Patent Publication Number: US-7715802-B2

Title: Frequency synthesizer and multi-band radio apparatus using said frequency synthesizer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-247703, filed Aug. 17, 2000, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a frequency synthesizer for generating signals having a plurality of desired frequencies, and also relates to a multi-band radio apparatus using the same. 
   2. Description of the Related Art 
   In general, mobile communication terminals are designed for the purpose of using in one communication system. Such a communication system may be a PDC (Personal Digital Cellular) mobile phone system, a mobile phone system conforming to IS-95, or PHS (Personal Handy-phone System). It is quite usual that one mobile communication terminal complies with only-one standard among others of various communication systems existing in the world. 
   Recently, demand is raised to provide another mobile communication terminal to cope with rapid diversifications of the mobile communication systems. Such a terminal can solely control transmission/reception in response to multiple, different communication systems. For example, so-called “multi-mode terminal” used for both the PDC mobile phone system and the PHS has already been proposed. 
   In most cases, different mobile communication systems use different frequency bands; therefore, a multi-mode terminal to deal with them should be provided with a “multi-band radio function”, i.e., a function of transmitting/receiving data within each of multiple frequency bands. 
   A direct conversion mode is known as an architecture suitable for realizing such a multi-band radio apparatus. In the apparatus using the direct conversion mode, received signals from an antenna are inputted to one of quadrature demodulators. 
   To the quadrature demodulator, a pair of local signals for receiver having phases different from each other by 90° are also inputted. They are generated by subjecting local signals output from a frequency synthesizer, to the phase shift by a π/2 phase shifter. Note that frequencies of the local signals are set with regard to frequencies of desired signals in the received signals. 
   Because the quadrature demodulator multiplies the received signals by the local signals, the desired signals are converted into baseband signals for an I (Inphase) channel and a Q (Quadrature phase) channel with a center frequency of 0 Hz, which are inputted to a baseband reception section for subsequent signal reproduction processing. 
   On the other hand, signals to be transmitted for the I channel and the Q channel generated by a baseband transmission section are inputted to the another quadrature modulator. 
   To a local input port of the quadrature modulator, local signals for transmitter having phases different from each other by 90°, which are generated by subjecting local signals output from the frequency synthesizer to the phase shift by the π/2 phase shifter are inputted. 
   Frequencies of the local signals are set to be equal to a transmission frequency. As this quadrature demodulator multiplies the transmission signals by the local signals, the frequencies of the transmission signals are converted into a predetermined transmission frequency. 
   The frequency synthesizer used in the multi-band radio apparatus must generate local signals in various frequency bands according to realization of the multi-band. Note that this requirement is not limited to the direct conversion mode. 
   Various modes such as GSM (global system mobile communication) using the 900 MHz band, DCS (digital cellular system) using the 1800 MHz band, PCS (personal communication services) using the 1900 MHz band, UMTS (universal mobile telecommunication system) using the 2 GHz band are extensively utilized in the world. Development of a four-band radio apparatus supposed to be used in all of these frequency bands is desired. 
   When the frequency synthesizer to cope with such a four-band radio apparatus is realized in compliance with, for instance, the direct conversion mode, there can be considered a method for preparing respective unit synthesizers for: GSM transmission, GSM reception, DCS transmission, DCS reception, PCS transmission, PCS reception, UMTS transmission and UMTS reception by analogy with the method for constituting the frequency synthesizer in the two-band radio apparatus which can cope with both PDS and PHS. 
   Since the reception frequency band of PCS and the transmission frequency band of UMTS are nearly equal to each other, one synthesizer can function for the both modes. That is, except special cases, unit synthesizers whose number corresponds to a plurality of necessary frequency bands are basically prepared. Therefore, when a number of bands is increased, a number of the unit synthesizers is also proportionately increased, which results in vast hardware. 
   In preparing the unit synthesizers according to the respective frequency bands in order to realize the multi-band radio apparatus, multiple unit synthesizers are required when a number of bands is increased. 
   Therefore, the scale of hardware become larger, which leads to increase in size of the multi-mode terminal and the price and the power consumption. 
   BRIEF SUMMARY OF THE INVENTION 
   In view of the above-described problems, it is an object of the present invention to provide a frequency synthesizer which comprises a small number of unit synthesizers and has a small circuit scale, and a multi-band radio apparatus using this frequency synthesizer. 
   To achieve this aim, according to the present invention, there is provided a frequency synthesizer comprising: 
   a first synthesizer which outputs signal of which frequency is within one of a plurality of frequency bands; 
   a second synthesizer which outputs a fixed frequency signal; 
   a first mixer which mixes the signal output from the first synthesizer with the fixed frequency signal output from the second synthesizer; 
   a first divider which divides a signal output from the first mixer by a first division ratio; 
   a second divider which divides the fixed frequency signal output from the second synthesizer by a second division ratio; 
   a second mixer which mixes the signal output from the first synthesizer with a signal output from the second divider; 
   a third divider which divides a signal output from the second mixer by a third division ratio to output a signal to be used as a first local signal; and 
   a switch which outputs either a signal output from the first divider or a signal output from the third divider as a second local signal. 
   As described above, in the frequency synthesizer according to the embodiment of the present invention, it is possible to generate signals in a plurality of frequency bands whose number is larger than that of the unit synthesizers by the small-scale circuit configuration in which the two unit synthesizers are combined with the arithmetic circuit comprising dividers and mixers for multiplication. 
   According to another embodiment of the present invention, there is provided a multi-band radio apparatus comprising: 
   a frequency synthesizer including:
         a first synthesizer which outputs signal of which frequency is within one of a plurality of frequency bands;   a second synthesizer which outputs a fixed frequency signal;   a first mixer which mixes the signal output from the first synthesizer with the fixed frequency signal output from the second synthesizer;   a first divider which divides a signal output from the first mixer by a first division ratio;   a second divider which divides the fixed frequency signal output from the second synthesizer by a second division ratio;   a second mixer which mixes the signal output from the first synthesizer with a signal output from the second divider;   a third divider which divides a signal output from the second mixer by a third division ratio to output a signal to be used as a first local signal; and   a switch which outputs either a signal output from the first divider or a signal output from the third divider as a second local signal;       

