Abstract:
A receiver IF circuit includes the following: a variable gain amplifier for amplifying an RF input signal; a frequency converter for mixing the amplified RF input signal and a local signal to generate polyphase intermediate-frequency signals that are used for suppressing an image component; a polyphase filter for receiving the polyphase intermediate-frequency signals and outputting an intermediate-frequency signal whose image component is suppressed; a frequency variable band-pass filter for selecting a channel of the intermediate-frequency signal while changing a frequency response in accordance with a supplied control signal; an IF demodulator for demodulating the intermediate-frequency signal; and an automatic gain control for detecting a level of an output signal of the IF demodulator and controlling a gain of the variable gain amplifier in accordance with the detected level. Using the polyphase filter and the frequency variable band-pass filter, the receiver IF circuit can achieve high-performance integration of an image rejection filter and a channel selection filter.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a receiver IF (intermediate frequency) circuit, particularly a receiver IF circuit in which a filter constituting an image rejection mixer is integrated with a filter having a channel selection function.  
         [0003]     2. Description of Related Art  
         [0004]      FIG. 7  shows a conventional superheterodyne radio receiver. When an RF signal is input, an RF filter  1  rejects an undesired signal including an image signal and transmits a desired signal. The RF signal that has passed through the RF filter  1  is amplified by a variable gain amplifier  2  and mixed with a local-frequency signal of an oscillator  4  by a frequency mixer  3 , so that the frequency is converted into an intermediate frequency (IF). Then, a band-pass filter (BPF)  6   b  removes an undesired signal from the output signal of the frequency mixer  3  and transmits only a desired IF signal. The band-pass filter  6   b  is composed mainly of an external passive component such as a ceramic filter. The output signal of the band-pass filter  6   b  is amplified by an IF amplifier  7  and subsequently is converted into a baseband signal by an IF demodulator  8 .  
         [0005]     The amplitude of the signal after demodulation is detected by an AGC (automatic gain control)  9 . The output of the AGC  9  is supplied to the variable gain amplifier  2  and the IF amplifier  7  as a gain control voltage for maintaining the amplitude of the baseband signal constant. The gains of the variable gain amplifier  2  and the IF amplifier  7  are controlled based on the gain control voltage so that an appropriate dynamic range is kept for the amplifiers and the filters. The region enclosed with a broken line represents an integrated block  10 . The indication is the same for the following description. The RF filter  1  and the band-pass filter  6   b  are located outside the integrated block  10 .  
         [0006]     Next, image interference that is a problem of the heterodyne system will be described.  FIG. 8  is a conceptual diagram of the image interference.  FIG. 8 ( a ) shows a frequency conversion by the mixer  3 . This figure shows a down-converting operation performed when the RF signal entering the mixer  3  through the RF filter  1  includes a desired wave V RF  and an image wave V IM . As shown in  FIG. 8 ( b ), the desired wave V RF  has a frequency (f LO +f IF ) that is higher than a local signal frequency f LO  by the amount equal to an intermediate frequency f IF . The image wave V IM  has a frequency (f LO −f IF ) that is lower than the local signal frequency f LO  by the amount equal to the intermediate frequency f IF .  
         [0007]     As shown in  FIG. 8 ( c ), whether the desired wave V RF  having a frequency (f LO +f IF ) or the image wave V IM  having a frequency (f LO −f IF ) is input to the receiver system, it is down-converted by the mixer  3  and transmitted by the band-pass filter  6   b  to generate a signal V OUT  with the same intermediate frequency f IF . Therefore, interference occurs due to the image signal and may degrade the reception quality. Accordingly, the RF filter  1  generally is used to reject the image wave beforehand.  
         [0008]     However, the external RF filter  1  increases the cost and makes it difficult to reduce the packaging density per substrate. In recent years, therefore, an image rejection mixer has been employed to deal with the image interference (e.g., JP 2001-513275, JP 2003-298356, or Sharzad Tadjpour and three others, “A 900-MHz Dual-Conversion Low-IF GSM Receiver in 0.35-μm CMOS” ISSCC, Vol. 36, No. 12, December, 2001). The image rejection mixer rejects the image wave with a circuit technology. Using this image rejection mixer can eliminate the function of rejecting the image wave from the external RF filter  1 .  FIG. 9  shows an example of the image rejection mixer.  
