Patent Publication Number: US-2009237036-A1

Title: Frequency synthesizer and loop filter used therein

Description:
TECHNICAL FIELD 
     The present invention relates to a frequency synthesizer and a loop filter using the same, and more particularly to a frequency synthesizer including a charge pump circuit and a loop filter. 
     BACKGROUND ART 
     In a wireless communicating apparatus such as a radio receiver, a television broadcast receiver or a portable telephone, generally, a frequency synthesizer using a PLL (Phase Locked Loop) is utilized as a local oscillating circuit.  FIG. 1  is a diagram showing a general structure of the frequency synthesizer using the PLL. As shown in  FIG. 1 , the frequency synthesizer includes a crystal oscillator  1 , a reference frequency divider  2 , a phase comparator  3 , a charge pump circuit  4 , a loop filter (LPF)  5 , a voltage controlled oscillator (VCO)  6  and a variable frequency divider  7 . 
     The crystal oscillator  1  generates a signal having a predetermined frequency. The reference frequency divider  2  divides a frequency of the signal output from the crystal oscillator  1  at a fixed dividing ratio and thus generates a reference signal having a reference frequency. The phase comparator  3  detects a phase difference between the reference signal output from the reference frequency divider  2  and a comparison signal output from the variable frequency divider  7  and outputs an error signal from Up and Down terminals corresponding to the result. 
     When the phase of the comparison signal is lagged from that of the reference signal, the phase comparator  3  outputs, from the Up terminal, an error signal having a pulse width corresponding to the phase difference. When the phase of the comparison signal is led from that of the reference signal, the phase comparator  3  outputs, from the Down terminal, an error signal having a pulse width corresponding to the phase difference. When the phase of the comparison signal is synchronized with that of the reference signal, the error signal is not output but a so-called floating state (a high impedance state) is brought. 
     The charge pump circuit  4  carries out a charging or discharging operation of a capacitor constituting the LPF  5  based on the error signal output from the Up terminal or the Down terminal of the phase comparator  3 . Consequently, a signal which is proportional to the phase difference detected by the phase comparator  3  is output from the LPF  5 . The error signal output from the phase comparator  3  is pulse-shaped and the LPF  5  serves to remove an AC component from the same signal to set a control voltage of the VCO  6 . 
     The VCO  6  is oscillated at a frequency which is proportional to a voltage of the signal output from the LPF  5 , and generates a local oscillating signal and outputs the local oscillating signal to an outside of the frequency synthesizer and the variable frequency divider  7 . The variable frequency divider  7  divides an output frequency of the VCO  4  at a specified dividing ratio and outputs the result as a comparison signal to the phase comparator  3 . The frequency synthesizer thus constituted is operated in such a manner that a frequency of the comparison signal gradually approximates to that of the reference signal through a negative feedback loop even if the frequency of the comparison signal is higher or lower than that of the reference signal. Consequently, the oscillating frequency of the VCO  6  is locked into a constant frequency. 
     In the frequency synthesizer having the structure described above, a time constant of the LPF  5  is determined by values of the capacitor and a resistor which are included in the LPF  5 . In order to increase the time constant, thereby carrying out a stable operation of the LPF  5 , it is necessary to increase a capacitance value of the capacitor or the value of the resistor. When the capacitance value of the capacitor is increased, however, it is hard to integrate the LPF  5  into a semiconductor chip. For this reason, there is a problem in that it is necessary to constitute the LPF  5  as an external component of the semiconductor chip. 
     On the other hand, when the capacitance value of the capacitor is decreased to easily carry out an integration, it is necessary to increase a resistance value in order to increase the time constant of the LPF  5 . When the resistance value is increased, however, there is a problem, in that a bad influence is exerted, for example, a thermal noise is generated so that C/N of VCO is deteriorated or a level of a spuriousness caused by a leakage of a reference frequency component is raised. 
     On the other hand, there has conventionally been proposed a PLL circuit which is suitable for an integration using a small integral capacitance and can improve a spurious suppression performance, thereby bringing out a performance of a digital oscillator sufficiently (for example, see Patent Document 1). 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 11-150735 
     In the technique described in the Patent Document 1, an output of a phase comparator is divided into two paths and a frequency control of a VCO is carried out through a gain control circuit for one of them and an LPF for the other. A gain of a gain control circuit and a time constant of the LPF are switched depending on a state of a signal to be processed. The time constant is switched by fixing a capacitance of a capacitor to be small and controlling a transconductance of a gm amplifier constituting an integrator. 
