Abstract:
A modulation mixer includes a low cost circuit for reducing a carrier frequency leak. An orthogonal modulator including the modulation mixer may be formed on a single silicon substrate and does not need to be connected to an external transformer to suppress carrier frequency leak.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an orthogonal modulator for use in a digital mobile communication device, such as a portable telephone, and, more particularly, to an orthogonal modulator less susceptible to the influence of a carrier leak. 
     FIG. 1 is a schematic block diagram of a conventional orthogonal modulator  11 . The orthogonal modulator  11  includes a frequency multiplier  12 , a phase shifter  13 , modulation mixers  14  and  15 , and an adder  16 , all formed on a single semiconductor substrate. 
     The orthogonal modulator  11  is connected to a transformer  17  called a balloon to suppress the occurrence of a carrier leak. The transformer  17  receives a carrier signal LOin and supplies a carrier signal LO in phase with the carrier signal LOin and a carrier signal LOx of the opposite phase to that of the carrier signal LOin, to the orthogonal modulator  11 . 
     The frequency multiplier  12  multiplies the frequencies of the complementary carrier signals LO and LOx by two and supplies frequency-doubled carrier signals  2 LO and  2 LOx to the phase shifter  13 . The phase shifter  13  divides the frequencies of the frequency-doubled carrier signals  2 LO and  2 LOx by two to generate four carrier signals LO 0 , LO 90 , LO 180  and LO 270  whose phases are shifted from one another by 90 degrees. The carrier signals LO 0  and LO 180  are complementary to each other, and the carrier signals LO 90  and LO 270  complementary to each other. 
     The modulation mixer  14  multiplies a baseband signal I or Ix by the carrier signal LO 0  or LO 180  to produce a first modulation signal VI. The modulation mixer  15  multiplies a baseband signal Q or Qx by the carrier signal LO 90  or LO 270  to produce a second modulation signal V 2 . The adder  16  adds the first and second modulation signals V 1  and V 2  together and outputs an output signal RFout. 
     A double mode phenomenon, which is caused by the carrier leak, makes the modulating operation of the orthogonal modulator  11  to be unstable. The double mode phenomenon includes a good mode which indicates the spectrum of the output signal RFout as shown in FIG. 3A and a bad mode indicating the spectrum of the output signal RFout as shown in FIG.  3 B. The mode (i.e., the good mode or the bad mode) is determined by the timing of powering on a portable device and the carrier signals LO and LOx. Referring to FIGS. 3A and 3B, a component CL appears at the frequencies of the carrier signals LO and LOx, and a component Pout appears at positions shifted on the high-frequency side from the frequencies of the carrier signals LO and LOx by the frequencies of the baseband signals I to Qx. A component IR appears at positions shifted on the low-frequency side from the frequencies of the carrier signals LO and LOx by the frequencies of the baseband signals I to Qx. 
     It is believed that the double mode phenomenon may attributed to the following two factors. The first factor is phase differences between the carrier signal LO or LOx and the carrier signals LO 0 , LO 90 , LO 180  and LO 270 . The phase shifter  13  operates to make the rising edge of the frequency-doubled carrier signal  2 LO match with the rising edge of the carrier signal LO 0 . However, the rising edge of the carrier signal LO 0  coincides with the rising edge of the carrier signal LO in some cases as shown in FIG.  2 A and coincides with the falling edge of the carrier signal LO in other cases as shown in FIG.  2 B. That is, the phase shifter  13  generates a carrier signal LO 0  having a phase difference of 0 degrees to the carrier signal LO or a carrier signal LO 0  having a phase difference of 180 degrees to the carrier signal LO. 
     The second factor is the generation of a direct current (DC) component on the output signal RFout. As the carrier signal LO or the output signal RFout has a high frequency, it easily leaks in space. As this leaked carrier enters the modulation mixers  14  and  15  through their input terminals, the DC component appears. 
     More specifically, the output signals Vout (output signals V 1  and V 2 ) of the modulation mixers  14  and  15  in an ideal state where the carrier signal LO does not leak are expressed by the following equation (1).                      V                 out     =                  cos        (     2      π                   f   LO        t     )       ×     cos        (     2      π                   f   BB        t     )                     =                  1   2          {       cos                 2        π        (       f   LO     +     f   BB       )          t     +     cos                 2        π        (       f   LO     -     f   BB       )          t       }                     (   1   )                                
     When the leaked carrier signal LO is input to the input terminal for the baseband signal I, Ix, Q or Qx, the output signals Vout of the modulation mixers  14  and  15  are given by the following equation (2).                      V                 out     =       cos        (       2      π                   f   LO        t     +     φ   1       )       ×     cos        (       2                 π                   f   LO        t     +     φ   2       )                     =       1   2          {       cos                 2        π        (       2        f   LO       +     φ   1     +     φ   2       )          t     +     cos        (       φ   1     -     φ   2       )         }                     (   2   )                                
     where φ 1  is the input phase of the original carrier signal LO and φ 2  is the leak phase (phase delay) of the leaked carrier signal LO, with the baseband signals ignored for the sake of convenience. The leak phase φ 2  is nearly constant, and the input phase φ 1  is 0 degree or 180 degrees (90 degrees or 270 degrees). Thus, the second term in the equation (2) that represents the DC component has two values. The two values for the second term causes the double mode phenomenon. Note that the first term in the equation (2) is hardly affected by the input phase φ 1  and the leak phase φ 2  because 2f LO  is sufficiently larger than those phases. 