         [0009]     In  FIG. 9 , a desired wave A RF cosω RF t and an image wave A IM cosω IM t are input (RF input). As local signals, sinω LO t is supplied to a mixer  3   a  and cosω LO t is supplied to a mixer  3   b . The high-frequency component included in the output signal of the mixer  3   a  is rejected by a LPF (low-pass filter)  50   a . Thus, the output signal of the LPF  50   a  is expressed by Formula (1). The signal that has passed through a 90-degree phase shifter  51  is expressed by Formula (2). 
 
( A   RF /2)·sin(ω LO −ω RF ) t +( A   IM /2)·sin(ω LO −ω IM ) t   Formula (1) 
 
( A   RF /2)·cos(ω RF −ω LO ) t −( A   IM /2)·cos(ω LO −ω IM ) t   Formula (2) 
 
 On the other hand, the signal that has been output from the mixer  3   b  and passed through a LPF  50   b  is expressed by Formula (3). 
 
( A   RF /2)·cos(ω RF −ω LO ) T +( A   IM /2)·cos(ω LO −ω IM ) t   Formula (3) 
 
 Consequently, the output of an adder  52  is A RF cos(ω RF −ω LO )t, and the image signal A IM cos(ω LO −ω IM )t can be removed. 
 
         [0010]     As the 90-degree phase shifter  51 , a CR/RC circuit that utilizes a 90-degree difference in phase between the voltage at both ends of a capacitor and the voltage at both ends of a resistor may be used. However, the image rejection characteristics are degraded because of a narrow bandwidth of the 90-degree phase sifter  51 , property variations of the capacitor and the resistor, and amplitude or phase errors of two signals with a 90-degree phase difference. Therefore, a polyphase filter has been used instead of the 90-degree phase shifter  51  (e.g., the above-mentioned JP 2003-298356 or Sharzad Tadjpour and three others, “A 900-MHz Dual-Conversion Low-IF GSM Receiver in 0.35-μm CMOS” ISSCC, Vol. 36, No. 12, December, 2001).  
         [0011]      FIG. 10  shows an example of the configuration of a passive polyphase filter. In the passive polyphase filter of  FIG. 10 , polyphase filters  53 - 1 ,  53 - 2 , . . . and  53 - n , each of which has four phases, are connected in n stages. The polyphase filter  53 - 1  includes resistors R 11  to R 14  and capacitors C 11  to C 14 . The polyphase filter  53 - 2  includes resistors R 21  to R 24  and capacitors C 21  to C 24 . The polyphase filter  53 - n  includes resistors Rn 1  to Rn 4  and capacitors Cn 1  to Cn 4 .  
         [0012]      FIG. 11  shows the image rejection characteristics of the passive polyphase filter of  FIG. 10 . In  FIG. 11 , a broken line  54  represents the characteristics when a desired signal is input, and a solid line  55  represents the characteristics when an image signal is input. A difference between the characteristics of the broken line  54  and the solid line  55  is image rejection. Since the polyphase filters are connected in multiple stages, the bandwidth becomes broader. Therefore, even if the elements vary, the image rejection characteristics are degraded less.  
         [0013]      FIG. 12  shows an example of an active polyphase filter for image rejection. In  FIG. 12 , input signals I, −I, Q and −Q have the same amplitude, but different phases of 0, −180, 90 and −90 degrees, respectively. Reference numerals  30 - 1 ,  30 - 2 , . . . and  30 - n  are BPFs and connected in n stages. The BPF  30 - 1  includes operational amplifiers  31 - 1  and  32 - 1 , resistors R 1   a , R 1   b  and R 1   c , and capacitor C 1   a . The BPF  30 - 2  includes operational amplifiers  31 - 2  and  32 - 2 , resistors R 2   a , R 2   b  and R 2   c , and capacitor C 2   a . The BPF  30 - n  includes operational amplifiers  31 - n  and  32 - n , resistors Rna, Rnb and Rnc, and capacitor Cna.  
