Abstract:
A semiconductor integrated circuit having a built-in PLL circuit which has two charge pump circuits for charging and discharging capacitive elements of a loop filter in response to signals generated by a phase comparator circuit. One of the two charge pump circuits has current sources which generate current values smaller than those generated by current sources of the other charge pump circuit. The loop filter has a first capacitive element connected to a charge/discharge node, and a second capacitive element connected to the charge/discharge node through a resistive element. The first capacitive element is charged and discharged by the one charge pump circuit, while the second capacitive element is charged and discharged by the other charge pump circuit. A charging current source of the one charge pump circuit operates simultaneously with a discharging current source of the other charge pump circuit, i.e., the charge pump circuits operate in opposite phase.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The present application is a continuation application of U.S. Ser. No. 11/241,995, filed Oct. 4, 2005, which is a continuation application of U.S. Ser. No. 10/739,117, filed Dec. 19, 2003 (now U.S. Pat. No. 7,015,735.  
         [0002]     The application U.S. Ser. No. 10/739,117, filed Dec. 19, 2003 relates to subject matter described in a co-pending application Ser. No. 10/253,922, filed Sep. 25, 2002 (now U.S. Pat. No. 6,906,596) assigned to the assignee of the present application. The disclosure of the co-pending application is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates to a phase locked loop (PLL) including a voltage controlled oscillator (VCO) circuit, and more particularly to techniques which are effectively applied to incorporation of a loop filter on a PLL into a semiconductor chip. More specifically, the present invention relates to techniques which are effectively utilized for a transmission PLL loop in a high frequency semiconductor circuit, for example, used in radio communication devices such as a portable telephone for modulating and upconverting a transmission signal.  
         [0004]     A radio communication device (mobile communication device) represented by a portable telephone typically comprises a semiconductor integrated circuit (generally referred to as a high frequency IC) which has functions of upconverting and modulating a transmission signal, downconverting and demodulating a received signal, and the like; a semiconductor integrated circuit (baseband IC) which has functions of converting a transmission signal to I, Q signals, and recovering reception data from demodulated I, Q signals; an electronic part referred to as a power module which contains a high frequency power amplifier, an associated bias circuit, an impedance matching circuit, and the like; and an electronic part referred to as a front end module which contains a transmission/reception switching circuit, a low pass filter, an impedance matching circuit and the like.  
         [0005]     In radio communication devices, with the recent trend of reducing the number of parts for a smaller size and a lower cost of the devices, conscious efforts have been made to incorporate as many circuits as possible into a single or several semiconductor integrated circuits. One of such efforts includes an attempt to provide a semiconductor chip with a built-in loop filter disposed on a loop of a transmission PLL within a high frequency IC.  
         [0006]     Generally, a second-order filter FLT as illustrated in  FIG. 1  has been used for the loop filter for a transmission PLL within a high frequency IC for providing a loop band characteristic required thereto. A charge pump is generally indicated by CP.  FIG. 2  is a graph showing the frequency characteristic of the filter FLT in  FIG. 1 , which has zero at frequency f 1  and a pole at frequency f 2 . The frequency f 1  and the frequency f 2  in the illustrated loop filter are expressed by the following equations: 
 
 f 1=1/{2 πC 1 R} 
 
 f 2=( C 1 +C 2)/2 πC 1 C 2 R   (1) 
 
         [0007]     Such a second-order filter employs two capacitive elements, wherein a loop filter for a transmission PLL handles relatively high frequencies, causing the larger capacitive element C 1  to have a high capacitive value on the order of nF (nanofarad), which makes it difficult to integrate the capacitive element C 1  itself on a chip. Thus, an external element is often used for the capacitive element C 1  to make up the loop filter.  
         [0008]     On the other hand, in order to reduce the capacitance values of capacitive elements used in a second-order loop filter, a technique has been developed and proposed for providing each capacitive element with a charge pump disposed in front thereof for charging and discharging the capacitive element, for example, as illustrated in  FIG. 3  (see, for example, 0018-9200/02 IEEE “A Fully Integrated CMOS Frequency Synthesizer With Charge-Averaging Charge Pump and Dual-Path Loop Filter for PCS- and Cellular-CDMA Wireless System”).  
       SUMMARY OF THE INVENTION  
       [0009]     The loop filter illustrated in  FIG. 3  comprises a charge pump CP 1  made up of a charging current source I 11  (=i 1 ) and a discharging current source I 12  (=i 1 ) for establishing a frequency characteristic having a cutoff frequency f 2 , as indicated by a one-dot chain line A in  FIG. 5 , together with a capacitor C 2  and a resistor R; and a charge pump CP 2  made up of a charging current source I 21  (=i 2 ) and a discharging current source I 22  (=i 2 ) for establishing a frequency characteristic, as indicated by a broken line B in  FIG. 5 , together with a capacitor C 1 . Then, these frequency characteristics are synthesized by an adder ADD to realize a frequency characteristic, as indicated by a solid line C in  FIG. 5 , which has a zero point at f 1  near the intersection of the curves A, B, and a pole at f 2 .  
         [0010]     It should be noted that when the charge-pumps CP 1 , CP 2  are not separated, a current flows from the current source I 11  to I 22  or from I 21  to I 12 , so that the adder ADD in  FIG. 3  may be actually represented as shown in  FIG. 4 . BFF in  FIG. 4  represents a buffer such as an emitter follower. The zero-point frequency f 1  and pole frequency f 2  in this loop filter are expressed by the following equations: 
 
 f 1=1/{2 πC 1( i 1 /i 2) R} 
 
 f 2=1/(2 πC 2 R )  (2) 
 