   a quadrature demodulator connected to the frequency synthesizer, which demodulates a received signal by use of the reception local signal; and 
   a quadrature modulator connected to the frequency synthesizer, which modulates a signal to be transmitted by use of the transmission local signal. 
   In a multi-band radio apparatus having in a radio portion a quadrature demodulator for demodulating a received signal by a pair of local signals having phases different from each other by 90° or 45° and a quadrature modulator for modulating a pair of transmission signals having phases different from each other by 90° by using a pair of local signals having phases different from each other by 90°, the frequency synthesizer is used to generate the local signals for receiver and the local signals for transmitter. With such a structure, for example, the multi-band radio apparatus adopting the direct conversion mode for both the transmission and reception systems can be realized in a small hardware scale. 
   Furthermore, in a multi-band radio apparatus having in a radio portion a quadrature demodulator for demodulating a received signal by a pair of local signals having phases different from each other by 90° or 45°, a quadrature modulator for modulating a pair of transmission signals having phases different from each other by 90° by using a pair of first local signals having phases different from each other by 90°, and a frequency converter for converting a frequency of an output signal from the quadrature modulator by using a second local signal, the frequency synthesizer is used to generate the local signals. With such a structure, for example, the multi-band radio apparatus using the direction conversion mode for the reception system and the super heterodyne mode for the transmission system can be realized in the small hardware scale. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  is a block diagrams showing a structure of a multi-band radio apparatus including a frequency synthesizer according to a first embodiment of the present invention; 
       FIG. 2  is a block diagram showing a structural example of an image suppression type mixer; 
       FIG. 3  is a block diagram showing a structure of a multi-band radio apparatus including a frequency synthesizer according to a second embodiment of the present invention; 
       FIG. 4A  is a block diagram showing an example of a divider also serving as a π/2 phase shifter; 
       FIG. 4B  is a timing chart of the divider also serving as a π/2 phase shifter; 
       FIG. 5  is a block diagram showing a structure of a multi-band radio apparatus including a frequency synthesizer according to a third embodiment of the present invention; 
       FIG. 6  is a block diagrams showing a structure of a multi-band radio apparatus including a frequency synthesizer according to a fourth embodiment of the present invention; 
       FIG. 7  is a block diagram showing a structure of a multi-band receiver including a frequency synthesizer according to a fifth embodiment of the present invention; 
       FIG. 8  is a block diagram showing a structure of a multi-band receiver including a frequency synthesizer according to a sixth embodiment of the present invention; 
       FIG. 9  is a block diagram showing a structure of a multi-band receiver including a frequency synthesizer according to a seventh embodiment of the present invention; 
       FIG. 10  is a block diagram showing a structure of a frequency synthesizer according to an eighth embodiment of the present invention; 
       FIG. 11  is a block diagram showing a structure of a frequency synthesizer according to a ninth embodiment of the present invention; 
       FIG. 12  is a block diagrams showing a structure of a multi-band receiver including a frequency synthesizer according to a tenth embodiment of the present invention; 
       FIG. 13  is a block diagram showing a structure of a multi-band receiver including a frequency synthesizer according to an eleventh embodiment of the present invention; and 
       FIG. 14  is a block diagram showing a structure of a multi-band receiver including a frequency synthesizer according to a twelfth embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   First Embodiment 
     FIG. 1  is a block diagram showing a structure of a multi-band radio apparatus including a frequency synthesizer according to a first embodiment of the present invention. The multi-band radio apparatus of this embodiment is a four-band radio apparatus adopting the direct conversion mode conform to GSM/DCS/PCS/UMTS. 
   A received signal from an antenna  1  is inputted to a quadrature demodulator  2  including two mixers  2 A and  2 B. When this received signal is multiplied by local signals having phases 0° and 90° inputted from a frequency synthesizer  10 A to local input ports of the mixers  2 A and  2 B through a π/2 phase shifter  4 , baseband received signals Ir and Qr for an I channel and a Q channel are generated. The baseband received signals Ir and Qr are inputted to a non-illustrated baseband processing stage. 
   On the other hand, baseband transmission signals It and Qt for the I channel and the Q channel output from the baseband processing stage are inputted to a quadrature modulator  3  including two mixers  3 A and  3 B. When these signals are multiplied by local signals having phases 0° and 90° inputted from the frequency synthesizer  10 A to local input ports of the mixers  3 A and  3 B through a π/2 phase shifter  5 , RF transmission signals for the I channel and the Q channel are generated. The RF signals for the I channel and the Q channel are combined with each other and transmitted through the antenna  1 . 
   The frequency synthesizer  10 A will now be described. 
   The frequency synthesizer  10 A comprises an HF synthesizer  11  for generating a first reference frequency signal having a variable frequency in a high-frequency band and an LF synthesizer  12  for generating a second reference frequency signal in a low-frequency band as unit synthesizers. Here, the terms “high-frequency band” and “low-frequency band” relatively mean that a frequency in the latter band is lower than that in the former band. The HF synthesizer  11  and the LF synthesizer  12  are constituted by using, for example, PLLs. 
   In the frequency synthesizer  10 A of this embodiment, by using the following arithmetic circuit to perform arithmetic operations including frequency division and multiplication to the reference frequency signals output from the HF synthesizer  11  and the LF synthesizer  12  as two unit synthesizers having different frequency bands, output signals having a plurality of necessary frequencies are generated as transmission/local signals in each of GSM/DCS/PCS/UMTS modes. 
   To a first mixer  13 , an output signal from the HF synthesizer  11  as the first reference frequency signal and an output signal from the LF synthesizer  12  as the second reference frequency signal are inputted. The output signal from the LF synthesizer  12  is also inputted to a first divider  14  having a division ratio “2”. An output signal from the HF synthesizer  11  and an output signal from the first divider  14  are inputted to a second mixer  15 . An output signal from the first mixer  13  is inputted to a second divider  16  having a division ratio “2”, and an output signal from the second mixer  15  is inputted to a third divider  17  whose division ratio can be switched between “2” and “4”. 
   A switch  18  switches an output signal from the second divider  16  and an output signal from the third divider  17 . An output signal from the switch  18  is outputted as a local signal and inputted to the quadrature demodulator  2  through the π/2 phase shifter  4 . An output signal from the third divider  17  is further outputted as a local signal and inputted to the quadrature modulator  3  through the π/2 phase shifter  5 . 
   An output signal frequency of the HF synthesizer  11 , an output signal frequency of the LF synthesizer  12 , enabling/disabling the operation of the second mixer  15 , the division ratio of the third divider  17  and the changeover operation of the switch  18  are controlled by a controller  19  in accordance with an operation mode of the multi-band radio apparatus. It is to be noted that the output signal frequency of the LF synthesizer  12  may be fixed in this embodiment and control executed by the controller  19  is not necessarily required except on/off switching of the LF synthesizer  12 . Moreover, a control signal line from the controller  19  to the second mixer  15  is omitted in  FIG. 1 . 
   The operation of the frequency synthesizer  10 A will now be concretely described in accordance with each operation mode of the multi-band radio apparatus hereinafter. For explaining the operation of the frequency synthesizer  10 A, Table 1 shows a concrete frequency structure of four bands, i.e., GSM/DCS/PCS/UMTS. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               GSM 
               DCS 
               PCS 
               UMTS 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
               Transmission 
               880-915 
               1710-1785 
               1850-1910 
               1920-1980 
             
