Patent Abstract:
A receiving apparatus comprising: a frequency conversion unit configured to convert a received radio frequency signal to an intermediate frequency signal; an automatic gain control unit configured to control an amplitude level of at least either one of the radio frequency signal and the intermediate frequency signal according to a gain control signal; a demodulation unit configured to demodulate an audio signal from the intermediate frequency signal; a correction level output unit configured to output a correction level signal corresponding to a difference between a demodulation unit input level and a predetermined reference level, the demodulation unit input level being an amplitude level of the intermediate frequency signal inputted to the demodulation unit; and an addition unit configured to add the gain control signal and the correction level signal, to be outputted as a signal strength signal indicating received signal strength of the radio frequency signal.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of priority to Japanese Patent Application No. 2009-112927, filed May 7, 2009, of which full contents are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a receiving apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    In radio communication, a received signal is subjected to filtering processing, frequency conversion processing, amplification processing and the like, to be demodulated to a baseband signal in general. Also, a receiving apparatus is generally known, which detects a reception status such as signal strength of the received signal or presence/absence of an interference signal, and which controls characteristics such as the filtering processing and the amplification processing according to the reception status, in order to improve a communication quality in the radio communication. 
         [0006]    For example, Japanese Patent Laid-Open Publication No. 2006-86685 discloses a receiving apparatus (AM car-radio receiver) including a signal strength determination means for determining received signal strength from an output control voltage of an AGC (Automatic Gain Control) circuit. In the receiving apparatus, strength of the received signal is determined by which is greater or smaller between the output control voltage of the AGC circuit and a predetermined reference value, to switch channel selection sensitivity of automatic channel selecting means according to such determination result. 
         [0007]    As mentioned above, the received signal strength can be determined by the output control voltage of the AGC circuit, and moreover, optimal channel selection sensitivity can be set using the received signal strength, so that communication quality of the receiving apparatus can be improved. 
         [0008]    In a common AGC circuit, a gain control signal for controlling a gain of an amplifier or an attenuation quantity (negative gain) of an attenuator is generated so that the higher an amplitude level of an output signal of the amplifier or the attenuator is, the smaller the gain becomes, and the lower the amplitude level of the output signal is, the greater the gain becomes. 
         [0009]    Here, there is illustrated in  FIG. 6  an example of a relationship between the amplitude level of the output signal and the gain control signal.  FIG. 6  shows a case where an amplitude level L 1  (short broken line) of the output signal is lowered once at the time of t=0 to become lower than a reference level Ref 1  (dashed-dotted line). As illustrated in  FIG. 6 , a gain control signal G 1  (solid line) is generated by smoothing a difference between the amplitude level L 1  and the reference level Ref 1 , and the amplitude level L 1  is gradually raised according to a smoothed gain control signal G 1 . Then, in  FIG. 6 , the gain control signal G 1  becomes stable at the time of t=t 1 . 
         [0010]    However, such an AGC circuit is required to be provided with a smoothing processing unit having a large time constant to such a degree that the gain control signal G 1  is not oscillated. Thus, a response speed of the gain control signal G 1  is limited by the time constant, and a longer time is required for the received signal strength, which can be determined by the gain control signal G 1 , to become stable. 
       SUMMARY OF THE INVENTION 
       [0011]    A receiving apparatus according to an aspect of the present invention, comprises: a frequency conversion unit configured to convert a received radio frequency signal to an intermediate frequency signal; an automatic gain control unit configured to control an amplitude level of at least either one of the radio frequency signal and the intermediate frequency signal according to a gain control signal; a demodulation unit configured to demodulate an audio signal from the intermediate frequency signal; a correction level output unit configured to output a correction level signal corresponding to a difference between a demodulation unit input level and a predetermined reference level, the demodulation unit input level being an amplitude level of the intermediate frequency signal inputted to the demodulation unit; and an addition unit configured to add the gain control signal and the correction level signal, to be outputted as a signal strength signal indicating received signal strength of the radio frequency signal. 
