Abstract:
A synthesizer receiver includes a variable bandpass filter having a variable width passband to which an intermediate frequency signal is supplied; a first detection circuit for detecting a level of an adjacent broadcast wave signal to enable supplying the detected output to the bandpass filter as a passband width control signal; a second detection circuit for detecting the received electric field intensity having a frequency counter for counting the frequency of the output signal of the variable bandpass filter; and a control circuit that checks the output detected by the second detection circuit during execution of a seek operation, controls the passband width of the variable bandpass filter to be switched to the wide band if the detected output of the second detection circuit indicates that the received electric field intensity is equal to or greater than a predetermined value, determines whether the detected output is tuned in to the broadcast wave signal based on the count value of the frequency counter, and stops the seek operation if the determination result indicates that the detected output is tuned in to the broadcast wave signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a synthesizer receiver. 
     2. Description of the Related Art 
     A synthesizer-type FM receiver is structured so that the received frequency (tuning frequency) is changed by changing the frequency dividing ratio of a variable frequency dividing circuit of phase locked loop (PLL). Therefore, the received frequency is sought (scanned) on FM broadcast band by changing the frequency dividing ratio N continuously. 
     When a broadcast wave signal is received by seeking, the seeking is stopped at the received frequency, the broadcast of this received frequency at the stop of seeking can be received continuously thereafter, and as the result the broadcast station is selected automatically. 
     If the pass band width of an intermediate frequency circuit of an FM receiver is narrowed, then the distortion factor or channel separation becomes poor and the sound quality becomes poor. To avoid the problem, the pass band width of the intermediate frequency circuit should be widened. 
     However, as shown in FIG. 3 with a solid line, in the case that the pass band width FIF of an intermediate frequency circuit is wide, for example, in the case that seeking is performed sequentially for lower to higher frequencies, the seeking stops at the frequency fRX which is, for example, not a broadcast wave signal when the seeking changes to a certain frequency fRX if a broadcast wave signal SNX of high received electric field intensity exists at the adjacent frequency (fRX+Δf) because the broadcast wave signal SNX is detected. 
     On the other hand, for example, as shown in FIG. 3 with a broken line, in the case that the pass band width FIF of an intermediate frequency circuit is narrow, the seeking does not stop sometimes at the frequency fRX even when the seeking reaches to the frequency fRX of the broadcast wave signal SRX. In other words, if the modulation is shallow at the broadcast wave signal SRX, the energy of the signal SRX is concentrated around the frequency fRX because the frequency spectrum component is concentrated around the frequency fRX. Therefore, the broadcast wave signal SRX can be detected sufficiently at that time even through the pass band width FIF of an intermediate frequency circuit is narrow, and the seeking can be stopped at the frequency fRX. 
     However, for example, as shown in FIG. 4 with a broken line, if the modulation is deep at the broadcast wave signal SRX, the energy of the signal SRX is dispersed because the frequency spectrum component is dispersed. As the result, the broadcast wave signal SRX cannot be detected at that time if the pass band width FIF of an intermediate frequency circuit is narrow, and as the result the seeking does not stop sometimes at the frequency fRX. 
     In summary, the seeking sometimes cannot be stopped at the frequency of a proper broadcast wave signal regardless of the magnitude of the pass band width FIF of an intermediate frequency circuit. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished to solve the above-mentioned problem. 
     For example, the present invention provides a synthesizer receiver, comprising: 
     a variable band pass filter having the variable pass band width to which an intermediate frequency signal is supplied; 
     a first detection circuit for detecting the level of an adjacent broadcast wave signal based on the output signal of the variable band pass filter, and enables supplying the detected output to the band pass filter as a control signal for controlling the pass band width; 
     a second detection circuit for detecting the received electric field intensity based on the output signal of the band pass filter; 
     a counter for counting the frequency of the output signal of the variable band pass filter; and 
     a control circuit to which an output signal according to the detected output of said second detection circuit and the count output of the counter is supplied, 
     wherein: 
     the control circuit performs processing comprising the steps of: 
     allowing the detection signal of the first detection circuit to control the pass band width of the variable band pass filter to thereby check the detected output of the second detection circuit during execution of the seek, 
     controlling the pass band width of the variable band pass filter to be switched to the wide band if the detected output of the second detection circuit indicates that the received electric field intensity is equal to or greater than a predetermined value as the checking result, and determining whether or not the detected output is tuned in to the broadcast wave signal based on the counter value of the counter, and 
     controlling the seeking to stop if the determination result indicates that the detected output is turned in to the broadcast wave signal. 