             
               frequency 
               MHz 
               MHz 
               MHz 
               MHz 
             
             
               Reception 
               925-960 
               1805-1880 
               1930-1990 
               2110-2170 
             
             
               frequency 
               MHz 
               MHz 
               MHz 
               MHz 
             
             
                 
             
          
         
       
     
   
   [GSM Transmission Mode] 
   At first, in case of performing transmission in the GSM mode, the output signal frequency of the HF synthesizer  11  is determined as a value within a frequency range of 3520 MHz to 3660 MHz in accordance with the transmission frequency, the second mixer  15  is disabled (allowing the output signal from the HF synthesizer  11  to pass without change), and the division ratio of the third divider  17  is determined as “4”. As a result, from the frequency synthesizer  10 A is outputted a local signal having a frequency of 880 MHz to 915 MHz obtained by dividing the frequency of 3520 MHz to 3660 MHz by four by the third divider  17 , and this local signal is inputted to the quadrature demodulator  3  through the π/2 phase shifter  5 . 
   [GSM Reception Mode] 
   At second, in case of performing reception in the GSM mode, the output signal frequency of the HF synthesizer  11  is determined as a value within a frequency range of 3700 MHz to 3840 MHz in accordance with the transmission frequency, the second mixer  15  is disabled (allowing the output signal from the HF synthesizer  11  to pass without change), the division ratio of the third divider  17  is determined as “4”, and the switch  18  is moved to the lower side (selecting the output signal of the third divider  17 ). As a result, a local signal having a frequency of 925 MHz to 960 MHz obtained by dividing the frequency of 3700 MHz to 3840 MHz by four by the third divider  17  is outputted from the frequency synthesizer  10 A through the switch  18 , and this local signal is inputted to the quadrature demodulator  2  through the π/2 phase shifter  4 . 
   In the GSM mode, since communication is carried out in the TDMA (time division multiple access) system, transmission and reception are not simultaneously carried out. Transmission and reception are changed over by switching the output signal frequency of the HF synthesizer  11  in accordance with the timing of transmission/reception as described above. 
   [DCS Transmission Mode] 
   At third, in case of performing transmission in the DCS mode, the output signal frequency of the HF synthesizer  11  is determined as a value within a frequency range of 3610 MHz to 3760 MHz in accordance with the transmission frequency, the output signal frequency of the LF synthesizer  12  is determined as 380 MHz, the second mixer  15  is enabled, and the division ratio of the third divider  17  is determined as “2”. An output signal of the LF synthesizer  12  is divided by two to be 190 MHz and then inputted to the second mixer  15 . 
   In the second mixer  15 , when the output signal from the HF synthesizer  11  and the output signal from the second divider  14  are multiplied together and a difference in frequency of the both signals is detected, an output signal having a frequency within a frequency range of 3420 MHz to 3570 MHz is obtained in accordance with the transmission frequency. By dividing the output signal having a frequency of 3420 MHz to 3570 MHz from the second mixer  15  by two in the third divider  17 , the frequency synthesizer  10 A outputs a local signal having a frequency of 1710 MHz to 1785 MHz, and this output signal is inputted to the quadrature modulator  3  through the π/2 phase shifter  5 . 
   [DCS Reception Mode] 
   Subsequently, in case of performing reception in the DCS mode, the output signal frequency of the HF synthesizer  11  is determined as a value in a frequency range of 3610 MHz to 3760 MHz in accordance with the reception frequency, the second mixer  15  is disabled (allowing the output signal from the HF synthesizer  11  to pass without change); the division ratio of the third divider  17  is determined as “2”, and the switch  18  is moved to the lower side (selecting the output signal from the third divider  17 ). As a result, the frequency synthesizer  10 A outputs through the switch  18  the local signal having a frequency of 1805 to 1880 MHz obtained by dividing the frequency of 3610 MHz to 3760 MHz by two in the second divider  17 , and this output signal is inputted to the quadrature demodulator  2  through the π/2 phase shifter  4 . 
   In the DCS mode, since communication is effected in the TDMA mode as similar to the GSM mode, transmission and reception are not simultaneously carried out. Transmission and reception are changed over by switching disabling/enabling of the second mixer  15  in accordance with the timing of transmission/reception as described above. 
   [PCS Transmission Mode] 
   Next, in case of carrying out transmission in the PCS mode, the output signal frequency of the HF synthesizer  11  is determined as a value within a frequency range of 3700 MHz to 3820 MHz in accordance with the transmission frequency, the second mixer  15  is disabled (allowing the output signal from the HF synthesizer to pass without change), and the division ratio of the third divider  17  is determined as “2”. As a result, the frequency synthesizer  10 A outputs a local signal having a frequency of 1850 MHz to 1910 MHz obtained by dividing the frequency of 3700 MHz to 3820 MHz by two in the third divider  17 , and this output signal is inputted to the quadrature modulator  3  through the π/2 phase shifter  5 . 
   [PCS Reception Mode] 
   Then, in case of performing reception in the PCS mode, the output signal frequency of the HF synthesizer  11  is determined as a value within a frequency range of 3860 MHz to 3980 MHz in accordance with the transmission frequency, the second mixer  15  is disabled (allowing the output signal from the HF synthesizer  11  to pass without change), the division ratio of the third divider  7  is determined as “2”, and the switch  18  is moved to the lower side (selecting the output signal from the third divider  17 ). As a result, the frequency synthesizer  10 A outputs through the switch  18  a local signal having a frequency of 1930 MHz to 1990 MHz obtained by dividing the frequency of 3860 MHz to 3980 MHz by two in the second divider  17 , and this output signal is inputted to the quadrature demodulator  2  through the π/2 phase shifter  4 . 
   Although there are several PCS modes, since communication is carried out in the TDMA system in case of a mode similar to the GSM mode, transmission and reception are not simultaneously performed. Transmission and reception are changed over by switching the output signal frequency of the HF synthesizer  11  in accordance with the timing of transmission/reception. 
   [UMTS Transmission Mode] 
