Abstract:
A reception IC comprising a received-signal-selecting circuit for selecting one of a plurality of received signals with reception bands different from each other, a frequency dividing circuit for dividing the frequency of an oscillation signal supplied thereto and outputting a plurality of divided-frequency signals having frequencies different from each other, a divided-frequency-signal-selecting circuit for selecting one of the divided-frequency signals output by the frequency dividing circuit as a local-oscillation signal, and a mixer circuit for being inputted the local-oscillation signal output by the divided-frequency-signal-selecting circuit and the received signal selected by the received-signal-selecting circuit, converting the frequency of the received signal and outputting the received signal with the converted frequency as an intermediate-frequency signal, wherein the received-signal-selecting circuit selects one of the received signals for a selected reception band, the divided-frequency-signal-selecting circuit selects one of the divided-frequency signals for the selected reception band, and by changing the frequency of the local oscillation signal for each selected reception band, the frequency of the received signal is changed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a reception IC and a receiving apparatus employing the reception IC.  
           [0002]    As a digital audio broadcasting standard, DAB specifications are adopted in Europe whereas ISDB-T specifications are embraced in Japan. The DAB specifications are a digital audio broadcasting standard conforming to Eureka 147 specifications.  
           [0003]    The ISDB-T specifications are listed as follows:  
           [0004]    Transmission band width: 432 kHz (for a narrowband ISDB-T standard)  
           [0005]    Modulation method: OFDM  
           [0006]    Multiplexing method: MPEG2  
           [0007]    In accordance with the above specifications, digital data including audio digital data of a plurality of channels is broadcasted at the same time. In the case of the narrow-band ISDB-T standard, the use of the contemporary VHF television broadcasting band in the digital broadcasting is planned.  
           [0008]    On the other hand, an analog receiver capable of receiving an audio signal of the VHF television broadcasting band in addition to signals of the AM and FM broadcasting is also put in the market. Moreover, the reception circuit of the analog receiver is implemented by an IC. Thus, for the ISDB-T standard, it is possible to realize an IC receiver.  
           [0009]    However, the bands of the VHF television broadcasting are given as follows:  
           [0010]    Low band (1 to 3 channels): 90 MHz to 108 MHz  
           [0011]    High band (4 to 12 channels): 170 MHz to 222 MHz  
           [0012]    Thus, for an intermediate frequency of 10.7 MHz, the ranges of the local oscillation frequency are given as follows:  
           [0013]    Low band: 100.7 MHz to 118.7 MHz  
           [0014]    High band: 180.7 MHz to 232.7 MHz  
           [0015]    Therefore, a local oscillation circuit employed in a receiver for receiving an audio signal of the VHF television broadcasting must be capable of varying the local oscillation frequency over a considerably wide range.  
           [0016]    In order to satisfy the above requirement, the local oscillation frequency is switched from a value for reception of a low-band broadcast to a value for reception of a high-band broadcast and vice versa.  
           [0017]    The following techniques are conceivable as a method for switching the local oscillation frequency:  
           [0018]    1: A technique to switch a coil and a variable-capacitance diode, which are used for oscillation in a local oscillation circuit, from components for reception of a low-band broadcast to those for reception of a high-band broadcast and vice versa.  
           [0019]    2: A technique to change a local oscillation circuit for reception of a low-band broadcast to a local oscillation circuit for reception of a high-band broadcast and vice versa.  
           [0020]    In the case of either technique, nevertheless, the variable-capacitance diodes, the coils and a switch device are required, inevitably increasing the number of external components attached to the IC. A large number of such external components entails an increase in receiver cost and serves as a disturbance to reduction of the size of the receiver.  
           [0021]    In addition, in the case of the first technique described above, the switch device is connected to the oscillation circuit, unavoidably lowering the Q value of the oscillation circuit. As a result, the oscillation circuit exhibits poor characteristics as evidenced by, among others, the fact that it becomes difficult to make the oscillation circuit to oscillate, the number of phase noises increases, the number of pulling phenomena increases and the magnitude of a consumed current rises.  
           [0022]    The same problem is also inevitably raised in a reception IC for digital audio broadcasting that makes use of a VHF television broadcasting band.  
         SUMMARY OF THE INVENTION  
         [0023]    It is thus an object of the present invention to solve the problems described above.  
           [0024]    To achieve the above object, according to the present invention, there is provided a reception IC including a received-signal-selecting circuit for selecting one of a plurality of received signals with reception bands different from each other, a frequency dividing circuit for dividing the frequency of an oscillation signal supplied thereto and outputting a plurality of divided-frequency signals having frequencies different from each other, a divided-frequency-signal-selecting circuit for selecting one of the divided-frequency signals output by the frequency dividing circuit as a local-oscillation signal, and a mixer circuit for being inputted the local-oscillation signal output by the divided-frequency-signal-selecting circuit and the received signal selected by the received-signal-selecting circuit, converting the frequency of the received signal and outputting the received signal with the converted frequency as an intermediate-frequency signal, wherein the received-signal-selecting circuit selects one of the received signals for a selected reception band, the divided-frequency-signal-selecting circuit selects one of the divided-frequency signals for the selected reception band, and by changing the frequency of the local oscillation signal for each selected reception band, the frequency of the received signal is changed.  
           [0025]    Thus, the reception frequency of a received signal can be changed without switching an oscillation circuit from a variable-capacity diode and a coil to others to change the oscillation frequency for the reception band.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 is a system diagram showing an embodiment of the present invention;  
         [0027]    [0027]FIG. 2 is a connection diagram showing a portion of the embodiment of the present invention;  
         [0028]    [0028]FIG. 3 is a connection diagram showing the continuation of FIG. 2;  
         [0029]    [0029]FIG. 4 is a system diagram showing another embodiment of the present invention;  
         [0030]    [0030]FIG. 5 is a connection diagram showing a portion of the other embodiment of the present invention;  
         [0031]    [0031]FIG. 6 is a connection diagram showing the continuation of FIG. 5;  
         [0032]    [0032]FIG. 7 is a system diagram showing a further embodiment of the present invention; and  
         [0033]    [0033]FIG. 8 is a system diagram showing a still further embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    Receiver . . . Part I  
         [0035]    [0035]FIG. 1 is a diagram showing a typical receiver adopting the super-heterodyne system to conform to the narrow-band ISDB-T standard as provided by the present invention. This receiver includes switch circuits  15 ,  44 A and  44 B for switching the reception band. In accordance with control executed by a system controller, which is not shown in the figure, the switch circuits  15  to  44 B are connected in a state of connection for receiving a low-band broadcast as shown in the figure. When receiving a high-band broadcast, however, the switch circuits  15  to  44 B are connected in a state of connection opposite to the state shown in the figure. Circuits enclosed by a dashed line are implemented as a single-chip IC.  
         [0036]    Next, configurations and operations for receiving low-band and high-band broadcasts are explained.  
         [0037]    Reception of low-band broadcasts  
         [0038]    An antenna  11  receives an ISDB-T broadcasted wave and the received signal is supplied to antenna tuning circuits  12 L and  12 H, which each adopt an electronic tuning technique. The antenna tuning circuit  12 L fetches a received signal SRX having a desired frequency in the low band. The received signal SRX is then supplied to mixer circuits  31 A and  31 B through signal lines by way of an AGC variable-gain amplifier  13 L, an inter-stage tuning circuit  14 L adopting the electronic tuning technique and the switch circuit  15 .  