         [0012]    Other features of the present invention will become apparent from descriptions of this specification and of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For more thorough understanding of the present invention and advantages thereof, the following description should be read in conjunction with the accompanying drawings, in which: 
           [0014]      FIG. 1  is a block diagram illustrating a configuration of a receiving apparatus including a signal strength detection unit according to a first embodiment of the present invention; 
           [0015]      FIG. 2  is a schematic diagram illustrating an example of a relationship between an input signal and an output signal in a correction level output unit  202 ; 
           [0016]      FIG. 3  is a schematic diagram illustrating an example of a relationship between an input signal and an output signal in an addition unit  203 ; 
           [0017]      FIG. 4  is a block diagram illustrating a configuration of a receiving apparatus including a signal strength detection unit according to a second embodiment of the present invention; 
           [0018]      FIG. 5  is a block diagram illustrating a configuration of a receiving apparatus including a signal strength detection unit according to a third embodiment of the present invention; and 
           [0019]      FIG. 6  is a schematic diagram illustrating an example of a relationship between an amplitude level of an output signal of an amplifier or an attenuator in a common AGC circuit and a gain control signal. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    At least the following details will become apparent from descriptions of this specification and of the accompanying drawings. 
       First Embodiment 
     Configuration of Receiving Apparatus 
       [0021]    There will hereinafter be described a configuration of a receiving apparatus including a signal strength detection unit according to a first embodiment of the present invention, referring to  FIG. 1 . 
         [0022]    The receiving apparatus illustrated in  FIG. 1  includes an antenna  1 , an RF (Radio Frequency) amplification unit  2 , a frequency conversion unit  3   a , IF (intermediate Frequency) amplification unit  6 , a demodulation unit  7 , an audio processing unit  8 , a speaker  9 , an AGC unit  11 , and a signal strength detection unit  20   a . The receiving apparatus is used for receiving FM radio broadcasting or AM radio broadcasting, for example. 
         [0023]    An RF signal outputted from the antenna  1  is inputted to the RF amplification unit  2 , and an RFa signal outputted from the RF amplification unit  2  is inputted to the frequency conversion unit  3   a . An IF 1  signal outputted from the frequency conversion unit  3   a  is inputted to the IF amplification unit  6 , and an IF 1   a  signal outputted from the IF amplification unit  6  is inputted to the demodulation unit  7 . Moreover, an AF (Audio Frequency) signal outputted from the demodulation unit  7  is inputted to the audio processing unit  8 , and an AFa signal outputted from the audio processing unit  8  is inputted to the speaker  9 . 
         [0024]    On the other hand, the IF 1   a  signal is inputted to the AGC unit  11  as well. The AGC unit  11  includes an amplitude level detection unit  111 , a comparison unit  112 , and a smoothing processing unit  113 , for example, and the IF 1   a  signal is inputted to the amplitude level detection unit  111 . An output level L 1  of the amplitude level detection unit  111  and a reference level Ref 1  are inputted to the comparison unit  112 , and a comparison result signal CP 1  outputted from the comparison unit  112  is inputted to the smoothing processing unit  113 . And a gain control signal G 1  outputted from the smoothing processing unit  113  is inputted to the IF amplification unit  6  and the signal strength detection unit  20   a.    
         [0025]    The signal strength detection unit  20   a  includes a correction level output unit  202  and an addition unit  203 , for example. The output level L 1  of the amplitude level detection unit  111  and the reference level Ref 1  are inputted to the correction level output unit  202 . A correction level signal OS outputted from the correction level output unit  202  and the gain control signal G 1  are inputted to the addition unit  203 . And then, a signal strength signal SDC outputted from the addition unit  203  is inputted to the audio processing unit  8 , for example. 
       ===Operation of Receiving Apparatus=== 
       [0026]    Subsequently, there will be described an operation of the receiving apparatus according to an embodiment of the present invention. 