   Then, in case of carrying out transmission in the UMTS mode, the output signal frequency of the HF synthesizer  11  is determined as a value within a frequency range of 3840 MHz to 3960 MHz in accordance with the transmission frequency, the second mixer  15  is disabled (allowing the output signal of the HF synthesizer  11  to pass without change), and the division ratio of the third divider  17  is determined as “2”. As a result, the frequency synthesizer  10 A outputs a local signal having a frequency of 1920 MHz to 1980 MHz obtained by dividing the frequency of 3840 MHz to 3960 MHz by two in the third divider  17 , and this output signal is inputted to the quadrature modulator  3  through the π/2 phase shifter  5 . 
   [UMTS Reception Mode] 
   Subsequently, in case of performing reception in the UMTS mode, the output signal frequency of the HF synthesizer  11  is determined as a value within a frequency range of 3840 MHz to 3960 MHz in accordance with the transmission frequency, the output signal frequency of the LF synthesizer  12  is determined as 380 MHz, the first mixer  13  is enabled, and the switch  18  is moved to the upper side (selecting the output signal of the second divider  16 ). In the first mixer  13 , a signal having a frequency of 4220 MHz to 4340 MHz is obtained by multiplying the output signal from the HF synthesizer  11  and the output signal from the LF synthesizer  12  together. Consequently, the frequency synthesizer  10 A outputs through the switch  18  a local signal having a frequency of 2110 MHz to 2170 MHz obtained by dividing the frequency of 4220 MHz to 4340 MHz by two in the second divider  17 , and this output signal is inputted to the quadrature demodulator  2  through the π/2 phase shifter  4 . 
   In case of the UMTS mode, since the CDMA/FDD (code division multiple access/frequency division duplex) system is adopted, transmission and reception are simultaneously carried out. According to the structure of this embodiment, it is possible to simultaneously output the local signals for receiver and transmitter having frequencies required for transmission/reception. 
   As mentioned above, in the frequency synthesizer  10 A of this embodiment, with the simple structure that the two unit synthesizers, i.e., the HF synthesizer  11  and the LF synthesizer  12  are prepared and the mixers  13  and  15 , the dividers  14 ,  16  and  17 , and the switch  18  are combined with these synthesizers, it is possible to generate all frequencies required for transmission/reception in each mode of GSM/DCS/PCS/UMTS. Therefore, when a number of unit synthesizers whose circuit scale is large is greatly reduced, the hardware scale can be considerably minimized. 
     FIG. 2  shows a structural example of an image suppression type mixer which is suitable as the first mixer  13  and the second mixer  15  depicted in  FIG. 1 . This mixer comprises π/2 phase shifters  21  and  22 , multipliers  23  and  24 , and an adder-subtracter  25 . This mixer basically multiplies an output signal from the HF synthesizer  11  and an output signal from the LF synthesizer  12  (or a signal obtained by further dividing an output signal from the LF synthesizer  12  by the divider  14 ) and outputs a signal having a frequency indicative of a sum or a difference of the output signals of the both synthesizers  11  and  12 . 
   In this case, as shown in  FIG. 2 , the π/2 phase shifters  21  and  22  are used to branch each of output signals from the both synthesizers  11  and  12  into two, and the two multipliers  23  and  24  are then used to carry out the above-described multiplication operation. In addition, the adder-subtracter  25  is used to add (or subtract) the output signals from the multipliers  23  and  24 . As a result, the image suppression effect can be obtained. Since approximately 30 dB can be obtained as an image suppression ratio, an image suppression filter which is usually required on a subsequent stage of the mixer can be eliminated in the mixer having the structure shown in  FIG. 2 . 
   Other embodiments according to the present invention will now be described. In each drawing of the following embodiments, like reference numerals denote the same constituent parts as those in  FIG. 1  to avoid tautological explanation, and a characteristic part of each embodiment will be mainly described. 
   Second Embodiment 
     FIG. 3  shows a structure of a multi-band radio apparatus including a frequency synthesizer according to a second embodiment of the present invention. In the frequency synthesizer  10 B of this embodiment, the second and third dividers  16  and  17  in the frequency synthesizer  10 A in  FIG. 1  are substituted by dividers  26  and  27  which also serve as the π/2 phase shifters, and a switch  28  capable of simultaneously switching signals for two channels is used in place of the switch  18 . 
     FIG. 4  shows an example of the circuit diagram of dividers also serving as the π/2 phase shifters which are used as the dividers  26  and  27 . This divider is realized with two D type flip flops DFF 1  and DFF 2  as main constituted parts as shown in  FIG. 4A . When clock signals are inputted to clock input terminals CK and _CK, a signal I and a signal Q obtained by dividing the clock signal by two are output from a terminal I, _I and a terminal Q, _Q. Although the clock signal, the signal I and the signal Q are treated as differential signals in  FIG. 4A , the signal I and the signal Q have a phase difference of 90° as shown in  FIG. 4B  illustrating only positive phase signals. That is, the divider shown in  FIG. 4A  also has a function of the π/2 phase shifter. 
   Therefore, when the divider shown in  FIG. 4A  is used for the dividers  26  and  27 , the signal I and the signal Q outputted from the dividers  26  and  27  can be used as the local signal inputted to the local input port of the quadrature demodulator  2  or the local signal inputted to the local input port of the quadrature modulator  3  as shown in  FIG. 3 . In addition, the π/2 phase shifters  4  and  5  shown in  FIG. 1  are no longer necessary. The switch  28  is constituted so as to be capable of simultaneously switching the signal I and the signal Q outputted from the dividers  26  and  27  also serving as the π/2 phase shifters. 
   Third Embodiment 
     FIG. 5  shows a structure of a multi-band radio apparatus including a frequency synthesizer according to a third embodiment of the present invention. In the first embodiment, description that the filter on the subsequent stage of the mixer can be eliminated by using such an image suppression type filter as shown in  FIG. 2  has been given. However, it is needless to say that inserting the filter to the subsequent stage of the mixer may be preferable depending on unnecessary spurious specifications of the output signal from the frequency synthesizer. 