         [0039]    A PLL circuit  41  generates an oscillation signal S 41  with a frequency varied in the range of 362 MHz to 434 MHz according to the reception frequency. The oscillation signal S 41  is supplied to a frequency dividing circuit  42  for multiplying the frequency of the oscillation signal S 41  by ½. To be more specific, the frequency dividing circuit  42  outputs a signal with a divided frequency varied in the range of 181 MHz to 217 MHz according to the reception frequency. The divided-frequency signal is further supplied to a frequency dividing circuit  43  at the following stage for similarly multiplying the divided frequency of the divided-frequency signal by ½. To be more specific, the frequency dividing circuit  43  outputs divided-frequency signals SLA and SLB, which each have a divided frequency varied in the range of 90.5 MHz to 108.5 MHz according to the reception frequency and have phases shifted from each other by 90 degrees. The divided-frequency signals SLA and SLB are then supplied to the mixer circuits  31 A and  31 B by way of the switch circuits  44 A and  44 B respectively as local-oscillation signals.  
         [0040]    The mixer circuits  31 A and  31 B mix the received signal SRX with the local-oscillation signals SLA and SLB respectively, converting the received signal SRX into  2  intermediate-frequency signals SIFA and SIFB, which have phases shifted from each other by 90 degrees. That is to say, the mixer circuits  31 A and  31 B convert the frequency of the received signal SRX to produce the in-phase and quadrature intermediate-frequency signals SIFA and SIFB, which are orthogonal to each other. It should be noted that the received signal SRX has a frequency in the range of 90 MHz to 108 MHz. At that time, the local-oscillation signals SLA and SLB are each generated with a divided frequency in the range of 90.5 MHz to 108.5 MHz for the frequency of the received signal SRX. As a result, the intermediate-frequency signals SIFA and SIFB each have a frequency of 500 kHz, which is equal to the difference between the frequency of the received signal SRX and the frequency of the local-oscillation signals SLA and SLB respectively.  
         [0041]    It should be noted that the PLL circuit  41  supplies a part of a control voltage applied to a variable-capacitance diode employed in a resonance circuit  41 C of a VCO not shown in the figure to the tuning circuits  12 L and  14 L as a tuning voltage for implementing tuning for the received signal SRX.  
         [0042]    The intermediate-frequency signals SIFA and SIFB generated by the mixer circuits  31 A and  31 B are supplied to phase shift circuits  33 A and  33 B by way of low-pass filters  32 A and  32 B respectively. The phase shift circuits  33 A and  33 B carry out phase processing on the intermediate-frequency signals SIFA and SIFB so that an original signal component of the intermediate-frequency signal SIFA has the same phase as an original signal component of the intermediate-frequency signal SIFB while an image component of the intermediate-frequency signal SIFA has a phase opposite to an image component of the intermediate-frequency signal SIF. Signals completed the phase processing at the phase shift circuits  33 A and  33 B are supplied to an adder  34  in which the image component of the intermediate-frequency signal SIFA and the image component of the intermediate-frequency signal SIFB cancel out each other. Thus, the adder  34  generates an intermediate-frequency signal SIF having the original signal component but with the image component canceled out.  
         [0043]    Then, the intermediate-frequency signal SIF is supplied to an output terminal  38  through a signal line passing through a band-pass filter  35  for intermediate-frequency filtering, an AGC variable-gain amplifier  36  and a low-pass filter  37 .  
         [0044]    The intermediate-frequency signal SIF supplied to the output terminal  38  is subjected to various kinds of demodulation processing corresponding to ISDB-T modulation processing carried out to transmit the ISDB-T broadcasted signal. Not shown in the figure, pieces of demodulation processing include complex Fourier transformation, frequency de-interleave, time de-interleave, selection of digital audio data of a desired channel among a plurality of channels, error correction, data decompression and D/A conversion. As a result of the demodulation processing, an audio signal of a desired program selected among a plurality of programs or a plurality of channels is generated.  
         [0045]    At that time, the intermediate-frequency signal SIF output by the low-pass filter  37  is supplied to an AGC detection circuit  45  for generating an AGC voltage V 45 . The AGC voltage V 45  is applied to the variable-gain amplifier  36  as a signal for controlling the gain of the amplifier  36 . The AGC voltage V 45  is also supplied to the variable-gain amplifier  13 L by way of an adder  47  as a signal for controlling the gain of the amplifier  13 L. The AGC voltage V 45  is used for performing AGC on the variable-gain amplifier  36  and the variable-gain amplifier  13 L.  
         [0046]    In addition, the intermediate-frequency signals SIFA and SIFB output by the low-pass filters  32 A and  32 B respectively are supplied to an AGC detection circuit  46  for generating an AGC voltage V 46 . The AGC voltage V 46  is also supplied to the variable-gain amplifier  13 L by way of the adder  47  as a signal for controlling the gain of the amplifier  13 L. The AGC voltage V 46  is used for reducing the gain of the variable-gain amplifier  13 L in case the levels of noise components included in the intermediate-frequency signals SIFA and SIFB exceed a prescribed value.  
         [0047]    The configuration and the operations for receiving low-band broadcasts have been explained so far. By having the system controller change a frequency division rate of a variable-rate frequency dividing circuit employed in the PLL circuit  41 , the frequency of the oscillation signal S 41  can be varied accordingly. It should be noted that the variable-rate frequency dividing circuit is not shown in the figure.  
         [0048]    Thus, by changing the oscillation frequency in the range of 362 MHz to 434 MHz according to a reception frequency as described above, the local-oscillation signals SLA and SLB will each have a frequency varied in the range of 90.5 MHz to 108.5 MHz according to the reception frequency. Accordingly, it is possible to obtain an intermediate-frequency signal SIF with an intermediate frequency of 500 kHz, that is, to receive a low-band broadcast.  
         [0049]    Reception of high-band broadcasts The antenna  11  receives an ISDB-T broadcasted wave. The antenna tuning circuit  12 H fetches a received signal SRX having a desired frequency in the high band. The received signal SRX is then supplied to the mixer circuits  31 A and  31 B through signal lines by way of an AGC variable-gain amplifier  13 H, an inter-stage tuning circuit  14 H adopting the electronic tuning technique and the switch circuit  15 .  
         [0050]    The PLL circuit  41  generates an oscillation signal S 41  with a frequency varied in the range of 341 MHz to 445 MHz according to the reception frequency. The oscillation signal S 41  is supplied to the frequency dividing circuit  42  for multiplying the frequency of the oscillation signal S 41  by ½. To be more specific, the frequency dividing circuit  42  outputs divided-frequency signals SHA and SHB, which each have a divided frequency varied in the range of 170.5 MHz to 222.5 MHz according to the reception frequency, and have phases shifted from each other by 90 degrees. The divided-frequency signals SHA and SHB are then supplied to the mixer circuits  31 A and  31 B by way of the switch circuits  44 A and  44 B respectively as local-oscillation signals.  