         [0027]    The antenna  1  receives a broadcast wave of the FM radio broadcasting or AM radio broadcasting, for example, to output an RF signal. The RF amplification unit  2  selectively amplifies a frequency band including a desired carrier wave in the RF signal, to be outputted as the RFa signal. The frequency conversion unit  3   a  frequency-converts the RFa signal, rejects an interference signal (interference signal) as appropriate, such as an image signal or an adjacent interference signal, using a BPF (Band-Pass Filter) or the like, and then outputs the IF 1  signal. As the intermediate frequency of the IF 1  signal, a frequency of 10.7 MHz or 455 kHz is used, for example. The IF 1  signal is outputted as an analog signal or a digital signal according to a configuration of the IF amplification unit  6  and subsequent stages. 
         [0028]    The IF amplification unit  6  amplifies the IF 1  signal by a gain corresponding to the gain control signal G 1 , to be outputted as the IF 1   a  signal. The demodulation unit  7  demodulates the IF 1   a  signal, to output the AF signal. Moreover, the audio processing unit  8  controls volume and sound quality of the AF signal according to the signal strength signal SDC, to be outputted as the AFa signal. For example, the audio processing unit  8  includes a stereo demodulation unit for demodulating the AF signal into a stereo signal with stereo separation (degree of separation) according to the signal strength signal SDC, and an LPF (Low-Pass Filter) for eliminating from the AF signal a component of a cutoff frequency or more according to the signal strength signal SDC. The speaker  9  converts the AFa signal to sound, to be outputted. 
         [0029]    The amplitude level detection unit  111  of the AGC unit  11  detects the amplitude level L 1  (hereinafter, referred to as a demodulation unit input level L 1 ) of the IF 1   a  signal to be inputted to the demodulation unit  7  using an envelope detection circuit, for example. The comparison unit  112  compares the demodulation unit input level L 1  with the reference level Ref 1 , to output the comparison result signal CP 1  indicating a difference between the two levels. The smoothing processing unit  113  smoothes the comparison result signal CP 1  using an integration circuit (LPF), for example, to generate the gain control signal G 1 . 
         [0030]    Here, the gain control signal G 1  has a waveform, which is inverted as appropriate according to a gain control method in the IF amplification unit  6 , however, in the following description, it is assumed as an example that the IF amplification unit  6  includes a fixed gain amplifier and a variable attenuator, and the higher the level of the gain control signal G 1  is, the greater an attenuation amount of the variable attenuator becomes (the smaller the gain of the IF amplification unit  6  becomes). In this case, the comparison result signal CP 1  has a level of L 1 -Ref 1 , and a relationship between the demodulation unit input level L 1  and the gain control signal G 1  is as illustrated in  FIG. 6 . 
         [0031]    The correction level output unit  202  of the signal strength detection unit  20   a  outputs a correction level signal OS corresponding to a difference between the demodulation unit input level L 1  detected by the amplitude level detection unit  111  and the reference level Ref 1  inputted to the comparison unit  112 . Here, there is illustrated in  FIG. 2  an example of the relationship between the demodulation unit input level L 1  and the correction level signal OS. As illustrated in  FIG. 2 , the correction level signal OS (solid line) has such a waveform as to be displaced from the demodulation unit input level L 1  (short broken line) by the reference level Ref 1 . As in the case of  FIG. 6 , the reference level Ref 1  is a negative level in  FIG. 2 . 
         [0032]    The addition unit  203  adds the gain control signal G 1  and the correction level signal OS, to output the signal strength signal SDC. Here, there is illustrated in  FIG. 3  an example of the relationship between the input signal and the output signal in the addition unit  203 . As illustrated in  FIG. 3 , the correction level signal OS (long broken line) is added to the gain control signal G 1  (short broken line), so that the signal strength signal SDC (solid line) becomes stable at the time of t=t 2 (&lt;t 1 ). 
         [0033]    As such, a signal obtained by adding the correction level signal OS to the gain control signal G 1  is used as the signal strength signal SDC indicating the received signal strength of the RF signal in the antenna  1 , and thus, a time required for the signal strength signal SDC to become stable can be reduced, as compared with a case where the gain control signal G 1  is used as it is. 
       Second Embodiment 
     Configuration of Receiving Apparatus 
       [0034]    There will hereinafter be described a configuration of a receiving apparatus including a signal strength detection unit according to a second embodiment of the present invention, referring to  FIG. 4 . 