     Therefore, the seeking will not be stopped erroneously even if there is a broadcast wave signal having high received electric field at the adjacent frequency. In other words, the seeking is stopped correctly and automatically even if there is a broadcast wave signal having high electric field intensity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating one embodiment of the present invention. 
     FIG. 2 is a flowchart describing one embodiment of the present invention. 
     FIG. 3 is a characteristic diagram describing the present invention. 
     FIG. 4 is a characteristic diagram describing the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the character  10  denotes an FM receiver having the synthesizer-type and double super heterodyne-type structure. A received signal received from an antenna  11  is supplied to an electronic tuning-type antenna tuning circuit  12 , and a broadcast wave signal SRX having a target frequency fRX is extracted. 
     The signal SRX is supplied to a first mixer circuit  13 , an oscillation signal SL 01  having a frequency fL 01   
     
       
           fL   01 = fRX−fIF   1   (1)  
       
     
     where fIF 1  is the first intermediate frequency, for example, fIF 1 =10.7 MHz 
     is extracted from VCO  21 , the signal SL 01  is supplied to a mixer circuit  13  as the first local oscillation signal, and the signal SRX is subjected to frequency conversion and converted to a first intermediate frequency signal SIF 1  (first intermediate frequency fIF 1 ). 
     Subsequently, the first intermediate frequency signal SIF 1  is supplied to a first intermediate frequency circuit  14 . The first intermediate frequency circuit  14  having a wide pass band width of, for example, fIF 1 ±90 kHz comprises, for example, a ceramic filter and an amplifier that are connected in cascade arrangement fashion. 
     The first intermediate frequency signal SIF 1  sent out from the first intermediate frequency circuit  14  is supplied to a second mixer circuit  15 , and a divided-frequency signal SL 02  having a frequency fL 02   
     
       
           fL   02 = fIF   1 − fIF   2   (2)  
       
     
     where fIF 2  is the second intermediate frequency, for example, fIF 2 =450 kHz 
     is extracted from the frequency dividing circuit  27 , the signal SL 02  is supplied to a mixer circuit  15  as the second local oscillation signal, and the signal SIF 1  is subjected to frequency conversion and converted to a second intermediate frequency signal SIF 2  (second intermediate frequency fIF 2 ). 
     Subsequently, the second intermediate frequency signal SIF 2  is supplied to an FM demodulation circuit  18  through a variable band pass filter served for the second intermediate frequency circuit and a limiter amplifier  17 , an audio signal is subjected to FM demodulation at that time, and the audio signal is supplied to a terminal  19 . The characteristic of the variable band pass filter  16  will be described hereinafter. 
     At that time, voltage controlled oscillator (VCO)  21  constitutes PLL  20  together with circuits  22  to  24 . In other words, the signal SL 01  supplied from VCO  21  is supplied to the variable frequency dividing circuit  22  and divided into frequencies of 1/N, and the divided-frequency signal is supplied to a phase comparison circuit  23 . Furthermore, at that time, an oscillation signal having a stable frequency is taken out from a quartz oscillation circuit  25 , the oscillation signal is supplied to a frequency dividing circuit  26  to form a divided-frequency signal having a reference frequency Δf, for example, frequency of 100 kHz, and the divided-frequency signal is supplied to the comparison circuit  23  as the reference signal. 
     The comparison output of the comparison circuit  23  is supplied to VCO  21  through the low pass filter  24  as the control voltage. The output voltage of the filter  24  is supplied to the tuning circuit  12  as the tuning voltage, and the oscillation signal of the oscillation circuit  25  is supplied to a frequency dividing circuit  27  as the frequency dividing input. 