         [0051]    The mixer circuits  31 A and  31 B mix the received signal SRX with the local-oscillation signals SHA and SHB respectively, converting the received signal SRX into 2 intermediate-frequency signals SIFA and SIFB, which have phases shifted from each other by 90 degrees. That is to say, the mixer circuits  31 A and  31 B convert the frequency of the received signal SRX to produce the in-phase and quadrature intermediate-frequency signals SIFA and SIFB, which are orthogonal to each other. It should be noted that the received signal SRX has a frequency in the range of 170 MHz to 222 MHz. At that time, the local-oscillation signals SHA and SHB are each generated with a divided frequency in the range of 170.5 MHz to 222.5 MHz for the frequency of the received signal SRX. As a result, the intermediate-frequency signals SIFA and SIFB each have a frequency of 500 kHz, which is equal to the difference between the frequency of the received signal SRX and the frequency of the local-oscillation signals SHA and SHB respectively.  
         [0052]    It should be noted that the PLL circuit  41  supplies a part of a control voltage applied to the variable-capacitance diode employed in the VCO to the tuning circuits  12 H and  14 H as a tuning voltage for implementing tuning for the received signal SRX.  
         [0053]    The intermediate-frequency signals SIFA and SIFB generated by the mixer circuits  31 A and  31 B are processed by the circuits  32 A to  37  in the same way as the reception of a low-band broadcast. An intermediate-frequency signal SIF resulting from the processing is supplied to the output terminal  38 . The intermediate-frequency signal SIF supplied to the output terminal  38  is then subjected to demodulation to produce an audio signal of a desired program.  
         [0054]    At that time, AGC is carried out on the circuits  45  to  47  in the same way as the reception of a low-band broadcast.  
         [0055]    The configuration and the operations for receiving high-band broadcasts have been explained so far. By having the system controller change a frequency division rate of a variable-rate frequency dividing circuit employed in the PLL circuit  41 , the oscillation frequency of the oscillation signal S 41  can be varied accordingly. Thus, by changing the oscillation frequency in the range of 341 MHz to 445 MHz according to a reception frequency as described above, the local-oscillation signals SHA and SHB will each have a frequency varied in the range of 170.5 MHz to 222.5 MHz according to the reception frequency. Accordingly, it is possible to obtain an intermediate-frequency signal SIF with an intermediate frequency of 500 kHz, that is, to receive a high-band broadcast.  
         [0056]    Summary  
         [0057]    The oscillation frequency of the oscillation signal S 41  generated by the PLL circuit  41  employed in the receiver shown in FIG. 1 has the following values:  
         [0058]    In reception of a low-band broadcast: In the range of 362 MHz to 434 MHz  
         [0059]    In reception of a high-band broadcast: In the range of 341 MHz to 445 MHz  
         [0060]    It is obvious that the frequency variation range for reception of a low-band broadcast is included in the frequency variation range for reception of a high-band broadcast. With those variation ranges of the oscillation frequency, it is not necessary to switch the frequency band from one to another when the PLL circuit  41  generates the oscillation signal S 41 .  
         [0061]    It is thus unnecessary to switch the resonance circuit  41 C employed in the PLL circuit  41  from a coil and a variable-capacitance diode to others. As a result, the number of external components attached to the IC can be reduced and, in addition, the decreased number of external components contributes to reduction of the cost and the size of the receiver.  
         [0062]    In addition, since it is not necessary to connect a switch device for switching the frequency band of the oscillation frequency from one to another to the oscillation circuit  41 C, the Q value of the oscillation circuit  41 C is not lowered. As a result, the oscillation circuit  41 C exhibits improved characteristics such as stable oscillation, fewer phase noises, fewer pulling phenomena and a lower current consumption.  
         [0063]    In addition, since the receiver needs only one oscillation circuit, which is prone to effects of surroundings, that is, the receiver needs only one VCO of the PLL circuit  41 , a layout is easy to make in the implementation of the IC, and it is hence possible to design an oscillation circuit that is proof against external disturbances. Moreover, since the tracking error characteristic is uniform independently of the reception band, the number of tracking errors can be reduced with ease.  
         [0064]    Furthermore, since the reception band can be switched from one to another with the VCO of the PLL circuit  41  kept in a state of oscillation as it is, a broadcast can be received fast even after band switching. On the top of that, it is possible to implement most of the circuits such as those enclosed by a dashed line in FIG. 1 as a single-chip IC.  
         [0065]    Switch circuit  15  and its peripheral circuits  
         [0066]    [0066]FIGS. 2 and 3 are diagrams showing a typical switch circuit  15  and typical mixer circuits  31 A and  31 B provided at a stage following the switch circuit  15 . It should be noted that portions denoted by reference numerals * 1  to * 5  shown in FIG. 2 are continued to portions denoted by reference numerals * 1  to * 5  shown in FIG. 3 respectively.  
         [0067]    The tuning circuit  14 L outputs balanced received signals ±SRX, which are supplied to the bases of transistors Q 151 L and Q 152 L respectively. The emitters of the transistors Q 151 L and Q 152 L are respectively connected to transistors Q 153  and Q 154  each serving as a constant-current source to form emitter followers.  
         [0068]    In the same way, the tuning circuit  14 H outputs balanced received signals ±SRX which are supplied to the bases of transistors Q 151 H and Q 152 H respectively. The emitters of the transistors Q 151 H and Q 152 H are respectively connected to the transistors Q 153  and Q 154  each serving as a constant-current source to form the emitter followers.  
         [0069]    In addition, control voltages V 15 L and V 15 H are generated. The control voltage V 15 L is supplied to the transistors Q 151 L and Q 152 L as a base bias voltage. On the other hand, the control voltage V 15 H is supplied to the transistors Q 151 H and Q 152 H as a base bias voltage. The system controller controls the control voltages V 15 L and V 15 H in accordance with the reception band. To put it in detail, the control voltage V 15 L is set at “H” while the control voltage V 15 H is reset to “L” for reception of a low-band broadcast. On the other hand, the control voltage V 15 L is reset at “L” while the control voltage V 15 H is set at “H” for reception of a high-band broadcast.  
         [0070]    The mixer circuit  31 A comprises a double-balanced multiplier  311  and current mirror circuits  312  to  314  for combining outputs of the multiplier  311 . The multiplier  311  inputs the received signals ±SRX generated by the switch circuit  15  and the local-oscillation signal SLA or SHA output by the switch circuit  44 A. The mixer circuit  31 B has the same configuration as the mixer circuit  31 A. It should be noted that the frequency dividing circuits  42  and thus the switch circuits  44 A and  44 B output balanced divided-frequency signals ±SHA and ±SHB respectively. By the same token, the frequency dividing circuits  43  and thus the switch circuits  44 A and  44 B output balanced divided-frequency signals ±SLA and ±SLB respectively.  
         [0071]    Thus, in reception of a low-band broadcast, the control voltage V 15 L turns on the transistors Q 151 L and Q 152 L while the control voltage V 15 H turns off the transistors Q 151 H and Q 152 H. Therefore, the reception signals ±SRX output by the tuning circuit  14 L are supplied to the mixer circuits  31 A and  31 B by way of the transistors Q 151 L and Q 152 L respectively.  
         [0072]    At that time, the divided-frequency signals ±SLA and ±SLB are supplied to the mixer circuits  31 A and  31 B by way of the switch circuits  44 A and  44 B respectively as local-oscillation signals. As a result, the mixer circuits  31 A and  31 B generate intermediate-frequency signals SIFA and SIFB respectively for the received low-band broadcast.  