         [0035]    The receiving apparatus illustrated in  FIG. 4  includes a signal strength detection unit  20   b  in place of the signal strength detection unit  20   a  in the receiving apparatus according to a first embodiment of the present invention. 
         [0036]    The signal strength detection unit  20   b  includes an amplitude level detection unit  201 , a correction level output unit  202 , and an addition unit  203 , for example. The IF 1   a  signal is inputted to the amplitude level detection unit  201 , while an output level L 1  of the amplitude level detection unit  201  and the reference level Ref 1  are inputted to the correction level output unit  202 . The correction level signal OS outputted from the correction level output unit  202  and the gain control signal G 1  are inputted to the addition unit  203 . The signal strength signal SDC outputted from the addition unit  203  is inputted to the audio processing unit  8 , for example. 
       ===Operation of Receiving Apparatus=== 
       [0037]    Subsequently, there will be described an operation of the receiving apparatus according to an embodiment of the present invention. 
         [0038]    The amplitude level detection unit  201  of the signal strength detection unit  20   b  detects the demodulation unit input level L 1  as in the case of the amplitude level detection unit  111  illustrated in  FIG. 1 . The correction level output unit  202  outputs the correction level signal OS corresponding to a difference between the demodulation unit input level L 1  detected by the amplitude level detection unit  201  and the reference level Ref 1  inputted to the AGC unit  11 . Moreover, the addition unit  203  adds the gain control signal G 1  and the correction level signal OS, to output the signal strength signal SDC. 
         [0039]    As is obvious from the above, in the receiving apparatus according to a first embodiment of the present invention, the demodulation unit input level L 1  detected in the AGC unit  11  is inputted to the signal strength detection unit  20   a , while in the receiving apparatus according to an embodiment of the present invention, the demodulation unit input level L 1  is detected in the signal strength detection unit  20   b . Therefore, the receiving apparatus according to an embodiment of the present invention can be applied even to a case where the AGC unit  11  does not have a function of outputting the demodulation unit input level L 1 . In the receiving apparatus according to a first embodiment of the present invention, it is not required that the signal strength detection unit is provided with an amplitude level detection unit independently, and thus, a circuit scale can be reduced. 
       Third Embodiment 
     Configuration of Receiving Apparatus 
       [0040]    There will hereinafter be described a configuration of a receiving apparatus including a signal strength detection unit according to a third embodiment of the present invention, referring to  FIG. 5 . 
         [0041]    The receiving apparatus illustrated in  FIG. 5  is configured as a double superheterodyne receiving apparatus, which includes the antenna  1 , the RF amplification unit  2 , frequency conversion units  3   b  and  5 , the IF amplification units  4  and  6 , the demodulation unit  7 , the audio processing unit  8 , the speaker  9 , the AGC units  11  to  13 , and a signal strength detection unit  20   c.    
         [0042]    The RF signal outputted from the antenna  1  is inputted to the RF amplification unit  2 , and the RFa signal outputted from the RF amplification unit  2  is inputted to the frequency conversion unit  3   b . An IF 2  signal outputted from the frequency conversion unit  3   b  is inputted to the IF amplification unit  4  and the AGC unit  13 , and an IF 2   a  signal outputted from the IF amplification unit  4  is inputted to the frequency conversion unit  5 . On the other hand, a gain control signal G 3  outputted from the AGC unit  13  is inputted to the RF amplification unit  2  and the signal strength detection unit  20   c.    
         [0043]    Moreover, the IF 1  signal outputted from the frequency conversion unit  5  is inputted to the IF amplification unit  6  and the AGC unit  12 , and the IF 1   a  signal outputted from the IF amplification unit  6  is inputted to the demodulation unit  7  and the AGC unit  11 . On the other hand, a gain control signal G 2  outputted from the AGC unit  12  is inputted to the IF amplification unit  4  and the signal strength detection unit  20   c , and the gain control signal G 1  outputted from the AGC unit  11  is inputted to the IF amplification unit  6  and the signal strength detection unit  20   c.    