     Referring now to FIG. 4, an improved orthogonal modulator  21  suppresses the doube mode phenomenon is shown. 
     The improved orthogonal modulator  21  has two ½ frequency dividers  22  and  23 , each of which includes a flip-flop type phase shifter. 
     The first ½ frequency divider  22  frequency-divides the carrier signal LO or LOx by two to yield frequency-divided signals of phases different by 90 degrees from each other. The second ½ frequency divider  23  frequency-divides the carrier signal LO or LOx by two to yield frequency-divided signals. A first modulation mixer  24  multiplies the first baseband signal I by the frequency-divided signal from the first ½ frequency divider  22 . A second modulation mixer  25  multiplies the second baseband signal Q by the frequency-divided signal from the first ½ frequency divider  22 . 
     An adder  26  combines the output signals, Iout and Qout, from the first and second modulation mixers  24  and  25 , amplifies the resultant signal and outputs the amplified signal Sout. A frequency multiplier  27  multiplies the amplified signal Sout from the adder  26  by the output signal of the second ½ frequency divider  23  to yield an output signal RFout. 
     The output signals Iout and Qout of the first and second modulation mixers  24  and  25 , the output signal Sout of the adder  26  and the output signal Vout of the frequency multiplier  27  are given by the following equations (3) to (6)                      I                 out     =       cos        (     2        π   ·       f   LO     /   2     ·   t       )       ×     cos        (     2      π                   f   BB        t     )                     =       1   2          {       cos                 2        π        (         f   LO     /   2     +     f   BB       )          t     +     cos                 2        π        (         f   LO     /   2     -     f   BB       )          t       }                     (   3   )                       Q                 out     =       cos        (       2        π   ·       f   LO     /   2     ·   t       -     90      °       )       ×     cos        (       2      π                   f   BB        t     +     90      °       )                     =       1   2          {       cos                 2        π        (         f   LO     /   2     +     f   BB       )          t     -     cos                 2        π        (         f   LO     /   2     -     f   BB       )          t       }                     (   4   )                       S                 out     =       I                 out     +     Q                 out                   =     cos                 2        π        (         f   LO     /   2     +     f   BB       )          t                   (   5   )                       V                 out     =     S                 out   ×     cos        (     2        π   ·       f   LO     /   2     ·   t       )                     =     cos                 2        π        (         f   LO     /   2     +     f   BB       )          t   ×     cos        (     2        π   ·       f   LO     /   2     ·   t       )                     =       1   2          {       cos                 2        π        (       f   LO     +     f   BB       )          t     +     cos                 2      π                   f   BB        t       }                     (   6   )                                
     It is apparent from those equations (3) to (6) that the influence of the carrier leak is repressed. Further, the provision of the frequency multiplier  27  on the output terminal side allows the frequencies of the output signals Iout and Qout of the modulation mixers  24  and  25  to be half the frequencies of the output signals V 1  and V 2  of the modulation mixers  14  and  15  in FIG.  1 . This ensures stable operation, and lower current consumption, of the orthogonal modulator  21 . 
     The first and second ½ frequency dividers  22  and  23  have relative large circuit scales. Accordingly, the chip size of the orthogonal modulator  21  becomes inevitably larger, which stands in the way of size reduction of portable devices. Further, the increased chip size increases the cost of the orthogonal modulator  21  and eventually the costs of portable devices using or incorporating the modulator  21 . 
     Accordingly, it is an objective of the present invention to provide a modulation mixer and an orthogonal modulator which reduce the influence of a carrier leak at a low cost. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the present invention provides a modulation mixer for use in an orthogonal modulator, for combining a carrier signal having a high frequency and a baseband signal having a low frequency and outputting a modulation signal. The modulation mixer includes a transistor receiving the baseband signal, and an element, connected to the transistor, for reducing a high-frequency component of a signal including a leaked carrier signal input together with the baseband signal to the transistor. 
     The present invention provides a modulation mixer for use in an orthogonal modulator, for combining a carrier signal having a high frequency and a baseband signal having a low frequency and outputting a modulation signal. The modulation mixer includes a transistor receiving the baseband signal, and an element, connected to the transistor, for reducing a frequency characteristic of the transistor with respect to a high-frequency component of a signal including a leaked carrier signal input together with the baseband signal to the transistor. 
     The present invention provides a modulation mixer for use in an orthogonal modulator, for combining a carrier signal having a high frequency and a baseband signal having a low frequency and outputting a modulation signal. The modulation mixer includes a first transistor receiving the baseband signal, and a second transistor receiving the carrier signal. The first transistor has a larger size than the second transistor to reduce a frequency characteristic of the first transistor with respect to a high-frequency component of a signal including a leaked carrier signal input together with the baseband signal to the first transistor. 
     The present invention provides an orthogonal modulator for producing a modulation signal. The modulator includes a frequency multiplier receiving a carrier signal having a high frequency and producing a complementary frequency-multiplied signal having a frequency about two times the frequency of the carrier signal. A phase shifter is connected to the frequency multiplier, receives the complementary frequency-multiplied signal and frequency-divides the complementary frequency-multiplied signal by two to produce an in-phase carrier signal having an in-phase component and an orthogonal carrier signal having an orthogonal component. A first modulation mixer is connected to the phase shifter, receives the in-phase carrier signal and a first baseband signal having a lower frequency than the in-phase carrier signal and combines the in-phase carrier signal and the first baseband signal to produce a first modulation signal. A second modulation mixer is connected to the phase shifter, receives the orthogonal carrier signal and a second baseband signal having a lower frequency than the orthogonal carrier signal and combines the orthogonal carrier signal and the second baseband signal to produce a second modulation signal. An adder is connected to the first and second modulation mixers, receives the first and second modulation signals from the first and second modulation mixers and adds the first and second modulation signals to generate the modulation signal of the orthogonal modulator. The first modulation mixer includes a first transistor receiving the first baseband signal, and a first element, connected to the first transistor, for reducing a high-frequency component of a signal including a leaked carrier signal input together with the first baseband signal to the first transistor. The second modulation mixer includes a second transistor receiving the second baseband signal, and a second element, connected to the second transistor, for reducing a high-frequency component of a signal including a leaked carrier signal input together with the second baseband signal to the second transistor. 