     Therefore, because the frequency of the divided-frequency signal supplied from the frequency dividing circuit  22  is equal to the frequency of the divided-frequency signal supplied from the frequency dividing circuit  26  in the steady state, the frequency fL 01  of the oscillation signal SL 01  is formulated as described herein under. 
     
       
           fL   01 = N×Δf   (3)  
       
     
     Based on the equations (1) and (3), 
     
       
           fRX=fL   01 + fIF   1 = N   x 0.1+10.7 (MHz)  
       
     
     Therefore, when the frequency dividing ratio is changed stepwise with step of 1 between 653 and 793, the local oscillation frequency fL 01  changes stepwise with step of 100 kHz between 65.3 MHz and 79.3 MHz, and the received frequency fRX changes stepwise with frequency step of 100 kHz (=Δf) over the frequency band ranging from 76 MHz to 90 MHz correspondingly to the frequency dividing ratio N. Therefore, the FM broadcast band is sought by changing the frequency dividing ratio N stepwise with step of 1 in the above-mentioned range. 
     A microcomputer  31  is provided for controlling the system, various key operations (operation switch)  32  are connected, and the frequency dividing ratio N is supplied from the microcomputer  31  to the variable frequency dividing circuit  22 . When the key  32  is operated, the microcomputer  31  functions to change the frequency dividing ratio N of the frequency dividing circuit  22  correspondingly to the operated key, and the received frequency fRX is changed thereby. 
     Furthermore, the microcomputer  31  is provided with, for example, a seeking control routine  100  as shown in FIG. 2 as a part of the program to be executed by a central processing unit (CPU), not shown in the drawing. The routine  100  is served to realize seeking and seeking stop in the present invention. Though the detail of the routine  100  will be described hereinafter, only the part of the routine  100  that relates to the present invention is shown in FIG.  2 . 
     Furthermore, the intermediate frequency signal SIF 2  sent out from the band pass filter  16  is supplied to a detection circuit  41 , a detection signal S 41  for indicating whether the level of the intermediate frequency signal SIF 1  is equal to or greater than a predetermined value or not, namely a detection signal S 41  for indicating whether the received electric field intensity of the received signal SRX is equal to or greater than a predetermined value or not, is extracted, and the detection signal S 41  is supplied to the microcomputer  31 . Furthermore, the intermediate frequency signal SIF 2  sent out from the limiter amplifier  17  is supplied to a waveform shaping circuit  42  so as to shape it to generate a pulse P 42 , and the pulse P 42  is supplied to the microcomputer  31 . 
     Furthermore, the demodulated output sent out from the demodulation circuit  18  is supplied to a band pass filter  43 . The band pass filter  43  is served to detect a signal (beat frequency component) having the frequency adjacent to the received frequency fRX being received at that time. To perform the detection, the pass band of the band pass filter  43  ranges over a predetermined frequency range having the center at 100 kHz (=Δf), for example, range from 50 kHz to 200 kHz. The filter output is supplied to the detection circuit  44 , and the level of the filter output is detected and extracted. 
     Therefore, for example, as shown in FIG. 3, when the frequency coincides with a certain received frequency fRX after seeking, if the pass bandwidth of the variable band pass filter  16  is wide (shown in FIG. 3 with a solid line) and a broadcast wave signal SNX having high received electric field intensity exists on the adjacent frequency (fRX+Δf), a part of the broadcast wave signal SNX is extracted from the band pass filter  43  as the filter output. 
     However, if the pass band width of the variable band pass filter  16  is narrow (for example, shown in FIG. 3 with a broken line), the output cannot be obtained from the band pass filter  43  even if a broadcast wave signal having high received electric field intensity exists on the adjacent frequency (fRX+Δf). Furthermore, even if the pass band width of the variable band pass filter  15  is wide, the output cannot be obtained from the band pass filter  43  if a broadcast wave signal having high electric field intensity does not exist. 
     Furthermore, at that time, the detection signal S 44  that indicates the output level of the band pass filter  43  is extracted from the detection circuit  44 . 