   In the frequency synthesizer  10 C of this embodiment, band-pass filters  31  and  32  are inserted to the subsequent stages of the mixers  13  and  15 . These filters  31  and  32  may be constituted by combining discrete components such as a coil (L), a capacitor (C) or a resistor (R), or by using filter components formed as modules such as an LC laminated filter, a dielectric filter, or an SAW (surface acoustic wave) filter. Additionally, these filters can be realized in a simpler structure by constituting the band-pass filters  31  and  32  by low-pass filters or high-pass filters depending on the frequency concern. 
   Fourth Embodiment 
   In the direct conversion mode, in order to suppress deterioration of the reception characteristic caused due to generation of the DC offset, a harmonic mixer may be used in the quadrature demodulator on the reception side. The harmonic mixer is different from a regular mixer, and a signal having a frequency which is ½ of the reception frequency is used as a local signal. 
     FIG. 6  shows a structure in case of using the harmonic mixer in the quadrature demodulator  2  as a fourth embodiment according to the present invention. In the frequency synthesizer  10 D of this embodiment, a fourth divider  33  is inserted on the subsequent stage of the switch  18 . The division ratio of the fourth divider  33  is “2” and used for generating a local signal having a frequency which is ½ of the reception frequency and required in the quadrature demodulator  2  having the harmonic mixer structure. 
   Incidentally, when utilizing the harmonic mixer, since a phase difference of the local signals to be supplied to the two mixers must be 45° in the quadrature demodulator  2 , the π/2 phase shifter  6  is used in place of the π/2 phase shifter  4  shown in  FIG. 1 . 
   Fifth Embodiment 
     FIG. 7  shows a structure according to a fifth embodiment of the present invention obtained by improving the fourth embodiment illustrated in  FIG. 6 . In the frequency synthesizer  10 E of this embodiment, second and third dividers  26  and  27  also serving as the π/2 phase shifters and the switch  18  capable of simultaneously switching signals for two channels are used as similar to the second embodiment illustrated in  FIG. 3 , and a fifth divider  34  having the division ratio of “2” which also functions as the π/2 phase shifter is added as well as the fourth divider  33  depicted in  FIG. 6 . 
   By dividing each of signals having a phase 0° outputted from the second and third dividers  26  and  27  by two in the fourth divider  33  through the switch  28 , these signals are outputted as the local signals having a phase 0°. Further, by dividing each of signals having a phase 90° outputted from the dividers  26  and  27  by two by the fifth divider  34  which also functions as the π/2 phase shifter through the switch  28 , these signals are outputted as the local signals having a phase 45°. 
   As described above, according to this embodiment, since the two local signals having a phase difference of π/4 in total are obtained, the  FIG. 6  π/4 phase shifter  6  used in the fourth embodiment can be eliminated. 
   Sixth Embodiment 
     FIG. 8  shows another structural example in case of using the harmonic mixers in the quadrature demodulator  2  as a sixth embodiment according to the present invention. In the frequency synthesizer  10 F of this embodiment, the first divider  26  having the division ratio “2” depicted in  FIG. 7  is substituted by a divider  41  having the division ratio “4”, and a fourth divider  42  having the division ratio “2” is inserted between the third divider  27  and the switch  28 . Moreover, the dividers  33  and  34  illustrated in  FIG. 7  are removed. 
   According to this embodiment, since the dividers  41  and  42  can also function as the π/4 phase shifters, the effects similar to those of the fifth embodiment shown in  FIG. 7  can be obtained because the π/4 phase shifter required for the harmonic mixers can be eliminated. 
   Seventh Embodiment 
   All of the first to sixth embodiments mentioned above are examples in which the present invention is applied to the multi-band radio apparatus using the direct conversion mode in both the transmission system and the reception system. Description will now be given as an example that the present invention is applied to the multi-band radio apparatus in which the direct conversion mode is used only in the reception system and the super heterodyne mode is used in the transmission system as shown in  FIG. 9  as a seventh embodiment according to the present invention. 
   In  FIG. 9 , a frequency converter  7  is inserted between the quadrature modulator  3  of the transmission system and the antenna  1 . In this case, the quadrature modulator  3  is used as an intermediate frequency converter. That is, baseband transmission signals It and Qt for the I channel and the Q channel are converted into intermediate frequency signals by the quadrature modulator  3 , then up-converted by the frequency converter  7 , and supplied to the antenna  1 . 
   The frequency converter  7  is constituted by phase comparators  71   a  to  71   c , down converters  72   a  to  72   c , an up converters  72   d  and VCOs (voltage control oscillators)  73   a  to  73   c . Suffixes a, b and c indicate systems for GSM, DCS and PCS, and the up converter  72   d  is used for UMTS. The phase comparators  71   a  to  71   c  compare output signals from VCOs  73   a  to  73   c  with output signals from down converters  72   a  to  72   c , and output signals indicative of phase differences between these signals. Oscillation frequencies of the VCOs  73   a  to  73   c  are controlled by the output signals from the phase comparators  71   a  to  71   c . The down converters  72   a  to  72   c  down-convert output signals from the VCOs  73   a  to  73   c  by using a first local signal inputted from the later-described frequency synthesizer  100 A. 
   This structure seems to be complicated transmission system structure as compared with the multi-band radio apparatus adopting the direct conversion mode, but the quadrature modulator  3  of the transmission system can be shared by all the modes. Further, when the direct conversion mode is used in both transmission and reception, although the output signal frequency of the quadrature modulator  3  coincides with the transmission frequency, the output frequency of the quadrature modulator  3  becomes the intermediate frequency in this embodiment. 
   When the super heterodyne mode is adopted in the transmission mode in this way, the structure of the frequency synthesizer is changed by the intermediate frequency of the transmission system. However, if the intermediate frequency is 380 MHz in GSM/DCS and 190 MHz in PCS/UMTS, the frequency synthesizer can be realized by the most simplest structure. Output signal frequencies of the frequency synthesizer in this case will be shown in Table 2 in order. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
               GSM 
               DCS 
               PCS 
               UMTS 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
               Transmission 
               380 
               380 
               190 
               190 
             