         [0011]     As can be understood from a comparison of Equation (1) with Equation (2), the frequencies f 1  and f 2  are determined by the ratio of the capacitors C 1  and C 2  in the loop filter of  FIG. 1 , whereas the frequencies f 1  and f 2  are determined by the ratio of the currents i 1  and i 2  in the loop filter of  FIG. 3 . Therefore, when the current i 2  is reduced, for example, to one tenth of i 1 , the result is equivalent to the ratio of the capacitors C 1 , C 2  equal to or higher than 10:1 in the loop filter of  FIG. 1 , even if the capacitors C 1 , C 2  have similar capacitance values. More specifically, the capacitor C 1  in the loop filter of  FIG. 1  has a capacitance value on the order of nF, whereas the capacitance values of the capacitors C 1 , C 2  in the loop filter of  FIG. 3  can be reduced to several hundred pF (picofarad) which is one order of magnitude smaller. As a result, the loop filter can be integrated on a chip.  
         [0012]     However, in an actual circuit implemented as illustrated in  FIG. 4 , noise generated by the buffer BFF causes an increase in phase error of a PLL loop. Furthermore, when the buffer BFF is applied to a transmission PLL, noises generated from the buffer BFF under operation intrudes into the supply voltage and causes a problem of increasing side-band spurious.  
         [0013]     Also, in the charge pumps CP 1 , CP 2 , fluctuations in the supply voltage may cause a change in the current values i 1 , i 2  of the current sources, depending on the type of circuit, resulting in a deviation of the loop gain from a desired value. To avoid this deviation, it is contemplated to provide a voltage regulator through which the charge pumps are provided with the supply voltage. However, although a voltage regulator is typically provided with a band gap reference circuit for preventing the generated voltage from varying due to the fluctuating supply voltage and a change in temperature, the band gap reference circuit can be a source of noise in a radio communication system to increase a phase error of the PLL loop and side-band spurious.  
         [0014]     Further, since variation of the supply voltage also cause a voltage controlled oscillator (VCO) circuit to vary an oscillation frequency, the VCO is desirably supplied with a supply voltage through a voltage regulator. However, in doing so, similar problems arise as is the case in the charge pumps, i.e., the band gap reference circuit within the voltage regulator acts as a noise source to increase the phase error of the PLL loop and side-band spurious.  
         [0015]     It is an object of the present invention to provide a loop filter which comprises a second- or higher order filter including capacitive elements, the capacitance ratio of which can be reduced, allowing the integration of the loop filter on a chip.  
         [0016]     It is another object of the present invention to provide a PLL circuit which is free from an increase in phase error even if a loop filter is integrated on a chip.  
         [0017]     It is a further object of the present invention to provide a PLL circuit which is free from an increase in side-band spurious even if a loop filter is integrated on a chip when the PLL circuit is applied for use in a transmission PLL.  
         [0018]     It is a further object of the present invention to provide a voltage regulator for supplying a supply voltage to charge pumps and a transmission VCO while protecting the charge pumps and transmission VCO from adverse effects due to noise generated by a band gap reference circuit.  
         [0019]     Representative features of the present invention herein disclosed will be generally described as follows.  
         [0020]     Specifically, in a first aspect of the present invention, a semiconductor integrated circuit has a built-in PLL circuit which includes two charge pump circuits for charging and discharging capacitive elements of a loop filter in response to signals generated by a phase comparator circuit. One of the two charge pump circuits has current sources which generate current values smaller than those generated by current sources of the other charge pump circuit. The loop filter has a first capacitive element connected to a charge/discharge node, and a second capacitive element connected to the charge/discharge node through a resistive element. The first capacitive element is charged and discharged by the one charge pump circuit, while the second capacitive element is charged and discharged by the other charge pump circuit. A charging current source of the one charge pump circuit operates simultaneously with a discharging current source of the other charge pump circuit. In other words, the two charge pump circuits are configured to operate in opposite phase.  
         [0021]     According to the semiconductor integrated circuit described above, two capacitive elements having smaller capacitance values can be employed by reducing the ratio of the current values supplied from the current sources which form part of the one charge pump circuit to the current values supplied from the current sources which form part of the other charge pump circuit. The reduction in the capacitance values of the capacitive elements permits the loop filter to be integrated on the semiconductor IC chip. In addition, since the one charge pump circuit operates in opposite phase to the other charge pump circuit, a need is eliminated for disposing a buffer, which can be a noise source, between the resistive element and second capacitive element, thereby making it possible to prevent an increased phase error of the PLL loop and increased side-band spurious.  
         [0022]     In a second aspect of the present invention, a semiconductor integrated circuit having a built-in PLL circuit includes a voltage controlled oscillator circuit which operates with a supply voltage generated by a voltage regulator that relies on a base-emitter voltage of a bipolar transistor to generate a predetermined voltage with less source voltage dependency. The voltage regulator includes a voltage regulating circuit which utilizes the base-emitter voltage of the bipolar transistor as a reference voltage for generating the predetermined voltage. The voltage regulator also includes a first resistive element connected in series with the bipolar transistor for generating the base-emitter voltage, a series connection of a second resistive element and a switching element connected in parallel with the first resistive element, and a temperature detector circuit. The switching element is controlled by an output signal of the temperature detector circuit to switch the voltage generated by the voltage regulator in accordance with the temperature detected by the temperature detector circuit.  
         [0023]     According to the Semiconductor Integrated circuit in the second aspect described above, since the voltage regulator relies on the base-emitter voltage of the bipolar transistor to generate the predetermined voltage, the voltage regulator eliminates a reference voltage generator circuit which can be a noise source. Thus, the resulting voltage regulator will not adversely affect the charge pumps and transmission VCO with noise which would be otherwise generated by the reference voltage generator circuit. Also, since the base-emitter voltage has a negative temperature characteristic, the voltage generated by the voltage regulator has temperature dependency. However, the switching element is controlled by the output signal from the temperature detector circuit to switch the generated voltage in accordance with the detected temperature, so that the charge pump and transmission VCO can be supplied with a voltage with less temperature dependency.  
