Abstract:
An FM radio receiver comprises a radio frequency tuning circuit for selecting a frequency modulated signal from an antenna and a local oscillator tuning circuit for tuning to an oscillating signal so as to convert the frequency modulated signal to an intermediate frequency. In the radio frequency tuning circuit, a capacitance variable ratio is set so as to be capable of receiving continuously the frequencies of both a first FM receiving band used in Japan and a second FM receiving band used in the US and Europe. The local oscillator tuning circuit is so constructed that an inductance value and a capacitance variable ratio can be switched so that the receiver is set to lower heterodyne for reception in Japan or upper heterodyne for reception in the US or Europe. As a result, a single receiver design can be configured for reception of FM radio broadcasting in either Japan or the US or Europe using a common front-end circuit, without increasing the number of components.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an FM radio receiver containing a radio frequency tuning circuit for selecting a signal from an antenna and a local oscillator tuning circuit tunable to an oscillating signal for converting the frequency of a receiving signal to an intermediate frequency. 
     In such a kind of super-heterodyne FM radio receiver, a desired broadcast signal received at an antenna is converted to a frequency called an intermediate frequency (IF), which is then detected and demodulated. For FM reception, 10.7 MHz is normally used as this intermediate frequency. A local oscillator circuit is oscillated at a frequency always apart from the frequency of a desired signal (that is, receiving frequency) by 10.7 MHz. A 10.7 MHz IF signal is generated as a beat component as a result of mixing the oscillating signal of this local oscillator circuit and the received signal so as to convert the received signal to the intermediate frequency. At this conversion, if the local oscillator circuit is oscillated at a frequency higher by 10.7 MHz than the receiving frequency, it is called the upper heterodyne method. On the other hand, if the local oscillator circuit is oscillated at a frequency lower by 10.7 MHz than the receiving frequency, it is called the lower heterodyne method. 
     Whether an FM radio receiver is designed in the upper heterodyne method or lower heterodyne method is determined depending on the frequency allocation for FM radio broadcasting in each country and the presence of interference signals at image frequencies. In a super-heterodyne FM radio receiver, if an interference signal is located at the image frequency which is 10.7 MHz apart from a local oscillating frequency in the opposite direction to a receiving frequency, (i.e., 21.4 MHz apart from the receiving frequency,) image signals are mixed into the intermediate frequency signal resulting in interference. In Japan, the frequency allocation for FM radio broadcasting is 76-90 MHz, and channel 2 of TV broadcasting is allocated at frequencies 21.4 MHz higher than the FM band. In the USA, the frequency allocation for FM radio broadcasting is 88-108 MHz, and channels 4 to 6 of TV broadcasting are allocated at frequencies 21.4 MHz lower than the FM band. Accordingly, to avoid image frequency interference by strong TV broadcasting signals with a high electric field strength, the lower heterodyne method is used in Japan and the upper heterodyne method is used in the US (and also in Europe). 
     A conventional FM radio receiver will now be described referring to a car radio receiver with a front-end circuit as shown in FIG.  6 . 
     The front-end circuit of the conventional FM radio receiver comprises a radio frequency tuning circuit containing an antenna tuning circuit  61  and an RF tuning circuit  63 , an RF amplifier circuit  62 , a local oscillator circuit containing a local oscillator tuning circuit  64 , and a mixer circuit  65 . The antenna tuning circuit  61  comprises a tuning coil L 9 , a pair of variable capacitance diodes D 13 , D 14  which cathodes are connected to each other, and a capacitor C 11 . The RF tuning circuit  63  comprises a tuning coil L 10 , a pair of variable capacitance diodes D 15 , D 16  which cathodes are connected to each other, and a capacitor C 12 . The local oscillator tuning circuit  64  comprises an oscillator coil L 11 , a pair of variable capacitance diodes D 17 , D 18  which cathodes are connected to each other, and a capacitor C 13 . The variable capacitance diodes D 13 , D 14 , D 15 , D 16 , D 17 , D 18  have the same voltage versus capacitance characteristic. A common tuning voltage from a source  66  is applied to each pair of the variable capacitance diodes of the antenna tuning circuit  61 , RF tuning circuit  63 , and local oscillator tuning circuit  64 . Generally, this tuning voltage is set in a range of from 1 to 8 volts which can be supplied stably from a vehicle battery. 
     For an FM radio receiver which is intended for use in Japan, a value of each element is determined so that, within a tuning voltage range (e.g., 1-8 V) applied to the pair of the variable capacitance diodes of each tuning circuit, the antenna tuning circuit  61  and RF tuning circuit  63  tune to FM radio broadcasting signals of 76-90 MHz and the local oscillator tuning circuit  64  tunes to oscillating signals (i.e., 65.3-79.3 MHz) which are lower by 10.7 MHz than the tuning frequencies of the antenna tuning circuit  61  and RF tuning circuit  63 . 
     For an FM radio receiver which is intended for use in the US, a value of each element is determined so that, within a tuning voltage range (e.g., 1-8 V) applied to the pair of the variable capacitance diodes of each tuning circuit, the antenna tuning circuit  61  and RF tuning circuit  63  tune to FM radio broadcasting signals of 88-108 MHz and the local oscillator tuning circuit  64  tunes to oscillating signals (i.e., 98.7-118.7 MHz) which are higher by 10.7 MHz than the tuning frequencies of the antenna tuning circuit  61  and RF tuning circuit  63 . 
     For the FM radio receivers described above, the receiving bands are different between receivers for Japan and the US, and, moreover, the oscillating frequencies of the local oscillator circuits are entirely different because of a difference between upper heterodyne and lower heterodyne methods. Consequently, as shown in Table 1, the values of the coil elements and capacitance elements in the radio frequency tuning circuit and the local oscillator tuning circuit are significantly different for manufacturing the separate receivers. It is noted that, for both cases in Table 1, variable capacitance diodes used in each tuning circuit are those whose capacitance values change from 12 to 22.5 pF when the tuning voltage is changed from 1 to 8 V. 
     