     The present invention provides an orthogonal modulator for producing a modulation signal. The modulator includes a frequency multiplier for receiving a carrier signal having a high frequency and producing a complementary frequency-multiplied signal having a frequency about two times the frequency of the carrier signal. A phase shifter is connected to the frequency multiplier, receives the complementary frequency-multiplied signal, and frequency-divides the complementary frequency-multiplied signal by two to produce an in-phase carrier signal having an in-phase component and an orthogonal carrier signal having an orthogonal component. A first modulation mixer is connected to the phase shifter, receives the in-phase carrier signal and a first baseband signal having a lower frequency than the in-phase carrier signal and combines the in-phase carrier signal and the first baseband signal to produce a first modulation signal. A second modulation mixer is connected to the phase shifter, receives the orthogonal carrier signal and a second baseband signal having a lower frequency than the orthogonal carrier signal and combines the orthogonal carrier signal and the second baseband signal to produce a second modulation signal. An adder is connected to the first and second modulation mixers, receives the first and second modulation signals from the first and second modulation mixers and adds the first and second modulation signals to generate the modulation signal of the orthogonal modulator. The first modulation mixer includes a first transistor receiving the first baseband signal, and a first element, connected to the first transistor, for reducing a frequency characteristic of the first transistor with respect to a high-frequency component of a signal including a leaked carrier signal input together with the first baseband signal to the first transistor. The second modulation mixer includes a second transistor receiving the second baseband signal, and a second element, connected to the second transistor, for reducing a frequency characteristic of the second transistor with respect to a high-frequency component of a signal including a leaked carrier signal input together with the second baseband signal to the second transistor. 
     The present invention provides an orthogonal modulator for producing a modulation signal. The modulator includes a frequency multiplier for receiving a carrier signal having a high frequency and producing a complementary frequency-multiplied signal a frequency about two times the frequency of the carrier signal. A phase shifter is connected to the frequency multiplier, receives the complementary frequency-multiplied signal and frequency-divides the complementary frequency-multiplied signal by two to produce an in-phase carrier signal having an in-phase component and an orthogonal carrier signal having an orthogonal component. A first modulation mixer is connected to the phase shifter, receives the in-phase carrier signal and a first baseband signal having a lower frequency than the in-phase carrier signal, and combines the in-phase carrier signal and the first baseband signal to produce a first modulation signal. A second modulation mixer is connected to the phase shifter, receives the orthogonal carrier signal and a second baseband signal having a lower frequency than the orthogonal carrier signal, and combines the orthogonal carrier signal and the second baseband signal to produce a second modulation signal. An adder is connected to the first and second modulation mixers, receives the first and second modulation signals from the first and second modulation mixers and adds the first and second modulation signals to generate the modulation signal of the orthogonal modulator. The first modulation mixer includes a first transistor receiving the first baseband signal, and a second transistor receiving the in-phase carrier signal. The first transistor has a larger size than the second transistor to reduce a frequency characteristic of the first transistor with respect to a high-frequency component of a signal including a leaked carrier signal input together with the first baseband signal to the first transistor. The second modulation mixer includes a third transistor receiving the second baseband signal, and a fourth transistor receiving the orthogonal carrier signal. The third transistor has a larger size than the fourth transistor to reduce a frequency characteristic of the third transistor with respect to a high-frequency component of a signal including a leaked carrier signal input together with the second baseband signal to the third transistor. 
     The present invention provides an orthogonal modulator for producing a modulation signal. The modulator includes a phase shifter for receiving a carrier signal having a predetermined frequency and dividing the frequency of the carrier signal by two to generate an in-phase carrier signal having an in-phase component and an orthogonal carrier signal having an orthogonal component. A first modulation mixer is connected to the phase shifter, receives the in-phase carrier signal and a first baseband signal having a lower frequency than the in-phase carrier signal and combines the in-phase carrier signal and the first baseband signal to produce a first modulation signal. A second modulation mixer is connected to the phase shifter, receives the orthogonal carrier signal and a second baseband signal having a lower frequency than the orthogonal carrier signal and combines the orthogonal carrier signal and the second baseband signal to produce a second modulation signal. A first frequency multiplier is connected to the first modulation mixer and the phase shifter, receives the first modulation signal and one of the in-phase carrier signal and the orthogonal carrier signal and multiplies the first modulation signal by the one of the in-phase carrier signal and the orthogonal carrier signal to produce a third modulation signal. A second frequency multiplier is connected to the second modulation mixer and the phase shifter, receives the second modulation signal and one of the in-phase carrier signal and the orthogonal carrier signal and multiplies the second modulation signal by the one of the in-phase carrier signal and the orthogonal carrier signal to produce a fourth modulation signal. An adder is connected to the first and second frequency multipliers, receives the third and fourth modulation signals and adds the third and fourth modulation signals to produce the modulation signal of the orthogonal modulator. 