         [0024]     Advantages provided by representative aspects of the invention disclosed herein may be summarized as follows.  
         [0025]     Specifically, the loop filter can be integrated on a chip. One charge pump circuit is operated in opposite phase to the other charge pump circuit to eliminate the need for a buffer, which can be a noise source, disposed between the resistive element and second capacitive element, thereby making it possible to prevent an increased phase error of the PLL loop and increased side-band spurious.  
         [0026]     Also, the voltage regulator, which does not include a reference voltage generator circuit, will not adversely affect the charge pump or transmission VCO.  
         [0027]     Even if the voltage generated by the voltage regulator has temperature dependency, the switching element is controlled by the output signal from the temperature detector circuit to switch the generated voltage in accordance with the detected temperature, making it possible to supply the charge pumps and transmission VCO with a voltage with less temperature dependency.  
         [0028]     The above and other objects, novel features, and advantages of the present invention will become apparent from the following description of the specification with reference to the accompanying drawings.  
         [0029]     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  is a circuit diagram illustrating an exemplary configuration of a second-order loop filter provided on a conventional PLL loop;  
         [0031]      FIG. 2  is a characteristic graph showing the frequency characteristic of the loop filter illustrated in  FIG. 1 ;  
         [0032]      FIG. 3  is a circuit diagram illustrating another exemplary configuration of a conventional second-order loop filter;  
         [0033]      FIG. 4  is a circuit diagram illustrating the configuration of an actual loop filter in  FIG. 3 ;  
         [0034]      FIG. 5  is a characteristic graph showing the frequency characteristic of the loop filter illustrated in  FIG. 3 ;  
         [0035]      FIG. 6  is a circuit diagram illustrating a second-order loop filter according to one embodiment of the present invention;  
         [0036]      FIG. 7  is a circuit diagram illustrating an exemplary configuration of a loop filter according to another embodiment of the present invention, and a PLL loop which employs this loop filter;  
         [0037]      FIG. 8  is a circuit diagram illustrating an exemplary configuration of a loop filter according to a further embodiment of the present invention, and a PLL loop which employs this loop filter;  
         [0038]      FIG. 9  is a circuit diagram illustrating another exemplary configuration of a PLL loop which employs a loop filter according to one embodiment of the present invention;  
         [0039]      FIG. 10  is a circuit diagram illustrating one embodiment of a voltage regulator for supplying a supply voltage to a transmission oscillator circuit (TXVCO);  
         [0040]      FIG. 11  is a temperature characteristic graph showing a voltage outputted from the voltage regulator in the embodiment of  FIG. 10 , and the temperature dependency of the potential at an internal node within a temperature detector circuit;  
         [0041]      FIG. 12  is a temperature characteristic graph showing the temperature dependency of the output voltage in an exemplary modification to the voltage regulator in  FIG. 10 ;  
         [0042]      FIG. 13  is a circuit diagram illustrating another exemplary configuration of a temperature detector of the voltage regulator in the embodiment of  FIG. 10 ;  
         [0043]      FIG. 14  is a circuit diagram illustrating one embodiment of a phase comparator circuit which operates with a voltage applied from the voltage regulator in the foregoing embodiment; and  
         [0044]      FIG. 15  is a block diagram illustrating an exemplary configuration of a radio communication system including a high frequency IC (RF-IC) which employs the PLL comprising the loop filter and phase comparator circuit in the foregoing embodiments for a transmission PLL. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0045]     In the following, preferred embodiments of the present invention will de described with reference to the accompanying drawings.  
       First Embodiment  
       [0046]      FIG. 6  illustrates one embodiment of a loop filter which can be provided on a PLL loop. The loop filter in this embodiment comprises a first charge pump CP 1  which is made up of regulated current sources I 11 , I 12 , and switches SW 11 , SW 12  connected in series with the regulated current sources I 11 , I 12 , respectively; a capacitor C 2  connected between an output node N 1  of the charge pump CP 1  and a ground point; a resistor R and a capacitor C 1  connected between the output node N 1  and ground point in parallel with the capacitor C 2 ; and a second charge pump CP 2  which is made up of regulated current sources  121 ,  122 , and switches SW 21 , SW 22  connected in series with the regulated current sources I 21 , I 22 , respectively, and has an output node N 2  connected to a connection node N 3  at which the resistor R is connected to the capacitor C 1 .  
         [0047]     In the loop filter ( FIG. 6 ) of this embodiment, the switch SW 21  for charging the second charge pump CP 2  is controlled on/off by a down signal DOWN from a previous circuit (phase comparator circuit) in opposite phase to the switch SW 11  for charging the first charge pump CP 1 . The switch SW 22  for discharging the second charge pump CP 2  is controlled on/off by an up signal UP from the previous circuit in opposite phase to the switch SW 12  for discharging the first charge pump CP 1 .  
         [0048]     When used herein, the foregoing operation of the second charge pump CP 2  is referred to as the “opposite-phase operation.” In this embodiment, the second charge pump CP 2  operated in opposite phase eliminates the need for a buffer which is disposed between the resistor R and connection node N 3  in the loop filter illustrated in  FIG. 4 . It should be noted that in the loop filter of  FIG. 4 , the switch SW 21  associated with the second charge pump CP 2  is controlled on/off by a signal in phase with a signal applied to the switch SW 11  associated with the first charge pump CP 1 , and the switch SW 22  is controlled on/off by a signal in phase with a signal applied to the switch SW 12 .  
         [0049]     The output voltage Vout of the loop filter in  FIG. 6  has the same frequency characteristic as that shown in  FIG. 2 , where a zero point is present at f 1 , and a pole at f 2 . The zero point frequency f 1  and pole frequency f 2  are expressed by the following equation  
                 (   3   )     ⁢     :       ⁢     
     ⁢       f   1     =     1     2   ⁢           ⁢     π   ·       C   1       1   -   β       ·   R           ⁢     
     ⁢       f   2     =           C   1     +     C   2         2   ⁢     π   ⁢           ·     C   1     ·     C   2     ·   R         ≈     1     2   ⁢           ⁢     π   ·   C     ⁢           ⁢     1   ·   R             ⁢     
     ⁢     β   =       i   ⁢           ⁢   2       i   ⁢           ⁢   1                               
 