     The detection signal S 44  is supplied to the variable band pass filter  16  as a control signal for controlling the pass band width, and the pass band width is continuously changed correspondingly to the level of the detection signal S 44 . The pass band of the band pass filter  16  is controlled to be narrow, for example, fIF 2 ±10 kHz (for example, refer to the characteristic shown in FIG. 3 with a broken line) if the output of the filter  43  is large and the level of the detection signal S 44  is high, and on the other hand the pass band of the band pass filter  16  is controlled to be wide if the output of the filter  43  is small and the level of the detection signal S 44  is low, for example fIF 2 ±90 kHz (for example, the characteristic shown in FIG. 3 with a solid line). 
     Furthermore, a predetermined control signal S 31  is extracted from the microcomputer  31 , and the signal S 31  is supplied to the band pass filter  16 . In this case, the control signal S 31  sets any one of two modes described herein under to the pass band width of the variable band pass filter  16 . 
     Control by use of the detection signal S 44  is activated (referred to as “variable mode” hereinafter). 
     Control by use of the detection signal S 44  is inactivated, and wide band is set (referred to as “wide band mode” hereinafter). 
     In the structure as described hereinabove, when an operation that requires the seeking in the FM broadcast band, for example, automatic tuning, is indicated by means of key operation of the key  32 , the process of the CPU is started from step  101  of the routine  100  in the microcomputer  31 , the frequency dividing ratio N of the variable frequency dividing circuit  22  is set to the minimum value in the next step  102 . As the result, the received frequency fRX is set to the minimum frequency of 76 MHz. 
     Subsequently, whether the current received frequency fRX exceeds the maximum frequency 90 MHz or not is determined in step  103 , and the sequence proceeds from step  103  to step  104  because the routine  100  just started and the received frequency fRX is set to the minimum frequency 76 MHz in step  102  in this case. 
     The control signal S 31  functions to set the pass band width of the band pass filter  16  to the variable mode in step  104 , and in the next step the detection signal S 41  is checked to determine whether the received electric field strength at the current received frequency fRX is equal to or greater than a predetermined value or not. 
     If the received electric field intensity at the current received frequency fRX is lower than the predetermined value, then the sequence proceeds from step  105  to step  106 , and in step  106  the frequency dividing ratio N is incremented by 1 and the received frequency fRX is concomitantly incremented by 1 step, namely by 100 kHz, and the sequence thereafter returns to step  103 . Therefore, steps  103  to  106  are repeated until the broadcast having the received electric field intensity of the predetermined value is received. (Seeking state) 
     If the broadcast having the received electric field of the predetermined value is received, the value is determined in step  105 , and the sequence proceeds from step  105  to step  107 . The control signal S 31  functions to set the pass band width of the pass band filter  16  to the wide band mode in step  107 , and the number of pulse P 42  is thereafter counted to thereby determine whether the received frequency fRX tunes in to the transmission frequency (carrier frequency) of the broadcast wave signal or not in step  108 . 
     If the received frequency fRX does not tune in to the transmission frequency of the broadcast wave signal, then the sequence returns from step  108  to step  103  through step  106 . Therefore, thereafter, steps  103  to  108  are repeated until the received frequency fRX tunes in to the broadcast having the received electric field intensity equal to or greater than the predetermined value. 
     If the received frequency fRX tunes in to the broadcast having the received electric field intensity equal to or greater than the predetermined value, then the sequence proceeds from step  108  to step  111 , the control signal S 31  functions to set the pass band width of the band pass filter  16  to the variable mode in step  111 , and the routine  100  is thereafter brought to an end at step  112 . Therefore, at that time, the seeking is stopped, and a broadcast station is selected automatically. 
     If the received frequency fRX exceeds the maximum frequency before the broadcast having the received electric field intensity equal to or greater than the predetermined value is not tuned in spite of repeated steps  103  to  108 , then it is determined in step  103 , the sequence proceeds from step  103  to step  111 , and the routine  100  is brought to an end at step  112 . 