         [0014]     By using the polyphase filter, it is possible to reduce the degradation of the image rejection characteristics due to a variation in property of each element.  FIG. 13  shows an example of the image rejection characteristics of the active polyphase filter. In  FIG. 13 , a broken line  56  represents the frequency characteristics when a desired signal is input, and a solid line  57  represents the frequency characteristics when an image signal is input. A difference between the frequency characteristics of the broken line  56  and the solid line  57  is image rejection. The active polyphase filter is a band-pass filter and rejects signals outside of a certain band. Thus, the active polyphase filter also can be used as part of a channel filter.  
         [0015]     In order to reduce the cost, there has been an attempt to replace the passive components with the active components (e.g., the above-mentioned JP 2001-513275).  FIG. 14  shows an example of a receiver in which the band-pass filter  6   b  (passive band-pass filter) in  FIG. 7  is replaced with a band-pass filter  6   a  composed of a switched capacitor filter (SCF). The basic operation is the same as that in  FIG. 7 . In  FIG. 14 , reference numeral  58  is an anti-aliasing filter for preventing aliasing caused by using the SCF. The band-pass filter  6   a  removes an undesired signal from the signal that has passed through the anti-aliasing filter  58  after mixing and transmits only a desired intermediate-frequency signal. A frequency divider  11  divides the output signal of the oscillator  4  to generate a signal with a desired frequency, and supplies it as a clock for the SCF of the band-pass filter  6   a . A smoothing filter  42  removes a clock signal and its harmonic from the output of the band-pass filter  6   a.    
         [0016]     In the radio receiver, the input signal bands are broad, and signals with different modulation types such as AM or FM are input. Therefore, the radio receiver requires not only a channel filter that amplifies only a desired signal in various frequency bands, but also an image rejection filter for the heterodyne system. Thus, many receiving channel filters should be used, which increases the number of passive filters and makes it difficult to reduce the cost and the packaging area. Although the passive components may be replaced with the active components as disclosed in JP 2001-513275, many active filters are needed for each of the input signal bands or the types of signals. This may lead to an increase in circuit current, chip area, or noise.  
         [0017]     In the conventional examples of  FIGS. 7 and 14 , the RF filter  1  has both functions of the image rejection filter and the channel filter, and the band-pass filters  6   a ,  6   b  have the functions of rejecting an undesired signal after mixing and selecting only a desired IF signal. For the circuit in  FIG. 7 , these filters are composed mainly of a ceramic filter, SAW filter, or the like and should have enhanced selecting characteristics and image rejecting function. Therefore, when the filters are integrated into an active circuit, they must ensure higher precision, and are not likely to be stable against variations in element property. For the circuit in  FIG. 14 , which uses a switched capacitor filter (active filter) as the band-pass filter  6   a , the anti-aliasing filter  58  has to be provided in the preceding stage of the band-pass filter  6   a , although tuning is not necessary because the filter characteristics of the switched capacitor filter are synchronized with a clock. In order to achieve high-precision filters, the anti-aliasing filter also should have high precision and a large size. Thus, the chip cost, power consumption, or noise is increased to make integration difficult.  
         [0018]      FIG. 15  shows an example of a receiver system that receives two RF input signals in different signal bands. In this system, an RF1 signal enters an RF1 amplifier  2   a  through an RF1 filter  1   a , and an RF2 signal enters an RF2 variable gain amplifier  33  through an RF2 filter  1   b.    
         [0019]     The RF1 signal is subjected to double down-conversion. The output of a mixer  3  passes through an IF 1  band-pass filter  60 , and subsequently is mixed with a second local signal by a second mixer  61  and converted into IF 12 . The IF 12  is processed by an IF 12  band-pass filter  62 , an IF 12  amplifier  63 , and an IF 12  demodulator  64 , so that a baseband signal  1  is output.  
         [0020]     The output of the RF2 variable gain amplifier  33  is processed in the same manner as the circuit in  FIG. 7  by a mixer  3   c , an IF 2  band-pass filter  65 , an IF 2  amplifier  66 , an IF 2  demodulator  67 , and an AGC  68 , so that a baseband signal  2  is output.  