     The present invention provides an orthogonal modulator for producing a modulation signal. The modulator includes a phase shifter for receiving a carrier signal having a predetermined frequency and dividing the frequency of the carrier signal by two to generate an in-phase carrier signal having an in-phase component and an orthogonal carrier signal having an orthogonal component. A first modulation mixer is connected to the phase shifter, receives the in-phase carrier signal and a first baseband signal having a lower frequency than the in-phase carrier signal and combines the in-phase carrier signal and the first baseband signal to produce a first modulation signal. A second modulation mixer is connected to the phase shifter, receives the in-phase carrier signal and a second baseband signal having a lower frequency than the in-phase carrier signal and combines the in-phase carrier signal and the second baseband signal to produce a second modulation signal. A first frequency multiplier is connected to the first modulation mixer and the phase shifter, receives the first modulation signal and the in-phase carrier signal and multiplies the first modulation signal by the in-phase carrier signal to produce a third modulation signal. A second frequency multiplier is connected to the second modulation mixer and the phase shifter, receives the second modulation signal and the orthogonal carrier signal and multiplies the second modulation signal by the orthogonal carrier signal to produce a fourth modulation signal. An adder is connected to the first and second frequency multipliers, receives the third and fourth modulation signals and adds the third and fourth modulation signals to produce the modulation signal of the orthogonal modulator. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a schematic block diagram of a first conventional orthogonal modulator; 
     FIGS. 2A and 2B are waveform charts of signals generated by the orthogonal modulator of FIG. 1; 
     FIGS. 3A and 3B are spectrum waveform diagrams of signals output from the orthogonal modulator of FIG. 1; 
     FIG. 4 is a schematic block diagram of a second conventional orthogonal modulator; 
     FIG. 5 is a schematic block diagram showing an orthogonal modulator according to a first embodiment of this invention; 
     FIG. 6 is a circuit diagram of a modulation mixer of the orthogonal modulator of FIG. 5; 
     FIG. 7 is a circuit diagram of a first modification of the modulation mixer of FIG. 5; 
     FIG. 8 is a circuit diagram showing a second modification of the modulation mixer of FIG. 5; 
     FIG. 9 is a circuit diagram depicting a third modification of the modulation mixer of FIG. 5; 
     FIG. 10 is a circuit diagram of a fourth modification of the modulation mixer of FIG. 5; 
     FIG. 11 is a schematic block diagram illustrating an orthogonal modulator according to a second embodiment of this invention; 
     FIG. 12 is a circuit diagram of a frequency multiplier of the orthogonal modulator of FIG. 11; 
     FIG. 13 is a circuit diagram of a phase shifter of the orthogonal modulator of FIG. 11 which has the same structure as a frequency divider of FIG. 4; 
     FIG. 14 is a schematic block diagram of a first modification of the orthogonal modulator of FIG. 11; and 
     FIG. 15 is a schematic block diagram of a second modification of the orthogonal modulator of FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 5 is a schematic block diagram of an orthogonal modulator  31  according to a first embodiment of the present invention. The orthogonal modulator  31  comprises a frequency multiplier  12 , a phase shifter  13 , first and second modulation mixers  32  and  33 , and an adder  16 , preferably all formed on a single semiconductor substrate. 
     The frequency multiplier  12  receives complementary carrier signals LO and LOx and multiplies their frequencies by two to produce frequency-doubled complementary carrier signals  2 LO and  2 LOx, which are sent to the phase shifter  13 . 
     The phase shifter  13  divides the frequencies of the frequency-doubled carrier signals  2 LO and  2 LOx by two to generate four carrier signals LO 0 , LO 90 , LO 180  and LO 270  whose phases are shifted from one another by 90 degrees. The carrier signals LO 0  and LO 180  are complementary to each other, and are the in-phase components of the carrier signals LO and LOx (hereinafter called in-phase carrier signals). The carrier signals LO 90  and LO 270  are complementary to each other, and are the orthogonal components of the carrier signals LO and LOx (hereinafter called orthogonal carrier signals LO 90  and LO 270 ). 
     The first modulation mixer  32  receives the in-phase carrier signal LO 0  or LO 180  from the phase shifter  13  and multiplies a first digital baseband signal I or Ix by the in-phase carrier signal LO 0  or LO 180  to produce a first modulation signal V 1  or V 1 x. 
     The second modulation mixer  33  receives the orthogonal carrier signal LO 90  or LO 270  from the phase shifter  13  and multiplies a second digital baseband signal Q or Qx by the orthogonal carrier signal LO 90  or LO 270  to produce a second modulation signal V 2  or V 2 x. The adder  16  receives the first modulation signal V 1  or V 1 x and the second modulation signal V 2  or V 2 x from the modulation mixers  32  and  33 , and adds the second modulation signal V 2  or V 2 x to the first modulation signal V 1  or V 1 x to yield an output signal RFout. 
     FIG. 6 is a circuit diagram of the first modulation mixer  32 . As the second modulation mixer  33  preferably has the same structure as the first modulation mixer  32 , a diagram and detailed description thereof will be omitted. The first modulation mixer  32  is a double balanced mixer (DBM) which includes transistors Tr 1  to Tr 6 , resistors R 1  to R 3 , constant current sources  35  and  36 , and capacitors C 1  and C 2 . 