         [0050]     It can be understood from the equation (3) above that when the ratio β (=i 2 /i 1 ) of the current i 2  of the current source associated with the second charge pump CP 2  to the current i 1  of the current source associated with the first charge pump CP 1  is set to 0.9, the resulting filter can have the frequencies f 1 , f 2  as if it had the capacitor C 1 , the capacitance value of which is reduced by a factor of ten.  
         [0051]     It is therefore possible to reduce the capacitance value of the capacitor C 1  to a similar capacitance value to the capacitor C 2 . In a transmission PLL contained in a high frequency IC intended by the inventors for application, since C 2  has several hundred picofarads, C 1  can be reduced as well to approximately several hundred picofarads. Then, since capacitors having capacitance values as low as the foregoing can be sufficiently formed on a semiconductor chip, the loop filter can be integrated on the semiconductor chip. Further, since this embodiment eliminates the need for the buffer which would be required in the loop filter of  FIG. 4  that controls the switches in phase, it is possible to prevent noise generated in the buffer from introducing into the output Vout to increase a phase error of the PLL loop. Moreover, the elimination of the buffer can result in a corresponding reduction in the circuit size.  
         [0052]     Next, other embodiments of the loop filter according to the present invention will be described with reference to  FIGS. 7 and 8 . These embodiments permit the PLL to rapidly pull in upon start of a voltage controlled oscillator (VCO) to enable a high speed lockup operation. A phase comparator, designated by PD, compares the phase of an oscillating signal from the VCO or a signal resulting from a frequency division of the oscillating signal with the phase of a reference signal φref to generate on/off control signals UP, DOWN for the current sources of the charge pump CP 1 , CP 2 .  
         [0053]     The embodiment illustrated in  FIG. 7  comprises a series connection of a resistor R 1  and a switch SW 1  in parallel with the resistor R in the embodiment of  FIG. 6 . The switch SW 1  is temporarily turned on by a control signal upon start of the VCO.  
         [0054]     Though not apparent in  FIG. 7 , the current sources associated with the charge pumps CP 1 , CP 2  are also controlled by the control signals to increase their current values. The switch SW 1  is turned off after the PLL is locked up, and the current values of the current sources associated with the charge pumps CP 1 , CP 2  are also returned to normal values. For scaling up/down the current values of the current sources associated with the charge pumps CP 1 , CP 2 , an auxiliary current source and switch, for example, may be provided in parallel with a reference current source, such that the switch is turned on/off to change the current value.  
         [0055]     By temporarily turning on the switch SW 1  within the loop filter upon start of the VCO in the foregoing manner, the capacitors C 1 , C 2  can be rapidly charged to speed up the pull-in of the PLL. It should be noted that the switch SW 1  alone could be disposed in parallel with the resistor R only for purposes of speeding up the pull-in of the PLL, but this may reduce a phase margin to result in oscillations. For this reason, the resistor R 1  is additionally disposed in series with the switch SW 1  to ensure a sufficient phase margin. In addition, a plurality of sets of the switch SW 1  and resistor RF may be provided in parallel with the resistor R to correct a discrepancy in the characteristic of the filter due to variations in the course of manufacturing and the like.  
         [0056]     On the other hand, the embodiment in  FIG. 8  additionally provides a switch SW 1  in parallel with the resistor R, and a switch SW 2  and a regulated voltage source VS 1  in parallel with the capacitor C 1  in the embodiment of  FIG. 6 . These switches SW 1 , SW 2  are temporarily turned on before the VCO is started, and turned off after the PLL is locked up by the associated control signals. A voltage selected for the regulated voltage source VS 1  is close to a VCO control voltage which corresponds to a frequency at which the PLL is locked.  
         [0057]     In the embodiment of  FIG. 8 , since the VCO is started with a control voltage terminal of the VCO applied with a voltage which is supposed to be applied when the PLL is locked, the PLL can be more rapidly pulled in than when the voltage applied to the control voltage terminal is gradually increased from zero volt.  
         [0058]      FIG. 9  illustrates an exemplary modification to the PLL which employs the loop filter of the foregoing embodiment.  
         [0059]     In the PLL in the embodiments of  FIGS. 7 and 8 , the charge pumps CP 1 , CP 2  for charging and discharging the capacitors of the loop filter is controlled by the signals from the single phase comparator PD. In such a configuration, since the distance from the phase comparator PD to the charge pump CP 1  differs from the distance from the phase comparator PD to the charge pump CP 2 , there is a difference between signal delay times of the two parts, which causes a discrepancy between the switching timing of the charge pump CP 1  and the switching timing of the charge pump CP 2 , possibly resulting in unwanted spurious.  
         [0060]     To address this problem, in the embodiment of  FIG. 9 , a phase comparator PD 2  is provided for controlling the charge pump CP 2  separately from the phase comparator PD 1  for controlling the charge pump CP 1 , so that the signal delay amounts can be individually adjusted. In this way, the switching timing of the charge pump CP 1  can be matched with the switching timing of the charge pump CP 2  to prevent the unwanted spurious.  
       Second Embodiment  
       [0061]     Generally, the voltage controlled oscillator (VCO) suffers from a varying oscillation frequency as the supply voltage fluctuates. To prevent the varying oscillation frequency, the present invention supplies a transmission oscillator circuit (TXVCO) with a supply voltage from a voltage regulator.  FIG. 10  illustrates one embodiment of a voltage regulator for supplying a transmission oscillator circuit (TXVCO) with a supply voltage.  
         [0062]     The voltage regulator illustrated in  FIG. 10  comprises a regulator unit  110  for generating a desired supply voltage using a base-emitter voltage Vbe of a bipolar transistor; and a temperature detector unit  120  for generating a control signal to the regulator unit  110 . Conventional voltage regulators typically have a band gap reference circuit. The use of the band gap reference circuit permits the voltage regulator to generate a stable supply voltage without source voltage dependency regardless of changes in temperature.  
         [0063]     However, in a particular voltage regulator for generating a supply voltage which is supplied to a charge pump that forms part of a transmission oscillator circuit or a transmission PLL, noise generated in the band gap reference circuit is transferred to the transmission oscillator circuit or charge pump through the generated voltage, causing a lower CN ratio.  
         [0064]     In the voltage regulator of the embodiment illustrated in  FIG. 10 , since the regulator unit  110  does not include a band gap reference circuit, no noise will be transferred to the transmission oscillator circuit or charge pumps through a generated voltage. While the temperature detector unit  120  is provided with a voltage regulator circuit which comprises a band gap reference circuit, the temperature detector unit  120  utilizes the band gap reference circuit for generating the control signal which turns on/off a MOS switch within the regulator unit  110  to switch a generated voltage, so that noise generated by the band gap reference circuit will not be transferred to the transmission oscillator circuit or charge pumps.  
         [0065]     Consequently, in a PLL circuit having a transmission oscillator circuit and charge pumps which is operated with a voltage generated by the voltage regulator, it is possible to prevent an increased phase error and increased side-band spurious.  
         [0066]     The regulator unit  110  is made up of a voltage regulator circuit  111  which comprises MOS transistors M 1 , M 2  having their source terminals connected to a supply voltage terminal Vcc and their gates coupled to each other to form a current mirror circuit; a bipolar transistor Q 1  connected in series with the MOS transistor M 1 ; a bipolar transistor Q 2  connected in series with the MOS transistor M 2 ; and resistors R 1 , R 2 , and a voltage converter circuit  112  using a differential amplifier.  
         [0067]     In the voltage regulator circuit  111 , the transistor Q 1  has a collector connected to a base of the transistor Q 2 , and a base applied with a voltage divided by the resistors R 1 , R 2  connected in series with the transistor Q 2 . Thus, the voltage regulator circuit  111  generates a regulated voltage Va which is determined by the base-emitter voltage Vbe of the transistor Q 1  and the ratio of the resistors R 1 , R 2 . In addition, a series connection of a switch MOS transistor M 3  and a resistor R 3  is connected in parallel with the resistor R 1 , such that the generated voltage Va is switched in two steps in accordance with an on- or off-state of the switch MOS transistor M 3 .  
         [0068]     The voltage converter circuit  112  comprises a pair of differential input transistors Q 3 , Q 4 ; a resistor R 4  connected between a common emitter of the transistors Q 3 , Q 4  and a ground point; active load MOS transistors M 4 , M 5  connected to collectors of the transistors Q 3 , Q 4 , respectively; an output transistor M 6  having a gate terminal connected to the collector of one differential input transistor Q 3 ; and a transistor Q 5  and a resistor R 6  connected in series between the supply voltage Vcc and ground point. The transistor Q 5  is applied with an output voltage Vreg at the base terminal, while the differential input transistor Q 3  is applied with the regulated voltage Va generated by the voltage regulator circuit  111  at the base terminal, and an emitter voltage of the transistor Q 5  is fed back to the base terminal of the other differential input transistor Q 4 , so that the voltage converter circuit  112  operates in such a manner that the emitter voltage Vb of the transistor Q 5  matches the regulated voltage Va from the voltage regulator circuit  111 .  
         [0069]     The voltages Va, Vb, Vreg generated by the appropriate components in the voltage regulator of  FIG. 10  are expressed as follows:  
       Va   =       {         (       r   ⁢           ⁢   1     +     r   ⁢           ⁢   2       )     /   r     ⁢           ⁢   2     }     ×   VbeQ   ⁢           ⁢   1         
       Vb   =       Va   -     VbeQ   ⁢           ⁢   3     +     VbeQ   ⁢           ⁢   4       ≈   Va         
             Vreg   =       ⁢     Vb   +     VbeQ   ⁢           ⁢   5                   =       ⁢     Va   +     VbeQ   ⁢           ⁢   5                   =       ⁢         {         (       r   ⁢           ⁢   1     +     r   ⁢           ⁢   2       )     /   r     ⁢           ⁢   2     }     ×   VbeQ   ⁢           ⁢   1     +     VbeQ   ⁢           ⁢   5                 
 