         [0073]    In reception of a high-band broadcast, on the other hand, the control voltage V 15 L turns off the transistors Q 151 L and Q 152 L while the control voltage V 15 H turns on the transistors Q 151 H and Q 152 H. Thus, the reception signals ±SRX output by the tuning circuit  14 H are supplied to the mixer circuits  31 A and  31 B by way of the transistors Q 151 H and Q 152 H respectively.  
         [0074]    At that time, the divided-frequency signals ±SHA and ±SHB are supplied to the mixer circuits  31 A and  31 B by way of the switch circuits  44 A and  44 B respectively as local-oscillation signals. As a result, the mixer circuits  31 A and  31 B generate intermediate-frequency signals SIFA and SIFB respectively for the received high-band broadcast.  
         [0075]    Receiver . . . Part II  
         [0076]    [0076]FIG. 4 is a diagram showing another typical receiver adopting the super-heterodyne system as provided by the present invention. In this receiver, the functions of the switch circuits  15 ,  44 A and  44 B employed in the receiver shown in FIG. 1 are included in mixer circuits.  
         [0077]    For the above reason, there are provided 2 pairs of mixer circuits. One of the pair consists of mixer circuits  31 LA and  31 LB while the other pair consists of mixer circuits  31 HA and  31 HB. The system controller controls direct-current bias voltages for the mixer circuits  31 LA,  31 LB,  31 HA and  31 HB in accordance with the reception band. To put it in detail, bias voltages for transistors composing the mixer circuits  31 LA,  31 LB,  31 HA and  31 HB are controlled so that, when a low-band broadcast is received, the mixer circuits  31 LA and  31 LB are operating effectively and, when a high-band broadcast is received, on the other hand, the mixer circuits  31 HA and  31 HB are operating effectively.  
         [0078]    Next, configurations and operations for receiving low-band and high-band broadcasts are explained.  
         [0079]    Reception of low-band broadcasts  
         [0080]    An antenna  11  receives an ISDB-T broadcasted wave. An antenna tuning circuit  12 L fetches a received signal SRX having a desired frequency in the low band. The received signal SRX is then supplied to the mixer circuits  31 LA and  31 LB through signal lines by way of an AGC variable-gain amplifier  13 L and an inter-stage tuning circuit  14 L adopting the electronic tuning technique.  
         [0081]    A PLL circuit  41  generates an oscillation signal S 41  with a frequency varied in the range of 362 MHz to 434 MHz according to the reception frequency. The oscillation signal S 41  is supplied to a frequency dividing circuit  42 , being converted into a signal with a divided frequency varied in the range of 181 MHz to 217 MHz according to the reception frequency. The divided-frequency signal is further supplied to a frequency dividing circuit  43  at the following stage, being converted into 2 divided-frequency signals SLA and SLB, which each have a divided frequency varied in the range of 90.5 MHz to 108.5 MHz according to the reception frequency and have phases shifted from each other by 90 degrees. The divided-frequency signals SLA and SLB are then supplied to the mixer circuits  31 LA and  31 LB respectively as local-oscillation signals.  
         [0082]    The mixer circuits  31 LA and  31 LB mix the received signal SRX with the local-oscillation signals SLA and SLB respectively, converting the received signal SRX into 2 intermediate-frequency signals SIFA and SIFB, which have phases shifted from each other by 90 degrees. That is to say, the mixer circuits  31 A and  31 B convert the frequency of the received signal SRX to produce the in-phase and quadrature intermediate-frequency signals SIFA and SIFB, which are orthogonal to each other. It should be noted that the intermediate-frequency signals SIFA and SIFB each have an intermediate frequency of 500 kHz.  
         [0083]    The intermediate-frequency signals SIFA and SIFB are supplied to an adder  34  by way of low-pass filters  32 A and  32 B and phase shift circuits  33 A and  33 B respectively. The adder  34  generates an intermediate-frequency signal SIF having the original signal component but with the image component canceled out.  
         [0084]    Then, the intermediate-frequency signal SIF is supplied to an output terminal  38  through signal lines passing through a band-pass filter  35  for intermediate-frequency filtering, an AGC variable-gain amplifier  36  and a low-pass filter  37 . The intermediate-frequency signal SIF supplied to the output terminal  38  is then supplied to a demodulation circuit not shown in the figure. As a result of demodulation, an audio signal of a desired program selected among a plurality of programs is generated.  
         [0085]    At that time, AGC voltages V 45  and V 46  are generated from the intermediate-frequency signals SIF, SIFA and SIFB to be used in AGC in the same way as the receiver shown in FIG. 1.  
         [0086]    The configuration and the operations for receiving low-band broadcasts have been explained so far. By having the system controller change a frequency division rate of a variable-rate frequency dividing circuit employed in the PLL circuit  41 , the frequency of the oscillation signal S 41  can be varied accordingly. Thus, by changing the oscillation frequency in the range of 362 MHz to 434 MHz according to a reception frequency as described above, the local-oscillation signals SLA and SLB will each have a frequency varied in the range of 90.5 MHz to 108.5 MHz according to the reception frequency. Accordingly, it is possible to obtain an intermediate-frequency signal SIF with an intermediate frequency of 500 kHz, that is, to receive a low-band broadcast.  
         [0087]    Reception of high-band broadcasts  
         [0088]    The antenna  11  receives an ISDB-T broadcasted wave. An antenna tuning circuit  12 H fetches a received signal SRX having a desired frequency in the high band. The received signal SRX is then supplied to the mixer circuits  31 HA and  31 HB through signal lines by way of an AGC variable-gain amplifier  13 H and an inter-stage tuning circuit  14 H adopting the electronic tuning technique.  
         [0089]    The PLL circuit  41  generates an oscillation signal S 41  with a frequency varied in the range of 341 MHz to 445 MHz according to the reception frequency. The oscillation signal S 41  is supplied to the frequency dividing circuit  42 , being converted into 2 divided-frequency signals SHA and SHB, which each have a divided frequency varied in the range of 170.5 MHz to 222.5 MHz according to the reception frequency and have phases shifted from each other by 90 degrees. The divided-frequency signals SHA and SHB are then supplied to the mixer circuits  31 HA and  31 HB respectively as local-oscillation signals.  
         [0090]    The mixer circuits  31 HA and  31 HB mix the received signal SRX with the local-oscillation signals SHA and SHB respectively, converting the received signal SRX into 2 intermediate-frequency signals SIFA and SIFB, which have phases shifted from each other by 90 degrees. That is to say, the mixer circuits  31 A and  31 B convert the frequency of the received signal SRX to produce the in-phase and quadrature intermediate-frequency signals SIFA and SIFB, which are orthogonal to each other. It should be noted that the intermediate-frequency signals SIFA and SIFB each have an intermediate frequency of 500 kHz.  
         [0091]    The intermediate-frequency signals SIFA and SIFB are processed in the circuits  32 A to  37  in the same way as the reception of a low-band broadcast to produce an intermediate-frequency signal SIF supplied to the output terminal  38 . The intermediate-frequency signal SIF supplied to the output terminal  38  is then supplied to a demodulation circuit provided at the next stage. As a result of demodulation, an audio signal of a desired program selected among a plurality of programs is generated.  
         [0092]    At that time, the circuits  45  to  47  carry out AGC in the same way as the reception of a low-band broadcast.  