     The first transistor Tr 1  has an emitter, a collector connected via the resistor R 1  to the power supply line of a high-potential power supply Vcc, and a base which receives the in-phase carrier signal LO 0 . The second transistor Tr 2  has an emitter connected to the emitter of the first transistor Tr 1 , a collector connected via a resistor R 2  to the power supply line of the high-potential power supply Vcc, and a base which receives the in-phase carrier signal LO 180 . The first and second transistors Tr 1  and Tr 2  form a first differential pair  37 . 
     The third transistor Tr 3  has an emitter, a collector connected via the resistor R 1  to the power supply line of the high-potential power supply Vcc, and a base which receives the in-phase carrier signal LO 180 . The fourth transistor Tr 4  has an emitter connected to the emitter of the third transistor Tr 3 , a collector connected via the resistor R 2  to the power supply line of the high-potential power supply Vcc, and a base which receives the in-phase carrier signal LO 0 . The third and fourth transistors Tr 3  and Tr 4  form a second differential pair  38 . Also, the bases of second and third transistors are connected to each other. 
     The fifth transistor Tr 5  has an emitter connected via a constant current source  35  to the power supply line of a low-potential power supply (ground GND), a collector connected to the emitters of the first and second transistors Tr 1  and Tr 2 , and a base which receives the baseband signal I. The base of the fifth transistor Tr 5  is connected to a baseband input terminal  28 , and a node between this base and the input terminal  28  is grounded via a capacitor C 1 . The capacitor C 1  reduces the high-frequency component included in the signal applied to the base of the transistor Tr 5 . 
     The sixth transistor Tr 6  has an emitter connected via a constant current source  36  to the power supply line of the low-potential power supply (ground GND), a collector connected to the emitters of the third and fourth transistors Tr 3  and Tr 4 , and a base which receives the baseband signal Ix. The base of the sixth transistor Tr 6  is connected to a baseband input terminal  29 , and a node between this base and the input terminal  29  is grounded via a capacitor C 2 , which reduces the high-frequency component included in the signal applied to the base of the transistor Tr 6 . The emitter of the fifth transistor Tr 5  is connected to the emitter of the sixth transistor Tr 6  via the resistor R 3 . The fifth and sixth transistors Tr 5  and Tr 6  form a third differential pair  39 . 
     The first modulation signal V 1 x is output from the collectors of the first and third transistors Tr 1  and Tr 3 , and the first modulation signal V 1  is output from the collectors of the second and fourth transistors Tr 2  and Tr 4 . 
     In the case of the orthogonal modulator  31  of a 1 GHz band, for example, the carrier signals LO and LOx (output signal RFout) have frequencies of approximately 900 MHZ, and the first and second baseband signals I to Qx have frequencies of approximately 100 KHz. The capacitors C 1  and C 2  have capacitances (about 10 pF to about 100 pF) large enough to reduce the frequency component of approximately 900 MHZ. The capacitors C 1  and C 2  having such capacitances hardly affect the first baseband signals I and Ix. 
     When the leaked carrier signal LO or LOx, or the output signal RFout is input together with the first baseband signal I or Ix to the associated baseband input terminal  28  or  29 , the high-frequency component of the carrier signal LO or LOx, or the output signal RFout is decreased by the associated capacitor C 1  or C 2 . As a result, the bases of the fifth and sixth transistors Tr 5  and Tr 6  mostly receive the first baseband signals I and Ix respectively. This reduces the influence of the carrier leak on the first modulation signal V 1  or V 1 x output from the first modulation mixer  32  or the output signal RFout output from the orthogonal modulator  31 . 
     FIG. 7 is a circuit diagram of a first alternative modulation mixer  41 , which is a first modification of the modulation mixer  21 . As shown in FIG. 7, the collectors of the fifth and sixth transistors Tr 5  and Tr 6  are grounded via the capacitors C 1  and C 2 . The capacitors C 1  and C 2  decrease the high-frequency components included in signals that are amplified by the fifth and sixth transistors Tr 5  and Tr 6 . Consequently, the influence caused by the carrier leak is reduced. 
     FIG. 8 is a circuit diagram of a second alternative modulation mixer  42 . As shown in FIG. 8, a negative feedback resistor R 11  is provided between the base and collector of the fifth transistor Tr 5 , and a negative feedback resistor R 12  is provided between the base and collector of the sixth transistor Tr 6 . The resistors R 11  and R 12  provide the fifth and sixth transistors Tr 5  and Tr 6  with DC and AC feedbacks. The amplification factors for the high-frequency signals of the fifth and sixth transistors Tr 5  and Tr 6  decrease when the resistors R 11  and R 12  have low resistances. That is, the resistors R 11  and R 12  degrade the high frequency characteristics of the fifth and sixth transistors Tr 5  and Tr 6 , reducing the amplification factors for their high-frequency signals. This decreases the conversion gains of the modulation mixers  32  and  33 , making the value of the second term (DC component) in the equation (2) smaller, which reduces the influence of the carrier leak on the output signal RFout. 
     It is preferable that the resistances of the resistors R 11  and R 12  are set smaller than that of the resistor R 3  and small enough to avoid a diode connection of the fifth and sixth transistors Tr 5  and Tr 6 . When the resistor R 3  has a resistance of about 1000 ohms to about 100 ohms, for example, the resistors R 11  and R 12  should preferably have resistances of about 100 ohms to about 10 ohms. 
