Abstract:
A radio-frequency receiver includes an RF amplification circuit which amplifies a received RF signal and generates an amplified RF signal, a mixing circuit which converts the amplified RF signal into an intermediate-frequency signal, an IF amplification circuit which generates an amplified IF signal, a first level detection circuit which detects a level of the amplified RF signal, a second level detection circuit which detects a level of the IF signal, a third level detection circuit which detects a level of the amplified IF signal, a RF reference level generation circuit which generates an RF reference level based on one of respective detection signal levels of the first and second level detection circuits, and an RF gain control circuits which controls an amplification gain of the RF amplification circuit so that a detection signal level of the third level detection circuit becomes equal to the RF reference level.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to the reception of RF (radio-frequency) signals, and more particularly, the present invention relates to an RF receiver device equipped with an automatic gain control function that controls an amplification gain (amplification factor) of a RF amplification stage. 
     Herein, “radio-frequency” is broadly defined and is not particularly limited. Non-limiting examples include high-signal frequencies for broadcasting of high-definition television (HDTV) signals, such as the ultrahigh frequency (UHF) bands utilized in the Integrated Service Digital Broadcasting—Terrestrial (ISDB-T) standard. 
     2. Description of the Related Art 
     A conventional ISDB-T type RF receiver is illustrated in  FIG. 1 , and includes an RF variable gain amplifier  1 , a mixer  2 , an intermediate-frequency (IF) variable gain amplifier  3 , a filter  4 , an ADC (analog-digital converter)  5 , power detectors  6  and  7 , and an IF gain controller  8 . Receivers of this type are disclosed, for example, in Japanese Patent Application Laid-Open No. 2002-290178 and Japanese Patent Application Laid-Open No. 2007-019900. 
     The RF variable gain amplifier  1  includes a gain control terminal for varying an amplification gain of an input RF signal received via an antenna (not shown). The resultant amplified RF signal is supplied to the mixer  2 , which is configured to mix the supplied RF signal with an oscillation signal generated from a local oscillator to convert the RF signal into an intermediate-frequency (IF) signal. The IF variable gain amplifier  3  includes a gain control terminal for varying the gain the IF signal received from the mixer  2 . The filter  4  extracts (passes) intermediate frequency signal components from the output of the IF variable gain amplifier  3 . The ADC  5  is configured to convert an analog IF signal output from the filter  4  into a digital IF signal. The digital IF signal is then supplied, for example, to an OFDM decoder circuit (not shown). 
     The power detector  6  is configured to detect a power level of the IF signal output from the mixer  2 , and supply a corresponding gain control signal to the gain control terminal of the RF variable gain amplifier  1 . The RF variable gain amplifier  1  changes the amplification gain thereof in accordance with the gain control signal. 
     Likewise, the power detector  7  is configured to detect a power level of the digital IF signal output from the ADC  5  and to supply corresponding power level detection signal to the IF gain controller  8 . The IF gain controller  8  is configured to generate an gain control signal in accordance with the power level detection signal and to supply the gain control signal to the gain control terminal of the IF variable gain amplifier  3 . The IF variable gain amplifier  3  changes the amplification gain thereof in accordance with the gain control signal. 
     By adjusting the respective amplification gains of the RF variable gain amplifier  1  and the IF variable gain amplifier  3 , the IF signal can be controlled to a suitable level and supplied to a decoder circuit when an RF signal of a desired channel is received. 
     In the conventional RF receiver device, a reception frequency band of the RF variable gain amplifier  1  contains therein a broadcast wave component for each of a plurality of channels, and a broadcast wave component of a desired channel is selected by the mixer  2  from among the plural channels. The broadcast wave component of the desired channel is centered on an intermediate frequency. As illustrated in  FIG. 2 , the situation can occur in which the reception frequency band of the RF variable gain amplifier  1  further contains therein broadcast wave components of channels adjacent to the broadcast wave component of the desired channel. In such a case, as also illustrated in  FIG. 2 , in the frequency domain of an output IF signal of the mixer  2 , the broadcast wave component of the desired channel, i.e., a broadcast wave component of an intermediate frequency, and broadcast wave components of plural adjacent channels in the vicinity of the desired channel are present. Since the broadcast wave components of the plural adjacent channels are included in a power detection range of the power detector  6 , respective power levels of the broadcast wave components within the power detection range are reflected on the power level detection of the power detector  6 . Therefore, even when a broadcast wave exhibits a high reception power level, the amplification gain of the RF variable gain amplifier  1  is controlled. 