         [0021]     The RF filters  1   a  and  1   b , the IF 1  band-pass filter  60 , and the IF 2  band-pass filter  65  are needed for each of the RF1 and RF2 signals. These filters have to be used depending on the functions and frequency characteristics of the image rejection filter or the channel selection filter.  
       SUMMARY OF THE INVENTION  
       [0022]     Therefore, with the foregoing in mind, it is an object of the present invention to provide a receiver IF circuit that can achieve high-performance integration of an image rejection filter and a channel selection filter at low cost, and thus can reduce the cost of a receiver and the area of a substrate for reception.  
         [0023]     A receiver IF circuit of the present invention includes the following: a variable gain amplifier for amplifying an RF input signal; a frequency converter for mixing the amplified RF input signal and a local signal to generate polyphase intermediate-frequency signals that are used for suppressing an image component; a polyphase filter for receiving the polyphase intermediate-frequency signals and outputting an intermediate-frequency signal whose image component is suppressed; a frequency variable band-pass filter for selecting a channel of the intermediate-frequency signal while changing a frequency response in accordance with a supplied control signal; an IF demodulator for demodulating the intermediate-frequency signal; and an automatic gain control for detecting a level of an output signal of the IF demodulator and controlling a gain of the variable gain amplifier in accordance with the detected level. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIG. 1  is a block diagram showing a receiver IF circuit of Embodiment 1 of the present invention.  
         [0025]      FIG. 2  shows an example of a SCF used as an active filter of a frequency variable band-pass filter.  
         [0026]      FIG. 3  is a block diagram showing a receiver IF circuit of Embodiment 2 of the present invention.  
         [0027]      FIG. 4  is a block diagram showing a receiver IF circuit of Embodiment 3 of the present invention.  
         [0028]      FIG. 5  shows a specific example of a multistage biquadratic active LPF.  
         [0029]      FIG. 6  is a block diagram showing a receiver IF circuit of Embodiment 4 of the present invention.  
         [0030]      FIG. 7  is a block diagram showing a conventional receiver IF circuit.  
         [0031]      FIG. 8  is a diagram for explaining interference caused by an image signal.  
         [0032]      FIG. 9  is a block diagram schematically showing a conventional image rejection mixer.  
         [0033]      FIG. 10  is a circuit diagram showing an example of a passive polyphase filter.  
         [0034]      FIG. 11  shows the image rejection characteristics of the passive polyphase filter.  
         [0035]      FIG. 12  is a circuit diagram showing an example of an active polyphase filter.  
         [0036]      FIG. 13  shows the image rejection characteristics of the active polyphase filter.  
         [0037]      FIG. 14  is a block diagram showing another example of a conventional receiver IF circuit.  
         [0038]      FIG. 15  is a block diagram showing yet another example of a conventional receiver IF circuit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]     The receiver IF circuit of the present invention uses a polyphase filter for image rejection and a frequency variable band-pass filter composed of a switched capacitor filter (SCF) or the like, and thus allows the polyphase filter to function as an anti-aliasing filter for the frequency variable band-pass filter. Therefore, the receiver IF circuit can achieve high-performance integration at low cost.  
         [0040]     It is preferable that the receiver IF circuit further includes a reference signal generator for generating a reference signal, and the local signal and the control signal are generated based on the reference signal. Accordingly, the frequency variable band-pass filter and the frequency converter can be synchronized to operate with high precision.  
         [0041]     The receiver IF circuit further may include the following: a plurality of the frequency converters corresponding to a plurality of RF input signals for generating a plurality of signal groups of the polyphase intermediate-frequency signals; and a plurality of switches for switching the signal groups to be supplied to the polyphase filter, whereby the plurality of RF input signals in different frequency bands are received.  