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 radio frequency tuning 
                 local oscillator tuning 
               
               
                   
                 circuit 
                 circuit 
               
             
          
           
               
                   
                   
                 Capacitance 
                 Oscillator 
                 Capacitance 
               
               
                   
                 tuning coil 
                 element 
                 coil 
                 element 
               
               
                   
                 L9, L10 
                 C11, C12 
                 L11 
                 C13 
               
               
                   
                   
               
             
          
           
               
                 for Japan 
                 120.476 nH 
                 14.03 pF 
                 182.127 nH 
                 10.117 pF 
               
               
                 for US 
                 104.258 nH 
                  8.77 pF 
                  76.42 nH 
                 11.525 pF 
               
               
                   
               
             
          
         
       
     
     Therefore, conventional FM radio receivers need to be designed and manufactured separately for Japan and the US. Consequently, more design work is involved and the handling of components used in manufacturing the FM radio receivers becomes complicated since more components are required, thereby spoiling productivity. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an FM radio receiver wherein both a circuit necessary for receiving FM radio broadcasting signals in Japan and a circuit necessary for receiving FM radio broadcasting signals in US can be incorporated by means of a common front-end circuit without increasing the number of components, thereby preventing an increase of design work and a decline of productivity. 
     To achieve the above object, the present invention provides an FM radio receiver comprising a radio frequency tuning circuit for selecting a frequency modulated signal from an antenna and a local oscillator tuning circuit for tuning to an oscillating signal so as to convert the frequency of a receiving signal to an intermediate frequency. The radio frequency tuning circuit and the local oscillator tuning circuit each have variable capacitance diode elements. The variable capacitance diode elements of the radio frequency tuning circuit and variable capacitance diode element of the local oscillator tuning circuit are connected to a tuning control voltage source. The radio frequency tuning circuit has a capacitance variable ratio set so as to be capable of receiving continuously a first receiving band and a second receiving band. The local oscillator tuning circuit is so constructed that its inductance value and capacitance variable ratio can be switched according to the receiving band, so that the local oscillator circuit is set to lower heterodyne when the first receiving band is selected and to upper heterodyne when the second receiving band is selected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing a first embodiment of a front-end circuit of the FM radio receiver of the present invention. 
     FIG. 2 is a circuit diagram showing a second embodiment of a front-end circuit of the FM radio receiver of the present invention. 
     FIG. 3 is a circuit diagram showing a third embodiment of a front-end circuit of the FM radio receiver of the present invention. 