     On the other hand, as illustrated in  FIG. 3 , the situation can occur where the reception frequency band of the RF variable gain amplifier  1  contains therein broadcast wave components of channels distant from, and not adjacent to, the broadcast wave component of the desired channel. In such a case, as also illustrated in  FIG. 3 , in the frequency domain of the output IF signal of the mixer  2 , broadcast wave components of plural channels outside the power detection range of the power detector  6  and distant from the broadcast wave component of the desired channel are present. Since only the broadcast wave component of the desired channel is included in the power detection range of the power detector  6 , the power level of only the broadcast wave component of the desired channel is reflected on the power level detection of the power detector  6 . Therefore, the amplification gain of the RF variable gain amplifier  1  is controlled in accordance with the power level detection of only the broadcast wave component of the desired channel. 
     When the reception signal power of the broadcast wave component of a channel distant from the desired channel is large, the RF variable gain amplifier  1  might be saturated with the reception signal power of the distant channel. As a result, the signal component of the desired channel may be distorted, and thus, reception performance may deteriorate. 
     To overcome this problem, the gain of the RF variable gain amplifier  1  may be set to a low value. However, in this case, when other channels adjacent to the desired channel are present, power levels of other channels other than the desired channel are reflected on the power detection. Therefore, the power of the signal component of the desired channel after having passed through the filter  4  becomes very low, and thus, there may be a case where it is difficult to obtain sufficient signal amplitude for decoding. 
     On the other hand, in a case where the power detector is provided at an input of the mixer  2  instead of the output of the mixer  2 , the gain of the RF variable gain amplifier  1  is controlled by the entire reception power present in the reception frequency band. Therefore, as the sum of the power of other channels present in a region where saturation rarely occurs increases, an amplification factor of the desired channel decreases. As a result, the case can arise in which is difficult to obtain a signal amplitude required for decoding. In an effort to minimize this problem, the gain of the RF variable gain amplifier  1  may be set to a high value. However, in this case, it is not practically possible to detect saturation occurring in the mixer  2 , and thus, the RF variable gain amplifier may enter into an unintended saturation state. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide an RF receiver device which is capable of controlling a signal component of a desired channel to a suitable level while avoiding or minimizing saturation of a high-frequency amplification stage such as an RF variable gain amplifier. 
     In accordance with an aspect of the present invention, a radio-frequency (RF) receiver is provide which includes an RF amplification circuit which amplifies a received RF signal and generates an amplified RF signal, a mixing circuit which converts the amplified RF signal into an intermediate-frequency (IF) signal, an IF amplification circuit which amplifies the IF signal and generates an amplified IF signal, a first level detection circuit which detects a signal level of the amplified RF signal, a second level detection circuit which detects a signal level of the IF signal, a third level detection circuit which detects a signal level of the amplified IF signal, a RF reference level generation circuit which generates an RF reference level based on at least one of respective detection signal levels of the first and second level detection circuits, and an RF gain control circuits which controls an amplification gain of the RF amplification circuit so that a value corresponding to a detection signal level of the third level detection circuit becomes equal to the RF reference level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspect and features of the present invention with become readily apparent from the detailed description that follows, with reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a conventional RF RF receiver; 
         FIG. 2  illustrate reception frequencies and mixer output frequencies of channels in the conventional RF receiver of  FIG. 1 ; 
         FIG. 3  illustrate reception frequencies and mixer output frequencies of channels in the conventional RF receiver of  FIG. 1 ; 
         FIG. 4  is a block diagram of RF receiver device according to an embodiment of the present invention; 
         FIG. 5  is a block diagram of an IF gain controller of the RF receiver device of  FIG. 4  according to an embodiment of the present invention; 
         FIG. 6  is a block diagram of a RF reference level generator of the RF receiver device of  FIG. 4  according to an embodiment of the present invention; 
         FIG. 7  illustrates an output value table of a selector according to an embodiment of the present invention; 
         FIG. 8  is a block diagram of an RF gain controller of the RF receiver device of  FIG. 4  according to an embodiment of the present invention; 
         FIG. 9  illustrates reception frequencies and mixer output frequencies of channels in the RF receiver of  FIG. 4  according to an embodiment of the present invention; and 
         FIG. 10  illustrates reception frequencies and mixer output frequencies of channels in the RF receiver of  FIG. 4  according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will be described below by way of preferred, but non-limiting, embodiments of the invention. The embodiments presented herein are considered examples of various implementations of the invention, and are not intended to limit or specifically define the overall scope of the invention. 