         [0042]     In this configuration, the receiver IF circuit further may include the following: a phase-locked loop for controlling a frequency of a voltage-controlled oscillator by using a signal of a quartz oscillator as a reference signal; a plurality of dividers for dividing an output signal of the voltage-controlled oscillator and supplying the resultant signals as the local signals to each of the frequency converters corresponding to the RF input signals; and a variable divider for dividing the signal of the quartz oscillator and supplying the resultant signal as the control signal to the frequency variable band-pass filter. By using an accurate clock, different characteristics can be achieved precisely with a single basic filter, so that the necessary chip area can be reduced.  
         [0043]     The receiver IF circuit further may include the following: the frequency converter corresponding to some of a plurality of RF input signals for generating the polyphase intermediate-frequency signals; a frequency converter corresponding to the other of the RF input signals for generating a single-phase intermediate-frequency signal; and a plurality of switches for switching the polyphase intermediate-frequency signals and the single-phase intermediate-frequency signal to be supplied to the polyphase filter, whereby the plurality of RF input signals in different frequency bands are received.  
         [0044]     In this configuration, the receiver IF circuit further may include the following: a phase-locked loop for controlling a frequency of a voltage-controlled oscillator by using a signal of a quartz oscillator as a reference signal; a plurality of dividers for dividing an output signal of the voltage-controlled oscillator and supplying the resultant signals as the local signals to each of the frequency converters corresponding to the RF input signals; and a variable divider for dividing the signal of the quartz oscillator and supplying the resultant signal as the control signal to the frequency variable band-pass filter.  
         [0045]     The polyphase filter may be composed of a combination of passive polyphase filters or active polyphase filters.  
         [0046]     The frequency variable band-pass filter may be composed of a switched capacitor filter. In this case, it is preferable that the polyphase filter also functions as an anti-aliasing filter for the switched capacitor filter composing the frequency variable band-pass filter.  
         [0047]     It is preferable that a clock frequency of the control signal supplied to the switched capacitor filter composing the frequency variable band-pass filter is higher than an RF input signal band.  
         [0048]     The receiver IF circuit further may include a plurality of variable gain amplifiers.  
         [0049]     Hereinafter, the present invention will be described by way of illustrative embodiments with reference to the drawings.  
       Embodiment 1  
       [0050]      FIG. 1  shows a receiver IF circuit of Embodiment 1 of the present invention. An RF filter  1  selects the frequency of an RF input signal, and then a variable gain amplifier  2  amplifies the RF signal. The output signal of the variable gain amplifier  2  is supplied to a mixer  3   a  and a mixer  3   b , in each of which the signal is mixed with an orthogonal local signal generated by an oscillator  4 . Consequently, the RF signal is converted into polyphase intermediate-frequency signals, i.e., quadrature phase signals of I, −I, Q and −Q for suppressing the image component. The I, −I, Q and −Q signals are supplied to a polyphase filter  5 . An image rejection mixer is formed by a block  12  enclosed with a broken like that includes the mixers  3   a  and  3   b , the oscillator  4 , and the polyphase filter  5 .  
         [0051]     The output of the polyphase filter  5  is supplied to a frequency variable band-pass filter  6  to select only a desired IF signal. The frequency variable band-pass filter  6  is controlled based on a control signal obtained by dividing the output signal of the oscillator  4  with a frequency divider  11   a , and thus the selecting frequency is adjusted. The output of the frequency variable band-pass filter  6  is amplified by an IF amplifier  7  and subsequently is converted into a baseband signal by an IF demodulator  8 . The output of the IF demodulator  8  is applied to an AGC  9 , and a control voltage is supplied from the AGC  9  to the variable gain amplifier  2  and the IF amplifier  7 , so that their gains are controlled to maintain the signal level constant based on the control voltage.  
         [0052]     In this circuit, the frequency of the control signal supplied to the frequency variable band-pass filter  6  is varied by changing a dividing ratio of the frequency divider  11   a , and thus the frequency selection characteristics can be changed.  