     FIG. 4 is a circuit diagram showing a fourth embodiment of a front-end circuit of the FM radio receiver of the present invention. 
     FIG. 5 is a characteristic diagram showing a tracking characteristic of the FM radio receiver of the present invention. 
     FIG. 6 is a circuit diagram of a front-end circuit of a conventional FM radio receiver. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The embodiments of an FM radio receiver according to the present invention will be described with reference to FIGS.  1 - 5 . 
     FIG. 1 is a circuit diagram showing a first embodiment of a front-end circuit of the FM radio receiver of the present invention. Reference numeral  11  denotes an antenna tuning circuit, numeral  12  denotes an RF amplifier circuit, numeral  13  denotes an RF tuning circuit, numeral  14  denotes a local oscillator tuning circuit and numeral  15  denotes a mixer circuit. The antenna tuning circuit  11  comprises a tuning coil L 1 , variable capacitance diodes D 1 , D 2  and a capacitor C 1 . The cathodes of the variable capacitance diodes D 1 , D 2  are connected to each other. The anode of variable capacitance diode D 1  is connected to one end of tuning coil L 1 , and the anode of variable capacitance diode D 2  is connected to the other end of tuning coil L 1 . A capacitor C 1  is connected in parallel to tuning coil L 1 . The RF tuning circuit  13  comprises a tuning coil L 2  having a tap T 2 , variable capacitance diodes D 3 , D 4 , and a capacitor C 2 . The cathodes of variable capacitance diodes D 3 , D 4  are connected to each other. The anode of variable capacitance diode D 3  is connected to one end of tuning coil L 2  and the anode of variable capacitance diode D 4  is connected to the other end of tuning coil L 2 . A capacitor C 2  is connected in parallel to the tuning coil L 2 . Tap T 2  of tuning coil L 2  is grounded. 
     The local oscillator tuning circuit  14  comprises an inductance switching circuit  16  and variable capacitance diodes D 5 , D 6 . Circuit  16  includes an oscillator coil L 3  with a coil L 3 A and a coil L 3 B formed by providing a tap T 3 . One end of oscillator coil L 3  is grounded through a switch SW 1  and the other end of oscillator coil L 3  is grounded through a switch SW 2 . The cathodes of variable capacitance diodes D 5 , D 6  are connected to each other. The anode of variable capacitance diode D 5  is connected to tap T 3  of oscillator coil L 3 , and the anode of variable capacitance diode D 6  is grounded. A switched capacitor circuit  17  for switching a capacitance variable ratio is connected to a junction of variable capacitance diode D 5  and tap T 3  of oscillator coil L 3 . The switching circuit  17  is formed by connecting the series circuit of a capacitor C 4  and a switch SW 3  in parallel with a capacitor C 3 . 
     Variable capacitance diodes D 1 , D 2 , D 3 , D 4 , D 5 , and D 6  have the same voltage versus capacitance characteristic. The cathodes of variable capacitance diodes D 1 , D 2  of the antenna tuning circuit  11 , the cathodes of the variable capacitance diodes D 3 , D 4  of the RF tuning circuit  13 , and the cathodes of the variable capacity diodes D 5 , D 6  of the local oscillator tuning circuit  14  are connected to a common tuning control voltage source  18 . 
     In this radio frequency tuning circuit, values of circuit elements are determined so as to be capable of covering from 76 MHz which is a lower limit of FM radio broadcasting band in Japan, to 108 MHz which is an upper limit of FM radio broadcasting band in the US. To cover this range, a variable ratio of capacitance necessary for the radio frequency tuning circuit is expressed in equation (1). 
     