             
               first LO 
               MHz 
               MHz 
               MHz 
               MHz 
             
             
               Transmission 
               500-535 
               2090-2165 
               2040-2100 
               2110-2170 
             
             
               second LO 
               MHz 
               MHz 
               MHz 
               MHz 
             
             
               Reception LO 
               925-960 
               1805-1880 
               1930-1990 
               2110-2170 
             
             
                 
               MHz 
               MHz 
               MHz 
               MHz 
             
             
                 
             
          
         
       
     
   
   The frequency synthesizer  101 A of this embodiment shown in  FIG. 9  is configured to generate such frequencies. The frequency synthesizer  100 A comprises an HF synthesizer  101  for generating a first reference frequency signal in a high-frequency band and an LF synthesizer  102  for generating a second reference frequency signal in a low-frequency band for unit synthesizers. In the frequency synthesizer  10 A, output signals having necessary frequencies are generated by performing arithmetic operations including multiplication and frequency division by the following arithmetic circuit with respect to the reference frequency signals outputted from the HF synthesizer  101  and the LF synthesizer  102  as two unit synthesizers having different frequency bands. 
   An output signal from the LF synthesizer  102  is divided by the first divider  103  having the division ratio “4”. The first mixer  104  multiplies an output signal from the HF synthesizer  101  and an output signal from the first divider  103  together. An output signal from the first mixer  104  is divided by a second divider  105  having the division ratio “2” and then inputted to the quadrature demodulator  2  through the π/2 phase shifter  4  as a local signal. 
   The output signal from the LF synthesizer  102  is also divided by a third divider  17  which can switch the division ratio between “2” and “4”, and then inputted to the quadrature modulator  3  as a transmission first local signal. Furthermore, the output signal from the HF synthesizer  101  is inputted to the frequency converter  7  through a fourth divider  107  having the division ratio “4” as a transmission second local signal. 
   The output signal frequency of the HF synthesizer  101 , the output signal frequency of the LF synthesizer  102 , enabling/disabling the first mixer  104 , enabling/disabling the second divider  105 , the division ratio of the third divider  106 , and enabling/disabling the fourth divider  107  are controlled by a controller  110  in accordance with an operation mode of the multi-band radio apparatus. The output signal frequency of the LF synthesizer  102  may be fixed in this embodiment, and control effected by the controller  110  is not necessarily required. Moreover, a control signal line from the controller  110  to the mixer  104  is eliminated in  FIG. 9 . 
   The operation of the frequency synthesizer  100 A will now be concretely described in accordance with each operation mode of the multi-band radio apparatus. 
   [GSM Transmission Mode] 
   At first, in case of performing transmission in the GSM mode, the output signal frequency of the HF synthesizer  101  is determined as a value within a frequency range of 2000 MHz to 2140 MHz in accordance with the transmission frequency, the output signal frequency of the LF synthesizer  102  is determined as 760 MHz, and the division ratio of the third divider  106  is determined as “2”. In this case, the frequency synthesizer  100 A outputs a signal having a frequency of 380 MHz obtained by dividing 760 MHz by two in the third divider  106 , and this signal is inputted to the quadrature modulator  3  as a transmission first local signal. 
   In addition, a signal having a frequency of 500 MHz to 535 MHz obtained by dividing the output signal frequency 2000 MHz to 2140 MHz of the HF synthesizer  101  by four using the fourth divider  107  is outputted as a transmission second local signal and inputted to the frequency converter  7 . 
   [GSM Reception Mode] 
   Subsequently, in case of performing reception in the GSM mode, the output signal frequency of the HF synthesizer  101  is determined as a value within a frequency range of 2040 MHz to 2110 MHz in accordance with the transmission frequency, the output signal frequency of the LF synthesizer  102  is determined as 760 MHz, and the mixer  104  is enabled. The output signal from the LF synthesizer  102  is divided by two to be 190 MHz by the first divider  103  and then inputted to the mixer  104 . 
   In the mixer  104 , the output signal from the HF synthesizer  101  and the output signal from the first divider  103  are multiplied together, and a difference frequency component of the both signals is extracted. As a result, an output signal having a frequency in a frequency range of 1850 MHz to 2300 MHz is obtained in accordance with the transmission frequency. When the output signal having a frequency of 1850 MHz to 2300 MHz from the second mixer  15  is divided by in the second divider  105 , a signal having a frequency of 925 MHz to 1785 MHz is outputted from the frequency synthesizer  100 A as the local signal and then inputted to the quadrature demodulator  2  through the π/2 phase shifter  4 . 
   In the GSM mode, since communication is effected in the TDMA system, transmission and reception are not simultaneously carried out. Transmission and reception are changed over by switching the output signal frequency of the HF synthesizer  101  in accordance with the timing of transmission/reception. 
   [DCS Transmission Mode] 
   Then, in case of carrying out transmission in the DCS mode, the output signal frequency of the HF synthesizer  101  is determined as a value in a frequency range of 2090 MHz to 2165 MHz in accordance with the transmission frequency, the output signal frequency of the LF synthesizer  102  is determined as 760 MHz, the division ratio of the third divider  106  is determined as “2”, and the fourth divider  107  is disabled (allowing the output signal from the HF synthesizer  101  to pass without being divided). In this case, the frequency synthesizer  100 A outputs a signal having a frequency of 380 MHz obtained by dividing 760 MHz by two in the third divider  106  as a transmission first local signal, and this signal is inputted to the quadrature modulator  3 . 
   Moreover, the output signal having a frequency of 2090 MHz to 2165 MHz from the HF synthesizer  101  is output as a transmission second local signal without being divided by the fourth divider  107  and inputted to the frequency converter  7 . 
   [DCS Reception Mode] 
   Subsequently, in case of performing reception in the DCS mode, the output signal frequency of the HF synthesizer  101  is determined as a value in a frequency range of 1995 MHz to 2070 MHz in accordance with the reception frequency, the output signal frequency of the LF synthesizer  102  is determined as 760 MHz, the third divider  103  is enabled, the mixer  104  is enabled, and the second divider  105  is disabled (allowing the output signal from the mixer  104  to pass without being divided). The output signal from the LF synthesizer  102  is divided by four to be 190 MHz in the second divider  103  and then inputted to the mixer  104 . 
   In the mixer  104 , by multiplying the output signal from the HF synthesizer  101  and the output signal from the first divider  103  together, an output signal having a frequency in a frequency range of 1805 MHz to 1880 MHz is obtained in accordance with the reception/transmission frequency. The output signal having a frequency of 1805 MHz to 1880 MHz from the mixer  105  is outputted from the frequency synthesizer  101 A as a local signal without being divided by the second divider  105 , and inputted to the quadrature demodulator  2  through the π/2 phase shifter  4 . 