         [0053]     As an example of the polyphase filter  5 , the passive polyphase filter in  FIG. 10  may be used. The quadrature phase signals I, −I, Q and −Q that are output from the mixers  3   a  and  3   b  enter the passive polyphase filter with the same amplitude. The polyphase filters  53 - 1 ,  53 - 2 , . . . and  53 - n  have a center frequency of f01=1/(2πR 11 ×C 11 ), f02=1/(2πR 21 ×C 21 ), and f0n=1/(2πRn 1 ×Cn 1 ), respectively.  FIG. 11  shows the frequency characteristics of the passive polyphase filter as a whole. The passive polyphase filter exhibits the notch characteristics of the solid line  55  for an image signal and the substantially all-pass characteristics of the broken line  54  for a desired signal. The polyphase filters are connected in multiple stages. Therefore, even if the capacitors or resistors vary, the desired image rejection characteristics can be achieved.  
         [0054]     As another example of the polyphase filter  5 , the active polyphase filter in  FIG. 12  may be used. The quadrature phase signals I, −I, Q and −Q enter the active polyphase filter with the same amplitude. The BPFs  30 - 1 ,  30 - 2 , . . . and  30 - n  have a center frequency of f01=1/(2πC 1   a ×R 1   c ), f02=1/(2πC 2   a ×R 2   c ), and f0n=1/(2πCna×Rnc) and a−3 dB bandwidth of BW1=2/(2πC 1   a ×R 1   b ), BW2=2/(2πC 2   a ×R 2   b ), and BWn=2/(2πCna×Rnc), respectively.  FIG. 13  shows the frequency characteristics of the active polyphase filter. The active polyphase filter exhibits the BPF characteristics of the broken line  56  for a desired signal and those of the solid line  57  for an image signal. Therefore, the image rejection characteristics can be achieved.  
         [0055]     In the circuit of  FIG. 1 , the signal after passing through the polyphase filter  5  is supplied to the frequency variable band-pass filter  6 , thereby providing the following effects. When the frequency variable band-pass filter  6  is composed of a switched capacitor filter (SCF), the anti-aliasing filter  58  generally has to be provided in the preceding stage of the SCF, as shown in the conventional example of  FIG. 14 . In contrast, the active polyphase filter in  FIG. 12  can serve as an anti-aliasing filter because it has the image rejection function and acts at the same time as an active BPF, as shown in  FIG. 13 . This allows the polyphase filter  5  also to be used as an anti-aliasing filter, and thus can reduce the chip area and the electric power.  
         [0056]     In order to achieve necessary attenuation at fs/2, where fs is the sampling frequency of the SCF, the polyphase BPFs can be connected in multiple stages to obtain the necessary attenuation. Moreover, the SCF has another great advantage of changing the frequency characteristics by varying the clock frequency.  
         [0057]      FIG. 2  shows an example of the SCF that can be used as the frequency variable band-pass filter  6 . This SCF includes capacitor selection networks  20  to  23  and an operational amplifier  24 . The capacitor selection network  20  includes capacitors C 1   ap  to Cnap and switches SW. The capacitor selection network  21  includes capacitors C 1   an  to Cnan and switches SW. The capacitor selection network  22  includes capacitors C 2   a  and switches SW. The capacitor selection network  23  includes capacitors C 2   a  and switches SW. The capacitance is selected by the selection mode of a necessary frequency, and a clock selected with respect to the necessary frequency is supplied to the switch SW. The SCF in  FIG. 2  can constitute an integrator or a first-order basic filter. Thus, it is possible to form a filter with desired selecting characteristics by selecting the capacitor network and the clock frequency. The SCF also can constitute a second- or higher-order filter. All the filters can share the operational amplifier  24 , thereby reducing the electric power and the cost.  
       Embodiment 2  
       [0058]      FIG. 3  shows a receiver IF circuit of Embodiment 2 of the present invention. Two RF input signals, i.e., RF1 and RF2 in different frequency bands are input to this circuit. After the frequency is selected by an RF filter  1   a , the RF1 signal enters a variable gain amplifier  2 . The signal amplified by the variable gain amplifier  2  is supplied to a mixer  3   a  and a mixer  3   b . Similarly, after the frequency is selected by an RF2 filter  1   b , the RF2 signal enters an amplifier  13 . The signal amplified by the amplifier  13  is supplied to a mixer  3   c  and a mixer  3   d.    