       
         C max/C min=(F max/F min) 2   (1) 
       
     
     where C max is a maximum value of total capacitance of the radio frequency tuning circuit, C min is a minimum value of total capacitance of the radio frequency tuning circuit, F max is a maximum value of tuning frequency and F min is a minimum value of tuning frequency. 
     If the upper limit (that is, upper limit of US FM radio broadcasting band) of the tuning frequency and the lower limit (that is, lower limit of FM radio broadcasting band of Japan) of the tuning frequency are substituted in the equation (1), the variable ratio of the capacitance of the radio frequency tuning circuit is 2.02. Since this variable ratio of the capacitance of the radio frequency tuning circuit is calculated in terms of the total capacitance including a capacitance of a capacitance element connected to the tuning coil and any stray capacitance, the variable ratio of the variable capacitance diode is required to be larger than this variable ratio of the capacitance of the radio frequency tuning circuit, and is preferred to be more than about 2.5. 
     On the other hand, the local oscillator tuning circuit  14  is designed so as to be tuned at oscillating frequencies of 65.3-79.3 MHz for use in Japan and 98.7-118.7 MHz for use in the US. Therefore, two kinds of inductance values are switched in the inductance switching circuit  16  so as to be tuned to two discontinuous bands of oscillating signals. Further, in the local oscillator tuning circuit  14 , the ratio between the variable ratio of receiving frequency and variable ratio of oscillating frequency for receiver use in Japan is 1.025, while the ratio between the variable ratio of receiving frequency and variable ratio of oscillating frequency for receiver use in the US is 0.980. Therefore, two distinct capacitance values are switched in capacitance ratio switching circuit  17  to achieve that difference. 
     In this FM radio receiver, if the variable capacitance diodes having a capacitance variable ratio of about 3.0 for the tuning voltage of 1 to 8 V are used as D 1 , D 2 , D 3 , D 4 , D 5 , D 6 , then the preferred value of each circuit element is as shown in Table 2. 
     
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Radio frequency tuning 
                 Local oscillator tuning 
               
               
                   
                 circuit 
                 circuit 
               
             
          
           
               
                   
                 Tuning 
                 Capacitance 
                 Oscillator 
                 Capacitance 
               
               
                   
                 coil 
                 element 
                 coil L3 
                 element 
               
             
          
           
               
                   
                 L1, L2 
                 C1, C2 
                 L3A 
                 L3B 
                 C3 
                 C4 
               
               
                   
                   
               
             
          
           
               
                 for Japan 
                 75 nH 
                 13 pF 
                   
                 114 nH 
                 7 pF 
                   
               
               
                 for US 
                 74 nH 
                 13 pF 
                 55 nH 
                   
                 7 pF 
                 10 pF 
               
               
                   
               
             
          
         
       
     