         [0093]    The configuration and the operations for receiving high-band broadcasts have been explained so far. By having the system controller change a frequency division rate of a variable-rate frequency dividing circuit employed in the PLL circuit  41 , the frequency of the oscillation signal S 41  can be varied accordingly. Thus, by changing the oscillation frequency of the oscillation signal S 41  in the range of 341 MHz to 445 MHz according to a reception frequency as described above, the local-oscillation signals SHA and SHB will each have a frequency varied in the range of 170.5 MHz to 222.5 MHz according to the reception frequency. Accordingly, it is possible to obtain an intermediate-frequency signal SIF with an intermediate frequency of 500 kHz, that is, to receive a high-band broadcast.  
         [0094]    Summary  
         [0095]    Much like the receiver shown in FIG. 1, the oscillation frequency of the oscillation signal S 41  generated by the PLL circuit  41  employed in the receiver shown in FIG. 4 has the following values:  
         [0096]    In reception of a low-band broadcast: In the range of 362 MHz to 434 MHz  
         [0097]    In reception of a high-band broadcast: In the range of 341 MHz to 445 MHz  
         [0098]    It is thus unnecessary to switch the resonance circuit  41 C employed in the PLL circuit  41  from a coil and a variable-capacitance diode to others. As a result, the number of external components attached to the IC can be reduced and, in addition, the decreased number of external components contributes to reduction of the cost and the size of the receiver.  
         [0099]    In addition, since it is not necessary to connect a switch device for switching the frequency band of the oscillation frequency from one to another to the oscillation circuit  41 C, the Q value of the oscillation circuit  41 C is not lowered. As a result, the oscillation circuit  41 C exhibits improved characteristics.  
         [0100]    In addition, since the reception band is switched from one to another by controlling direct-current bias voltages of the mixer circuits  31 LA,  31 LB,  31 HA and  31 HB, the current consumption can be reduced. To put it in detail, in the case of the receiver shown in FIG. 1, the switch circuits  15 ,  44 A and  44 B directly switch the high-frequency signals, namely, the received signal SRX and the local-oscillation signals SLA to SHB from ones to others. Thus, the switch circuits  15 ,  44 A and  44 B consume large currents. In the case of the receiver shown in FIG. 4, on the other hand, such high-frequency signals are not subjected to switching so that the current consumption can be reduced.  
         [0101]    Mixer circuits  31 LA to  31 HB  
         [0102]    [0102]FIGS. 5 and 6 are diagrams showing typical mixer circuits  31 LA to  31 HB. It should be noted that portions denoted by reference numerals * 1  to * 7  shown in FIG. 5 are continued to portions denoted by reference numerals * 1  to * 7  shown in FIG. 6 respectively. The mixer circuit  31 LA comprises a double-balanced multiplier  311 L and current mirror circuits  312  to  314  for combining outputs of the multiplier  311 L.  
         [0103]    The tuning circuit  14 L outputs balanced received signals ±SRX of a low-band broadcast, which are supplied to the multiplier  311 L. At that time, a control voltage V 31 L is used for controlling the effect of the received signals ±SRX by being supplied to transistors employed in the multiplier  311 L as a base bias voltage. The frequency dividing circuit  43  outputs balanced divided-frequency signals ±SLA, which are each supplied to the mixer circuit  31 LA as a local-oscillation signal.  
         [0104]    On the other hand, a double-balanced multiplier  311 H is connected to the current mirror circuits  312  and  313 . The double-balanced multiplier  311 H and the current mirror circuits  312  to  314  compose the mixer circuit  31 HA.  
         [0105]    The tuning circuit  14 H outputs balanced received signals ±SRX of a high-band broadcast. The received signals ±SRX are supplied to the multiplier  311 H. At that time, a control voltage V 31 H is used for controlling the effect of the received signals ±SRX by being supplied to transistors employed in the multiplier  311 H as a base bias voltage. The frequency dividing circuit  42  outputs balanced divided-frequency signals ±SHA, which are each supplied to the mixer circuit  31 HA as a local-oscillation signal.  
         [0106]    The mixer circuits  31 LB and  31 HB have the same configurations as the mixer circuits  31 LA and  31 HA respectively. In the case of the mixer circuit  31 LB, the tuning circuit  14 L outputs balanced received signals ± SRX of a low-band broadcast. At that time, the control voltage V 31 L is used for controlling the effect of the received signals ±SRX by being supplied to the mixer circuit  31 LB along with the received signals ±SRX. The frequency dividing circuit  43  outputs balanced divided-frequency signals ±SLB, which are each supplied to the mixer circuit  31 LB as a local-oscillation signal.  
         [0107]    In the case of the mixer circuit  31 HB, on the other hand, the tuning circuit  14 H outputs balanced received signals ±SRX of a high-band broadcast. At that time, the control voltage V 31 H is used for controlling the effect of the received signals ±SRX by being supplied to the mixer circuit  31 HB along with the received signals ±SRX. The frequency dividing circuit  42  outputs balanced divided-frequency signals ±SHB, which are each supplied to the mixer circuit  31 HB as a local-oscillation signal.  
         [0108]    The system controller controls the control voltages V 31 L and V 31 H in accordance with the reception band. To put it in detail, the control voltage V 31 L is set at “H” while the control voltage V 31 H is reset to “L” for reception of a low-band broadcast. On the other hand, the control voltage V 31 L is reset to “L” while the control voltage V 31 H is set at “H” for reception of a high-band broadcast.  
         [0109]    Thus, when a low-band broadcast is being received, the control voltage V 31 L puts the multipliers  311 L employed in the mixer circuits  31 LA and  31 LB in an operating state while the control voltage V 31 H puts the multipliers  311 H employed in the mixer circuits  31 HA and  31 HB in a non-operating state. As a result, the multipliers  311 L convert the received signals ±SRX of the low-band broadcast into intermediate-frequency signals ±SIFA and ±SIFB by using the local-oscillation signals ±SLA and ±SLB respectively. The current mirror circuits  312  to  314  convert the intermediate-frequency signals ±SIFA and ±SIFB into intermediate-frequency signals SIFA and SIFB respectively, and output the intermediate-frequency signals SIFA and SIFB.  
         [0110]    In the same way, when a high-band broadcast is being received, the control voltage V 31 H puts the multipliers  311 H employed in the mixer circuits  31 HA and  31 HB in an operating state while the control voltage V 31 L puts the multipliers  311 H employed in the mixer circuits  31 LA and  31 LB in a non-operating state. As a result, the multipliers  311 H convert the received signals ±SRX of the high-band broadcast into intermediate-frequency signals ±SIFA and ±SIFB by using the local-oscillation signals ±SHA and ±SHB respectively. The current mirror circuits  312  to  314  convert the intermediate-frequency signals ±SIFA and ±SIFB into intermediate-frequency signals SIFA and SIFB respectively, and output the intermediate-frequency signals SIFA and SIFB.  
         [0111]    In this way, the circuits shown in FIGS. 5 and 6 convert the frequency of the received signal SRX of a low-band or high-band broadcast to produce the intermediate-frequency signals SIFA and SIFB respectively. In this case, it is not necessary to switch the received signal SRX as well as the local-oscillation signals from SLA to SHA and from SLB to SHB or vice versa in accordance with the reception band. As a result, it is possible to eliminate consumption of currents that would otherwise be required for the switching operations in the conventional receiver.  