   In the DCS mode, since communication is carried out in the TDMA system as similar to the GSM mode, transmission and reception are not simultaneously performed. Transmission and reception is changed over by switching the frequency of the HF synthesizer  10  in accordance with the timing of transmission/reception. 
   [PCS Transmission Mode] 
   Then, in case of effecting transmission in PCS, the output signal frequency of the HF synthesizer  101  is determined as a value within a frequency range of 2040 MHz to 2100 MHz in accordance with the transmission frequency, the output signal frequency of the LF synthesizer  102  is determined as 760 MHz, the division ratio of the third divider  106  is determined as “4”, and the fourth divider  107  is disabled (allowing the output signal from the HF synthesizer  101  to pass without being divided). In this case, the frequency synthesizer  100 A outputs a signal having a frequency of 190 MHz obtained by dividing 760 MHz by four in the third divider  106  as a transmission first local signal, and this signal is inputted to the quadrature modulator  3 . 
   Furthermore, the output signal having a frequency of 2040 MHz to 2100 MHz from the HF synthesizer  101  is outputted as a transmission second local signal without being divided by the fourth divider  107 , and inputted to the frequency converter  7 . 
   [PCS Reception Mode] 
   Then, in case of carrying out reception in PCS, the output signal frequency of the HF synthesizer  101  is determined as a value within a frequency range of 2120 MHz to 2180 MHz in accordance with the transmission frequency, the output signal frequency of the LF synthesizer  102  is determined as 760 MHz, the mixer  104  is enabled, the second divider  105  is enabled, and the fourth divider  107  is disabled (allowing the output signal of the HF synthesizer  101  to pass without being divided). The output signal from the LF synthesizer  102  is divided by four to be 190 MHz in the first divider  103  and then inputted to the mixer  104 . 
   In the mixer  104 , by multiplying the output signal from the HF synthesizer  101  and the output signal of the first divider  103  together, there is obtained an output signal having a frequency within a frequency range of 1930 MHz to 2100 MHz in accordance with the transmission frequency. The output signal having a frequency of 1930 MHz to 2100 MHz from the second mixer  104  is outputted from the frequency synthesizer  100 A as a local signal without being divided by the second divider  105 , and inputted to the quadrature demodulator  2  through the π/2 phase shifter  4 . 
   In the PCS mode, since communication is carried out in the TDMA system as similar to the GSM mode, transmission and reception are not simultaneously performed. Transmission and reception are changed over by switching the output signal frequency of the HF synthesizer  101  in accordance with the timing of transmission/reception. 
   [UMTS Transmission Mode] 
   Subsequently, in case of effecting transmission in the UMTS mode, the output signal frequency of the HF synthesizer  101  is determined as a value within a frequency range of 2110 MHz to 2170 MHz in accordance with the transmission frequency, the output signal frequency of the LF synthesizer  102  is determined as 760 MHz, the division ratio of the third divider  106  is determined as “4”, and the fourth divider  107  is disabled (allowing the output signal from the HF synthesizer  101  to pass without being divided). In this case, the frequency synthesizer  100 A outputs a signal having a frequency of 190 MHz obtained by dividing 760 MHz by four in the third divider  106  as a transmission first local signal, and this signal is inputted to the quadrature modulator  3 . 
   Moreover, the output signal having a frequency of 2110 MHz to 2170 MHz from the HF synthesizer  101  is outputted as a transmission second local signal without being divided by the fourth divider  107 , and inputted to the frequency converter  7 . 
   [UMTS Reception Mode] 
   Then, in case of performing reception in the UMTS mode, the output signal frequency of the HF synthesizer  101  is determined as 2110 MHz to 2170 MHz similarly as in transmission, the mixer  104  is disabled (allowing the output signal from the mixer  104  to pass without modification), and the second divider  105  is disabled (allowing the output signal of the mixer  104  to pass without being divided). The output signal of the LF synthesizer  102  is divided by four to be 190 MHz in the first divider  103 , and then passes through the mixer  104  without modification. In addition, this signal is outputted from the frequency synthesizer  100 A as a local signal without being divided by the second divider  105 , and inputted to the quadrature demodulator  2  through the π/2 phase shifter  4 . 
   In case of the UMTS mode, since communication is carried out in the CDMA/FDD system, transmission and reception are simultaneously effected. According to the structure of this embodiment, the transmission first and second local signals and the local signal for receiver which have frequencies required for transmission/reception can be simultaneously outputted at this moment. 
   As described above, in the frequency synthesizer  100 A in this embodiment, with the structure that only the HF synthesizer  101  and the LF synthesizer  102  are prepared as the unit synthesizers and the dividers  103 ,  105 ,  106  and  107  and the mixer  104  are combined with these synthesizers, it is also possible to generate all frequencies required for transmission/reception in each mode of GMS/DCS/PCS/UMTS. Therefore, great reduction in a number of the unit synthesizers having a large circuit scale can considerably decrease the hardware scale. 
   In addition, since the transmission first local signal having 0° and 90° obtained by necessarily dividing the output signal from the LF synthesizer  102  by four or two in the third divider  106  is supplied to the quadrature modulator  3  in the transmission system, the divider  106  can also serve as the π/2 phase shifter. 
   Eighth Embodiment 
     FIG. 10  shows a structure of a frequency synthesizer according to an eighth embodiment of the present invention. In the eighth embodiment, although a filter on the subsequent stage of the mixer  104  can be eliminated by using such an image suppression type filter as shown in  FIG. 2 , it is needless to say that insertion of the filter to the subsequent stage of the mixer  104  or the subsequent stage of the first divider  103  may be preferable depending on unnecessary spurious specifications of the output signal of the frequency synthesizer. 
   In the frequency synthesizer  100 B of this embodiment, band-pass filters  108  and  109  are inserted to the subsequent stage of the first divider  103  and the subsequent stage of the mixer  104 , respectively. These filters  108  and  109  may be configured by combining discrete components such as a coil (L), a capacitor (C) or a resistor (R), or filter components formed into modules such as an LC laminated filter, a dielectric filter or an SAW (surface acoustic wave) filter may be used for these filters. Additionally, the present invention can be realized with a simpler structure by configuring the band-pass filters  31  and  32  by low-pass filters or high-pass filters depending on frequency concerns. 
   Ninth Embodiment 
     FIG. 11  shows a structure of a frequency synthesizer according to a ninth embodiment of the present invention. In the frequency synthesizer  100 C of this embodiment, a second LF synthesizer  120  is added. In the frequency synthesizer  100 A according to the seventh embodiment depicted in  FIG. 9 , a signal having a frequency of 190 MHz is generated by dividing an output signal having a frequency of 760 MHz from the LF synthesizer  102  by the first divider  103 . On the other hand, in this embodiment, the newly provided second LF synthesizer  120  is used to generate a signal having a frequency of 190 MHz. 
   Although an output signal from the divider  103  shown in  FIG. 9  has rectangular waves, an output signal from the second LF synthesizer  120 , i.e., a signal inputted to the mixer  104  can have sinusoidal waves according to this embodiment. As compared with the case where the output signal of the divider  103  is inputted to the mixer  104  as shown in  FIG. 9 , it is possible to reduce the necessity of adding the band restriction by the filter  108  such as shown in  FIG. 10 . 