     FIG. 9 is a circuit diagram of a third alternative modulation mixer  43 . As shown in FIG. 9, a negative feedback capacitor C 11  is provided between the base and collector of the fifth transistor Tr 5 , and a negative feedback capacitor C 12  is provided between the base and collector of the sixth transistor Tr 6 . The capacitors C 11  and C 12  provide the fifth and sixth transistors Tr 5  and Tr 6  with AC feedbacks. The amplification factors of the fifth and sixth transistors Tr 5  and Tr 6  decrease as the frequency of the input signal gets higher. That is, the capacitors C 11  and C 12  deteriorate the high frequency characteristics of the fifth and sixth transistors Tr 5  and Tr 6 . 
     FIG. 10 is a circuit diagram of a fourth alternative modulation mixer  44 . The modulation mixer  44  in FIG. 10 includes seventh and eighth transistors Tr 7  and Tr 8  which are larger in size than the fifth and sixth transistors Tr 5  and Tr 6 . The sizes of the seventh and eighth transistors Tr 7  and Tr 8  are greater than those of the first to sixth transistors Tr 1 -Tr 6  by about ten times to about twenty times. The high frequency characteristics of the seventh and eighth transistors Tr 7  and Tr 8  are lower than those of the first to fourth transistors Tr 1 -Tr 4 , which are smaller in size. The seventh and eighth transistors Tr 7  and Tr 8  therefore amplify the leaked carrier signal LO or LOx, or the output signal RFout with a lower amplification factor, which results in reduced influence of the carrier leak on the output signal RFout. 
     FIG. 11 is a schematic block diagram of an orthogonal modulator  51  according to a second embodiment of the present invention. The orthogonal modulator  51  comprises a phase shifter  13 , first and second modulation mixers  32  and  33 , first and second frequency multipliers  52  and  53 , and an adder  54 , all formed on a semiconductor chip. The first and second modulation mixers  32  and  33  may be replaced with the modulation mixers  14  and  15  in FIG. 1 or the modulation mixers  24  and  25  in FIG.  4 . 
     The phase shifter  13 , which is preferably a flip-flop type phase shifter, receives complementary carrier signals LO and LOx and frequency-divides the carrier signals LO and LOx to generate carrier signals LO 0 , LO 90 , LO 180  and LO 270  whose phases are shifted from one another by 90 degrees. 
     The first modulation mixer  32  receives the in-phase carrier signal LO 0  or LO 180  from the phase shifter  13  and the second modulation mixer  33  receives the orthogonal carrier signal LO 90  or LO 270  from the phase shifter  13 . The frequencies of the carrier signals received by the first and second modulation mixers  32  and  33  are half the frequencies of the carrier signals received by the first and second modulation mixers  14  and  15  in FIG.  1 . Therefore, the current consumption of the first and second modulation mixers  32  and  33  of the orthogonal modulator  51  is reduced. 
     The first modulation mixer  32  multiplies the first baseband signal I or Ix by the in-phase carrier signal LO 0  or LO 180  to yield a first modulation signal V 1  or V 1 x. The second modulation mixer  33  multiplies the second baseband signal Q or Qx by the orthogonal carrier signal LO 90  or LO 270  to yield a second modulation signal V 2  or V 2 x. 
     The first frequency multiplier  52 , connected between the adder  54  and the first modulation mixer  32 , multiplies the first modulation signal V 1  or V 1 x by the in-phase carrier signal LO 0  or LO 180  to produce a third modulation signal V 3  or V 3 x. The first modulation signals V 1  and V 1 x have substantially the same frequencies as the in-phase carrier signals LO 0  and LO 180 . Therefore, the first frequency multiplier  52  outputs the third modulation signal V 3  or V 3 x which has double the frequency of the in-phase carrier signal LO 0  or LO 180 . 
     The second frequency multiplier  53 , connected between the adder  54  and the second modulation mixer  33 , multiplies the second modulation signal V 2  or V 2 x by the in-phase carrier signal LO 0  or LO 180  to produce a fourth modulation signal V 4  or V 4 x. The second modulation signals V 2  and V 2 x have substantially the same frequencies as the orthogonal carrier signals LO 90  and LO 270 . Therefore, the second frequency multiplier  53  outputs the fourth modulation signal V 4  or V 4 x which has double the frequency of the orthogonal carrier signal LO 90  or LO 270 . 
     The adder  54  adds the third and fourth modulation  5  signals V 3  or V 3 x, and V 4  or V 4 x to produce complementary output signals RO and ROx. 
     FIG. 12 is a schematic circuit diagram of the first frequency multiplier  52  including transistors Tr 11  to Tr 18 , resistors R 11  and R 12  and constant current sources  64  to  66 . 
     The first and second transistors Tr 11  and Tr 12  have emitters connected together, bases connected respectively to input terminals  61   a  and  61   b , and collectors connected to the power supply line of the high-potential power supply via the respective resistors R 11  and R 12 , and form a differential amplifier. The third and fourth transistors Tr 13  and Tr 14  have emitters connected together, bases connected respectively to the input terminals  61   b  and  61   a , and collectors connected to the power supply line of the high-potential power supply via the respective resistors R 11  and R 12 , and also form a differential amplifier. The base of the fourth transistor Tr 14  is also connected to the base of the first transistor Tr 11 . The fifth and sixth transistors Tr 15  and Tr 16  have emitters connected together, bases connected respectively to input terminals  62   a  and  62   b , and collectors connected to the power supply line of the low-potential power supply (ground GND) via the constant current source  64 , and form another differential amplifier. The collector of the fifth transistor Tr 15  is connected to the emitters of the first and second transistors Tr 11  and Tr 12 . The collector of the sixth transistor Tr 16  is connected to the emitters of the third and fourth transistors Tr 13  and Tr 14 . 