     For ease of understanding and to avoid redundancy, like reference numbers refer to the same or similar elements throughout the drawings. Also, while the drawings contain a number of circuit elements, it will be understood from the nature of electrical circuits that when an element is described as being connected to another element, it can be directly connected the other element or one or more intervening elements may be present. In contrast, if an element is referred to as being “directly connected to” another element, then no intervening elements are present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” “connected” versus “directly connected,” etc.). 
     As is traditional in the field of the present invention, embodiments of the invention may be described at least partially in terms of functional blocks or units. It will be readily understood that the functional blocks or units denote electronic circuits which are configured (e.g., by dedicated and/or programmable circuitry) to execute the signaling and/or computational operations described herein. Also, it will be readily understood that one or more functional blocks may be physically combined into complex circuitry without departing from the spirit and scope of the present invention. 
       FIG. 4  illustrates an RF receiver device according to an embodiment of the present invention. The RF receiver device may, for example, be utilized in ISDB-T type terrestrial digital broadcasting. 
     The RF receiver device of  FIG. 4  includes an RF variable gain amplifier  11 , a mixer  12 , an IF variable gain amplifier  13 , a filter  14 , an ADC  15 , power detectors  16 ,  17  and  18 , an IF gain controller  19 , an RF reference level generator  20 , and an RF gain controller  21 . 
     The RF variable gain amplifier  11 , the mixer  12 , the IF variable gain amplifier  13 , the filter  14 , and the ADC  15  function in the same manner as the RB variable gain amplifier  1 , the mixer  2 , the IF variable gain amplifier  3 , the filter  41  and the ADC  5  described previously in connection with the conventional device illustrated in  FIG. 1 . Accordingly, a detailed description of these element is omitted here to avoid redundancy. 
     The power detector  16  is configured to detect a power level of an output RF signal of the RF variable gain amplifier  11 , and to supply a corresponding power level detection signal to the RF reference level generator  20 . The power detector  17  is configured to detect a power level of an output IF signal of the mixer  12 , and to supply a corresponding power level detection signal to the RF reference level generator  20 . The power detector  18  is configured to detect a power level of the digital IF signal output from the ADC  15 , and to supply a corresponding power level detection to the IF gain controller  19 . 
     The IF gain controller  19  is configured to generate a gain control signal in accordance with the power level detection signal output from the power detector  18 , and to supply the gain control signal to the gain control terminal of the IF variable gain amplifier  13 . 
     The IF gain controller  19  according to an embodiment of the present invention is shown in  FIG. 5 . According to the example of  FIG. 5 , the IF gain controller  19  includes a comparator  191 , an integrator  192 , a control value converter  193 , and a holder  194 . The comparator  191  is configured to compare the power level detection signal (from power detector  18 ) with a reference level R 1 , and to generate a gain increase signal when the power level detection signal is smaller than the reference level R 1 , and to generate a gain decrease signal when the power level detection signal is not smaller than the reference level R 1 . The integrator  192  is configured to integrate an output signal of the comparator  191  at a predetermined time interval. In the example of this embodiment, the gain increase signal output from the comparator  191  is +1 and the gain decrease signal is −1. The control value converter  193  is configured to convert a value of an integration result by the integrator  192  to a control value for the IF variable gain amplifier  13  to thereby generate the gain control signal. For example, when the IF variable gain amplifier  13  has a control voltage range of 0.5 to 2.5 V and a control amount of 20 dB/V and the integrator  192  has an output value range of 0 to 128, a gain control step of the IF variable gain amplifier  13  is adjusted to 0.3125 dB/step (40/128) so that an output value of 0 corresponds to a control voltage of 0.5 V, while an output value of 128 corresponds to a control voltage of 2.5 V. 
     The holder  194  is normally in a through state (OFF state) where it relays and supplies the gain control signal output from the control value converter  193  to the gain control terminal of the IF variable gain amplifier  13 . However, upon reception of an ON control signal from other non-illustrated communication functional blocks within a device containing the RF receiver device, the holder  194  holds the gain control signal that existed immediately before reception of the ON control signal, so that the held gain control signal is supplied to the gain control terminal of the IF variable gain amplifier  13 . For example, when other communication functional blocks are in a transmission mode, a gain of the IF variable gain amplifier  13  is fixed to a gain immediately before reception in response to the ON control signal. 
