Abstract:
A center frequency F 0  of an IF filter is effectively adjusted. The IF filter filters a down-converted signal centering around the center frequency F 0 . A pseudo sine wave generation circuit generates a pseudo sine wave having a level change of at least two steps respectively on both positive and negative sides. The pseudo sine wave is made to pass through the IF filter by a switch circuit, and in the state, an F 0  adjustment circuit adjusts the center frequency F 0  in the IF filter  14  by comparing a phase of the pseudo sine wave with a phase of a signal after passing through the IF filter.

Description:
FIELD OF THE INVENTION 
     The present disclosure relates to a tuner which down-converts an RF signal to an IF signal and performs signal processing. 
     BACKGROUND OF THE INVENTION 
     In a radio tuner or the like, by mixing a local signal separated by an intermediate frequency (IF) from a desired station frequency by a mixer with an RF signal for which an arriving radio wave is received by an antenna, the RF signal of a desired station is down-converted to an IF signal, and the obtained IF signal is filtered in an IF filter to take out a desired station signal. 
     Then, the IF filter is requested to appropriately take out the desired station signal. Thus, it is desired to accurately set a passing band of the IF filter, and a center frequency F 0  of the IF filter is adjusted. Although a ceramic filter whose passing band is accurate is also used, it cannot be incorporated in an IC and is externally attached. 
     Conventionally, in F 0  adjustment of the IF filter, an adjustment method utilizing a replica oscillation circuit is used. In the adjustment method, a replica circuit simulating an IF filter circuit in a tuner is oscillated and the oscillation frequency is counted by a frequency counter. Then, by adjusting the oscillation frequency of the replica oscillation circuit to be an F 0  value and reflecting the adjustment result on the F 0  adjustment of the IF filter, the IF filter is adjusted. 
     RELATED ART 
     
         
         Japanese Patent Laid-Open Publication No. 2009-105727 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved 
     In the above-described adjustment method of the IF filter, since the circuit simulating the IF filter is oscillated and adjustment is made on the basis of the oscillation frequency and an actual IF filter circuit is not utilized, an error tends to occur. 
     Also, the tuner is provided with an S meter circuit which detects a level of an output signals of the IF filter. For the S meter, a signal is inputted from the outside to a radio wave reception part of the tuner, and detection output of the S meter is adjusted such that an output result of the S meter at the time becomes a certain specific value. In such an adjustment method, there are problems of requiring an environment for external adjustment to be constructed and of increased man-hours due to the external adjustment. 
     Means for Solving the Problems 
     In the present invention, a tuner which down-converts an RF signal to an IF signal and performs signal processing includes an IF filter which filters the down-converted signal centering around a center frequency F 0  to take out the IF signal, a pseudo sine wave generation circuit which generates a pseudo sine wave having a level change of at least two steps respectively on both positive and negative sides, a switch circuit which supplies the pseudo sine wave to the IF filter instead of the RF signal, and an F 0  adjustment circuit which adjusts the center frequency F 0  of the IF filter, and in the tuner, the pseudo sine wave is made to pass through the IF filter by the switch circuit, and in this state, the F 0  adjustment circuit compares a phase of the pseudo sine wave with a phase of the signal after passing through the IF filter, thereby adjusting the center frequency F 0  in the IF filter. 
     Further, preferably, an S meter which detects a signal level of output of the IF filter and an S meter adjustment circuit which adjusts output of the S meter are provided, the pseudo sine wave is made to pass through the IF filter by the switch circuit, and in this state, a detection level of the S meter is adjusted depending on a level of an output signal of the S meter circuit. 
     According to the present invention, by making a signal pass through the IF filter and performing adjustment work, adjustment accuracy may be improved. Also, by adjusting level detection of the S meter by an internally generated signal, dispersion of the S meter may be adjusted without input of an external signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a main section of a tuner; 
         FIG. 2  is a view describing a waveform of a pseudo sine wave; 
         FIG. 3  is a conceptual diagram illustrating F 0  adjustment; 
         FIG. 4  is a view illustrating a configuration of an F 0  adjustment circuit; 
         FIG. 5  is a conceptual diagram illustrating S meter adjustment; and 
         FIG. 6  is a view describing detection level characteristics of an S meter. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, the embodiment of the present invention will be described on the basis of drawings. 
       FIG. 1  is a block diagram illustrating a main section of a tuner. An RF signal received by an antenna is supplied to a mixer  10 . To the mixer  10 , a local signal separated from a desired station signal by a frequency of an IF signal is supplied, and the IF signal before filtering, for which the desired station signal is down-converted to the frequency of the IF signal, is obtained. The IF signal is amplified as prescribed in an IF amplifier  12  and then supplied to an IF filter  14 . 