     The seventh transistor Tr 17  has a base connected to the collectors of the first and third transistors Tr 11  and Tr 13 , a collector connected to the power supply line of the high-potential power supply, an emitter connected to an output terminal  63   a  and to the power supply line of the low-potential power supply via the constant current source  65 . The eighth transistor Tr 18  has a base connected to the collectors of the second and fourth transistors Tr 12  and Tr 14 , a collector connected to the power supply line of the high-potential power supply, an emitter connected to an output terminal  63   b  and to the power supply line of the low-potential power supply via the constant current source  66 . 
     The first modulation signals V 1  and V 1 x from the first modulation mixer  32  are respectively input to the input terminals  61   a  and  61   b , and the in-phase carrier signals LO 0  and LO 180  from the phase shifter  13  are respectively input to the input terminals  62   a  and  62   b . Alternatively, the in-phase carrier signals LO 0  and LO 180  may respectively be input to the input terminals  61   a  and  61   b , and the first modulation signals V 1  and V 1 x may respectively be input to the input terminals  62   a  and  62   b . The modulation signals V 3  and V 3 x are output from the output terminals  63   a  and  63   b.    
     The operation of the orthogonal modulator  51  will now be described with reference to equations given below. The output signal Iout (the first modulation signal V 1  or V 1 x) of the first modulation mixer  32  is given by the following equation (7).                      I                 out     =       cos        (     2        π   ·       f   LO     /   2     ·   t       )       ×     cos        (     2      π                   f   BB        t     )                     =       1   2          {       cos                 2        π        (         f   LO     /   2     +     f   BB       )          t     +     cos                 2        π        (         f   LO     /   2     -     f   BB       )          t       }                     (   7   )                                
     where f LO  indicates the frequencies of the carrier signals LO and LOx and f BB  indicates the frequencies of the first and second baseband signals I, Ix, Q and Qx. 
     The output signal Iout 2  (the third modulation signal V 3  or V 3 x) of the first frequency multiplier  52  is given by the following equation (8).                      I                 out2     =                  1   2          {       cos                 2        π        (         f   LO     /   2     +     f   BB       )          t     +     cos                 2        π        (         f   LO     /   2     -     f   BB       )          t       }     ×                              cos        (     2        π   ·       f   LO     /   2     ·   t       )                   =                  1   4          {       cos                 2        π        (       f   LO     +     f   BB       )          t     +     cos                 2      π                   f   BB        t     +     cos                 2        π        (       f   LO     -     f   BB       )          t     +                                  cos                 2      π                   f   BB        t     }                 (   8   )                                
     The output signal Qout (the second modulation signal V 2  or V 2 x) of the second modulation mixer  33  is given by the following equation (9).                      Q                 out     =       cos        (       2        π   ·       f   LO     /   2     ·   t       -     90      °       )       ×     cos        (       2      π                   f   BB        t     +     90      °       )                     =       1   2          {       cos                 2        π        (         f   LO     /   2     +     f   BB       )          t     -     cos                 2        π        (         f   LO     /   2     -     f   BB       )          t       }                     (   9   )                                
     The output signal Qout 2  (the fourth modulation signal V 4  or V 4 x) of the second frequency multiplier  53  is given by the following equation (10).                      Q                 out2     =                  1   2          {       cos                 2        π        (         f   LO     /   2     +     f   BB       )          t     -     cos                 2        π        (         f   LO     /   2     -     f   BB       )          t       }     ×                              cos        (     2        π   ·       f   LO     /   2     ·   t       )                   =                  1   4     [       {       cos                 2        π        (       f   LO     +     f   BB       )          t     +     cos                 2      π                   f   BB        t       }     -     {       cos                 2        π        (       f   LO     -     f   BB       )          t     +                                      cos                 2      π                   f   BB        t     }     ]                 (   10   )                                
     The output signal Out (the output signal RO or ROx) of the adder  54  given by the following equation (11).                    Out   =       I                 out2     +     Q                 out2                   =       1   2          {       cos                 2        π        (       f   LO     +     f   BB       )          t     +     cos                 2      π                   f   BB        t       }                     (   11   )                                
     When the baseband frequency f BB  is approximately 100 KHz and the carrier frequency f LO  is about 1 GHz, for example, the value of the second term in the equation (11) becomes significantly small and hardly affects the output signal Out. That is, the value of the second term can be ignored. The output signal of the adder  54  can essentially be considered to have the value of the first term alone. The influence of the carrier leak on the output signal Out is thus reduced. 
     The orthogonal modulator  51  in FIG. 11 according to the second embodiment has a smaller circuit area than the conventional orthogonal modulator  21  in FIG.  4 . Referring to FIG. 11 in comparison with FIG. 4, the circuit area of the phase shifter  13  is the same as the first or second ½ frequency divider  22  or  23 . FIG. 13 is a circuit diagram of the phase shifter  13  or the first or second ½ frequency divider  22  or  23 . The total circuit area of the first and second frequency multipliers  52  and  53  is substantially the same as the circuit area of the first or second ½ frequency divider  22  or  23 . The orthogonal modulator  51  therefore has a smaller circuit area than the orthogonal modulator  21  in FIG. 4 by the circuit area of the frequency multiplier  27 . This structure reduces the size and cost of the orthogonal modulator  51 , and thus, a portable device using the same. 