     Returning to  FIG. 4 , the RF reference level generator  20  is configured to set a reference level R 2  of the RF gain controller  21  based on the respective power level detection signals of the power detectors  16  and  17 . In particular, the RF reference level generator  20  is configured to avoid allowing the RF variable gain amplifier  11  to enter into a saturation state. 
       FIG. 6  illustrates an embodiment of the RF reference level generator  20 . As shown in  FIG. 6 , the RF reference level generator  10  of this example is configured to include first and second increase/decrease determiners  31  and  32 , a selector  33 , and an integrator  34 . The first increase/decrease determiner  31  is provided with comparators  311 ,  313 ,  314  and  316 , counters  312  and  315 , and an average value calculator  317 . Although not shown, the second increase/decrease determiner  32  is configured in the same manner as the first increase/decrease determiner  31 . 
     The comparator  311  is configured to compare the power level detection signal of the power detector  16  with a first threshold th 1  at a predetermined time interval n. Assuming that the power level detection is S 1 , the comparator  311  generates an output value S 2  representing “1” when S 1 &gt;th 1 , and an output value S 2  representing “0” is generated when S 1 ≦th 1 . The counter  312  is configured to count the output value S 2  of the comparator  311  obtained at the predetermined time interval n, and to output a count value S 3  at a predetermined time interval m. The comparator  313  is configured to compare the count value S 3  of the counter  312  with a second threshold th 2  at the predetermined time interval m. The comparator  313  generates an output value S 4  representing “−1” when S 3 &gt;th 2 , and an output value S 4  representing “+1” is generated when S 3 ≦th 2 . In this embodiment, the predetermined time interval m is an integer multiple of the predetermined time interval n. 
     The comparator  314  is configured to compare the power level detection signal S 1  of the power detector  16  with a third threshold th 3  at the predetermined time interval n. IN this embodiment, the first and third thresholds satisfy a relationship of th 1 &gt;th 3 . The comparator  314  generates an output value S 5  representing “1” when S 1 &gt;th 3 , and an output value S 5  representing “0” is generated when S 1 ≦th 3 . The counter  315  is configured to count the output value S 5  of the comparator  314  obtained at the predetermined time interval n, and to output a count value S 6  at the predetermined time interval m. The comparator  316  is configured to compare the count value S 6  of the counter  315  with a fourth threshold th 4  at the predetermined time interval m. The comparator  316  generates an output value S 7  representing “−1” when S 6 &gt;th 4 , and an output value S 7  representing “+1” is generated when S 6 ≦th 4 . 
     The average value calculator  317  is configured to calculate an average value (S 4 +S 7 )/2 of the output value S 4  of the comparator  313  and the output value S 7  of the comparator  316  at the predetermined time interval m, and to output the average value as an output value S 8  to the increase/decrease determiner  31 . The output value S 8  has three values of “−1,” “0” and “+1.” The output value S 8  of “−1” is a gain increase request, “+1” is a gain decrease request, and “0” is a neutral request, i.e., a do-nothing request. 
     The above-described output values S 2  to S 8  can be expressed by the following formulae. Here, N is an arbitrary integer. 
     
       
         
           
             
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     As mentioned above, the second increase/decrease determiner  32  has the same configuration as the first increase/decrease determiner  31 , except that the increase/decrease determiner  32  is supplied with the power level detection of the power detector  17 . 
     The selector  33  is configured to output a selection value in response to the output value of the first increase/decrease determiner  31  and the output value of the second increase/decrease determiner  32  at the predetermined time interval m. The selection value is either the output value of the first increase/decrease determiner  31  or the output value of the second increase/decrease determiner  32  at an arbitrary time point. 
       FIG. 7  is a table illustrating an output value table of the selector  33  with respect to the respective output values S 8  of the first increase/decrease determiner  31  and the respective output values S 10  of the second increase/decrease determiner  32  according to an embodiment of the present invention. In the illustrated example, “−1” is a gain increase request, “+1” is a gain decrease request and “0” is a neutral request, i.e., a do-nothing request. 