     Then, in the IF filter  14 , the desired station signal around the center frequency of the IF signals is extracted, and it is supplied to a subsequent stage, demodulated, and reproduced. 
     Also, the IF signal after filtering, which is output of the IF filter  14 , is supplied to an S meter  16 . The S meter detects a level of the IF signal after filtering, and outputs a detection result as S meter output. The S meter output indicates a reception strength level of the desired station signal, and is utilized in stereo/monaural switching control or the like. 
     In the present embodiment, a pseudo sine wave generation circuit  20  is provided, and a pseudo sine wave is generated there. Then, a switch  22  is provided between the IF amplifier  12  and the mixer  10 . By switching the switch  22 , output of the mixer  10  or the pseudo sine wave from the pseudo sine wave generation circuit  20  can be switched and inputted to the IF amplifier  12 . 
     In the present embodiment, in the state that the switch  22  is switched and the pseudo sine wave is supplied to the IF amplifier  12 , the center frequency F 0  of the IF filter  14  is adjusted and a detection level of the S meter  16  is adjusted. 
     The IF signal after filtering, which is the output of the IF filter  14 , is supplied to an F 0  adjustment circuit  24 . To the F 0  adjustment circuit  24 , the pseudo sine wave which is an input signal to the IF amplifier  12  is also supplied. The F 0  adjustment circuit prepares an F 0  adjustment signal for adjusting the center frequency F 0  of the IF filter  14  from comparison between the IF signal after filtering and the pseudo sine wave. The F 0  adjustment signal is supplied to the IF filter  14 , and the bandpass center frequency F 0  of the IF filter is adjusted. 
     Also, the S meter output is supplied to an S meter adjustment circuit  26  as well. Ideally, the S meter output would be a prescribed value determined beforehand when the pseudo sine wave is inputted, but an error is caused by variation. The S meter adjustment circuit  26  generates a signal for S meter adjustment. According to the signal for S meter adjustment, the detection level in the S meter  16  is adjusted. That is, the detection level of the S meter  16  is adjusted such that the S meter output when the pseudo sine wave is inputted becomes the prescribed value. 
     A circuit of the tuner in the present embodiment is built inside a semiconductor integrated circuit (IC or LSI). Thus, the IF filter  14  is not a ceramic filter, and requires adjustment for optimizing filtering characteristics. The IF filter  14  may be active or passive. 
     &lt;Pseudo Sine Wave&gt; 
     The pseudo sine wave generated in the pseudo sine wave generation circuit  20  will be described here. The pseudo sine wave is the one illustrated in  FIG. 2(   c ) for instance, is not just a rectangular wave, and has a level change of two steps on one side at least. Such a pseudo sine wave is generated by composition of rectangular wave signals by a digital circuit. For instance, by adding a rectangular wave in  FIG. 2(   a ) and a rectangular wave in  FIG. 2(   b ) prepared using the same clock frequency, the pseudo sine wave with a two-step level change on one side in  FIG. 2(   c ) is generated. Digital data obtained by such a logical operation may be supplied to the IF amplifier  12  as an analog pseudo sine wave by a DAC. 
     &lt;Adjustment of F 0 &gt; 
     In F 0  adjustment of the IF filter  14 , the switch  22  is switched, and the pseudo sine wave generated in the pseudo sine wave generation circuit  20  is inputted to the IF amplifier  12  and the F 0  adjustment circuit  24 . In the F 0  adjustment circuit  24 , phases are compared with the output of the IF filter  14 , and the center frequency F 0  of the IF filter  14  is adjusted on the basis of the comparison. 
     A conceptual diagram for adjustment of the center frequency F 0  of the IF filter  14  is illustrated in  FIG. 3 . A mode of inputting the RF signal is switched to a mode of inputting the pseudo sine wave by the switch  22 . Thus, the pseudo sine wave is inputted to the IF amplifier  12 . As a result, for the output of the IF filter  14 , the signal which passes through the IF amplifier  12  and the IF filter  14  is outputted. For the signal, a change part is made dull by filtering and the signal is turned to a waveform close to a sine wave. 
     Then, the pseudo sine wave and the output signal of the IF filter  14  are inputted to the F 0  adjustment circuit  24 . The F 0  adjustment circuit  24  adjusts the bandpass center frequency F 0  of the IF filter  14 . 