 where r 1 , r 2  represent the resistance values of the resistors R 1 , R 2 ; and VbeQ 1 , VbeQ 3 , VbeQ 4 , VbeQ 5  represent the base-emitter voltages of the bipolar transistors Q 1 , Q 3 , Q 4 , Q 5 , respectively. 
 
         [0070]     Further, when VbeQ 1 =VbeQ 5  stands, the foregoing equation can be transformed as follows supposing that VbeQ 1 =VbeQ 5 =Vbe stands: 
 
 Vreg ={( r 1+2 r 2)/ r 2}× Vbe  
 
         [0071]     It can be understood from this equation that the voltage regulator of  FIG. 10  can generate a voltage which is determined by the base-emitter voltage Vbe of the bipolar transistor and the resistance ratio of the resistors R 1 , R 2 . As is well known, the base-emitter voltage Vbe of the bipolar transistor has a negative temperature characteristic. Therefore, the output voltage Vreg generated by the voltage regulator of  FIG. 10  would fluctuate following a change in temperature unless appropriate actions were taken therefor (Vreg is lower as the temperature is higher).  
         [0072]     In the embodiment of  FIG. 10 , to address fluctuations in the output voltage Vreg, a series connection of the switch MOS transistor M 3  and resistor R 3  is connected in parallel with the resistor R 1  of the voltage regulator circuit  111 , such that the generated voltage Va is switched in two steps in accordance with an on- or off-state of the switch MOS transistor M 3 .  
         [0073]     In response to the voltage Va switched in two steps, the voltage regulator also switches the output voltage Vreg in steps in accordance with the on- or off-state of the switch MOS transistor M 3 .  
         [0074]     Specifically, as the switch MOS transistor M 3  is turned on, which is equivalent to a reduction in the resistance value r 1  of the resistor R 1  in the aforementioned equation which expresses the output voltage Vreg, the voltage Va is reduced so that the voltage regulator generates a lower output voltage Vreg. Therefore, the switch MOS transistor M 3  is turned on at lower than a proper temperature Tc to reduce the output voltage Vreg, while the switch MOS transistor M 3  is turned off at the proper temperature Tc or higher to increase the output voltage Vreg.  
         [0075]     In this way, the output voltage Vreg of the voltage regulator can be controlled to decrease as the temperature becomes higher, once rise up at the temperature Tc at one step, and again gradually decrease, as indicated by a solid line in  FIG. 11 . As a result, by changing the output voltage Vreg through the switching of the switch MOS transistor M 3 , it is possible to reduce the amount of fluctuations in the output voltage Vreg of the voltage regulator with respect to the temperature. If the switch MOS transistor M 3  is not switched, the output voltage Vreg of the voltage regulator will be largely reduced as the temperature rises, as indicated by a broken line in  FIG. 11 .  
         [0076]     The temperature detector unit  120  in the voltage regulator of  FIG. 10  is provided for generating a signal for switching the switch MOS transistor M 3 .  
         [0077]     The temperature detector unit  120  in this embodiment comprises a bias circuit  121  composed of MOS transistors M 7 -M 9 ; a differential circuit  122  composed of bipolar transistors Q 6 , Q 7 , which differ in the emitter size from each other, current mirror MOS transistors M 10 , M 11  for supplying collector currents to the bipolar transistors Q 6 , Q 7 , respectively, and resistors R 6 , R 7  connected between emitters of the bipolar transistors Q 6 , Q 7  and a ground point; an emitter follower circuit  123   a  composed of a bipolar transistor Q 8  for applying a current in accordance with a collector voltage of the bipolar transistor Q 7 , and a resistor R 8 ; a source follower circuit  123   b  composed of a MOS transistor M 12  for applying a current in accordance with a collector voltage of the bipolar transistor Q 6 , and a resistor R 9 ; and a voltage comparator circuit  124  composed of bipolar transistors Q 9 , Q 10  which are applied with a drain voltage of the MOS transistor M 12  and an emitter voltage of the bipolar transistor Q 8 , respectively, at their bases, and have their emitters coupled to each other to perform a differential operation, current mirror MOS transistors M 13 -M 17  which are connected to collectors of the bipolar transistors Q 9 , Q 10 , and MOS transistors M 17 , M 18  which are connected in series with the MOS transistors M 13 , M 17  and have their gate terminals connected to each other to form a current mirror; and an inverter circuit  125  which has a hysteresis characteristic. The transistors Q 6 , Q 7  in the temperature detector circuit  122  are set such that the former has an emitter size eight times larger than the latter.  
         [0078]     In the temperature detector circuit  122  in  FIG. 10 , the following equations are derived: 
 
 I 1=( VbeQ 7 −VbeQ 6)/ r 6 
 
 Vc =( I 1 +I 2) r 7 +VbeQ 7 
 
 Vd=I 3 r 9 
 
 where I 1 , I 2  represent collector currents flowing into the bipolar transistors Q 6 , Q 7 , respectively; Vc represents a voltage at the emitter of the bipolar transistor Q 8 ; Vd represents a drain voltage of the MOS transistor M 12 ;  13  represents a current flowing through the resistor R 9 ; and r 6 , r 7 , r 9  are resistance values of the resistors R 6 , R 7 , R 9 , respectively. 
 