     By being designed as above, the values of the tuning coil and capacitance element of the radio frequency tuning circuit can be substantially equal for manufacturing receivers for use in Japan and the US. Therefore, the components to be used for both can be made common. 
     On the other hand, the values of the oscillator coil and capacitance element of the local oscillator tuning circuit are quite different depending on receiver manufacture for Japan or the US. Therefore, switching of the values is conducted as described below. 
     For manufacturing the FM radio receiver for the US, in the local oscillator tuning circuit  14 , SW 1  of inductance switching circuit  16  and SW 3  of capacitance ratio switching circuit  17  are set ON and SW 2  of inductance switching circuit  16  is set OFF. Thus, local oscillator circuit  14  is tuned to oscillating signals of 98.7-118.7 MHz since coil L 3 A, variable capacitance diodes D 5 , D 6  and capacitors C 3 , C 4  are active. For manufacturing the FM receiver for use in Japan, in the local oscillator tuning circuit  14 , SW 1  of inductance switching circuit  16  and SW 3  of capacitance ratio switching circuit  17  are set OFF and SW 2  of inductance switching circuit  16  is set ON. Thus, local oscillator tuning circuit  14  is tuned to oscillating signals of 65.3-79.3 MHz since coil L 3 B, variable capacitance diodes D 5 , D 6 , and capacitor C 3  are active. 
     Such switching of the inductance and the capacitance variable ratio of the local oscillator tuning circuit is electronically controllable by driving switching elements such as transistors or diodes according to a band switch signal output from a micro-computer, so that selectable manufacturing for shipment to Japan or the US can be made easily in any case. 
     FIG. 2 is a circuit diagram showing a second embodiment of a front-end circuit of a FM radio receiver according to the present invention. Reference numeral  21  denotes an antenna tuning circuit, numeral  22  denotes an RF amplifier circuit, numeral  23  denotes an RF tuning circuit, numeral  24  denotes a local oscillator tuning circuit and numeral  25  denotes a mixer circuit. 
     The antenna tuning circuit  21  and RF tuning circuit  23  each contains a tuning coil, a pair of variable capacitance diodes, and a capacitance element. The values of the circuit elements are determined so as to be capable of covering from the lower limit of 76 MHz of FM radio broadcasting band in Japan to the upper limit of 108 MHz of FM radio broadcasting band in the US. 
     Local oscillator tuning circuit  24  comprises an oscillator coil L 4  having a tap T 4  and variable capacitance diodes D 7 , D 8 . In oscillator coil L 4 , one end thereof is grounded through a switch SW 4  and tap T 4  is also grounded through a switch SW 5 . Oscillator coil L 4  and switches SW 4 , SW 5  form an inductance switching circuit  26 . In this inductance switching circuit  26 , two inductance values are obtained according to a ratio of a number of windings between the other end of the oscillator coil L 4  and tap T 4  to a total number of windings of the oscillator coil L 4 . The cathodes of variable capacitance diodes D 7 , D 8  are connected to each other, an anode of variable capacitance diode D 7  is connected to the other end of the oscillation coil L 4 , and an anode of the variable capacitance diode D 8  is grounded. A capacitance ratio switching circuit  27  is connected to a junction of variable capacitance diode D 7  and oscillator coil L 4 . Capacitance ratio switching circuit  27  is formed by connecting a capacitor C 5  in parallel to a series circuit of capacitor C 6  and switch SW 6 . 
     The variable capacitance diodes of the antenna tuning circuit  21 , RF tuning circuit  23 , and local oscillator tuning circuit  24  have the same voltage versus capacitance characteristic. The cathodes of the variable capacitance diodes of the antenna tuning circuit  21 , the cathodes of the variable capacitance diodes of the RF tuning circuit  23 , and the cathodes of the variable capacitance diodes of local oscillator tuning circuit  24  are connected to a common tuning power supply. 
     In case of shipment of the FM radio receiver having such a structure to the US, in local oscillator tuning circuit  24 , switch SW 5  of inductance switching circuit  26  and switch SW 6  of capacitance ratio switching circuit  27  are set ON and switch SW 4  of inductance switching circuit  26  is set OFF. Therefore, active components in local oscillator tuning circuit  24  consist of the windings between the other end of oscillator coil L 4  and tap T 4 , variable capacitance diodes D 7 , D 8 , and capacitors C 5 , C 6 . In case of shipment to Japan, in local oscillator tuning circuit  24 , switch SW 5  of inductance switching circuit  26  and switch SW 6  of capacitance ratio switching circuit  27  are set OFF and switch SW 4  of inductance switching circuit  26  is set ON. Therefore, active components in local oscillator tuning circuit  24  consist of oscillator coil L 4 , variable capacitance diodes D 7 , D 8 , and capacitor C 5 . 
     FIG. 3 shows a circuit diagram showing a third embodiment of a front-end circuit of the FM radio receiver according to the present invention. In the antenna tuning circuit and RF tuning circuit, the value of each circuit element is determined so as to be capable of receiving frequency modulated signals of 76 MHz-108 MHZ. 