     DISCLOSURE OF THE INVENTION 
     In the technique described in the Patent Document 1, however, the time constant of the LPF is simply switched to be decreased or increased depending on a synchronizing state of the PLL circuit. For this reason, a fundamental solution cannot be obtained. More specifically, there is a problem in that a C/N characteristic or a spurious characteristic cannot be improved when the capacitance value of the capacitor is decreased to reduce the time constant of the LPF. 
     In order to solve the problem, it is an object of the present invention to enable an integration of a loop filter to be easily carried out with a decrease in a capacitance value of a capacitor and an improvement in a C/N characteristic and a spurious characteristic irrespective of a synchronizing state of a frequency synthesizer. 
     In order to attain the object, in the present invention, a loop filter is constituted by a plurality of capacitors connected in parallel, a switch for carrying out switching to perform a charging or discharging operation of the capacitors as a pipeline processing, and a capacitor connected between an output of a parallel circuit including the capacitors and a ground. 
     According to the present invention having the structure described above, the charging or discharging operation is carried out on a pipeline basis for each of the capacitors which are connected in parallel. More specifically, when the charging operation for one of the capacitors is ended, the charging operation is carried out for the next capacitor and is then carried out for the subsequent capacitor. Consequently, results charged respectively are sequentially stored in the capacitors connected to an output of a parallel circuit. Thus, it is possible to equivalently implement a great time constant as a whole through a group of the capacitors even if the time constant of each of the capacitors is reduced with a decrease in the capacitance values of the capacitors connected in parallel. Accordingly, it is possible to decrease the capacitance value of the capacitor, thereby carrying out the integration easily. In addition, it is possible to improve the C/N characteristic and the spurious characteristic irrespective of the synchronizing state of the frequency synthesizer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a diagram showing a general structure of a frequency synthesizer using a PLL, 
         FIG. 2  is a diagram showing an example of a structure of a frequency synthesizer according to the present embodiment, 
         FIG. 3  is a diagram showing an example of a structure of a charge pump circuit according to the present embodiment, 
         FIG. 4  is a diagram showing an example of a structure of a loop filter according to the present embodiment, and 
         FIG. 5  is a diagram showing an example of a clock signal generated by a clock generator according to the present embodiment. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment according to the present invention will be described below with reference to the drawings.  FIG. 2  is a diagram showing an example of a structure of a frequency synthesizer according to the present embodiment. In  FIG. 2 , components having the same functions as those of the components shown in  FIG. 1  have the same reference numerals. As shown in  FIG. 2 , a frequency synthesizer according to the present embodiment includes a crystal oscillator  1 , a reference frequency divider  2 , a phase comparator  3 , a charge pump circuit  4 , a loop filter (LPF)  15 , a voltage controlled oscillator (VCO)  6 , a variable frequency divider  7 , a frequency divider  11  and a clock generator  12 . 
     The crystal oscillator  1  generates a signal having a predetermined frequency. The reference frequency divider  2  divides a frequency of the signal output from the crystal oscillator  1  at a fixed dividing ratio and thus generates a reference signal having a reference frequency. A reference generator according to the present invention is constituted by the crystal oscillator  1  and the reference frequency divider  2 . The phase comparator  3  detects a phase difference between the reference signal output from the reference frequency divider  2  and a comparison signal output from the variable frequency divider  7  and outputs an error signal from Up and Down terminals corresponding to the result. 
     When the phase of the comparison signal is lagged from that of the reference signal, the phase comparator  3  outputs, from the Up terminal, an error signal having a pulse width corresponding to the phase difference. When the phase of the comparison signal is led from that of the reference signal, the phase comparator  3  outputs, from the Down terminal, an error signal having a pulse width corresponding to the phase difference. When the phase of the comparison signal is synchronized with that of the reference signal, the error signal is not output but a so-called floating state (a high impedance state) is brought. 
     The charge pump circuit  4  carries out a charging or discharging operation of a capacitor constituting the LPF  15  based on the error signal output from the Up terminal or the Down terminal of the phase comparator  3 .  FIG. 3  is a diagram showing an example of a structure of the charge pump circuit  4 . The charge pump circuit  4  includes a first switch  4   a  connected to a power supply and a second switch  4   b  connected to a ground, and either of the first and second switches  4   a  and  4   b  is turned ON depending on a phase lead/lag of the comparison signal with respect to the reference signal. 
     More specifically, in the charge pump circuit  4 , when the phase of the comparison signal is lagged from that of the reference signal, the first switch  4   a  is turned ON in response to the error signal supplied from the Up terminal of the phase comparator  3  so that an electric charge is supplied to (stored in) the capacitor of the LPF  15 . On the other hand, when the phase of the comparison signal is led from that of the reference signal, the second switch  4   b  is turned ON in response to the error signal supplied from the Down terminal of the phase comparator  3  so that the electric charge stored in the capacitor of the LPF  15  is discharged (pumped). 