         [0079]     Assuming herein that the MOS transistors MNb, M 11 , M 12  connected to form a current mirror have the same size, I 1 =I 2 =I 3  is established. In this event, paying attention to the bipolar transistors Q 6 , Q 7 , VbeQ 7 =VbeQ 6 +I 1  r 6  is established. From this equation, the corrector current I 1  of the bipolar transistor Q 6  is expressed by I 1 =(VbeQ 7 −VbeQ 6 )/r 6 . Therefore, the voltages Vc, Vd are transformed as expressed by the following equations:  
                   Vc   =       ⁢       2   ⁢           ⁢   I   ⁢           ⁢   1   ⁢           ⁢   r   ⁢           ⁢   7     +     VbeQ   ⁢           ⁢   7                   =       ⁢       {     2   ⁢     (       VbeQ   ⁢           ⁢   7     -     VbeQ   ⁢           ⁢   6       )     ⁢   r   ⁢           ⁢     7   /   r     ⁢           ⁢   6     }     +     VbeQ   ⁢           ⁢   7                     Equation   ⁢           ⁢   3.1             
  Vd =( VbeQ 7 −VbeQ 6) r 9 /r 6  Equation 3.2  
         [0080]     The base-emitter voltages VbeQ 6 , VbeQ 7  of the bipolar transistors Q 6 , Q 7  have negative temperature characteristics. Moreover, in this embodiment, the bipolar transistors Q 6 , Q 7  are set to have the emitter sizes in the ratio of 8:1. Therefore, the absolute values of their base-emitter voltages are in a relationship expressed by |VbeQ 6 |&lt;|VbeQ 7 |. From this relationship, (VbeQ 7 −VbeQ 6 ) in the foregoing Equation 3.2 is positive. In conclusion, a temperature coefficient of the voltage Vc can be brought closer to zero if the resistance values r 6 , r 7  of the resistors R 6 , R 7  are set such that the absolute value of a temperature coefficient in the first term is equal to the absolute value of a temperature coefficient in the second term.  
         [0081]     On the other hand, it can be understood from Equation 3.2 that a temperature coefficient of the voltage Vd can be adjusted by changing a resistance ratio r 9 /r 6  of the resistors R 6  and R 9 .  
         [0082]     In the temperature detector circuit in this embodiment, the resistance values r 6 , r 7  are appropriately set for the resistors R 6 , R 7  to bring the temperature coefficient of the voltage Vc closer to zero, and the resistance ratio r 9 /r 6  is appropriately adjusted for the resistors R 6 , R 9  such that the voltages Vc, Vd intersect near 40° C., as shown in  FIG. 11 .  
         [0083]     The voltage comparator circuit  124  compares the voltages Vd, Vc, and changes its output voltage if Vd is higher than Vc, causing the output of the inverter  125  to transition to a high level which turns off the switch MOS transistor M 3  in the regulator unit  110 . The resistance ratio r 9 /r 6  can be changed for the resistors R 6 , R 9  to arbitrarily set a temperature at which the switch MOS transistor M 3  switches from on-state to off-state.  
         [0084]     It should be noted that the temperature detector unit  120  in this embodiment can suffer from noise generated in the temperature detector circuit  122 , but the noise, if any, will be blocked by the inverter  125 , and therefore will never be transferred to the voltage regulator unit  110 . It is therefore possible to prevent a degradation in the characteristic of a circuit which operates with the voltage generated by the voltage regulator. In addition, the voltage comparator circuit  124  may be provided with a hysteresis characteristic to prevent the switch MOS transistor M 3  from repeatedly turning on and off due to fluctuations in temperature.  
         [0085]     The foregoing embodiment has been described in connection with a circuit which is configured to switch the voltage generated by the voltage regulator in two steps in accordance with the temperature.  
         [0086]     Alternatively, the regulator unit  110  in  FIG. 10  may comprise a plurality of sets of the switch MOS transistor M 3  and resistor R 3 , and the temperature detector unit  120  may comprise a plurality of sets of the source follower circuit  123   b  composed of the MOS transistor M 12  and resistor R 9 , and the comparator circuit  124  composed of a pair of the differential transistors Q 9 , Q 10 , to switch the voltage generated by the voltage regulator in multiple steps, for example, as illustrated in  FIG. 12 , in accordance with the temperature.  
         [0087]      FIG. 13  illustrates an exemplary modification to the voltage regulator according to the second embodiment. This modified voltage regulator comprises normal-on switching elements SW 1 , SW 2 , SW 3  connected between the source terminal of the MOS transistor M 8  and the supply voltage terminal Vcc, between the gate and drain of the MOS transistor M 10 , and between the gate terminals of the MOS transistors M 10  and M 12 , and normal-off switching elements SW 4 , SW 5  connected between the gate terminals of the MOS transistors M 10 , M 11  and the supply voltage terminal Vcc, and between the gate terminal of the MOS transistor M 12  and the supply voltage terminal Vcc, respectively, in the temperature detector unit  120 . With the provision of these switching elements SW 1 -SW 5 , the temperature detector circuit  120  is operated only when temperature information is required, such as upon start of the PLL circuit, and is otherwise made inoperative to save the power consumption. The normal-off switching elements SW 4 , SW 5  are provided for preventing a through current from flowing through the circuit due to a floating potential state at an internal node in the circuit during an inoperative state.  
         [0088]     Also, in this modified voltage regulator, the inverter circuit  125  in the embodiment of  FIG. 10  is replaced with a flip-flop  126  for latching the output of the temperature detector circuit  120 . This flip-flop  126  latches the output of the temperature detector circuit  120  immediately before the VCO is started, so that the VCO can be obviated from instable operations caused by variations in the output of the temperature detector circuit  120  after the start of the VCO due to fluctuations in temperature.  
       Third Embodiment  
       [0089]      FIG. 14  illustrates one embodiment of a phase comparator circuit which is applied with the voltage from the voltage regulator in the foregoing embodiment for operations. The phase comparator circuit  236  in this embodiment comprises a signal comparator unit (left-hand side circuit)  361  and an output unit (right-hand side circuit)  362 . As illustrated in  FIG. 14 , the signal comparator unit  361  comprises a cascaded configuration of differential circuits, wherein a current regulating transistor Q 30  is connected to emitters of a lower differential transistor pair Q 31 , Q 32 , and differential transistor pairs Q 33 , Q 34  and Q 35 , Q 36  are connected to collectors of the lower differential transistor pair Q 31 , Q 32 . The transistors Q 33 , Q 35  have their collectors coupled to each other, which are then connected to a common load transistor Q 37 . Likewise, the transistors Q 34 , Q 36  also have their collectors coupled to each other, which are connected to a common load transistor Q 38 .  
         [0090]     In this embodiment, differential reference signals φref, /φref are applied to base terminals of the lower differential transistor pair Q 31 , Q 32 , respectively, while a signal φTX from the VCO is applied to base terminals of the transistors Q 33 , Q 36 , and a signal /φTX having the phase opposite to that of the signal φX is applied to base terminals of the transistors Q 34 , Q 35  in the upper differential transistor pairs Q 33 -Q 36 , so that the transistors Q 33 -Q 36  generate voltages at their collectors in accordance with a phase difference between φref and φTX.  