   Tenth Embodiment 
     FIG. 12  shows a structure of a multi-band radio apparatus including a frequency synthesizer according to a tenth embodiment of the present invention. Giving description as to a difference of this embodiment from the seventh to ninth embodiments, in the frequency synthesizer  100 D of this embodiment, an HF synthesizer  111  for generating a signal having a frequency which is twice as high as that of the HF synthesizer  101  is used; the first divider for dividing an output signal from the LF synthesizer  102  is changed from the divider  103  having the division ratio “4” to the divider  113  having the division ratio “2”; the second divider for dividing an output signal from the mixer  104  is changed to the divider  115  capable of switching the division ratio between “2” and “4”; and the fourth divider for dividing an output signal from the HF synthesizer  111  is changed to the divider  117  capable of switching the division ratio between “2” and “8”. 
   In the frequency synthesizer  100 A having the structure shown in  FIG. 9 , since a signal to be outputted to the reception side is not necessarily divided by two, the π/2 phase shifter  4  is required on the local signal input side of the quadrature demodulator  2 . However, since the divider for dividing a frequency by two is also necessarily provided in the reception system in the structure of the frequency synthesizer  100  according to this embodiment, this divider can also function as the π/2 phase shifter. 
   Eleventh Embodiment 
     FIG. 13  shows a structure of a multi-band radio apparatus including a frequency synthesizer according to an eleventh embodiment of the present invention. In the structure using the direct conversion mode in the reception system and-the super heterodyne mode in the transmission system as similar to the seventh to tenth embodiments, this embodiment corresponds to a case of using the harmonic mixers in the quadrature demodulator  2  in the reception system similarly as described in the fourth embodiment. In this case, the frequency synthesizer  101 E can be realized by a fewer constituent elements. 
   The frequency synthesizer  100 E in this embodiment is different from the frequency synthesizer  100 A shown in  FIG. 9  of the seventh embodiment in that the second divider  105  is substituted by the divider  115  capable of switching the division ratio between “2”. and “4”. Further, in case of utilizing the harmonic mixers, since a phase difference of the local signals supplied to the two mixers in the quadrature demodulator  2  must be set to 45°, the π/2 phase shifter  4  shown in  FIG. 9  is substituted by the π/4 phase shifter  6 . 
   As to the operation of this frequency synthesizer  10 E, in description of the operation of the seventh embodiment, the divider  115  in  FIG. 13  is operated with the division ratio “4” when the divider  105  is activated, and the divider  115  is operated with the division ratio “2” when the divider  105  is disabled (allowing the output signal from the mixer  104  to pass without being divided). As a result, it is possible to obtain from the divider  115  the local signal having a frequency which is ½ of the reception frequency required in the quadrature demodulator  2  having the harmonic mixer structure. 
   Twelfth Embodiment 
     FIG. 14  shows a structure of a multi-band radio apparatus including a frequency synthesizer according to a twelfth embodiment of the present invention. Although all of the frequency synthesizers  100 A to  100 E described in the seventh to eleventh embodiments are configured to adopt the super heterodyne mode in the transmission system, the frequency synthesizer  100 F of this embodiment corresponds to an example in which the super heterodyne mode is adopted in the transmission system for each mode of GSM/DCS/PCS and the direct conversion mode is adopted for only the UMTS mode. 
   The structure of the frequency synthesizer  100 F of this embodiment is similar to that of the frequency synthesizer  100 A shown in  FIG. 9  but very different in that switches  121  and  122  and a second mixer  123  are added. Further, the output signal frequency of the LF synthesizer  102  is changed from 760 MHz to 380 MHz, and hence the first divider is changed to the divider  115  having the division ratio “2” and the third divider is changed to the divider  116  having the division ratio “2”, respectively. 
   The added second mixer  123  multiplies an output signal from the HF synthesizer  101  by a signal transmitted through the fourth divider  107 , and multiplies an output signal from the LF synthesizer  102  by a signal subjected to division by two in the third divider  116 . The switches  121  and  122  are provided for switching an output signal from the divider  116  and an output signal from the second mixer  123  and outputting a resulting signal as a transmission first local signal. 
   In this frequency synthesizer  100 F, although the same operation as that of the frequency synthesizer  100 A shown in  FIG. 9  is carried out in the three modes of GSM/DCS/PCS, the local signal matched with the transmission frequency required for the direct conversion mode can be obtained by the added second mixer  123  in the UMTS mode. That is, the switches  121  and  123  are changed over so as not to energize the mixer  123  in case of the GSM/DCS/PCS modes, and they are changed over so as to energize the mixer  123  in the UMTS mode. 
   Giving further concrete description as to the operation in case of performing transmission in the UMTS mode, the output signal frequency of the HF synthesizer  101  is determined as a value within a frequency range of 2110 MHz to 2170 MHz in accordance with the transmission frequency, the output signal frequency of the LF synthesizer  102  is determined as 380 MHz, and the fourth divider  107  is disabled (allowing the output signal from the HF synthesizer  101  to pass without being divided). 
   In this case, the frequency synthesizer  100 F outputs the local signal having the same frequency as the transmission frequency of 1920 MHz to 1990 MHz (see Table 1) obtained by multiplying in the second mixer  123  a signal having a frequency of 190 MHz obtained by dividing 380 MHz by two in the third divider  116  and a signal having a frequency of 2110 MHz to 2170 MHz from the HF synthesizer  101  which has passed through the fourth divider  107 . This output signal is inputted to the quadrature modulator  3 . At this moment, the frequency converter  7  is controlled to be disabled (allowing the output signal from the quadrature modulator  3  to pass without modification). 
   Although the above has described the case the present invention is applied to the multi-band radio apparatus conform to the four modes of GSM/DCS/PCS/UMTS in the foregoing embodiments, the present invention can be also applied to the multi-band radio apparatus conform to arbitrary two modes or three modes of these four modes. Furthermore, the present invention includes a multi-band radio apparatus conform to five communication modes that another communication mode is added to these four modes, or any other apparatus as long as it has a structure for generating signals (local signals) in a plurality of (three or more) frequency bands exceeding a number of unit synthesizers by combining at least two unit synthesizers including the HF synthesizer and the LF synthesizer with the arithmetic circuit comprising of the dividers and the mixers. 
   As described above, according to the frequency synthesizer of the present invention, it is possible to generate signals in a plurality of frequency bands exceeding a number of unit synthesizers with a small circuit scale structure comprising two unit synthesizers for basically producing reference frequency signals in a high-frequency band and a low-frequency band. 
   Furthermore, this frequency synthesizer can be used to realize a multi-band radio apparatus which can be utilized in two or more frequency bands with a small hardware scale. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.