         [0044]    The AF signal outputted from the demodulation unit  7  is inputted to the audio processing unit  8  and the AFa signal outputted from the audio processing unit  8  is inputted to the speaker  9 . 
         [0045]    The signal strength detection unit  20   c  includes the correction level output unit  202  and the addition unit  203 , for example. As is the case with the signal strength detection unit  20   a  according to a first embodiment of the present invention, the demodulation unit input level L 1  and the reference level 
         [0046]    Ref 1  are inputted to the correction level output unit  202 . The correction level signal OS outputted from the correction level output unit  202  and the gain control signals G 1  to G 3  are inputted to the addition unit  203 . Then, the signal strength signal SDC outputted from the addition unit  203  is inputted to the audio processing unit  8 , for example. 
       ===Operation of Receiving Apparatus=== 
       [0047]    Subsequently, there will be described an operation of the receiving apparatus according to an embodiment of the present invention. 
         [0048]    The antenna  1  outputs the RF signal as in the case of the receiving apparatus according to a first embodiment of the present invention, and the RF amplification unit  2  selectively amplifies a frequency band including a desired carrier wave in the RF signal by a gain corresponding to the gain control signal 
         [0049]    G 3 , to be outputted as the RFa signal. The frequency conversion unit  3   b  frequency-converts the RFa signal, rejects an interference signal, as appropriate, such as an image signal or an adjacent interference signal using the BPF or the like, and then outputs the IF 2  signal. Moreover, the IF amplification unit  4  amplifies the IF 2  signal by a gain corresponding to the gain control signal G 2 , to be outputted as the IF 2   a  signal. The frequency conversion unit  5  further frequency-converts the IF 2   a  signal, to output the IF 1  signal. The (intermediate) frequency of the IF 2  signal is set at a frequency at least higher than the intermediate frequency of the IF 1  signal. Particularly, in order to reliably reject the image signal, the frequency may be set at a frequency higher than the radio frequency of the RF signal. 
         [0050]    The IF amplification unit  6  amplifies the IF 1  signal by a gain corresponding to the gain control signal G 1 , to be outputted as the IF 1   a  signal. The demodulation unit  7  demodulates the IF 1   a  signal, to output the AF signal. Moreover, the audio processing unit  8  controls volume and sound quality of the AF signal according to the signal strength signal SDC, to be outputted as the AFa signal. Then, the speaker  9  converts the AFa signal to sound, to be outputted. 
         [0051]    The AGC unit  13  has a configuration similar to that of the AGC unit  11  illustrated in  FIG. 1 , for example, and smoothes a difference between the amplitude level of the IF 2  signal and a reference level Ref 3 , to generate the gain control signal G 3 . Similarly, the AGC unit  12  smoothes a difference between the amplitude level of the IF 1  signal and the reference level Ref 2 , to generate the gain control signal G 2 . Moreover, similarly, the AGC unit  11  smoothes a difference between the amplitude level (demodulation unit input level L 1 ) of the IF 1   a  signal and the reference level Ref 1 , to generate the gain control signal G 1 . 
         [0052]    Here, the gain control signals G 1  to G 3  have waveforms, which are inverted as appropriate according to gain control methods in the RF amplification unit  2  and the IF amplification units  4  and  6 , however, in the following description, it is assumed as an example that the higher the levels of the gain control signals are, the smaller the gains of the respectively corresponding amplification units become. Moreover, it is assumed that the relationships between the levels of the gain control signals and the gains of the amplification units are substantially equivalent. In this case, the sum of the levels of the gain control signals indicates the total gain of the amplification units, and the sum of the three gain control signals G 1  to G 3  in the receiving apparatus according to an embodiment of the present invention can be considered as with the single gain control signal G 1  in the receiving apparatus according to a first embodiment of the present invention. Even if the relationship between the levels of the gain control signals and the gains of the amplification units is different from the above, the gain control signals are multiplied by an positive or negative coefficient as appropriate, to be inputted to the signal strength detection unit  20   c , so that the sum of the levels of the gain control signals indicates the total gain of the amplification units, and the sum of the three gain control signals G 1  to G 3  in the receiving apparatus according to an embodiment of the present invention can be considered as with the single gain control signal G 1  in the receiving apparatus according to a first embodiment of the present invention. 