     A local oscillator tuning circuit  34  comprises oscillator coils L 5 , L 6  and a pair of variable capacitance diodes D 9 , D 10 . In an oscillator coil L 5 , one end thereof is grounded through a switch SW 7  and the other end is connected to the one end of an oscillator coil L 6 . The other end of the oscillator coil L 6  is grounded through a switch SW 8 . An inductance switching circuit  36  is thus comprised of oscillator coils L 5 , L 6  and switches SW 7 , SW 8 . An anode of variable capacitance diode D 9  is connected to a junction of oscillator coil L 5  and oscillator coil L 6 . An anode of variable capacitance diode D 10  is grounded. A capacitance ratio switching circuit  37  is connected to a junction of variable capacitance diode D 9  and inductance switching circuit  36 . The capacitance ratio switching circuit  37  is formed by connecting a capacitor C 7  and capacitor C 8  in series while connecting a switch SW 9  in parallel to capacitor C 8 . 
     The variable capacitance diodes of the antenna tuning circuit, the RF tuning circuit, and local oscillator tuning circuit  34  have the same voltage versus capacitance characteristic and are connected to a common tuning control voltage source. 
     In case of shipment of the FM radio receiver having such a structure to the US, in local oscillator tuning circuit  34 , switch SW 7  of inductance switching circuit  36  and switch SW 9  of capacitance ratio switching circuit  37  are set ON and switch SW 8  of inductance switching circuit  36  is set OFF. In case of shipment to Japan, switch SW 7  of inductance switching circuit  36  and switch SW 9  of capacitance ratio switching circuit  37  are set OFF and switch SW 8  of inductance switching circuit  36  is set ON. Therefore, in case of shipment to US, the active components in local oscillator tuning circuit  34  consist of oscillator coil L 5 , variable capacitance diodes D 9 , D 10 , and capacitor C 7 . In case of shipment to Japan, the active components in local oscillator tuning circuit  34  consist of oscillator coil L 6 , variable capacitance diodes D 9 , D 10 , and capacitors C 7 , C 8 . 
     It is noted that oscillator coils L 5  and L 6  may be formed as a single component in which a winding of oscillator coil L 5  and a winding of oscillator coil L 6  are wound on a common magnetic core. They may also be formed separately. 
     FIG. 4 is a circuit diagram showing a fourth embodiment of the front-end circuit of the FM radio receiver according to the present invention. The component values of the radio frequency tuning circuit are determined so as to be capable of tuning to frequency modulated signals of 76 MHz-108 MHz. 
     A local oscillator tuning circuit  44  comprises oscillator coils L 7 , L 8  connected in series and a pair of variable capacitance diodes D 11 , D 12  connected in series. 
     One end of oscillator coil L 8  is grounded and a junction of oscillator coil L 7  and oscillator coil L 8  is grounded through a switch SW 10 . An inductance value switching circuit  46  consists of the oscillator coils L 7 , L 8  and switch SW 10 . Variable capacitance diode D 11  is connected to the other end of oscillator coil L 7 . A capacitance ratio switching circuit  47  is connected to a junction of variable capacitance diode D 11  and inductance switching circuit  46 . Capacitance ratio switching circuit  47  is formed by connecting capacitors C 9  and C 10  in series while connecting a switch SW 11  in parallel to capacitor C 10 . 
     The variable capacitance diodes of the radio frequency tuning circuit and local oscillator tuning circuit  44  have the same voltage versus capacitance characteristic and are connected to a common tuning control voltage source. 
     Likewise, in case of shipment of the FM radio receiver having such a structure to the US, in the local oscillator tuning circuit  44 , switch SW 10  of inductance switching circuit  46  and switch SW 11  of capacitance ratio switching circuit  47  are set ON. In case of shipment to Japan, switch SW 10  of inductance switching circuit  46  and switch SW 11  of capacitance ratio switching circuit  47  are set OFF. 
     In this embodiment, the oscillator coils L 7  and L 8  are formed separately to avoid magnetic coupling between coils L 7  and L 8 . 
     FIG. 5 is a characteristic diagram showing a tracking characteristic of the FM radio receiver of the present invention. The horizontal axis indicates reception frequency and the vertical axis indicates tracking error. 
     In case of a receiver configured for shipment to the US, the FM radio receiver is set to upper heterodyne as shown by  51 . In case of a receiver configured for shipment to Japan, it is set to lower heterodyne as shown by  52 . 
     While preferred embodiments of the FM radio receiver of the present invention have been described above, this invention is not limited to these embodiments. For example, instead of using a common tuning control voltage source, this invention can also be effective when the tuning control voltage supplied to the radio frequency tuning circuit is derived through a D/A converter circuit where a precisely corrected tuning control voltage is produced based on digital data stored in a memory device.