         [0112]    Receiver . . . Part III  
         [0113]    [0113]FIG. 7 is a diagram showing a typical receiver adopting the direct-conversion system for the narrow-band ISDB-T standard as provided by the present invention. Much like the receiver shown in FIG. 1, this receiver includes switch circuits  15 ,  44 A and  44 B for switching the reception band from one to another. In accordance with control executed by a system controller, which is not shown in the figure, the switch circuits  15  to  44 B are connected in a state of connection for receiving a low-band broadcast as shown in the figure. When receiving a high-band broadcast, however, the switch circuits  15  to  44 B are connected in a state of connection opposite to the state shown in the figure. Circuits enclosed by a dashed line are implemented as a single-chip IC.  
         [0114]    Next, configurations and operations for receiving low-band and high-band broadcasts are explained.  
         [0115]    Reception of low-band broadcasts  
         [0116]    An antenna  11  receives an ISDB-T broadcasted wave. An antenna tuning circuit  12 L fetches a received signal SRX having a desired frequency in the low band. The received signal SRX is then supplied to mixer circuits  31 A and  31 B through signal lines by way of an AGC variable-gain amplifier  13 L, an inter-stage tuning circuit  14 L adopting the electronic tuning technique and the switch circuit  15 .  
         [0117]    A PLL circuit  41  generates an oscillation signal S 41  with a frequency varied in the range of 360 MHz to 432 MHz according to the reception frequency. The oscillation signal S 41  is supplied to a frequency dividing circuit  42  for multiplying the frequency of the oscillation signal S 41  by ½. To be more specific, the frequency dividing circuit  42  outputs a signal with a divided frequency varied in the range of 180 MHz to 216 MHz according to the reception frequency. The divided-frequency signal is further supplied to a frequency dividing circuit  43  at the following stage for similarly multiplying the divided frequency of the divided-frequency signal by ½. To be more specific, the frequency dividing circuit  43  outputs divided-frequency signals SLA and SLB, which each have a divided frequency varied in the range of 90 MHz to 108 MHz according to the reception frequency and have phases shifted from each other by 90 degrees. The divided-frequency signals SLA and SLB are then supplied to the mixer circuits  31 A and  31 B by way of the switch circuits  44 A and  44 B respectively as local-oscillation signals.  
         [0118]    The mixer circuits  31 A and  31 B mix the received signal SRX with the local-oscillation signals SLA and SLB respectively, converting the received signal SRX into 2 intermediate-frequency signals SIFA and SIFB, which have phases shifted from each other by 90 degrees. That is to say, the mixer circuits  31 A and  31 B convert the frequency of the received signal SRX to produce the in-phase and quadrature intermediate-frequency signals SIFA and SIFB, which are orthogonal to each other. It should be noted that the received signal SRX has a frequency in the range of 90 MHz to 108 MHz. At that time, the local-oscillation signals SLA and SLB are each generated with a divided frequency in the range of 90 MHz to 108 MHz for the frequency of the received signal SRX. As a result, the intermediate-frequency signals SIFA and SIFB each have a frequency of 0 since the reception frequency is equal to the local-oscillation frequency.  
         [0119]    It should be noted that the PLL circuit  41  supplies a part of a control voltage applied to a variable-capacitance diode employed in a resonance circuit  41 C of a VCO not shown in the figure to the tuning circuits  12 L and  14 L as a tuning voltage for implementing tuning for the received signal SRX.  
         [0120]    The intermediate-frequency signal SIFA generated by the mixer circuit  31 A is supplied to an output terminal  38 A through signal lines by way of a low-pass filter  32 A, an AGC variable-gain amplifier  36 A and a low-pass filter  37 A. In the same way, the intermediate-frequency signal SIFB generated by the mixer circuit  31 B is supplied to an output terminal  38 B through signal lines by way of a low-pass filter  32 B, an AGC variable-gain amplifier  36 B and a low-pass filter  37 B.  
         [0121]    The intermediate-frequency signals SIFA and SIFB supplied to the output terminals  38 A and  38 B respectively are each subjected to various kinds of demodulation processing corresponding to ISDB-T modulation processing carried out to transmit the ISDB-T broadcasted signal. Pieces of demodulation processing include complex Fourier transformation, frequency de-interleave, time de-interleave, selection of digital audio data of a desired channel among a plurality of channels, error correction, data decompression and D/A conversion. As a result of the demodulation processing, an audio signal of a desired program selected among a plurality of programs or a plurality of channels is generated. It should be noted that the pieces of demodulation processing are not shown in the figure.  
         [0122]    At that time, the intermediate-frequency signals SIFA and SIFB output by the low-pass filters  37 A and  37 B respectively are supplied to an AGC detection circuit  45  for generating an AGC voltage V 45 . On the other hand, the intermediate-frequency signals SIFA and SIFB output by the mixer circuits  31 A and  31 B respectively are supplied to an AGC detection circuit  46  for generating an AGC voltage V 46 . The AGC voltages V 45  and V 46  are used for carrying out AGC in the same way as the receiver shown in FIG. 1.  
         [0123]    The configuration and the operations for receiving low-band broadcasts have been explained so far. By having the system controller change a frequency division rate of a variable-rate frequency dividing circuit employed in the PLL circuit  41 , the oscillation frequency can be varied accordingly. Thus, by changing the oscillation frequency in the range of 360 MHz to 432 MHz according to a reception frequency as described above, the local-oscillation signals SLA and SLB will each have a frequency varied in the range of 90 MHz to 108 MHz according to the reception frequency. Accordingly, it is possible to obtain intermediate-frequency signals SIFA and SIFB each with an intermediate frequency of 0, that is, to receive a low-band broadcast.  
         [0124]    Reception of high-band broadcasts  
         [0125]    The antenna  11  receives an ISDB-T broadcasted wave. An antenna tuning circuit  12 H fetches a received signal SRX having a desired frequency in the high band. The received signal SRX is then supplied to the mixer circuits  31 A and  31 B through signal lines by way of an AGC variable-gain amplifier  13 H, an inter-stage tuning circuit  14 H adopting the electronic tuning technique and the switch circuit  15 .  
         [0126]    The PLL circuit  41  generates an oscillation signal S 41  with a frequency varied in the range of 340 MHz to 444 MHz according to the reception frequency. The oscillation signal S 41  is supplied to the frequency dividing circuit  42  for multiplying the frequency of the oscillation signal S 41  by ½. To be more specific, the frequency dividing circuit  42  outputs divided-frequency signals SHA and SHB, which each have a divided frequency varied in the range of 170 MHz to 222 MHz according to the reception frequency, and have phases shifted from each other by 90 degrees. The divided-frequency signals SHA and SHB are then supplied to the mixer circuits  31 A and  31 B by way of the switch circuits  44 A and  44 B respectively as local-oscillation signals.  
         [0127]    The mixer circuits  31 A and  31 B mix the received signal SRX with the local-oscillation signals SHA and SHB respectively, converting the received signal SRX into 2 intermediate-frequency signals SIFA and SIFB, which have phases shifted from each other by 90 degrees. That is to say, the mixer circuits  31 A and  31 B convert the frequency of the received signal SRX to produce the in-phase and quadrature intermediate-frequency signals SIFA and SIFB, which are orthogonal to each other. It should be noted that the received signal SRX has a frequency in the range of 170 MHz to 222 MHz. At that time, the local-oscillation signals SHA and SHB are each generated with a divided frequency in the range of 170 MHz to 222 MHz according to the frequency of the received signal SRX. As a result, the intermediate-frequency signals SIFA and SIFB each have a frequency of 0 since the reception frequency is equal to the local oscillation frequency.  