         [0091]     The output unit  362  comprises transistors Q 41 , Q 44  connected in a current mirror configuration with the load transistor Q 37  in the signal comparator unit; transistors Q 42 , Q 43  connected in a current mirror configuration with the load transistor Q 38 ; transistors Q 45 -Q 48  connected in series with the transistors Q 41 - 44 , respectively; and transistors Q 49 , Q 50  for applying bias voltages to base terminals of the transistors Q 45 , Q 46  and Q 47 , Q 48 , respectively. The transistors Q 45 , Q 46  have their bases connected in common, the transistors Q 47 , Q 48  also have their bases connected in common, the transistors Q 49 , Q 50  have their collectors connected to a supply voltage terminal, and their bases connected to collectors of Q 41 , Q 43 , respectively, such that bias voltages are applied from an emitter of the transistor Q 49  to the common base of the transistors Q 45 , Q 46 , and from an emitter of the transistor Q 50  to the common base of the transistors Q 47 , Q 48 , respectively.  
         [0092]     In the phase comparator circuit of this embodiment, the transistors Q 41 , Q 44  and transistors Q 42 , Q 43  are respectively set to have their emitter sizes in a ratio of 10:9. The transistors Q 45 , Q 46  have the same emitter size, and the transistors Q 47 , Q 48  also have the same emitter size. Consequently, the transistors Q 44 , Q 48  are applied with 9/10 of a current applied to the transistors Q 42 , Q 46 .  
         [0093]     Further, the transistor Q 48  is applied at its base with a signal in phase with a signal applied to a base of the transistor Q 42 , while the transistor Q 44  is applied at its base with a signal in phase with a signal applied to a base of the transistor Q 41 , so that the pairs of transistors Q 42 , Q 48  and Q 46 , Q 44  are controlled in phase, respectively. Thus, the transistor Q 42  corresponds to the regulated current source I 11 , while the transistor Q 46  corresponds to the regulated current source I 12  in  FIG. 6 . Also, the transistor Q 44  corresponds to the regulated current source I 21 , while the transistor Q 48  corresponds to the regulated current source  122 . The phase comparator circuit of the embodiment in  FIG. 14  configured in the foregoing manner operates with a supply voltage which is the voltage Vreg supplied from the regulator unit  110  with less source voltage dependency and temperature dependency, allowing the regulated current sources to maintain consistent current values against fluctuations in the supply voltage and changes in temperature for charging and discharging a loop filter.  
       Fourth Embodiment  
       [0094]     Referring next to  FIG. 15 , description will be made on an exemplary configuration of a radio communication system including a high frequency IC (RF-IC) which employs the PLL comprising the loop filter and phase comparator circuit according to the foregoing embodiments for a transmission PLL.  
         [0095]     As illustrated in  FIG. 15 , the radio communication system in this embodiment comprises an antenna  400  for transmitting and receiving signal radiowaves; a switch  410  for switching between a transmission and a reception mode; bandpass filters  420   a - 420   d  each comprised of a SAW filter for removing unwanted waves from a received signal, and the like; a high frequency power amplifier circuit (power module)  430  for amplifying a transmission signal; a high frequency IC  200  for demodulating a received signal and modulating a transmission signal; and a baseband circuit  300  for converting transmission data into I, Q signals and controlling the high frequency IC  200 . In this embodiment, the high frequency IC  200  and baseband circuit  300  are implemented on separate semiconductor chips, respectively, as individual semiconductor integrated circuits.  
         [0096]     Though not particularly limited, the high frequency IC  200  in this embodiment is configured to be capable of modulating and demodulating signals in four frequency bands conforming to the following communication schemes: GSM850 and GSM900, DCS1800, and PCS1900. Accordingly, the bandpass filters provided in the radio communication system include the filter  420   a  for passing therethrough a received signal in the frequency band of GSM 850; filter  420   b  for passing therethrough a received signal in the frequency band of GSM 900; filter  420   c  for passing therethrough a received signal in the frequency band of DCS1800; and filter  420   d  for passing therethrough a received signal in the frequency band of PCS1900.  
         [0097]     The high frequency IC  200  in this embodiment is generally divided into a reception-related circuit RXC; a transmission-related circuit TXC; and a control-related circuit which comprises circuits common to the transmission- and reception-related circuits such as a control circuit, a clock generator circuit, and the like.  
         [0098]     The reception-related circuit RXC comprises low noise amplifiers  210   a - 210   d  for amplifying received signals in the respective frequency bands of GSM850, GSM900, DCS1800, and PCS1900, respectively; a frequency divider/phase shifter circuit  211  for dividing a local oscillating signal φRF generated by a high frequency oscillator circuit (RFVCO)  250  and generating orthogonal signals which are 90° out of phase from each other; mixer circuits  212   a ,  212   b  for mixing received signals amplified by the low noise amplifiers  210   a - 210   d  with the orthogonal signals generated by the frequency divider/phase shifter circuit  211  to demodulate and downconvert an I-signal and a Q-signal; high gain amplifier units  220 A,  220 B common to the respective frequency bands for amplifying the demodulated I- and Q-signals for delivery to the baseband LSI  300 ; and an offset cancel circuit  213  for canceling input DC offsets of amplifiers in the high gain amplifier units  220 A,  220 B.  
         [0099]     The high gain amplifier unit  220 A comprises a plurality of low pass filters LPF 11 , LPF 12 , LPF 13 , LPF 14  and gain control amplifiers PGA 11 , PGA 12 , PGA 13  which are alternately connected in series; and an amplifier AMP 1  connected at the final stage, and amplifies a demodulated I-signal to a predetermined amplitude level while removing unwanted waves. Likewise, the high gain amplifier unit  220 B comprises a plurality of low pass filters LPF 21 , LPF 22 , LPF 23 , LFP 24  and gain control amplifiers PGA 21 , PGA 22 , PGA 23  which are alternately connected in series; and an amplifier AMP 2  connected at the final stage, and amplifies a demodulated Q-signal to a predetermined amplitude level.  
         [0100]     The offset cancel circuit  213  comprises A/D converter circuits (ADC) provided in correspondence to the respective gain control amplifiers PGA 11 -PGA 23  for converting output potential differences of the gain control amplifiers PGA 11 -PGA 23  when their input terminals are short-circuited; D/A converter circuits (DAC) each for generating an input offset voltage based on the result of the conversion made by an associated A/D converter, such that the resulting input offset voltage reduces a DC offset to zero in the output of the corresponding gain control amplifier PGA 11 -PGS 23 , and applying the generated input offset voltage to a differential input of the corresponding gain control amplifier; and a control circuit for controlling the A/D converter circuits (ADC) and D/A converter circuits (DAC) to perform an offset cancel operation.  