     The integrator  34  is configured to integrate the output value of the selector  33  output at the predetermined time interval m. A value of an integration result by the integrator  34  is used as the reference level R 2 , which is supplied to the RF gain controller  21  as an output of the RF reference level generator  20 . 
     Returning to  FIG. 4 , the RF gain controller  21  is configured to generate a gain control signal in accordance with the output value of the integrator  192  within the IF gain controller  19 , and to supply the gain control signal to the gain control terminal of the RF variable gain amplifier  11 . 
     An embodiment of the RF gain controller  21  is illustrated in  FIG. 8 . That is, the RF gain controller  21  of this example is configured to include a comparator  211 , an integrator  212 , a control value converter  213 , and a holder  214 . The comparator  211  is supplied with the output value of the integrator  192  as the value corresponding to the power level detection signal of the IF signal while being supplied with the output value of the integrator  34  as the reference level R 2 . The gain control signal is supplied from the holder  214  to the gain control terminal of the RF variable gain amplifier  11 . 
     The holder  214  is normally in a through state (OFF state) where it relays and supplies the gain control signal output from the control value converter  213  to the gain control terminal of the RF variable gain amplifier  11 . However, upon reception of an ON control signal from other aforementioned communication functional blocks, the holder  214  holds the gain control signal that existed immediately before the reception of the ON control signal so that the held gain control signal is supplied to the gain control terminal of the RF variable gain amplifier  11 . When other communication functional blocks are in a transmission mode, a gain of the RF variable gain amplifier  11  is fixed to a gain immediately before the reception in response to the ON control signal. 
     In the RF receiver device described above, the power level of the digital IF signal output from the ADC  15  is detected by the power detector  18 . The power level detection of the power detector  18  is supplied to the IF gain controller  19 . In the IF gain controller  19 , the power level detection and the reference level R 1  are compared with each other by the comparator  191 , a comparison result is integrated by the integrator  192 , and the output value of the integrator  192  is converted by the control value converter  193 , whereby the gain control signal is generated. The gain control signal is supplied to the gain control terminal of the IF variable gain amplifier  13  via the holder  194 . Since the IF variable gain amplifier  13  changes its gain in accordance with the gain control signal, the power level of the digital IF signal output from the ADC  15  is adjusted accordingly. In this manner, a first feedback loop is formed, and thus, the power level of the digital IF signal is controlled so as to be equal to the reference level R 1  of the comparator  191  by the first feedback loop. 
     Further, the output value of the integrator  192  is supplied to the RF gain controller  21 . In the RF gain controller  21 , the output value of the integrator  192  and the reference level R 2  generated by the RF reference level generator  20  are compared with each other by the comparator  211 , a comparison result is integrated by the integrator  212 , and the output value of the integrator  212  is converted by the control value converter  213 , whereby the gain control signal is generated. The gain control signal is supplied to the gain control terminal of the RF variable gain amplifier  11  via the holder  214 . Since the RF variable gain amplifier  11  changes its gain in accordance with the gain control signal, the power level of the digital IF signal output from the ADC  15  is adjusted accordingly. In this manner, a second feedback loop is formed. A feedback gain of the second feedback loop is smaller than a feedback gain of the first feedback loop. By the second feedback loop, the output value of the integrator  192  is controlled so as to be equal to the reference level R 2 : that is, the gain of the IF variable gain amplifier  13  is controlled so as to be constant. 
     In the second feedback loop, the reference level R 2  is generated by the RF reference level generator  20 . As described above, the RF reference level generator  20  adjusts the reference level R 2  based on both or either one of the power level on an input side of the mixer  12  and the power level on an output side thereof. 
     As the output power level of the RF variable gain amplifier  11  increases so that the RF variable gain amplifier  11  enters into a saturation state, the analog output value S 1  of the power detector  16  detecting the power level of the input side of the mixer  12  increases. With the increase in the output value S 1  of the power detector  16 , the output value S 5  of the comparator  314  (in some cases, including the output value S 2  of the comparator  311 ) is inverted from “0” representing a non-saturation state to “1” representing a saturation state. In this way, when both the output value S 4  of the comparator  313  and the output value S 7  of the comparator  316  become “1,” the output value S 8  of the increase/decrease determiner  31  becomes “+1”. When the analog output value S 9  of the power detector  17  detecting the power level on the output side of the mixer  12  increases, the output value S 10  of the increase/decrease determiner  32  becomes “+1” by the same operation. 
