division ratio. 
     In step S 710 , the judging interval unit  530  sets the total comparison time STOP. In step S 715 , the counters of the first frequency detecting unit  510  and the second frequency detecting unit  520  are reset. In step S 720 , the state machine  550  resets an index i, for example, resets the index i to 1. In step S 730 , the judging interval unit  530  defines at least one judging period in the total comparison time STOP, and the comparator  540  detects and compares the reference frequency REF and the output frequency DIV of the frequency dividing unit  40  during the judging period. In the total comparison time STOP, if the comparison result shows that the reference frequency REF matches the output frequency DIV of the frequency dividing unit  40 , the current capacitor configuration is determined to be an optimal capacitor configuration (step S 750 ), and the calibration mode is ended (step S 755 ). 
     If the comparison result of the step S 730  shows that the reference frequency REF does not match the output frequency DIV of the frequency dividing unit  40 , the state machine  550  determines whether any unselected candidate capacitor configuration exists in the capacitor configurations of the VCO  100  (step S 740 ), and selects another one of the capacitor configurations (step S 745 ), and the steps S 715 -S 740  are repeated. In the step S 745 , the state machine  550  uses the binary search rule to select the next candidate capacitor configuration. In the step S 740 , if it is determined that there is none of the possible capacitor configuration, the state machine  550  can confirm that the previous selected capacitor configuration is already the optimal capacitor configuration (step S 750 ). After the capacitor configuration of the VCO  100  is determined, the state machine  550  ends the calibration mode and resumes the normal operation mode (step S 755 ). 
     The step S 730  includes steps S 731 -S 735 . In the step S 731 , the judging interval unit  530  defines at least one judging period in the total comparison time STOP, and sets the judging period to be 2 CYCLE+i . Since the state machine  550  obtains a modified bits L from the fractional part M of the target division ratio according to the precision CYCLE, the modulus controller  42  requires 2 CYCLE+1  periods (or an integer multiple of the 2 CYCLE+1  periods) to generate a correct equivalent fractional division ratio, so that the judging period set by the step S 731  has 2 CYCLE+i  periods. 
     In the step S 732 , the counters of the first frequency detecting unit  510  and the second frequency detecting unit  520  respectively count the pulse number N 1  of the reference frequency REF and the pulse number N 2  of the frequency-divided output frequency DIV during the judging period set by the step S 731 . In the step S 733 , the comparator  540  transmits a difference of the pulse numbers N 1  and N 2  (i.e. |N 1 −N 2 |) to the state machine  550  after the judging period is ended, and the state machine  550  detects and compares the difference |N 1 −N 2 | with a threshold. 
     Since the first frequency detecting unit  510  and the second frequency detecting unit  520  are not synchronized, the state machine  50  changes the capacitor configuration of the VCO  100  when the difference between the pulse number N 1  of the reference frequency REF and the pulse number N 2  of the frequency-divided output frequency DIV is greater than or equal to 2, so as to avoid misjudgment caused by phase difference. Therefore, in the step S 733 , it is determined whether the difference |N 1 −N 2 | is smaller than 2, and if it is determined that the difference |N 1 −N 2 | is greater than or equal to 2 according to the step S 733 , it represents that the currently selected capacitor configuration is not the optimal capacitor configuration, so that the step S 740  is executed to find a next capacitor configuration. If it is determined that the difference |N 1 −N 2 | is smaller than 2 according to the step S 733 , the index i is added by 1 (the step S 735 ), and then the steps S 731 -S 733  are repeated until 2 CYCLE+i  is equal to the total comparison time STOP. Namely, a smaller judging period is first used to detect the difference between the pulse numbers N 1  and N 2 , and under a condition that |N 1 −N 2 | is smaller than 2, the judging period is gradually increased to again detect the difference between the pulse numbers N 1  and N 2  until the judging period 2 CYCLE+i  is equal to the total comparison time STOP. Therefore, if the current capacitor configuration is incorrect, the automatic frequency calibration circuit  500  can immediately end the detection operation to select a next capacitor configuration. If a situation that the difference between the pulse numbers N 1  and N 2  is greater than or equal to 2 is not occurred after the total comparison time STOP, it represents that the automatic frequency calibration circuit  500  has found the most suitable tuning curve (the capacitor configuration) (the step S 750 ). In this case, the switch SW 1  is turned on and the switches SW 2  and SW 3  are turned off, so that the frequency synthesizer  400  ends the calibration mode and enters a phase-locked loop (PLL) stage (step S 755 ). 
     In summary, the exemplary embodiment of the present disclosure provides the automatic frequency calibration circuit  500  for a fractional-N frequency synthesizer. The bit number n required to be considered in the calibration mode during the automatic frequency calibration can be obtained according to the reference frequency REF of the frequency synthesizer  400  and the control voltage range V TP  of the VCO  100 . Therefore, the VCO curve (the capacitor configuration) suitable for locking the synthesizer  400  can be quickly and effectively found within the total comparison time STOP. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claims and their equivalents.