     The LPF  15  serves to remove an AC component from the error signal output from the phase comparator  3  and passing through the charge pump circuit  4 . More specifically, the error signal output from the phase comparator  3  is pulse-shaped and the LPF  15  serves to remove the AC component from the same signal to set a control voltage of the VCO  6 . A signal which is proportional to the phase difference detected by the phase comparator  3  is output from the LPF  15 . 
       FIG. 4  is a diagram showing an example of a structure of the LPF  15  according to the present embodiment. As shown in  FIG. 4 , the LPF  15  according to the present embodiment includes a plurality of capacitors C 1  to C n  connected in parallel between an input terminal A and an output terminal B and a plurality of switches SW 11  to SW 1n  and SW 21  to SW 2n  for carrying out switching to perform a charging or discharging operation of the capacitors C 1  to C n  as a pipeline processing (which will be described below in detail). 
     Some of the capacitors C 1  to C n  and the switches SW 11  to SW 1n  and SW 21  to SW 2n  which have the same subscripts (1 to n) of the designations constitute a single path of a parallel circuit. For example, a single path is constituted by the capacitor C 1  and the switches SW 11  and SW 21  connected therebefore and thereafter. Similarly, another path is constituted by the capacitor C 2  and the switches SW 12  and SW 22  connected therebefore and thereafter. n paths are connected in parallel so that the parallel circuit is constituted. 
     Thus, the LPF  15  according to the present embodiment further includes a capacitor C H  on an output side of the parallel circuit including the capacitors C 1  to C n  and the switches SW 11  to SW 1n  and SW 21  to SW 2n  (between an output end of the parallel circuit and the ground). The capacitor C H  serves to hold electric charge&#39;s stored in the capacitors C 1  to C n  and output sequentially. For this reason, the capacitor C H  to be used has a greater capacitance value than capacitance values of the capacitors C 1  to C n  constituting the parallel circuit. 
     The VCO  6  in  FIG. 2  is oscillated at a frequency which is proportional to a voltage of the signal output from the LPF  15 , and generates a local oscillating signal and outputs the local oscillating signal to an outside of the frequency synthesizer and the variable frequency divider  7 . The variable frequency divider  7  divides an output frequency of the VCO  4  at a specified dividing ratio and outputs the result as a comparison signal to the phase comparator  3 . The frequency synthesizer thus constituted is operated in such a manner that a frequency of the comparison signal gradually approximates to that of the reference signal through a negative feedback loop even if the frequency of the comparison signal is higher or lower than that of the reference signal. Consequently, the oscillating frequency of the VCO  6  is locked into a constant frequency. 
     The frequency divider  11  divides a frequency of the reference signal output from the reference frequency divider  2  at a fixed dividing ratio. The clock generator  12  generates clock signals φ 1  to φ n  from the signal subjected to the frequency-division through the frequency divider  11 . A clock generating circuit according to the present invention is constituted by the frequency divider  11  and the clock generator  12 . The respective switches SW 11  to SW 1n  and SW 21  to SW 2n  in the LPF  15  are controlled to be switched based on the clock signals φ 1  to φ n  generated by the clock generator  12 . 
       FIG. 5  is a diagram showing an example of the clock signals φ 1  to φ n  generated by the clock generator  12 . As shown in  FIG. 5 , the clock generator  12  sequentially generates the clock signals φ 1  to φ n  without a mutual overlap in such a manner that one of the clock signals falls and the next clock then rises immediately. Thereafter, the generated clock signals φ 1  to φ n  are sequentially supplied to the respective switches SW 11  to SW 1n  and SW 21  to SW 2n  in the LPF  15 . 
     At this time, the clock generator  12  supplies clock signals φ i  and φ i+1  (n+1=1 is set with i=n) which is shifted in order by one to two switches SW 1i  and SW 2i  (i=1 to n) constituting a single path. More specifically, the clock signal φ i  is supplied to the switch SW 1i  connected before the capacitor C 1 , and furthermore, the clock signal φ i+1  lagged by one is supplied to the switch SW 2i  connected after the capacitor C i . In consideration of the paths, when the clock signals φ i  and φ i+1  are supplied to the switches SW 1i  and SW 2i  of an i-th path, the clock signals φ i+1  and φ i+2  lagged by one are supplied to switches SW 1i+1  and SW 2i+1  of a next path. 
     Description will be given to an operation of the LPF  15  thus constituted. For example, an electric charge is supplied to the capacitor C i  while the switch SW 1i  connected before the capacitor C i  in the i-th path is turned ON in response to the clock signal φ i . When the clock signal φ i  falls so that the switch SW 1i  is turned OFF, the switch SW 2i  connected after the capacitor C i  is immediately turned ON in response to the clock signal φ i+1  so that the electric charge stored in the capacitor C i  in the ON state of the switch SW 1i  is supplied to the capacitor C 1i  connected to the output side of the parallel circuit. 