         [0128]    It should be noted the PLL circuit  41  supplies a part of a control voltage applied to the variable-capacitance diode employed in the VCO to the tuning circuits  12 H and  14 H as a tuning voltage for implementing tuning for the received signal SRX.  
         [0129]    The intermediate-frequency signals SIFA and SIFB generated by the mixer circuits  31 A and  31 B respectively are processed by the circuits  32 A to  37  in the same way as the reception of a low-band broadcast. Intermediate-frequency signals SIFA and SIFB resulting from the processing are supplied to the output terminals  38 A and  38 B respectively. The intermediate-frequency signals SIFA and SIFB supplied to the output terminals  38 A and  38 B are then subjected to demodulation to produce an audio signal of a desired program.  
         [0130]    At that time, AGC is carried out on the circuits  45  to  47  in the same way as the reception of a low-band broadcast.  
         [0131]    The configuration and the operations for receiving high-band broadcasts have been explained so far. By having the system controller change a frequency division rate of a variable-rate frequency dividing circuit employed in the PLL circuit  41 , the oscillation frequency of the oscillation signal S 41  can be varied accordingly. Thus, by changing the oscillation frequency in the range of 340 MHz to 444 MHz according to a reception frequency as described above, the local-oscillation signals SHA and SHB will each have a frequency varied in the range of 170 MHz to 222 MHz according to the reception frequency. Accordingly, it is possible to obtain intermediate-frequency signals SIFA and SIFB with an intermediate frequency of 0, that is, to receive a high-band broadcast.  
         [0132]    Summary  
         [0133]    The oscillation frequency of the oscillation signal S 41  generated by the PLL circuit  41  employed in the receiver shown in FIG. 7 has the following values:  
         [0134]    In reception of a low-band broadcast: In the range of 360 MHz to 432 MHz  
         [0135]    In reception of a high-band broadcast: In the range of 340 MHz to 444 MHz  
         [0136]    Thus, this receiver exhibits the same effects as the receiver shown in FIG. 1.  
         [0137]    Receiver . . . Part IV  
         [0138]    [0138]FIG. 8 is a diagram showing another typical receiver adopting the direct-conversion system as provided by the present invention. Much like the receiver shown in FIG. 4, in this receiver, the functions of the switch circuits  15 ,  44 A and  44 B are included in mixer circuits. It should be noted that FIG. 8 shows neither the antenna tuning circuits  12 L and  12 H nor the oscillation circuit  41 C due to page-size limitation.  
         [0139]    For the above reason, there are provided 2 pairs of mixer circuits. One of the pair consists of mixer circuits  31 LA and  31 LB while the other pair consists of mixer circuits  31 HA and  31 HB. The system controller controls direct-current bias voltages for the mixer circuits  31 LA,  31 LB,  31 HA and  31 HB in accordance with the reception band. To put it in detail, bias voltages for transistors composing the mixer circuits  31 LA,  31 LB,  31 HA and  31 HB are controlled so that, when a low-band broadcast is received, the mixer circuits  31 LA and  31 LB are operating effectively and, when a high-band broadcast is received, on the other hand, the mixer circuits  31 HA and  31 HB are operating effectively.  
         [0140]    Next, configurations and operations for receiving low-band and high-band broadcasts are explained.  
         [0141]    Reception of low-band broadcasts  
         [0142]    A received signal of an ISDB-T broadcasted wave is supplied to the mixer circuits  31 LA and  31 LB through signal lines by way of an antenna tuning circuit, an AGC variable-gain amplifier  13 L and an inter-stage tuning circuit  14 L adopting the electronic tuning technique. It should be noted that the antenna tuning circuit is not shown in the figure.  
         [0143]    A PLL circuit  41  generates an oscillation signal S 41  with a frequency varied in the range of 360 MHz to 432 MHz according to a reception frequency. The oscillation signal S 41  is supplied to a frequency dividing circuit  42 , being converted into a signal with a divided frequency varied in the range of 180 MHz to 216 MHz according to the reception frequency. The divided-frequency signal is further supplied to a frequency dividing circuit  43  at the following stage, being converted into 2 divided-frequency signals SLA and SLB, which each have a divided frequency varied in the range of 90 MHz to 108 MHz according to the reception frequency and have phases shifted from each other by 90 degrees. The divided-frequency signals SLA and SLB are then supplied to the mixer circuits  31 LA and  31 LB respectively as local-oscillation signals.  
         [0144]    The mixer circuits  31 LA and  31 LB mix the received signal SRX with the local-oscillation signals SLA and SLB respectively, converting the received signal SRX into 2 intermediate-frequency signals SIFA and SIFB, which have phases shifted from each other by 90 degrees. That is to say, the mixer circuits  31 A and  31 B convert the frequency of the received signal SRX to produce the in-phase and quadrature intermediate-frequency signals SIFA and SIFB, which are orthogonal to each other. It should be noted that the intermediate-frequency signals SIFA and SIFB each have an intermediate frequency of 0.  
         [0145]    The intermediate-frequency signal SIFA generated by the mixer circuit  31 A is supplied to an output terminal  38 A through signal lines by way of a low-pass filter  32 A, an AGC variable-gain amplifier  36 A and a low-pass filter  37 A. In the same way, the intermediate-frequency signal SIFB generated by the mixer circuit  31 B is supplied to an output terminal  38 B through signal lines by way of a low-pass filter  32 B, an AGC variable-gain amplifier  36 B and a low-pass filter  37 B.  
         [0146]    It should be noted that the intermediate-frequency signals SIFA and SIFB supplied to the output terminals  38 A and  38 B respectively are then subjected to modulation. As a result of demodulation, an audio signal of a desired program is generated.  
         [0147]    In addition, AGC detection circuits  45  and  46  generate AGC voltages V 45  and V 46  respectively, which are used for carrying out AGC of the AGC variable-gain amplifiers  36 A,  36 B and  13 L.  
         [0148]    The configuration and the operations for receiving low-band broadcasts have been explained so far. By having the system controller change a frequency division rate of a variable-rate frequency dividing circuit employed in the PLL circuit  41 , the frequency of the oscillation signal S 41  can be varied accordingly. It should be noted that the variable-rate frequency dividing circuit itself is not shown in the figures. Thus, by changing the oscillation frequency in the range of 360 MHz to 432 MHz according to a reception frequency as described above, the local-oscillation signals SLA and SLB will each have a frequency varied in the range of 90 MHz to 108 MHz according to the reception frequency. Accordingly, it is possible to obtain intermediate-frequency signals SIFA and SIFB each having an intermediate frequency of 0, that is, to receive a low-band broadcast.  
         [0149]    Reception of high-band broadcasts  
         [0150]    A received signal of an ISDB-T broadcasted wave is supplied to the mixer circuits  31 HA and  31 HB through signal lines by way of an antenna tuning circuit, an AGC variable-gain amplifier  13 H and an inter-stage tuning circuit  14 H adopting the electronic tuning technique. It should be noted that the antenna tuning circuit is not shown in the figure.  