         [0053]    As is the case with the signal strength detection unit  20   a  according to a first embodiment of the present invention, the correction level output unit  202  of the signal strength detection unit  20   c  outputs the correction level signal OS corresponding to a difference between the demodulation unit input level L 1  detected in the AGC unit  11  and the reference level Ref 1  inputted to the AGC unit  11 . The addition unit  203  adds the sum of the gain control signals G 1  to G 3  and the correction level signal OS, to output the signal strength signal SDC. 
         [0054]    As such, in the receiving apparatus according to an embodiment of the present invention including a plurality of the AGC units, a signal obtained by adding the correction level signal OS to the sum of the gain control signals G 1  to G 3  is used as the signal strength signal SDC indicating the received signal strength of the RF signal in the antenna  1 , and thus, the time required for the signal strength signal SDC to become stable can be reduced, as compared with a case where the sum of the gain control signals G 1  to G 3  is used as it is. 
         [0055]    As mentioned above, as the signal strength signal SDC indicating the received signal strength of the RF signal in the antenna  1 , a signal obtained by adding to the gain control signal G 1  the correction level signal OS corresponding to a difference between the demodulation unit input level L 1  and the reference level Ref 1  is used, so that the time required for the signal strength signal SDC to become stable can be reduced, as compared with the case where the gain control signal G 1  is used as it is. 
         [0056]    Moreover, the demodulation unit input level L 1  is inputted to the correction level output unit  202  from the amplitude level detection unit  111  of the AGC unit  11 , and thus, it is not required that the signal strength detection unit is provided with the amplitude level detection unit independently, so that the circuit scale can be reduced. 
         [0057]    Furthermore, even if the receiving apparatus includes the plurality of AGC units, since a signal obtained by adding the correction level signal OS to the sum of the gain control signals G 1  to G 3  is used as the signal strength signal SDC, the time required for the signal strength signal SDC to become stable can be reduced, as compared with the case where the sum of the gain control signals G 1  to G 3  is used as it is. 
         [0058]    Furthermore, even if the receiving apparatus includes the plurality of AGC units, since the demodulation unit input level L 1  is inputted to the correction level output unit  202  from the AGC unit  11 , which allows the IF 1   a  signal to be inputted thereto as well as the demodulation unit  7 , it is not required that the signal strength detection unit is provided with the amplitude level detection unit independently, and thus, the circuit scale can be reduced. 
         [0059]    Furthermore, the audio processing unit  8  demodulates the AF signal into a stereo signal with stereo separation according to the signal strength signal SDC, and thus, specific noise of the stereo signal in a state of a weak electric field with low received signal strength can be reduced. 
         [0060]    Furthermore, since the audio processing unit  8  eliminates from the AF signal a component at the cutoff frequency or more according to the signal strength signal SDC, the noise of a high frequency in the sate of the weak electric field can be suppressed and the ratio of an effective signal to noise can be improved. 
         [0061]    In embodiments of the present invention described above, the receiving apparatus is configured as a (single) superheterodyne receiving apparatus including a single AGC unit or a double superheterodyne receiving apparatus including three AGC units, however, this is not limitative. The receiving apparatus is only required to include one or more AGC units in a stage prior to the demodulation unit  7 , and the number of AGC units and the number of stages of intermediate frequency conversion is not limited. 
         [0062]    In embodiments of the present invention described above, the receiving apparatus controls volume and sound quality of the AF signal according to the signal strength signal SDC in the audio processing unit  8 , however, this is not limitative. The receiving apparatus may include a detection unit for detecting other reception statuses such as presence or absence of an adjacent interference signal or a multipass interference signal. Furthermore, each amplification unit may include a filter such as a BPF whose passband is changed according to a combination of these reception statuses. 
         [0063]    The above embodiments of the present invention are simply for facilitating the understanding of the present invention and are not in any way to be construed as limiting the present invention. The present invention may variously be changed or altered without departing from its spirit and encompass equivalents thereof.

Technology Classification (CPC): 7