     A configuration example of the F 0  adjustment circuit  24  is illustrated in  FIG. 4 . In this example, the F 0  adjustment circuit  24  includes a phase comparator  242 , a logic circuit  244 , and a DAC  246  for F 0  adjustment. Then, the pseudo sine wave to be a reference and an output signal after passing through the IF filter are inputted to the phase comparator  242 , and phases of both are compared there. That is, when frequencies of both signals are the same, the phases should be coincident, and a signal for the phase comparison result is supplied to the logic circuit  244 . The logic circuit  244  rewrites the DAC  246  for F 0  adjustment on the basis of the phase comparison result (phase shift). Thus, the DAC  246  for the F 0  adjustment circuit supplies the F 0  adjustment signal according to a rewritten value to the IF filter  14 . Thus, according to the phase comparison result, the center frequency F 0  of the IF filter is changed. The adjustment is repeated to drive the center frequency F 0  of the IF filter  14  to be an optimum value, and the center frequency F 0  of the IF filter  14  can be set to the optimum value finally. 
     While such an F 0  adjustment operation may be performed in a plant before shipping, it may be also performed every time power is turned on, readjustment may be made at desired timing on the basis of environmental information on temperature or the like, or readjustment may be made periodically. Also, final output of the DAC  246  for F 0  adjustment is preferably written in a nonvolatile memory such as an EEPROM. Then, preferably, a value stored in the nonvolatile memory is supplied to the DAC for F 0  adjustment normally and the value in the nonvolatile memory is rewritten when the above-described adjustment operation is performed. 
     Here, the pseudo sine wave generated in the pseudo sine wave generation circuit  20  is the one illustrated in  FIG. 2(   c ) for instance. It is not just a rectangular wave, and has a level change of two steps on one side at least. By using such a pseudo sine wave, a harmonic of a prescribed order is offset and the harmonic can be suppressed. When the rectangular wave is used, the harmonic becomes relatively large and thus an IF frequency tends to fluctuate. On the other hand, by using the pseudo sine wave, accurate F 0  adjustment can be made. Also, the number of levels may be increased further to be close to the sine wave preferably, but the waveform in  FIG. 2(   c ) is preferable in terms of easiness of generation. 
     &lt;Adjustment of S Meter&gt; 
     In detection level adjustment of the S meter  16  as well, the generated pseudo sine wave is inputted to a signal processing route. 
     A conceptual diagram for the detection level adjustment of the S meter  16  is illustrated in  FIG. 5 . First, the switch  22  is switched, and a mode of receiving the RF signal is switched to a mode of inputting the pseudo sine wave from the pseudo sine wave generation circuit  20  to the IF amplifier  12 . Thus, the pseudo sine wave is inputted to the IF amplifier  12 , and the signal for which the output is filtered in the IF filter  14  is obtained. 
     The signal after filtering is inputted to the S meter  16 , and a result of the level detection of the signal after passing through the IF filter  14  is outputted. For the pseudo sine wave, the signal level thereof is recognized beforehand. Then, the S meter adjustment circuit  26  adds adjustment such that the result of the level detection at the time becomes a certain fixed value. 
     For instance, it is assumed that a characteristic indicated by a solid line in  FIG. 6  is a correct characteristic. When a characteristic in the case of inputting the pseudo sine wave is a characteristic indicated by a broken line, the S meter adjustment circuit  26  generates the meter adjustment signal so as to turn it to the characteristic indicated by the solid line. 
     While such an adjustment operation of the S meter may be performed in a plant before shipping, it may be also performed every time power is turned on, readjustment may be made at desired timing on the basis of environmental information on temperature or the like, or readjustment may be made periodically. Also, preferably, final output of the S meter adjustment circuit  26  is written in a nonvolatile memory such as an EEPROM, a value stored in the nonvolatile memory is used normally, and the value is rewritten when the above-described adjustment operation is performed. 
     In such a manner, according to the present embodiment, as described above, the F 0  adjustment of the IF filter  14  and the detection level adjustment of the S meter  16  can be made. 
     In the F 0  adjustment of the IF filter  14 , compared to a conventional indirect adjustment method using a replica oscillation circuit, by making the signal pass through the actual IF filter  14  and performing adjustment work, accuracy can be improved. 
     Also, in the adjustment of the detection level of the S meter  16 , by adjusting the level detection of the S meter  16  by an internally generated signal, dispersion of the S meter  16  is adjusted without input of an external signal and dispersion of the S meter output is reduced. 
       10  Mixer,  12  Amplifier,  14  IF filter,  16  S meter,  20  Pseudo sine wave generation circuit,  22  Switch,  24  F 0  adjustment circuit,  26  S meter adjustment circuit,  242  Phase comparator,  244  Logic circuit,  246  DAC for F 0  adjustment.