         [0101]     The transmission-related circuit TXC comprises an oscillator circuit (IFVCO)  230  for generating an oscillating signal φIF at an intermediate frequency such as 640 MHz, for example; a phase shifter/frequency divider circuit  232  for dividing the oscillating signal φIF generated by the oscillator circuit  230  and generating orthogonal signals which are 90° out of phase from each other; modulator circuits  233   a ,  233   b  each comprised of a mixer for modulating the generated orthogonal signal with the I-signal or Q-signal supplied from the baseband circuit  300 ; an adder  234  for adding the modulated signals; transmission oscillator circuits (TXVCO)  240   a ,  240   b  each for generating a transmission signal φTX at a predetermined frequency; an offset mixer  235   a  for synthesizing a feedback signal which is the transmission signal φTX outputted from the transmission oscillator circuit  240   a ,  240   b , extracted by a coupler or the like, with a signal φRF′ divided from the oscillating signal φRF generated by the high frequency oscillator circuit (RFVCO)  250  to generate a signal at a frequency corresponding to a difference in frequency between the two signals; a phase comparator circuit  236  for comparing the output of the offset mixer  235   a  with a signal TXIF resulting from the synthesis in the adder  234  to detect a frequency difference and a phase difference; a charge pump/loop filter  237  for generating a voltage in accordance with the output of the phase detector circuit  236 ; and buffer circuits  238   a ,  238   b  each for converting differential outputs of the associated TXVCO  240   a ,  240   b  into a single signal for delivery to the outside.  
         [0102]     One of the transmission oscillator circuits  240   a ,  240   b  generates a signal in a band from 850 to 900 MHz for GSM, and the other generates a signal in a band from 1800 to 1900 MHz for DCS and PCS.  
         [0103]     The high frequency IC chip  200  in this embodiment further comprises a control circuit  260  for controlling the overall chip; an RF synthesizer  261  which makes up an RF PLL circuit together with the high frequency oscillator circuit (RFVCO)  250 ; an IF synthesizer  262  which makes up an IF PLL circuit together with the intermediate frequency oscillator circuit (IFVCO)  230 ; and a reference oscillator circuit (TCXO)  264  for generating a clock signal φref which serves as a reference signal for these synthesizers  261 ,  262 . The synthesizers  261 ,  262  are each composed of a phase comparator circuit, a charge pump, a loop filter, and the like.  
         [0104]     Since the reference oscillating signal φref is required to be accurate in frequency, an external quartz vibrator is connected to the reference oscillator circuit  264 . The frequency selected for the reference oscillating signal φref may be 26 MHz or 13 MHz. This is because quartz vibrators at such frequencies are general purpose parts and are readily available on the market.  
         [0105]     The control circuit  260  comprises a control register which is set based on a signal from the baseband IC  300 . Specifically, the high frequency IC  200  is supplied with a clock signal CLK for synchronization, a data signal SDATA, and a load enable signal LEN as a control signal from the baseband IC  300 . As the load enable signal LEN is asserted to a valid level, the control circuit  260  sequentially captures the data signal SDATA transmitted thereto from the baseband IC  300  in synchronism with the clock signal CLK, and sets the captured data signal SDATA into the control register. Though not particularly limited, the data signal SDATA may be transmitted in series. The baseband IC  300  may be based on a microprocessor or the like. The data signal SDATA includes a command issued from the baseband IC  300  to the high frequency IC  200 .  
         [0106]     In this embodiment, the phase detector circuit  236 , charge pump/loop filter  237 , transmission oscillator circuits (TXVCO)  240   a ,  240   b , and offset mixer  235  make up a transmission PLL circuit (TXPLL) for performing a frequency conversion. In the multi-band type radio communication system according to this embodiment, the control circuit  260  changes the frequency φRF of the oscillating signal from the high frequency oscillator circuit  250  in accordance with a particular channel in use, and changes the frequency of a signal supplied to the offset mixer  235   a  in accordance with a GSM mode or a DCS/PCS mode to switch the transmission frequency, for example, in response to a command from the baseband IC  300  during transmission and reception.  
         [0107]     On the other hand, different values are set for the oscillation frequency of the high frequency oscillator circuit (RFVCO)  250  in the reception mode and transmission mode, respectively. In the transmission mode, the oscillation frequency fRF of the high frequency oscillator circuit (RFVCO)  250  is set, for example, in a range of 3,616 to 3,716 MHz for GSM850, in a range of 3,840 to 3,980 MHz for GSM900, in a range of 3,610 to 3,730 MHz for DCS, or in a range of 3,860 to 3,980 MHz for PCS. Then, the frequency divider circuit divides the oscillation frequency fRF by four for GSM, or by two for DCS and PCS, and the resulting oscillation frequency is supplied to the offset mixers  235   a ,  235   b.    
         [0108]     The offset mixer  235   a  outputs a signal which corresponds to a difference in frequency between the oscillating signal φRF from the RFVCO  250  and the transmission oscillating signal φTX from the transmission oscillator circuits (TXCVO)  240   a ,  240   b  (fRF-fTX), and the transmission PLL (TXPLL) operates such that this difference signal matches the modulated signal TXIF in frequency. Stated another way, the TXVCOs  240   a ,  240   b  are controlled to oscillate at a frequency which corresponds to the difference in frequency between the oscillating signal φRF from the RFVCO  250  (fRF/4 for GSM, and fRF/2 for DCS and PCS) and the modulated signal TXIF.  
         [0109]     While the invention created by the inventors has been described in specific manner with reference to several embodiments thereof, it should be understood that the present invention is not limited to the foregoing embodiments but can be modified in various manners without departing from the spirit and scope of the invention. For example, while the foregoing embodiments have been described in connection with a second-order loop filter to which the present invention is applied, the present invention can be applied as well to a third-order loop filter.  
         [0110]     In the foregoing description, the present invention made by the inventors has been discussed oscillating signal φRF from the RFVCO  250  and the transmission oscillating signal φTX from the transmission oscillator circuits (TXCVO)  240   a ,  240   b  (fRF-fTX), and the transmission PLL (TXPLL) operates such that this difference signal matches the modulated signal TXIF in frequency. Stated another way, the TXVCOs  240   a ,  240   b  are controlled to oscillate at a frequency which corresponds to the difference in frequency between the oscillating signal φRF from the RFVCO  250  (fRF/4 for GSM, and fRF/2 for DCS and PCS) and the modulated signal TXIF.  
         [0111]     While the invention created by the inventors has been described in specific manner with reference to several embodiments thereof, it should be understood that the present invention is not limited to the foregoing embodiments but can be modified in various manners without departing from the spirit and scope of the invention. For example, while the foregoing embodiments have been described in connection with a second-order loop filter to which the present invention is applied, the present invention can be applied as well to a third-order loop filter.  
         [0112]     In the foregoing description, the present invention made by the inventors has been discussed mainly in connection with a transmission PLL which forms part of a radio communication system, and a high frequency IC which contains the transmission PLL, which belong to the field of utilization that underlies the