     On the other hand, as the output power level of the RF variable gain amplifier  11  decreases, the analog output value S 1  of the power detector  16  decreases. With the decrease in the output value S 1  of the power detector  16 , the output value  55  of the comparator  314  (in some cases, including the output value S 2  of the comparator  311 ) becomes “0” representing a non-saturation state. In this way, when both the output value S 4  of the comparator  313  and the output value  57  of the comparator  316  become “0”, the output value S 8  of the increase/decrease determiner  31  becomes “0” or “−1”. When the analog output value S 9  of the power detector  17  decreases, the output value S 10  of the increase/decrease determiner  32  becomes “0” or “−1” by the same operation. 
     As will be understood from  FIG. 7 , when the output value S 8  of the increase/decrease determiner  31  is “+1”, or when the output value S 8  of the increase/decrease determiner  31  is “0” and the output value S 10  of the increase/decrease determiner  32  is “+1”, the output value of the selector  33  becomes “+1”. Since “+1” is a gain decrease request, the value of the integration result of the integrator  34 , that is, the reference level R 2 , decreases. In response to the decrease in the reference level R 2 , the gain control signal of the RF gain controller  21  decreases the gain of the RF variable gain amplifier  11 . 
     On the other hands when the output value S 8  of the increase/decrease determiner  31  is “−1”, or when the output value S 8  of the increase/decrease determiner  31  is “0” and the output value S 10  of the increase/decrease determiner  32  is “−1”, the output value of the selector  33  becomes “−1”. Since “−1” is a gain increase request, the value of the integration result of the integrator  34 , that is, the reference level R 2 , increases. In response to the increase in the reference level R 2 , the gain control signal of the RF gain controller  21  decreases the gain of the RF variable gain amplifier  11 . 
     As illustrated in  FIG. 9 , a situation can occur where the reception frequency band of the RF variable gain amplifier  11  contains therein broadcast wave components of channels adjacent to the broadcast wave component of the desired channel. In such a case, as also illustrated in  FIG. 9 , in the frequency domain of an output IF signal of the mixer  12 , the broadcast wave component of the desired channel, i.e., a broadcast wave component of an intermediate frequency, and broadcast wave components of plural adjacent channels in the vicinity of the desired channel are present. That is, the broadcast wave component of the desired channel and the broadcast wave components of channels adjacent to the desired channel are included in the power detection frequency band of the power detector  16  detecting the power level on the input side of the mixer  12 . Further, the broadcast wave component of the desired channel and the broadcast wave components of channels adjacent to the desired channel are also included in the power detection frequency band of the power detector  17  detecting the power level on the output side of the mixer  12 . Therefore, even when there is a broadcast wave of an adjacent channel exhibiting a high reception power level, the amplification gain of the RF variable gain amplifier  11  is suitably controlled. 
     On the other hand, as illustrated in  FIG. 10 , another situation can occur in which the reception frequency band of the RF variable gain amplifier  11  contains therein broadcast wave components of channels distant from, and not adjacent to, the broadcast wave component of the desired channel. In such a case, as also illustrated in  FIG. 10 , in the frequency domain of the output IF signal of the mixer  12 , broadcast wave components of plural channels outside the power detection range of the power detector  17  and distant from the broadcast wave component of the desired channel are present. However, in this case, the power detection frequency band of the power detector  16  contains therein the broadcast wave component of the desired channel and the broadcast wave components of channels adjacent to the desired channel. Therefore, since the output value S 8  of the increase/decrease determiner  31  becomes “+1” based on at least the increase in the output value S 1  of the power detector  16 , the reference level R 2  decreases, and thus, the gain of the RF variable gain amplifier  11  is decreased. As a result, even when the reception signal power of the broadcast wave component of a channel distant from the desired channel is large, it is possible to prevent saturation of the RF variable gain amplifier  11  by the reception signal power of the distant channel. Accordingly, it is possible to provide favorable reception performance without causing distortion of the signal component of the desired channel. 
     In the embodiments described above, power detectors are utilized. However, other types of signal level detectors may be utilized, such a voltage detectors capable of detecting signal amplitudes. 
     Although the present invention has been described in connection with selected embodiments, it is not limited thereto. It will be apparent to those skilled in the art that various substitution, modifications and changes may be thereto without departing from the scope of the invention.