     According to the routine  100 , if the broadcast having the received electric field intensity equal to or greater than the predetermined value is tuned during the seeking, then the seeking is stopped at the received frequency fRX. In this case, because the pass band width of the band pass filter  16  used for selecting the second frequency signal SIF 2  is switched to the variable mode or wide band mode, the seeking will not be stopped or will not pass the frequency at which the seeking should be stopped erroneously. 
     In other words, in the case that the received electric field intensity is checked in step  105 , the pass band width of the band pass filter  16  is switched to the variable mode in step  104 . Therefore, as shown in FIG. 3, if there is a broadcast wave signal SNX having high received electric field intensity at the adjacent frequency (fRX+Δf) at a certain receiving frequency fRX, then the detection signal S 44  functions to narrow the pass band width of the band pass filter  16  as shown in FIG. 3 with a broken line. As the result, the broadcast wave signal SNX having high received electric field intensity of adjacent frequency (fRX+Δf) is removed from the detection signal S 41 . 
     Therefore, as shown in FIG. 3, even if there is a broadcast wave signal SNX having high received electric field intensity at the adjacent frequency (fRX+Δf), the broadcast wave signal SNX will not affect adversely on the step  104  and the received electric field intensity at the received frequency fRX is determined correctly, and as the result the seeking is not stopped erroneously at the received frequency fRX. 
     If there is no broadcast wave signal SNX having high received electric field intensity at the adjacent frequency (fRX+Δf) when the frequency comes to the received frequency fRX, the variable band pass filter  16  is switched to the wide band mode. Therefore, because the modulation of the broadcast wave signal SRX is deep, the frequency spectrum component is dispersed as shown in FIG. 4 with a broken line, and even if the energy of the broadcast wave signal SRX is dispersed, the detection signal S 41  indicates a correct received electric field intensity of the broadcast wave signal SRX, and as the result the seeking is stopped properly. 
     Furthermore, when whether the received frequency fRX tunes in to the transmission frequency of the broadcast wave signal SRX or not is checked by counting the number of the pulse P 42  in step  108 , because the pass band width of the band pass filter  16  is switched to the wide band mode in step  107 , the number of the pulse P 42  is counted without any error. 
     In detail, in the case that the pass band width of the band pass filter  16  is switched to the variable mode, if there is a broadcast wave signal SNX having high received electric field intensity at the adjacent frequency (fRX+Δf) when the frequency comes to a certain received frequency fRX, the detection signal S 44  functions to narrow the pass band width of the band pass filter  16  as shown in FIG. 3 with a broken line. As the result, the signal level at the received frequency fRX is lowered or reduced to 0, the number of the pulse P 42  is counted erroneously and the correct count value cannot be obtained. That is true in the case that the pass band width of the band pass filter  16  is fixed to the narrow band mode. 
     However, in the routine  100 , because the pass band width of the band pass filter  16  is switched to the wide band mode in step  107  and then the number of the pulse P 42  is counted, when there is a broadcast wave signal SNX having high received electric field intensity at the adjacent frequency (fRX+Δf) the pass band width of the band pass filter  16  is wide and the signal level at the received frequency fRX is high. Therefore, the number of the pulse P 42  is counted correctly. 
     As described hereinabove, according to the above-mentioned FM receiver, the seeking can be stopped correctly and automatically even if there is a broadcast wave signal SNX having high received electric field intensity. 
     The intermediate frequency signal SIF 2  sent out from the band pass filter  16  is supplied to the detection circuit  41  to thereby obtain the detection signal S 41  having the received electric field intensity in the above-mentioned embodiment, however, the intermediate frequency signal SIF 2  may be extracted out from the middle of the limiter amplifier  17  and supplied to the detection circuit  41  to thereby obtain the detection signal S 41 . 
     The control signal S 44  controls the band width so as to be switched by supplying the control signal S 31  to the variable band pass filter  16  in the above-mentioned embodiment, however, the control signal S 31  maybe supplied to the detection circuit  44  to control the band width so as to be switched similarly. Furthermore, for example, in the case that auto-preset is employed, in the step subsequent to step  111 , the data for indicating the received frequency fRX being received at that time, for example, the frequency dividing ratio N is stored in a nonvolatile memory, and then the sequence proceeds to step  108 .