         [0151]    The PLL circuit  41  generates an oscillation signal S 41  with a frequency varied in the range of 340 MHz to 444 MHz according to a reception frequency. The oscillation signal S 41  is supplied to the frequency dividing circuit  42 , being converted into 2 divided-frequency signals SHA and SHB, which each have a divided frequency varied in the range of 170 MHz to 222 MHz according to the reception frequency and have phases shifted from each other by 90 degrees. The divided-frequency signals SHA and SHB are then supplied to the mixer circuits  31 HA and  31 HB respectively as local-oscillation signals.  
         [0152]    The mixer circuits  31 HA and  31 HB mix the received signal SRX with the local-oscillation signals SHA and SHB respectively, converting the received signal SRX into 2 intermediate-frequency signals SIFA and SIFB, which have phases shifted from each other by 90 degrees. That is to say, the mixer circuits  31 A and  31 B convert the frequency of the received signal SRX to produce the in-phase and quadrature intermediate-frequency signals SIFA and SIFB, which are orthogonal to each other. It should be noted that the intermediate-frequency signals SIFA and SIFB each have an intermediate frequency of 0.  
         [0153]    The intermediate-frequency signals SIFA and SIFB are processed in the circuits  32 A to  37  in the same way as the reception of a low-band broadcast to produce intermediate-frequency signals SIFA and SIFB supplied to output terminals  38 A and  38 B respectively. The intermediate-frequency signals SIFA and SIFB supplied to the output terminals  38 A and  38 B are then modulated to produce an audio signal of a desired program.  
         [0154]    At that time, the circuits  45  to  47  carry out AGC in the same way as the reception of a low-band broadcast.  
         [0155]    The configuration and the operations for receiving high-band broadcasts have been explained so far. By having the system controller change a frequency division rate of a variable-rate frequency dividing circuit employed in the PLL circuit  41 , the frequency of the oscillation signal S 41  can be varied accordingly. Thus, by changing the oscillation frequency in the range of 340 MHz to 444 MHz according to a reception frequency as described above, the local-oscillation signals SHA and SHB will each have a frequency varied in the range of 170 MHz to 222 MHz according to the reception frequency. Accordingly, it is possible to obtain intermediate-frequency signals SIFA and SIFB each having an intermediate frequency of 0, that is, to receive a high-band broadcast.  
         [0156]    Summary  
         [0157]    The oscillation frequency of the oscillation signal S 41  generated by the PLL circuit  41  employed in the receiver shown in FIG. 8 has the following values:  
         [0158]    In reception of a low-band broadcast: In the range of 360 MHz to 432 MHz  
         [0159]    In reception of a high-band broadcast: In the range of 340 MHz to 444 MHz  
         [0160]    Thus, this receiver exhibits the same effects as the receiver shown in FIG. 4.  
         [0161]    Since the reception band is switched from one to another by controlling the direct-current bias voltages of the mixer circuits  31 LA,  31 LB,  31 HA and  31 HB, the magnitude of the consumed current can be reduced.  
         [0162]    Others  
         [0163]    The above description explains the configurations of narrow-band ISDB-T receivers. By the way, a receiver for receiving audio signals of FM and television broadcasts has the following reception bands:  
         [0164]    FM broadcasting band: 76 MHz to 90 MHz  
         [0165]    Television broadcasting low band: 90 MHz to 108 MHz (1 to 3 channels)  
         [0166]    Television broadcasting high band: 170 MHz to 222 MHz (4 to 12 channels)  
         [0167]    Thus, the reception band of a receiver for receiving audio signals of FM and television broadcasts can be divided into the following:  
         [0168]    1: Low band: 76 MHz to 108 MHz  
         [0169]    2: High band: 170 MHz to 222 MHz  
         [0170]    In this case, the oscillation signal S 41  generated by the PLL circuit  41  and the divided-frequency signals S 42  and S 43  used for converting frequencies need to be controlled as follows.  
         [0171]    In the case of a receiver with a configuration having a super-heterodyne system as shown in FIG. 1 or  4  to produce a typical intermediate frequency of 500 kHz, for the low band (1), the frequency range of the oscillation signal S 41  is set at 306 MHz to 434 MHz and the frequency of the received signal SRX is converted by using the divided-frequency signals SLA and SLB. For the high band (2), on the other hand, the frequency range of the oscillation signal S 41  is set at 341 MHz to 445 MHz and the frequency of the received signal SRX is converted by using the divided-frequency signals SHA and SHB.  
         [0172]    In the case of a receiver with a configuration adopting a direct-conversion system as shown in FIG. 7 or  8 , on the contrary, the frequency range is set as follows. For the low band (1), the frequency range of the oscillation signal S 41  is set at 304 MHz to 432 MHz and the frequency of the received signal SRX is converted by using the divided-frequency signals SLA and SLB. For the high band (2), on the other hand, the frequency range of the oscillation signal S 41  is set at 340 MHz to 444 MHz and the frequency of the received signal SRX is converted by using the divided-frequency signals SHA and SHB.  
         [0173]    A circuit for switching the reception band from (1) to (2) and vice versa for the above receivers may have the same configuration as that of the ISDB-T receiver shown in FIGS. 2 and 3 or FIGS. 5 and 6.  
         [0174]    In addition, in the receivers described above, the tuning circuit  12 L can be implemented as a band-pass filter having a low band as a pass band. By the same token, the tuning circuit  12 H can be implemented as a band-pass filter having a high band as a pass band. Moreover, the phase shift circuits  33 A and  33 B can each be implemented as a poly-phase filter.  
         [0175]    In accordance with the present invention, since it is not necessary to switch a PLL resonance circuit from a variable-capacitance diode and a coil to others corresponding to the reception band, the number of external components attached to the IC can be reduced. A decreased external-component count contributes to reduction of the cost and size of the receiver.  
         [0176]    Moreover, since it is not necessary to connect a switch device for switching the frequency band of the oscillation frequency from one to another to the PLL oscillation circuit, the Q value of the oscillation circuit is not lowered. As a result, the oscillation circuit exhibits improved characteristics such as stable oscillation, fewer phase noises, fewer pulling phenomena and a lower current consumption.  
         [0177]    In addition, since the receiver needs only one oscillation circuit, which is prone to effects of surroundings, that is, the receiver needs only one VCO of the PLL circuit, a layout is easy to make in the implementation of the IC, and it is hence possible to design an oscillation circuit that is proof against external disturbances. On the top of that, since the tracking error characteristic is uniform independently of the reception band, the number of tracking errors can be reduced with ease.  
         [0178]    Furthermore, since the reception band can be switched from one to another with the VCO of the PLL circuit  41  kept in a state of oscillation as it is, a broadcast can be received fast even after band switching. Moreover, it is possible to implement most of the circuits such as those enclosed by a dashed line in FIG. 1 as a single-chip IC.  
       [LIST OF ABBREVIATIONS USED IN THIS SPECIFICATION]  
       [0179]    AGC: Automatic Gain Control  
         [0180]    AM: Amplitude Modulation  
         [0181]    DAB: Digital Audio Broadcasting  
         [0182]    FM: Frequency Modulation  
         [0183]    IC: Integrated Circuit  
         [0184]    ISDB: Integrated Services Digital Broadcasting  
         [0185]    MPEG: Motion Picture Image Coding Experts Group  
         [0186]    OFDM: Orthogonal Frequency Division Multiplex  
         [0187]    PLL: Phase Locked Loop  
         [0188]    VCO: Voltage Controlled Oscillator  
         [0189]    VHF: Very High Frequency