         [0059]     Local signals obtained by dividing the output signal of an oscillator  4  with frequency dividers  14   a  and  14   b  are supplied to the mixers  3   a ,  3   b ,  3   c  and  3   d  to obtain a predetermined IF frequency. For image rejection, the local signals supplied to the mixers  3   a  and  3   b  differ in phase by 90 degrees. The local signals supplied to the mixers  3   c  and  3   d  also differ in phase by 90 degrees. Consequently, the mixers  3   a  and  3   b  output quadrature phase signals I 1 , −I 1 , Q 1  and −Q 1  having a frequency IF 1 , and the mixers  3   c  and  3   d  output quadrature phase signals I 2 , −I 2 , Q 2  and −Q 2  having a frequency IF 2 .  
         [0060]     These signals with two different frequencies are switched by switches  15  to  18  and then supplied to a polyphase filter  5 . The image rejection operation on the two RF input signals is performed by a block  12  that includes the mixers  3   a  to  3   d , the oscillator  4 , the frequency dividers  14   a  and  14   b , the switches  15  to  18 , and the polyphase filter  5 .  
         [0061]     The signal that has passed through the polyphase filter  5  is supplied to a frequency variable band-pass filter  6 . The frequency variable band-pass filter  6  operates based on a control signal supplied from a frequency divider  11  whose dividing ratio can be changed. Accordingly, the frequency variable band-pass filter  6  can be used for each of the different intermediate frequencies IF 1  and IF 2 .  
         [0062]     In the configuration of  FIG. 3 , after the signal has passed through the frequency variable band-pass filter  6 , the operations of the circuit for IF 1  are the same as those in  FIG. 1 . The circuit for IF 2  has no feedback loop. However, the circuit for IF 2  may perform an AGC operation like the circuit for IF 1 , and the amplifier  13  may be a variable gain amplifier.  
         [0063]     The receiver IF circuit of this embodiment allows the RF input signals with different frequencies to be processed by one polyphase filter  5  and one frequency variable band-pass filter  6 . Therefore, it is possible to suppress an increase in chip cost and to reduce the electric power. Although two different RF input signals are input in  FIG. 3 , the similar configuration also can be used for three or more RF input signals based on the following characteristics.  
         [0064]     (a) The control signal supplied to the frequency variable band-pass filter  6  can be selected optimally by the value of the frequency divider  11 .  
         [0065]     (b) The signal synchronized with each of the local frequencies that are supplied to the mixers  3   a  to  3   d  is supplied to the frequency variable band-pass filter  6 .  
         [0066]     (c) The polyphase filter  5  can perform the image rejection of the individual RF input signals and remove an undesired signal.  
       Embodiment 3  
       [0067]      FIG. 4  shows a receiver IF circuit of Embodiment 3 of the present invention. Like the circuit in  FIG. 3 , two RF input signals, i.e., RF1 and RF2 in different frequency bands are input to this circuit. The circuit in  FIG. 4  differs from the circuit in  FIG. 3  in that an RF2 filter  1   c  (external filter) performs the image rejection of the RF2 signal.  
         [0068]     For the RF1 signal, an image rejection mixer is formed by a block  12  that includes mixers  3   a  and  3   b , an oscillator  4 , a frequency divider  14   a , switches  25  to  28 , and a polyphase filter  5 . For the RF2 signal, the polyphase filter  5  serves as part of a selection filter of a frequency variable band-pass filter  6 . When the frequency variable band-pass filter  6  is composed of a SCF, the polyphase filter  5  serves as an anti-aliasing filter.  
         [0069]     Other operations are the same as those of the circuit in  FIG. 3 .  FIG. 5  shows an example of a circuit shared between an active polyphase BPF and an active biquadratic LPF. The shared circuit is formed by adding switches SW 1 , SW 2 , . . . and SWn to the active polyphase filter in  FIG. 12 . When the switches SW 1 , SW 2 , . . . and SWn are on, the circuit functions as the polyphase BPF. When the switches SW 1 , SW 2 , . . . and SWn are off, the circuit functions as the general biquadratic (bi-quad) LPF. Like the polyphase BPFs, the bi-quad LPFs can be connected in multiple stages to obtain the attenuation required for preventing aliasing.  