     The switch SW 1i+1  connected before the capacitor C i+1  in an (i+1)th path is simultaneously turned ON in response to the clock signal φ i+1  while the switch SW 2i  connected after the capacitor C i  in the i-th path is turned ON in response to the same clock signal φ i+1 . Consequently, the electric charge is supplied to the capacitor C i+1  in the (i+1)th path while the electric charge stored in the capacitor C i  in the i-th path is supplied to the capacitor C H . 
     When the clock signal φ i+1  falls so that the switch SW 1i+1  is turned OFF, the switch SW 2i+1  connected after the capacitor C i+1  is immediately turned ON in response to the clock signal φ i+2  so that the electric charge stored in the capacitor C i+1  in the ON state of the switch SW 1i+1  is supplied to the capacitor C H . At this time, the electric charge is simultaneously stored in a capacitor C i+2  in an (i+2)th path. 
     Thus, the electric charges are sequentially stored in the respective capacitors C 1  to C n , and the electric charges stored respectively are sequentially supplied to the capacitor C H . The capacitor C H  sequentially stores the electric charges supplied from the respective capacitors C 1  to C n . As described above, the error signal having a pulse width corresponding to the phase difference between the reference signal and the comparison signal is supplied from the phase comparator  3  to the charge pump circuit  4 . Therefore, the electric charges are supplied to the capacitor C H  corresponding to the pulse width of the error signal or the electric charges of the capacitor C H  are discharged corresponding to the pulse width of the error signal. 
     Description will be given to the capacitance value of the capacitor C H . As described above, the charging and discharging operations of the capacitors C 1  to C n  are sequentially carried out as the pipeline processing. As a result, the electric charges supplied from the respective capacitors C 1  to C n  are successively stored in the capacitor C H  provided on the output side of the capacitors C 1  to C n . Even if the capacitance value of the capacitor C H  is comparatively decreased so that the time constant is reduced, accordingly, the next electric charge is stored before the electric charge is lost due to a leakage current of the capacitor C H . Consequently, the capacitance value of the capacitor C H  can be set to be smaller than that of a capacitor required in the conventional LPF  5 . 
     For example, as the capacitor C H  which generates a very small leakage current, it is possible to use a PIP (polypropylene-insulator-polypropylene) capacitor, an MIM (metal-insulator-metal) capacitor, or an MOS (Metal Oxide Semiconductor) gate capacity, or the like. 
     As described above in detail, in the present embodiment, the LPF  15  is constituted by the capacitors C 1  to C n  connected in parallel, the switches SW 11  to SW 1n  and SW 21  to SW 2n  for carrying out switching to perform the charging or discharging operation of the capacitors C 1  to C n  as the pipeline processing, and the capacitor C H  connected to the output side of the parallel circuit including the capacitors C 1  to C n  and the switches SW 11  to SW 1n  and SW 21  to SW 2n . 
     By the structure, the charging or discharging operation is carried out on the pipeline basis for the respective capacitors C 1  to C n  which are connected in parallel. As a result, the electric charges stored sequentially in the capacitors C 1  to C n  are obtained as an output of the parallel circuit and are sequentially stored in the capacitor C H . Consequently, it is possible to implement a single capacitor having a great time constant as the whole circuit even if the time constants of the respective capacitors are reduced with a decrease in the capacitance values of the capacitors C 1  to C n  and C H . Accordingly, it is possible to easily carry out an integration with the decrease in the capacitance values of the capacitors C 1  to C n  and C H  and to improve a C/N characteristic and a spurious characteristic irrespective of a synchronizing state of the frequency synthesizer. 
     Although  FIG. 4  illustrates the structure for carrying out, as the pipeline processing, the charging or discharging operation of the capacitors C 1  to C n  connected in parallel in the embodiment described above, the present invention is not restricted thereto. It is also possible to employ any structure which can carry out the charging or discharging operation on the pipeline basis. 
     In addition, the embodiment is only illustrative for a concreteness to carry out the present invention and the technical range of the present invention should not be construed to be restrictive. In other words, the present invention can be carried out in various forms without departing from the spirit or main features thereof. 
     INDUSTRIAL APPLICABILITY 
     The present invention is useful for a frequency synthesizer (for example, a PLL circuit) including a charge pump circuit and a loop filter. 
     This application is based on Japanese Patent Application No. 2006-209426 filed on Aug. 1, 2006, the contents of which are incorporated hereinto by reference.