     FIG. 14 is a schematic block diagram of an orthogonal modulator  71  according to a first modification of the second embodiment. As shown in FIG. 14, the first frequency multiplier  52  of the orthogonal modulator  71  multiplies the first modulation signal V 1  or V 1 x output from the first modulation mixer  32  by the orthogonal carrier signal LO 90  or LO 270  to produce a third modulation signal V 3  or V 3 x. The second frequency multiplier  53  multiplies the second modulation signal V 2  or V 2 x output from the second modulation mixer  33  by the orthogonal carrier signal LO 90  or LO 270  to produce a fourth modulation signal V 4  or V 4 x. 
     In the first modification, the output signal Iout 2  (the third modulation signal V 3  or V 3 x) of the first frequency multiplier  52  is given by the following equation (12).                      Q                 out2     =                  1   2          {       cos                 2        π        (         f   LO     /   2     +     f   BB       )          t     +     cos                 2        π        (         f   LO     /   2     -     f   BB       )          t       }     ×                              cos        (       2        π   ·       f   LO     /   2     ·   t       -     90      °       )                   =                  1   4     [       cos        {       2        π        (       f   LO     +     f   BB       )          t     -     90      °       }       +     {       cos                 2      π                   f   BB        t     +     90      °       }     +                                  cos        {       2        π        (       f   LO     -     f   BB       )          t     -     90      °       }       +     cos        {       2      π                   f   BB        t     +     90      °       }         ]                 (   12   )                                
     The output signal Qout 2  (the fourth modulation signal V 4  or V 4 x) of the second frequency multiplier  53  is given by the following equation (13).                      Q                 out2     =                  1   2          {       cos                 2        π        (         f   LO     /   2     +     f   BB       )          t     -     cos                 2        π        (         f   LO     /   2     -     f   BB       )          t       }     ×                              cos        (       2        π   ·       f   LO     /   2     ·   t       -     90      °       )                   =                  1   4     [       cos        {       2        π        (       f   LO     +     f   BB       )          t     -     90      °       }       +     {       cos                 2      π                   f   BB        t     +     90      °       }     -                                {       cos        {       2        π        (       f   LO     -     f   BB       )          t     -     90      °       }       +     cos        (       2      π                   f   BB        t     +     90      °       )         ]                   (   13   )                                
     The output signal Out (the output signal RO or ROx) of the adder  54  is given by the following equation (14).                    Out   =       I                 out2     +     Q                 out2                   =       1   2          [       cos        {       2        π        (       f   LO     +     f   BB       )          t     -     90      °       }       +     cos        (       2      π                   f   BB        t     +     90      °       )         ]                     (   14   )                                
     It is apparent from the equation (14) that the orthogonal modulator  71  of the first modification reduces the influence of the carrier leak to the output signal Out. 
     FIG. 15 is a schematic block diagram of an orthogonal modulator  72  according to a second modification of the second embodiment. The first modulation mixer  32  multiplies the in-phase carrier signals LO 0  and LO 180  by the first baseband signals I and Ix to produce the first modulation signals V 1  and V 1 x. The first frequency multiplier  52  multiplies the first modulation signals V 1  and V 1 x by the in-phase carrier signals LO 0  and LO 180  to yield the third modulation signals V 3  and V 3 x. 
     The second modulation mixer  33  multiplies the in-phase carrier signals LO 0  and LO 180  by the second baseband signals Q and Qx to produce the second modulation signals V 2  and V 2 x. The second frequency multiplier  53  multiplies the second modulation signals V 2  and V 2 x by the orthogonal carrier signals LO 90  and LO 270  to yield the fourth modulation signals V 4  and V 4 x. 
     The output signal Iout (the first signal V 1  or V 1 x) of the first modulation mixer  32  is given by the aforementioned equation (7). The output signal Iout 2  (the third modulation signal V 3  or V 3 x) of the first frequency multiplier  52  is given by the equation (8). 
     The output signal Qout (the second modulation signal V 2  or V 2 x) of the second modulation mixer  33  is given by the following equation (15).                      Q                 out     =                cos                 2        π   ·       f   LO     /   2     ·   t     ×     cos        (       2      π                   f   BB        t     +     90      °       )                     =                  1   2     [       cos        {       2        π        (         f   LO     /   2     +     f   BB       )          t     +     90      °       }       +                                cos        {       2        π        (         f   LO     /   2     -     f   BB       )          t     -     90      °       }       ]                 (   15   )                                
     The output signal Qout 2  (the fourth modulation signal V 4  or V 4 x) of the second frequency multiplier  53  is given by the following equation (16).                      Q                 out2     =                  1   2     [       cos        {       2        π        (         f   LO     /   2     +     f   BB       )          t     +     90      °       }       +                                  cos        {       2        π        (         f   LO     /   2     -     f   BB       )          t     -     90      °       }       ]     ×     cos        (       2        π   ·       f   LO     /   2     ·   t       -     90      °       )                   =                  1   4     [       cos                 2        π        (       f   LO     +     f   BB       )          t     -     cos                 2      π                   f   BB        t     +     {         -   cos                   2        π        (       f   LO     -     f   BB       )          t     +                                      cos                 2      π                   f   BB        t     }     ]                 (   16   )                                
     The output signal Out (the output signal RO or ROx) of the adder  54  is given by the following equation (17).                    Out   =       I                 out2     +     Q                 out2                   =       1   2          {       cos                 2        π        (       f   LO     +     f   BB       )          t     +     cos                 2      π                   f   BB        t       }                     (   17   )                                
     It is apparent from the equation (17) that the orthogonal modulator  72  of the second modification reduces the influence of the carrier leak to the output signal Out. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiment are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.