       Embodiment 4  
       [0070]      FIG. 6  shows a receiver IF circuit of Embodiment 4 of the present invention. Like the circuit in  FIG. 3 , two RF input signals, i.e., RF1 and RF2 in different frequency bands are input to this circuit. The RF1 signal enters an RF1 variable gain amplifier  2  through an RF1 filter  1   a . The RF2 signal enters an RF2 variable gain amplifier  33  through an RF2 filter  1   b . After amplification of the signals, a first orthogonal mixer including mixers  3   a ,  3   b  and a second orthogonal mixer including mixers  3   c ,  3   d  generate quadrature phase signals having two different frequencies IF 1  and IF 2 , respectively.  
         [0071]     Local signals are supplied from a frequency divider  14   a  to the mixers  3   a  and  3   b . Similarly, local signals are supplied from a frequency divider  14   b  to the mixers  3   c  and  3   d . The output signal of a phase-locked loop using the output signal of a quartz oscillator  37  as a reference signal is supplied to the frequency dividers  14   a ,  14   b  and converted into necessary local frequencies. The phase-locked loop includes a voltage-controlled oscillator  34 , a phase comparator  35 , a LPF  36 , and the quartz oscillator  37 .  
         [0072]     The quadrature phase signals having frequencies IF 1 , IF 2  are switched by switches  38  to  41  and supplied to a polyphase filter  5 . The output of the polyphase filter  5  is supplied to a SCF channel filter  6   a  composed of a SCF. A control signal having a predetermined frequency is obtained by dividing the reference signal of the quartz oscillator  37  with a divider  11 , and then is supplied to the SCF channel filter  6   a . The divider  11  can switch the frequency of the control signal in accordance with the frequency to be selected by switching of the switches  38  to  41 .  
         [0073]     In this circuit, when the polyphase filter  5  has the configuration as shown in  FIG. 10  or  12 , it also can be used as an anti-aliasing filter for the SCF channel filter  6   a . That is, the polyphase filter  5  sufficiently attenuates the input signal of the SCF channel filter  6   a  to half the frequency of a clock of the SCF channel filter  6   a . Consequently, aliasing can be prevented, and the signals that have passed through the smoothing filters  42   a  and  42   b  become selected IF signals without any distortion.  
         [0074]     The signal through the smoothing filter  42   a  is processed by an IF 1  amplifier  7   a  and an IF 1  demodulator  8   a , so that a baseband signal  1  is output. Similarly, the signal through the smoothing filter  42   b  is processed by an IF 2  amplifier  7   b  and an IF 2  demodulator  8   b , so that a baseband signal  2  is output. The outputs of the IF 1  demodulator  8   a  and the IF 2  demodulator  8   b  are supplied to AGCs  9   a  and  9   b , respectively. Thus, the gains of the variable gain amplifiers  2  and  33 , the IF 1  amplifier  7   a , and the IF 2  amplifier  7   b  are controlled.  
         [0075]     In this embodiment, the image rejection function required for two different RF signals can be achieved by a common image rejection mixer (i.e., a block  12  enclosed with a broken line). Moreover, the selection filter of two different frequencies can be integrated as a common SCF channel filter  6   a  in the same chip. Thus, the number of external filters can be decreased to reduce cost and electric power.  
         [0076]     The switched capacitor filter is a discrete-time system and includes many harmonic contents of the clock frequency. Therefore, when the switched capacitor filter is integrated with a small RF circuit in the same chip, the harmonic contents may affect the RF circuit as noise, and also may be undesired components for the mixers. If the clock frequency is higher than the frequency of the RF input signal, the harmonic contents are attenuated during transmission over the circuit. At the same time, the effect of the harmonic contents as noise on the input signal band can be reduced. Accordingly, the clock frequency of the SCF is made higher than the frequency of the RF input signal to prevent the components of the signal processed by the SCF from being a disturbing wave of the RF circuit.  
         [0077]     The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.